A preliminary investigation of the corrosion and stress ... · PROGRAMELEMENT.PROJECT'as* AREA ftWORKUNITNUMBERS 12.REPORTDATE September1981 IS.NUMBEROFPAGES 84 IS.SECURITYCLASS,(ol

Calhoun: The NPS Institutional Archive

Theses and Dissertations Thesis Collection

1981-09

A preliminary investigation of the corrosion and

stress-corrosion susceptibility of thermomechanically

processed high magnesium aluminum magnesium alloys

Beberdick, Larry Edward

Monterey, California. Naval Postgraduate School

http://hdl.handle.net/10945/20649

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THESISA PRELIMINARY INVESTIGATION OF THE

CORROSION AND STRESS-CORROSION SUSCEPTIBILITYOF THERMOMECHANICALLY PROCESSED HIGH MAGNESIUM

ALUMINUM MAGNESIUM ALLOYS

by

Larry Edward Beberdick

September 1981

Thesis Advisor T. R. McNellev

Approved for public release; distribution unlimited

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REPORT DOCUMENTATION PAGEHFSJff NUMIIK 2. OOVT ACCESSION NO

4. title rmnsubutf) ^ Preliminary Investigationof the Corrosion and Stress - CorrosionSusceptibilitv of ThermomechanicallvProcessed High Magnesium, Aluminum Mag-nesium Allovs .

7. AuTHORi'tJ

Larrv Edward Beberdick

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IS. SUPPLEMENTARY NOTES

It. KEY WORDS (Contlnvo on rowori* .<<*• tl nocomoorr anW l4onilty or block ntmtbor)

Thermomechanicallv Processed, High Strength, Stress Corrosion,Aluminum, Corrosion, Aluminum Magnesium, Marine

20. ABSTRACT (Cantlnuo mn MMIM •<*• II noeooomrr mn4 Itontltf 07 block mmbot)

The stress corrosion cracking susceptibility and generalcorrosion characteristics of four thermomechanicallv processedhigh-Magnesium, Aluminum-Magnesium alloys were evaluated andcompared to those of "0"6-T6. Results obtained from stress-corrosion testing and from tension testing after stress-corrosexposure indicate that these 8 - 1 °i Mg alloys are less susceptibleto stress-corrosion cracking than 7075-T6. The addition of Cu

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or Cu and Mn to a 10% Mg alloy raises strength, homogenizes themicrostructure and reduces the tendency of such an alloy toexhibit intergranular cracking and exfoliation, especially ina sensitized condition. Results of accelerated general corrosiontesting and marine exposure both indicate that binary 83 Mg and10% Mg alloys are highly resistant to corrosion. Alloying withCu or Cu and Mn accelerates weight loss but to a lesser degreethan observed for 7075-T6.

DD Form 1473 :

1 Jan 73S/ N 0102-<>14-o601 itcu«i*w clami*ka*io»i o» ?hii »»afi^*»- o«t* i«i»»»*>


A Preliminary Investigation of the Corrosion andStress-Corrosion Susceptibility of Thermo-mechanically Processed High Magnesium,

Aluminum Magnesium Alloys

by

Larry Edward BeberdickLieutenant, United States Navy

B.S.E.E., Purdue University, 1974

Submitted in partial fulfillment of therequirements of the degree of

MASTER OF SCIENCE IN MECHANICAL ENGINEERING

from the

NAVAL POSTGRADUATE SCHOOLSeptember 1981

DUDL r

NAVi ,TE SCHOOLMONV- . V3S40

ABSTRACT

The stress corrosion cracking susceptibility and general

corrosion characteristics of four thermomechanically processed

high-Magnesium, Aluminum -Magnesium alloys were evaluated and

compared to those of 7076-T6. Results obtained from stress-

corrosion testing and from tension testing after stress-

corrosion exposure indicate that these 8-10% Mg alloys are

less susceptible to stress -corrosion cracking than 7075-T6.

The addition of Cu or Cu and Mn to a 10% Mg alloy raises

strength, homogenizes the microstructure and reduces the

tendency of such an alloy to exhibit intergranular cracking

and exfoliation, especially in a sensitized condition. Results

of accelerated general corrosion testing and marine exposure

both indicate that binary 8% Mg and 10% Mg alloys are highly

resistant to corrosion. Alloying with Cu or Cu and Mn acceler-

ates weight loss but to a lesser degree than observed for

7075-T6.

TABLE OF CONTENTS

I

.

INTRODUCTI ON - 12

A

.

BACKGROUND - 12

B. PREVIOUS WORK- 13

C. THIS WORK 15

II. EXPERIMENTAL PROCEDURE-- 17

A. PROCESSING AND SAMPLE PREPARATION 17

B. ANNEALING FOR SCC SENSITIZATION- ---18

C. TENSION TESTING 18

D. STRESS CORROSION TESTS--- ---19

E. GENERAL CORROSION TESTING-- 21

F. POST-EXPOSURE TESTING 22

III. RESULTS AND DISCUSSION- 24

A. THE AS -ROLLED AND AS -RECEIVED MATERIAL 2 4

B. THE INFLUENCE OF ANNEALING 26

C. STRESS CORROSION TESTING - 27

1. Effect of Stress Corrosion Exposure on theAs-Rolled and As-Received Condition 29

2. Effect of the Anneal on the Response tothe Stress -Corros ion Exposure 31

3. Summary of the Effect of Exposure:Consideration of the Alloys 35

D. THE GENERAL CORROSION TEST 57

IV. CONCLUSIONS AND RECOMMENDATIONS 41

A . CONCLUS I ONS 41

B . RE COMMENDAT I ONS 41

LIST OF REFERENCES 82

INITIAL DISTRIBUTION LIST 84

LIST OF FIGURES

1. A partial Aluminum -Magnesium binary phase diagramapplicable to the Aluminum-Magnesium alloys of thiswork. - 47

2. A schematic representation of the processing methodapplied to the high-Magnesium, Aluminum-Magnesiumalloys of this work. - 48

3. General and stress corrosion test tank. -49

4. Stress corrosion sample rack. 50

5. U-bend stress corrosion sample and holder. 50

6. Apparatus for making the guided U-bend. 51

7. Three-point bent-beam stress corrosion sampleand holder. 51

8. Apparatus for measuring the surface deflection ofthe three-point bent-beam samples. 52

9. General corrosion sample and holder.-- 52

10. Rim assembly with general corrosion sample andholder in place. 53

11. Spray nozzle. - 54

12. Typical installation of spray nozzle with associatedhoses for air and salt solution.- 54

13. Exposure rack and samples (3 of each alloy) usedaboard the R/V Acania. 55

14. The as-rolled material.-- 56

15. The as-rolled material after a seven-day, 100 C

sensitizing anneal

.

