TENSILE BEHAVIOR OF Mo POLYCRYSTALS UNDER ULTRAHIGH VACUUMS. Feuerstein, L. Rice, and H. Conrad Citation: Applied Physics Letters 4, 154 (1964); doi: 10.1063/1.1754010 View online: http://dx.doi.org/10.1063/1.1754010 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/4/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Insulation degradation behavior of multilayer ceramic capacitors clarified by Kelvin probe forcemicroscopy under ultra-high vacuum J. Appl. Phys. 113, 064103 (2013); 10.1063/1.4791714 Variation of surface morphology and electronic behavior under dynamic tensile conditions Appl. Phys. Lett. 88, 181902 (2006); 10.1063/1.2193048 Spontaneous emission of charged particles and photons during tensile deformation of oxidecoveredmetals under ultrahighvacuum conditions J. Appl. Phys. 48, 5262 (1977); 10.1063/1.323556 Apparatus for Determining Creep Behavior Under Conditions of Ultrahigh Vacuum Rev. Sci. Instrum. 37, 999 (1966); 10.1063/1.1720436 Adhesion of Polished Quartz Crystals under Ultrahigh Vacuum J. Appl. Phys. 36, 2326 (1965); 10.1063/1.1714478

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Volume 4, Number 8 APPLIED PHYSICS LETTERS 15 April 1964

TENSILE BEHAVIOR OF Mo POLYCRYSTALS UNDER ULTRAHIGH VACUUM

(fracture strain vs strain·rate;

room temperature; E)

Current interest in materials for space application has prompted studies of their mechanical behavior at extremely low pressures. Except for one reported investigation on aluminum (ref 1), this area has been relatively unexplored. This Letter reveals some recent observations on the stress-strain behavior

of bulk molybdenum polycrystals under ultrahigh vacuum.

Specimens were prepared from sintered Mo rod having a nominal purity of 99.93%. Following machining of gauge sections (5-mm diam x 50.8 mm

long), specimens were heat treated at 1600°C for 4 h, resulting in an average grain size of 0.12 mm. Electrolytic polishing of the samples in a 97% sulfuric acid solution was carried out prior to testing.

Experiments were performed at room temperature in an ion-pumped ultrahigh vacuum system positioned in an Instron tensile machine. Testing was accom­

plished over a range of constant strain rates from 4.2 x 10- 3 to 4.2 x 1O- S sec- 1• Pressure measure­

ments were made with an NRC Redhead gauge. Initial vacuums ranged from 2 to 0.5 x 10- 10 Torr, corrected (ref 2). Atmospheric pressure tests were carried out in situ after backfilling with dry nitrogen or aIr.

60'---~-----'-----'----'-----'----'I-_-~--_-_T~-_-1

50

'" ~40 .... '" '" g 30 w a:: .... (f)

w ii! 20 ....

10

o ULTRA-HIGH VACUUM

a ATMOSPHERE (N 2 )

• = 4.2 X 10-\ec-1

°0~--~=---~~-=~--~=-~0~.2~5~~0.~3~0--~0~.3~5--~0~40

TRUE STRAIN

Fig. 1. Effect of pressure on the room-temperature

tensile behavior of polycrystalline molybdenum.

154

60

50

'" E 40 E "-0>

'" ui f3 30 c: .... (I)

w ::> ~ 20

10

0

S. Feuerstein, L. Rice, and H. Conrad Materials Sciences Laboratory

Aerospace Corporation

2400 E. EI Segundo Boulevard

EI Segundo, California

(Received 17 March 1964)

0> :c E E

~IO-IO ::> (f) rn w a:: fl.

-4 -I • = 4.2 X 10 sec

TEST CHAMBER PRESSURE

10-11 L-____ -1-____ -1-____ ---L ____ --.L ____ -L ____ ~

o 0.10 0.20 030 TRUE STRAIN

Fig. 2. Typical variation of test chamber pressure

during the room·- temperature tensi Ie deformation of a

molybdenum polycrysta II ine specimen.

Figure 1 illustrates typical stress-strain behavior observed for specimens tested either at atmospheric

pressure or under ultrahigh vacuum. For any con­stant strain rate, the upper yield point, lower yield point, Liiders region, and strain hardening regions are almost invariant. Most prominent and consistent for some thirty test specimens was the fact that the vacuum tests exhibited approximately a 30% increase in strain to fracture (Ep)v ac over the fracture strains recorded for the atmospheric tests

(EP)atm;

that is, [(Ep) - (Ep) ]/(Ep) = 0.30. vac atm atm

The influence of strain rate (or test duration) on this behavior is