57

16. Histogram comparison of the mechanical propertiesof the Al-Mg alloys in the as-rolled conditionto the properties after a seven-day, 100°C anneal. --58

17. Percent of retained properties of the as-rolledmaterial after a seven-day, 100°C anneal. 59

18. The as-rolled as received material after stresscorros ion exposure .-- 60

19. Histogram comparison of the mechanical properties ofthe as-rolled/as-received material to the propertiesafter stress corrosion exposure. 63

20. Percent of retained properties of the as-rolled/as-received material after stress corrosionexposure .

- 64

21. The annealed alloys after stress corrosionexposure . 65

22. Histogram comparison of the mechanical propertiesof the annealed, polished and stress corrodedcondition to the properties in the annealedcondition. - 67

23. Percent of retained properties of the annealedmaterial after polishing and stress corrosionexposure. - --68

24. Histogram comparison of the mechanical propertiesof the annealed unpolished stress corroded condi-tion to the properties in the annealed condition. 69

25. Percent of retained properties of the annealedmaterial after stress corrosion exposure forthe unpolished samples. 70

26. Mechanical properties of the 8% Mg binary alloyin the as-rolled condition and after stresscorrosion exposure. 71

27. Mechanical properties of the 10% Mg , 0.4% Cu, 0.5%Mn alloy in the as-rolled condition and after stresscorrosion exposure . 72

28. Mechanical properties of the 10% Mg , 0.4% Cu alloyin the as-rolled condition and after stresscorrosion exposure . 73

29. xMechanical properties of the 10% Mg binary alloy inthe as-rolled condition and after stress corrosionexposure. - 74

30. Mechanical properties of 7075-T6 in the as-receivedcondition and after stress corrosion exposure. 75

31. Results of the first general corrosion salt spraytest. 76

32. Results of the second general corrosion saltspray test. - - 77

33. Surface topography of the general corrosion samplesafter 1000 hours of exposure. 78

34. The end sections of the general corrosion samplesafter 1000 hours exposure. - - --79

LIST OF TABLES

I. Alloy Composition 43

II. Mechanical Properties of the As Received 7075-T6and the Warm Rolled Alloys 44

III. Summary of Stress Corrosion Results in 3.51 NaClSolution, Alternate Immersion for 740 Hours 45

IV. Weight Loss Data Measured After ExposureTimes As Noted- 46

10

ACKNOWLEDGEMENT

It is appropriate at this time to express my most sincere

appreciation to those individuals who have provided help and

guidance, and especially those who supported me during this

ordeal. I want to thank Mr. Richard Schmidt of Naval Air

Systems Command for his support in this research. I want

to thank all of the shop and lab personnel who worked in

developing the test apparatus. I would also like to thank

Dr. J. Perkins for being my mentor in corrosion matters,

and Dr. T. McNelley my mentor in microstructural matters,

as well as being my advisor. The largest assist came from

fellow thesis students who were ever present and acted as

sounding boards for observational ideas. My most sincere

and heart felt appreciation go to my family, whom I neg-

lected during these last long months, for their constant

encouragement, and prayers. Without my wife Jan's loving

support, and my children's (Christine and Nathan) unwarranted

patience, this document would never have been produced.

11

I. INTRODUCTION

A. BACKGROUND

The alloys which are the subject of this research are

essentially higher Magnesium content alloys than the 3XXX

series Aluminum -Magnesium alloys currently in use. They are

wrought alloys, and derive their strength through inter-

mediate temperature thermomechanical processing. Wrought

Aluminum base alloys are generally classed as either heat

treatable or non-heat treatable. The three alloy systems

from which the heat treatable alloys are derived are the

Aluminum-Magnesium-Silicon system (6XXX) , the Aluminum-

Copper system (2XXX) and the Aluminum- Zinc-Magnesium system

(7XXX) . Strength generally increases in the order given,

the 7XXX alloys being presently the highest strength Aluminum

alloys in commercial use. The non-heat treatable alloys are

based on essentially unalloyed Aluminum (1XXX) , the Aluminum-

Manganese system (3XXX) or the Aluminum-Magnesium (Al-Mg)

system (5XXX) . In general these non-heat treatable alloys

derive their strength from solid-solution hardening and

possibly strain hardening.

Conventional wrought 5XXX alloys contain up to 6 percent

Mg and are generally considered to be low to medium strength

alloys possessing good corrosion resistance, fatigue resistance,

ductility and weldability. Their strength, however, is

12

substantially lower than the high strength alloys (e.g.

7XXX alloys) . The strength of such 5XXX alloys can be raised

by adding more Mg; however, using conventional processing

practice, hot working possibly followed by cold working and

annealing, numerous problems arise, including poor resistance

to corrosion and stress corrosion and difficulties in cold

working.

Conventional high strength alloys, such as the 7XXX alloys,

offer high strength but also often suffer from one or more of

such problems as poor fatigue resistance and fracture toughness

and poor stress corrosion characteristics. Efforts to improve

performance in one area (e.g. corrosion resistance) often lead

to a sacrifice in another area (e.g. strength). These high

Mg , Al-Mg alloys would be similarly subject to limitations

and trade-offs and should be viewed as complimenting rather

than replacing existing alloys.

B. PREVIOUS WORK

The thermomechanical processing (TMP) method used in this

research was developed and characterized in this laboratory

as discussed by Johnson [Ref. 1]. That work [Ref. 1] consid-

ered both binary Al-Mg alloys and Al-Mg alloys modified with

intentional additions of Cu, Mn and Cu together with Mn

.

Figure 1 is a partial Al-Mg phase diagram illustrating the

ranges of Mg content and temperature of interest in this pro-

cessing method and Figure 2 is a schematic of the essential TMP

.

13

The important features of this processing method are: 1)

solution treatment and hot working to dissolve all soluble

constituents and homogenize the microstructure ; 2) quenching

to retain Mg in solution; and 3) reheating to a temperature

below the solvus for Mg and extensive warm working to develop

a dislocation substructure and stable dispersion of the inter-

metallic beta (Alg Mgc) phase which precipitates under such

conditions. The warm rolling below the solvus temperature

typically about 300 C for Mg in the alloy results in a homo-

genous dispersion of equiaxed beta particles in the micro-

structure. Depending on the details of the TMP and alloy,

yield strengths vary from 50 Ksi (345 MPa) to 90 Ksi (620 MPa)

,

ultimate tensile strengths vary from 65 Ksi (450 MPa) to 100

Ksi (690 MPa) and ductility from 18% elongation down to 5%

elongation.

In a recent study, Cadwell [Ref. 2] evaluated the fatigue

characteristics of two alloys as a function of processing

history. His study was especially concerned with the quenching

step between the solution treatment and the hot working step

and the final warm rolling of the alloy. The significant

observation was that a slower quench (oil as opposed to water)

leads to improved fatigue behavior under high cycle fatigue

(HCF) conditions.

The S-N curves for a 10% Mg alloy, oil quenched between

solution treatment and warm working, suggested a fatigue limit

at a stress amplitude above 30 Ksi, corresponding to a fatigue-

14

strength to ultimate-strength ratio of 0.47. In contrast, the

alloy subjected to a water quench before warm rolling appeared

to exhibit a lower fatigue strength and less tendency for

flattening of the S-N surve under similar HCF conditions.

This observation was suggested to be the result of precipita-

tion of some relatively course beta particles in the slower

quench and these served to homogenize slip, postponing fatigue

crack initiation. The fatigue resistance of these alloys,

then may be considered as excellent although sensitive to

microstructural variations. This current work follows that

of Johnson [Ref. 1], as did that of Cadwell [Ref. 2], but

with regard to stress corrosion cracking (SCC) susceptibility

as the object of concern.

C. THIS WORK

It has been suggested [Ref s . 3,4,5,6] that preferential

precipitation along grain boundaries in the form of a continuous

precipitate film causes susceptibility to SCC in Al-Mg alloys.

Consequently, two ways have been proposed to produce SCC resist

ant microstructures : one, keep the grain boundaries free of

the continuous film of precipitates, or two, stimulate pre-

cipitation throughout the grains. The first approach is

successful if the Mg content of the alloy is below 3 ?j . In the

higher Mg , Al-Mg alloys a network of continuous grain boundary

precipitates can develop during precipitation at ambient temper

atures . To simulate long term ambient aging, a sensitizing

15

anneal, consisting of heating to 100 C for 7 days, is often

used. By comparison, it is not known exactly how fast contin-

uous networks develop at ambient temperatures, but the prob-

able time is as great as 50 years [Ref. 3].

One way of achieving a homogenization of the precipitates

is the TMP reported by Johnson [Ref. 1] . Hence a study into

the corrosion and stress corrosion characteristics of these

warm-worked high Mg alloys is indicated.

This work is intended to be a preliminary study of the

general corrosion and stress corrosion susceptibility of a

series of warm-worked Al-Mg alloys. Specific areas investi-

gated are: the SCC susceptibility of these alloys in the

as-rolled condition in comparison to 7075-T6 alloy; the effect

of the sensitizing anneal on the mechanical properties of

these Al-Mg alloys; the changes in the SCC susceptibility

resulting from the sensitizing anneal; and finally, how these

high strength Al-Mg alloys rank among themselves and how they

compare to the 7075-T6 alloy in a general corrosion environment

16

II. EXPERIMENTAL PROCEDURE

A. PROCESSING AND SAMPLE PREPARATION

Four Al-Mg alloys were studied in this research, their com-

positions are given in Table I. The alloys were obtained as

direct-chill castings from ALCOA, Inc., and are from the same

series used by Johnson [Ref. 1], who gave the details of the

original manufacture. The processing of these materials was

identical to that of Johnson [Ref. 1]. The alloys not contain-

ing Mn were solution treated at 440 C for 9 hours, forged, re-

heated to 440 C and then quenched. They were then reheated to

300 C and warm rolled from approximately 1.0 inch (25 mm) thick-

ness to a final thickness of 0.125 inch (3.2 mm). The material

containing Mn was given a second solution treatment prior to

warm rolling. This second treatment was at 490 C for 3 hours

and was intended to dissolve a Mn-containing phase not taken

into solution during the initial 440°C treatment. The 7075-T6

material was obtained as 0.10 inch (2.5 mm) thick sheet.

All subsequent test coupons were prepared from the as-rolled

or as-received sheet. These coupons were 3.0 in. x 0.0625 in.

x 0.05 in. (76.2 mm x 15.9 mm x 1.3 mm) in size. Due to the

limitations imposed by the processing equipment, all samples

were prepared with the long dimension parallel to the rolling

direct ion.

1"

Except as noted, all samples were buffed with 600 grit

paper and then polished with jeweler's rough. After polishing,

the samples were wiped clean with acetone, ultrasonically

cleaned in ethanol and then dried in warm air to ensure a

smooth, clean grease-free surface. In all cases the samples

were subject to the corrosive or stress corrosive environment

within an hour of final preparation.

B. ANNEALING FOR SCC SENSITIZATION

Conventionally processed Al-Mg alloys with more than 31

Mg are presumed to be susceptable to aging [Ref . 8] . In order

to determine if aging was a problem in these warm-worked high-

magnesium alloys, a series of machined test coupons for each

of the warm-rolled alloys were given a sensitizing anneal

[Ref. 9]. The 7075-T6 alloy was not annealed as the T6 temper

is already the most susceptible to SCC [Ref. 7]. The coupons

were annealed in a laboratory oven at 100 C for seven days

(168 hrs) . This anneal resulted in some oxidation of the

machined surface. One-half of the annealed samples for each

alloy were then tested, as described later, with this oxide

intact and the other half were polished as above, prior to

the SCC exposure.

C. TENSION TESTING

Stress -strain testing was conducted on test coupons repre-

senting several conditions examined in this research. Coupons

were tested without machining of a reduced gage section, both

18

for the as-rolled and for materials following SCC exposure.

This was done to avoid removing portions of the sample which

may have undergone degradation. Samples, then, which failed

within the grips of the test machine were disregarded. The

tension tests were accomplished using an Instron Model TT-1

test machine set at a crosshead speed of 0.2 in/min (5.1 mm/

min) for all tests.

D. STRESS CORROSION TESTS

Stress corrosion testing was accomplished by alternate

immersion in a 3.5% NaCl solution. The test cycle was 10 min-

utes immersed followed by 50 minutes air drying [Ref s . 10,11

and 12]. The test chamber, shown in Figure 3, was constructed

of Marine-grade plywood and painted with a water-sealing paint

Tank dimensions were 30 inches x 12.5 inches x 50 inches (762

mm x 317 mm x 1270 mm) . The salt water in the tank was main-

ained at a depth of 3.5 - 4.0 inches (89-102 mm), i.e. approx-

imately 20-24 gallons (76-91 1) of liquid.

The sample holder rack (Figure 4) was constructed of

Plexiglas and Lexan, and joining was accomplished using epoxy

cement and stainless steel screws. A s ilicone-adhesive caulk-

ing compound was used to isolate the screws from the environ-

ment. This rack was attached to a pneumatic actuator. This

actuator itself was attached to the rear of the tank and, with

appropriate timing controls, provided the up and down motion

of the rack to accomplish the alternate immersion of the test

samples

.

19

Test coupons were stressed in either of two ways. One

was a guided U-bend and the other was a three-point controlled

bending of the coupon. The U-bend samples (Fig- 5) were formed

into a "U" configuration with the procedure and apparatus

shown in Figure 6 following the method outlined in Ref. 13.

These samples were intended to provide a qualitative comparison

of the alloys in the stress corrosion environment. The stress

axis is the longitudinal axis of the material. Three-point

loaded, bent-beam samples are shown in Figure 7. Stress levels

employed were 0.65, 0.8 and 0.95 o'f the ultimate tensile

strength of the material, and were intended to provide a quanta

tive measure of the stress corrosion susceptibility of the

alloys tested. The desired stress level was obtained by mea-

suring the vertical deflection of the center of the three-point

loaded, bent-beam as shown in Figure 8 and then converting the

deflection to stress [Ref. 14].

Three stress corrosion tests were run, each for "40 hours

or until failure occurred, at which time only the failed sample

was removed. The first set of SCC samples exposed, after

polishing, were in the as-rolled/ as -received condition. The

second set of SCC samples exposed, also after polishing, were

in the annealed condition. The third run was done without re-

moving the protective oxide built up during the seven-day

anneal. These samples were, however, subjected to the same

acetone wipe and ethanol ultrasonic bath to ensure that the

only variable in this test was the presence of an oxide layer.

20

E. GENERAL CORROSION TESTING

The general corrosion test was conducted using a 3.5%

NaCl spray following the procedures given in Refs. 15 and 16.

The chamber used in this test was contained within the tank

used for the SCC tests. However, the spray chamber was iso-

lated from the alternate immersion test by Plexiglas baffel

plates. Plexiglas sheets also covered the top of the spray

chamber. Figure 3 also shows the 2.0 inch (51 mm) diameter

exhaust ducts placed at the rear of each spray chamber to

prevent an excessively humid environment in the room in which

the apparatus was operating. Within each chamber a 20 inch

(510 mm) diameter stainless steel rim rotated at 1/3 rpm

in a horizontal plane. The rim was painted with water sealing

paint to isolate it from the samples and salt spray. Test

samples were cleaned as before and mounted on Lexan holders

using a plastic screw (Fig. 9). These holders permitted in-

sulation between adjacent samples and allowed easy access with-

out interrupting the test. Figure 10 shows a rim assembly with

a sample and holder installed. Each chamber contained a glass

spray atomizer (Fig. 11), which was forced-fed with 3.5% NaCl

solution at 13 ml/min. Air was also supplied at a pressure

of 5 psig. A single stream of solution was supplied through

the center of the nozzle while air was injected in a conical

pattern to mix with the solution, forming a fine mist through

which the samples rotated. Figure 12 shows a spray nozzle

21

installation with its associated tubing for air and water.

The general-corrosion samples were removed after 5, 10, 50,

100, 500, and 1000 hour points for weight loss measurements.

Additionally, three samples of each alloy were cleaned as

before and subject to 1500 hours of actual marine exposure

on the Naval Postgraduate School research ship R/V ACANIA.

Figure 13 shows the samples and mount before installation

aboard the R/V ACANIA.

F. POST-EXPOSURE TESTING

Upon completion of testing all samples were immediately

rinsed in warm tap water and scrubbed with a soft bristle

brush to remove the salt deposits. They were then immersed

for three minutes in a 70% Nitric acid solution to remove

corrosion products. The general corrosion samples were then

weighted to determine weight loss during exposure. The 1000-

hour general corrosion samples were then sectioned for metal-

lographic examination.

The tensile properties of the bent-beam coupons which had

not failed in the stress corrosion test were obtained at the

conclusion of the stress corrosion exposure. This testing

was especially important in this research and was used to pro-

vide a ranking of these materials based on degradation of prop-

erties resulting from the stress corrosion exposure. This

method of evaluation of Al alloys after exposure to a stress

corrosive environment was suggested by Budd and Booth [Ref. 17

who conducted electro-mechanical stress corrosion tests on

22

various Aluminum alloys, and determined that the mechanical

properties retained after stress corrosion exposure were

indicative of the relative SCC susceptibility of the alloys

tested. Such rankings obtained in this research cannot be

used to predict actual service lives as appropriate long-term

testing was not undertaken here. Nonetheless, such rankings

would suggest the relative lifetimes of different materials

based on the degree of degradation observed after stress-

corrosion exposure.

23

III. RESULTS AND DISCUSSION

A. THE AS -ROLLED AND AS -RECEIVED MATERIALS

The microstructures and mechanical properties of the warm-

rolled alloys processed for this investigation are similar to

those reported by Johnson [Ref. 1] in his study of this thermo

mechanical processing method. Figure 14 shows micrographs of

longitudinal sections for the four warm-rolled alloys investi-

gated. The 8% Mg and 101 Mg alloys both exhibit highly elong-

ated grains with some precipitation of the beta (3) phase in

grain boundaries. Fewer and smaller 3 particles are present

in the 8% Mg alloy (Fig. 14a) when compared to the 101 Mg

alloy (Fig. 14b). The 8% Mg alloy exhibits generally less pre

cipitation within grains whereas the 10% Mg alloy appears to

have precipitation within grains and concentrated along slip

bands. Cadwell [Ref. 2], in his study of the fatigue charact-

eristics, also examined this 10% Mg alloy by transmission

electron microscopy. While still tentative, the results ob-

tained are consistent with the results of this investigation,

i.e. that precipitation occurs on slip bands in the 10% Mg

alloy. Cadwell [Ref. 2] also observed a fine (0.5 - 0.5 ^m)

dislocation cell structure in regions distant from the precip-

itated 3 particles.

The addition of 0.4% Cu to a 10% Mg alloy results in a

more homogenous microstructure after warm rolling (Fig. 14c).

24

The 3 phase is still evident in grain boundaries but precipita-

tion is more uniform within the grains, with much less tendency

to concentrate in slip bands. The further addition of 0.5%

Mn to a 10% Mg - 0.4% Cu alloy results in a still greater

degree of homogenizat ion as shown in Figure I4d. There is now

no optical microscopy evidence of either grain boundary or

slip band preference for precipitation. The size of the 3

particles in the Cu and the Cu - Mn containing alloys is about

the same as in the 10% Mg binary alloy, suggesting a homogeniz-

ing, as opposed to refining, effect of these additions. The

improved homogeneity evident in the alloy containing both Cu

and Mn may not be attributable to alloying effects alone.

An additional 3 hour solution treatment of this alloy was

necessary at 490 C to dissolve a Mn - containing phase present

in the cast condition. This increased solution treatment

temperature would contribute as well to a more homogeneous

precipitation in subsequent rolling.

The results of tension testing of the 7075-T6 alloy and

the four Al - Mg alloys are presented in Table II. This data

illustrates the strengthening effect of increased Mg content

under identical working conditions. The Cu addition results

in a further strength increase, most probably the result of

increased solid solution strengthening and more uniform sub-

structure with the more uniform 3 distribution. As of this

writing, transmission electron microscopy studies are being

undertaken to examine the influence of such Cu and Mn additions

25

on 3 precipitation and substructure formation in these alloys

.

The Mn addition results in no further increase in strength,

even though microstructural homogeneity is enhanced, and also

results in some ductility loss.

B. THE INFLUENCE OF ANNEALING

The microstructural effect of the seven-day, 100°C anneal

is presented in Figure 15. Annealing results in additional 3

precipitation in both the 8% Mg and 10% Mg alloys (Figs. 15a

and b, respectively). The most notable effect in the 8% Mg

alloy is increased precipitation on slip bands within grains,

although there is also some increased grain boundary precipita-

tion. The 10% Mg alloy is similarly affected, although in-

creased grain boundary precipitation appears to be more the

case with this material. In both alloys, the 3 particles have

coarsened. The 10% Mg - 0.4% Cu alloy appears to be somewhat

less affected by the anneal (Fig. 15c), although it now appears

to pit in the electrolytic etch. The 10% Mg - 0.4% Cu - 0.5%

Mn alloy (Fig. 15d) shows no discernable microstructural effect

of this anneal.

The anneal resulted in reduced strength and increased

ductility for the 8% Mg and 10% Mg alloys as illustrated in

Figure 16. The effect is particularly notable for the 10%

Mg alloy for which ductility increased by almost one-half

as a result of the anneal. In contrast, the alloys contain-

ing the Cu addition or the Cu and Mn additions exhibited little

26

loss of strength and some decrease in ductility. The decreased

strength and enhanced ductility noted for both the 8% Mg and

101 Mg alloys is consistent with the transmission electron

microscopy results reported by Cadwell [Ref. 2], A substruc-

ture not stabilized by the 3 particles, given their non-uniform

distribution, would be able to recover and coarsen, resulting

in reduced strength and improved ductility. The more uniform

distribution of the 3 particles in the Cu or Cu and Mn contain-

ing alloys would result in a more stable substructure, less

likely to coarsen during such an anneal. Figure 17 presents

the same data as Figure 16, but in terms of the percentage

of properties retained after the anneal.

The alloys may be ranked from those least affected to most

affected by this annealing treatment. From the microstructure

data such a ranking would be:

1 % Mg - 0.4% Cu - 0.51 Mn ; 1 % Mg - 0.4% Cu ; 8 % Mg ; 1 % Mg

Based on the mechanical property data as represented in Figure

17 , the ranking is

:

10% Mg - 0.4% Cu; 8% Mg ; 10% Mg ; 10% Mg - 0.4% Cu - 0.5% Mn

The latter ranking presumes a material to be more adversely

affected if ductility drops without gain in strength than if

strength decreases with a corresponding increase in ductility.

Hence these rankings are substantially different.

C. STRESS CORROSION TESTING

Susceptibility to stress corrosion cracking for the four

warm rolled alloys and 7075-T6 was evaluated by alternate

27

immersion of stressed coupons in a 3.5% NaCl solution. Beam-

type samples were loaded in three-point bending and additional

samples were subject to a guided U-bend prior to testing. The

alloys were all tested in the as-warm-rolled or as-received

condition. Two additional tests were conducted on the warm rolled

Al-Mg alloys after the annealing treatment. Prior to one test,

the oxide layer developed during the anneal was removed. In

the other test the oxide layer was left intact. The 7075-T6

was not annealed as the -T6 temper is the condition most prone

to stress -corrosion cracking [Uef. 7]. Total test duration

was 740 hours in all cases.

The results of these tests are summarized in Table III.

The most significant observation is that the only failures at-

tributable to stress corrosion occurred in three of the four

7075-T6 coupons subject to the guided U-bend. The 10% Mg - 0.4%

Cu - 0.5% Mn alloy possessed insufficient ductility to conduct

the guided U-bend test, either as-rolled or after the seven-

day, 100°C anneal. The ductility loss incurred in the 10% Mg

- 0.4% Cu alloy as a result of the anneal also precluded the

guided U-bend test of this alloy in the annealed condition.

From these data it can be concluded only that, for these test

conditions, the warm-rolled Al-Mg alloys are not more suscep-

tible to stress corrosion than the 7075-T6 alloy. In order to

determine the effect of the stress corrosion test exposure,

and to provide a ranking of the effect of this exposure, metal-

lographic examination and mechanical testing of the test coupons

was conducted at the conclusion of the 740 hour stress corrosion

exposure

.

28

1 . Effect of Stress Corrosion Exposure on the As-Rolledand As -Received Condition

Longitudinal metallographic sections, from beam-type

specimens of the five materials tested, are shown in Figure 18.

In all cases, the sections include the exposed surface of the

coupons in order to demonstrate the form of the stress corrosion

attack on these materials. The 8% Mg alloy (Fig. 18a) was

lightly affected, with a slight tendency for intergranular

cracking extending from pitting. The 10% Mg alloy (Fig. 18b)

exhibits a much more severe intergranular attack with extensive

cracking spreading from surface pitting in the manner of exfoli-

ation. This form of attack also occurs for the 10% Mg - 0.4%

Cu alloy, although the tips of the intergranular corrosion

cracks appear blunted in this alloy, as shown in Figure 18c.

The last of the warm-worked alloys examined, the 10% Mg - 0.4%

Cu - 0.5% Mg material, was subject to an entirely different

form of attack. Figure 18d shows shallow, rounded pits to

have formed, and no tendency for intergranular cracks to

spread from these pits. The 7075-T6 alloy (Fig. 18e) was

severely affected by the stress corrosion exposure, with

intergranular cracking spreading from irregular pitting in

the alloy.

The results of mechanical testing following this stress

corrosion test are summarized in Figure 19. The strength and

ductility of the 3% Mg and 10% Mg alloys were little affected

by the exposure, the 8% Mg alloy being the least affected of

29

all. The tensile properties, especially ductility, of the re-

maining alloys were more severely reduced by the exposure.

To aid in ranking the effect of the exposure and hence the

severity of the attack, these data were replotted in Figure

20 in terms of percentage of property (strength and ductility)

retained after exposure as compared to beforehand.

As before, the alloys may be ranked first on the basis

of the apparent severity of the microstructural degradation

from least to most severely degraded:

8% Mg; 10% Mg - 0.4% Cu -0.5% Mn; 101 Mg ; 10% Mg - 0.4% Cu;

7075-T6

Similarly, the ranking based on mechanical property degradation,

from least to most severely affected, is

:

8% Mg; 10% Mg; 10% Mg - 0.4% Cu - 0.5% Mn ; 10% Mg - 0.4% Cu

;

7075-T6

These rankings differ only in the reversal of the positions

of the 10% Mg alloy and the 10% Mg - 0.4% Cu - 0.5% Mn alloy.

The rankings given above are consistent with the result

of the stress -corros ion test itself, in that the 7075-T6 alloy,

most severely degraded by the stress corrosion exposure, was

also the only alloy for which any actual stress corrosion fail-

ure occurred. The warm rolled alloys which exfoliated during

exposure were subsequently pulled in tension parallel to the

original rolling direction, which is the long direction of the

grains. For this reason, the severity of the cracking mode

noted for the 10% Mg and 10% Mg - 0.4% Cu alloys is not reflected

30

in these rankings. Such cracking would certainly result in

a more pronounced effect on properties if measured in the

through-thickness direction. In contrast, the Mn containing

alloy, which exhibited only rounded pitting, would not likely

be as strongly affected in the through thickness orientation.

As noted previously, the Mn-containing alloy was pro-

cessed differently in that an additional, solution treatment

was necessary. The microstructural homogeneity evident in

this alloy may result in part from this as well as the Mn

addition itself. In turn, the altered form of environmental

attack may also be the result of the different process as

well as the Mn addition. This suggests study of the influence

of solution treatment temperature for the other alloys examined

here to determine the separate effects of this parameter and

the Mn addition. As noted by Johnson [Ref . 1] , this alloy may

also be processed to considerably higher tensile strength (up

to 100 KSI (690 MP a)) by reduced warm rolling temperatures.

The influence of this aspect of this processing method also

was not investigated here.

2 . Effect of the .Anneal on the Response to the Stress-Corrosion Exposure

This second series of tests was conducted in two parts.

As noted, half of the annealed test coupons were polished prior

to exposure while half were tested with the oxidation resulting

from the anneal left intact. Examination of metallographic

samples from both parts of this test revealed no difference

31

in the form of the attack resulting from the presence or

absence of the oxide. Figure 21 shows micrographs of longitu-

dinal sections like those discussed previously in Figure 18.

This series is from the test of the annealed and polished

series. Comparison of these figures reveals a similar form

of degradation for three of the four alloys. The 8% Mg alloy

is still relatively unaffected by exposure (Fig. 21a). The

10% Mg alloy (Fig. 21b) again exhibits intergranular cracking

and exfoliation although the cracking is somewhat blunted in

comparison to the as-rolled condition. This micrograph also

shows a section through a blister and the exfoliation occurring

underneath the blister. The 10% Mg - 0.4% Cu alloy (Fig. 21c)

also exhibits intergranular cracking as before.

In the as-rolled condition, the 10% Mg - 0.4% Cu -

0.5% Mn alloy was subject to formation of shallow rounded

pits but with no cracking extending from these pits. The

anneal before stress corrosion exposure results in better

surface retention during exposure with almost no pitting in

evidence and, again, no exfoliation (Fig. 21d) . This result

is consistent with the apparent stability of the microstructure

of this material during the anneal.

The absence or presence of the oxide layer during

stress corrosion testing did affect the resultant mechanical

properties, most notably those of the 10% Mg alloy. Figure

22 compares the annealed mechanical properties to those of the

annealed and exposed test coupons. The oxide layer formed in

32

the anneal had been removed prior to exposure for this sample

series. The 10% Mg alloy now has been severely degraded in

ductility. As noted microstructurally , this alloy also showed

evidence of blistering in addition to exfoliation and this

additional factor is likely involved in the large degradation

of ductility observed. The 8% Mg alloy exhibits little degrada-

tion of properties in contrast to the higher Mg alloy. The 10%

Mg - 0.4% Cu and the 10% Mg - 0.4% Cu-0.5% Mn alloys are

affected to a slightly lesser extent by exposure after anneal-

ing than by exposure in the as-rolled condition. This is better

seen by comparing Figure 23 to Figure 20. Both of these figures

represent data as percentage of property retained after exposure,

Figure 23 for the annealed (and polished) condition and Figure

20 for the as-received or as-rolled condition. This comparison

also illustrates the severe degradation experienced by the 10%

Mg alloy as a result of annealing.

Mechanical test results for the test series annealed

and exposed with the oxide layer intact are given in Figure 24.

Examination of these data reveal slightly better retention of

mechanical properties with the oxide layer intact when compared

to the data of Figure 22. The same conclusion is reached if

data is represented in terms of percentage of property retained

after exposure, as shown in Figure 25, and if comparison is

made to the data of Figure 23.

These results again suggest a ranking, now considering

only the four warm rolled alloys, of the effect of stress corrosion

33

exposure on the annealed condition. Based on the micro-

structural data a ranking from lease affected to most affected

is :

8% Mg; 10% Mg - 0.4% Cu - 0.5% Mn ; 10% Mg - 0.4% Cu;

10% Mg.

Ranking similarly based on the mechanical test results of

Figure 23, for the annealed and polished test series is:

8% Mg; 10% Mg; 10% Mg - 0.4% Cu; 10% Mg - 0.4% Cu - 0.5%

Mn; 10% Mg.

Only the position of the 10% Mg alloy is changed in such a

ranking if the annealed, unpolished series (Fig. 25) is con-

sidered. This alloy would now fall after the 8% Mg alloy:

8% Mg; 10% Mg; 10% Mg - 0.4% Cu; 10% Mg - 0.4% Cu - 0.5% Mn

As noted previously, the 7075-T6 alloy was not annealed

and therefore not tested in the annealed condition. The anneal

was intended to sensitize the warm-rolled alloys and the -T6

temper is already the most sensitive to stress corrosion for

the 7075 alloy [Ref . 7] . Comparison of the response of the

annealed materials can therefore be made to the data for the

7075-T6 previously given in Figures 19 and 20. Such comparison

immediately reveals that the warm-rolled alloys are still not

as severely degraded by exposure of the materials after the

sensitizing anneal as the 7075-T6 alloy, exposed in the as

-

received condition.

34

3. Summary of the Effect of Exposure: Considerationof the Alloys

Figures 26-30 summarize the results of the evaluation

of these alloys following the stress corrosion test. These

data were examined to provide a final, overall ranking of

the effect of stress corrosion exposure. These data are pre-

sented in order of least affected to most affected as:

8% Mg (Fig. 26); 10% Mg - 0.4% Cu - 0.5% Mn (Fig. 27);

10% Mg - 0.4% Cu (Fig. 28); 10% Mg (Fig. 29); 7075-T6 (Fig. 30)

The position of the 8% Mg alloy (Fig. 27) in these

rankings has remained the same throughout this study. This

alloy, in general, is little affected by the stress corrosion

exposure with tensile strength unaffected by exposure and with

loss of small fraction, typically less than one-tenth, of the

ductility possessed prior to exposure. It has been noted,

however, that the grain structure of this alloy is highly

elongated and that some precipitation has occurred in the

grain boundaries. However, this precipitation is discontin-

uous and it is possible that the extensive warm rolling has

resulted in lesser local concentration gradients in material

nearby grain boundaries than would be the case if such pre-

cipitation were the result of diffusional processes alone.

In this sense, such extensive warm working as utilized with

these materials may provide less stress corrosion sensitive

microstructures than might be expected in such high Mg alloys

by virtue of microstructural homogenization

.

35

The data for the 10% Mg - 0.4% Cu - 0.5% Mn alloy,

summarized in Figure 27, also reveals this materail as not

severely degraded by stress corrosion exposure. This alloy

is the highest strength, lowest ductility alloy of the series

of warm-rolled alloys evaluated and, as such, might have been

expected to show more severe degradation than actually observed

Two features distinguish this material. One is the alloying

additions, Cu and Mn in particular, and the other is the

modified solution treatment utilized. The combination of these

factors has resulted in a very homogeneous microstructure after

warm rolling. This observation suggests closer attention both

to the solution treatment prior to warm rolling and to the

effects of the Mn addition.

The 10% Mg - 0.4% Cu alloy (Fig. 28) exhibits an ano-

malous effect. In the as-rolled condition, the ductility of

this material was reduced from 7.5% elongation to about 2%

elongation as a result of the exposure. Even though annealing

reduced ductility slightly, to about 6.5% elongation, a higher

ductility (about 3.5%) was noted after exposure than after

exposure as-rolled. This suggests that the anneal had a de-

sensitizing, rather than a sensitizing, effect in this case.

This is only a very tentative observation at this point given

the relatively low ductilities noted after exposure. This

again, points out the possibilities of this warm-rolling

method for development of stress corrosion cracking resistant

microstructure in these high Mg , Al-Mg alloys, in conjunction

with relatively high strength.

36

The final warm-rolled alloy in this ranking is the 10%

Mg material (Fig. 29). This evaluation is dictated by the

severe degradation of properties noted by exposure after

annealing and as well the tendency toward intergranular crack-

ing and exfoliation. This alloy possesses consistently higher

ductility than others evaluated in this study. However, the

method of post-exposure testing does not reflect the severity

of degradation observed in that the coupons are stressed

parallel to the grain orientation and therefore the cracks

developed are also stressed parallel to rather than perpendic-

ular to the crack plane. A similar observation may be made

regarding the 7075-T6 alloy [Fig- 30) , however, in that post-

exposure testing also was parallel to the crack plane of the

intergranular cracks observed. This alloy was generally the

most severely affected in any case, especially with regard to

loss of ductility after exposure.

D. THE GENERAL CORROSION TEST

The effects of general corrosion exposure in a salt-spray

environment were evaluated by periodic weight loss measurement

during the exposure. Two separate, nominally identical, tests

were conducted. The results of these tests are presented in

Figures 31 and 32. These data show identical trends in beha-

vior for these two tests; however, weight losses are larger

in Run 2, suggesting some possible differences in the actual

spray environment.

5"

The 8% Mg and 101 Mg exhibited almost no weight loss over

the 1000 hour duration of the test. The alloys with the Cu

and Cu-Mn additions behave in a similar manner with the onset

of attack at about 100 hours and an acceleration in weight loss

thereafter. These latter alloys are intermediate in behavior

to the binary (8 or 10% Mg) alloys and the 7075-T6, which

consistently exhibits the greatest weight loss.

An additional test of these alloys consisted of exposure

to an actual marine atmosphere by exposing a series of test

coupons aboard the R/V ACANIA, a research vessel with home

port in Monterey, California. This test was run for 1500

hours, at which point weight loss measurements were made.

These data are presented in Table IV along with the tabular

data for the accelerated salt-spray test. These alloys show

an identical trend in weight loss to that observed in the

salt-spray test are equivalent to weight loss in the acceler-

ated test at times between 50 and 100 hours.

The general appearance of a series of the salt-spray test

coupons is shown in the macrophotographs of Figure 33. These

were obtained using oblique lighting; hence, unaffected

regions of the originally-polished coupons appear dark. The

8% Mg and 10% Mg alloys exhibit the least corrosive attack,

although the 10 Mg alloy is somewhat more degraded than the

3% Mg alloy. The 10% Mg - 0.4% Cu and the 10% Mg - 0.4 Cu -

0.5% Mn alloys both show more extensive surface degradation

and the 7073-T6 alloy shows the most extensive degradation

with numerous, deep pits.

38

Figure 34 shows a series of longitudinal metallographic

sections including the ends as well as surfaces of these

salf-spray test coupons. Again, the 8% Mg alloy (Fig. 34a)

is only lightly attacked. However, the 10% Mg alloy (Fig. 34b)

is seen to experience intergranular attack from the end near

the surface, leading to exfoliation. Close examination also

reveals numerous intergranular cracks penetrating from the

exposed end of the sample and some cracking from blisters.

The 10% Mg - 0.4% Cu alloy exhibits no intergranular attack

(as shown in Fig. 34c) whereas under stress -corros ion exposure

such attack was observed. Further, no pitting is observed;

given that weight loss has occurred, the corrosion attack is

taking place uniformly for this alloy. Figure 34d illustrates

intergranular attack from the exposed ends of the test coupon

for the 10% Mg - 0.4% Cu - 0.5% Mn alloy, however, the crack-

ing is characterized by very blunt crack tips. The 7075-T6

alloy is seen to exhibit extensive intergranular attack from

the exposed ends of the test coupon as shown in Figure 34d,

and extensive pitting with some intergranular cracking

spreading from this pitting.

The above observations may be used, as before, to rank

these alloys, now with regard to their resistance to general

corrosion. Based on weight loss measurements, the ranking,

from most resistant to least is:

8% Mg and 10% Mg ; 10% Mg - 0.4% Cu and 10% Mg - 0.4% Cu -

0.5% Mn; ^075 -T6

39

Since raetallographic study indicates intergranular attack in

two cases, these rankings would necessarily change to:

8% Mg; 10% Mg - 0.4% Cu; 10% Mg ; 101 Mg - 0.4% Cu -

0.5% Mn; 7075-T6

The alteration in order reflects the observation of blistering

and intergranular attack in the 10% Mg alloy, even given the

small weight loss noted for this alloy.

The addition of Cu to Al-Mg alloys is done to enhance

general corrosion resistance and retard pitting [Ref. 18].

In these alloys, however, the binary 8% Mg and 10% Mg exhibit

the greatest resistance to weight loss and pitting in the

salt-spray test. It should be noted that Cu additions to con-

ventional Al-Mg alloys are typically less than 0.2% while the

Cu addition in both alloys of this study is about 0.4%.

Similarly, Tomashov [Ref. 19] notes that Mn additions up to

2.0% to Al alloys result in highly corrosion and stress-

corrosion resistant alloys. In this research, 0.5% Mn addition

to the 10% Mg - 0.4% Cu composition resulted in a material less

resistant to stress-corrosion degradation than a binary 8% Mg

alloy, although considerably higher in strength. This alloy

similarly is less corrosion resistant than the binary 8% Mg

and 10% Mg alloys.

40

IV. CONCLUSIONS AND RECOMMENDATIONS

A. CONCLUSIONS

1. The alloys evaluated in this test are less susceptible

to general and stress corrosion than the control alloy 7075-T6.

2. The Thermomechanically processed 8% Mg alloy exhibits

the best overall resistance to general and stress corrosion.

3. The Thermomechanically processed 10 % Mg alloy exhibits

blistering and exfoliation in both general and stress corrosion

environments, and it is severely degraded by the sensitizing

anneal. This is the result of insufficient homogenization of

the 8 phase.

4. The addition of Cu has a homogenizing and stabilizing

effect on the Thermomechanically processed 10°5 Mg alloy.

5. The addition of Cu and Mn in conjunction with the

additional solution treatment at 490°C for 3 hours results

in the most homogeneous and stable microstructure in the

Thermomechanically processed 10% Mg alloys.

B. RECOMMENDATIONS

1. Further study of processing and alloying variables,

especially the solution treatment and Mn addition, should be

conducted. In the solution treatment stage, effects of in-

creased time and temperature should be considered. Final

rolling temperature as well should be included as it has a

strong effect on resultant strength.

41

2. Extended testing to failure should be conducted to

validate these results. This should be in conjunction with

environmental exposure of these materials.

42

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LIST OF REFERENCES

1. Johnson, R. B., The Influence of Alloy Composition andThermo-mechanical Processing in Microstructures andMechanical Properties of High Magnesium, Aluminum-Magnesium Alloys, W. S~! Thesis , Naval PostgraduateSchool, Monterey, CA, June 19 80.

2. Cadwell, C. A., Jr., Fatigue Characteristics and Micro-structural Analysis of Thermo -mechanically processed

"

High Magnesium, Aluminum-Magnesium Alloys , M. S~! Thesis

,

Naval Postgraduate School, Monterey, CA, June 1981.

3. Hyatt, M. V. and Seidel, M. 0., in Stress CorrosionCracking in High Strength Steels and in Titanium andAluminum Alloys , ed. B . F . Brown, pp 148-244, Phys icalMetallurgy Branch, Metallurgy Division, Naval ResearchLaboratory, 1972.

4. Patterson, R. A., Stress Corrosion Testing of 5083Aluminum, Corros ion, Vol. 37, No. 8, p 455, Aug 19 81

.

5. Brown, J. A., and Pratt, J. N., The ThermodynamicProperties of Solid Al-Mg Alloys" MetallurgicalTransactions, Vol. 1, pp 2743-2750, October 1970.

6. Conserva, M. , and Leoni , M. , Effects of Thermal andThermo-mechanical Process ing on Properties of Al-MgAllovs , Metallurgical Transactions A, Vol . 6A, op189-195, January 19 75.

7. ASM, Metals Handbook, 9th Edition, Vol. 2, p 32, 1979.

8. Corrosion , 2nd edition, Vol. 1, pp 8:86 - 8:95,The Butterworth Group, 1977.

9. Campbell, H. S., in Handbook of Corrosion Testing andEvaluation , ed. W. H. Ailor, pp 18-20, Reynolds MetalsCo. Richmond, Virginia, 1977.

10. American Society for Testing and Materials (ASTM)Specification G-64, Resistance to Stress CorrosionCracking, of High Strength Aluminum Allovs, 1980

.

11. ASTM Specification G-4 7, Determining Susceptibility to

Stress Corrosion Crac"

ATToy Products, 1979~Stress Corrosion Cracking of High Strength Aluminum

82

12. ASTM, Specification, G-44, Alternate Immersion StressCorrosion Testing in 3.5% Sodium Cloride Solution ,

1~9~7 5

13. ASTM, Specification, G-30, Making and Using U-BendStress Corrosion Test Specimens ,1979 .

14. ASTM, Specification, G-39, Preparation and Use of Bent-Beam Stress Corrosion Test Specimens , 1979

.

15. ASTM, Specification, G-117, Salt Spray Testing , 1973.

16. ASTM, Specification, G-43, Acidified Synthetic SeaWater (fog) Testing , 1975.

17. Budd, M. K., and Booth, F. F., Structure CorrosionTesting of Aluminum Alloys by Electrical Methods 7~*

Aluminum Laboratories Limited, Banbury England, 1 961.

18. Mondolfo, L. F. , Aluminum Alloys: Structure andProperties

,

pp 806-819, Butterworth Inc., 1976.

19. Tomashov, N. D., Theory of Corrosion and Protection ofMetals: The Science of Corrosion, Translated fromRussian by: Boris H. Tyell , Isidore Geld, and HermanS. Preiser, pp 619-622, The MacMillan Co., 1966.

83

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3. Department Chairman, Code 69Mx 1

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4. Professor T. R. McNelley, Code 69Mc 5

Department of Mechanical EngineeringNaval Postgraduate SchoolMonterey, California 93940

5. Professor M. Edwards 2

Metallurgy BranchThe Royal Military College of ScienceShivenham, Swindon, Wilts SN6-8LA

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84

BeberdickA preliminary lnves.

tigatxon of the cor-rosion and stress-corrosion suscepti-

lltul°f the™-echan-ically processed high

magnesium aluminummagnesium alloys.

2 9 12 2J o 3 i n

Thesis

B'31£

clBeberdick *

W«*«X>»>

A preliminary inves-tigation of the cor-rosion and stress-corrosion suscepti-bility of thermomechan-ically processed highmagnesium aluminummagnesium alloys.

Documents

A preliminary investigation of the corrosion and stress ... · PROGRAMELEMENT.PROJECT'as* AREA ftWORKUNITNUMBERS 12.REPORTDATE September1981 IS.NUMBEROFPAGES 84 IS.SECURITYCLASS,(ol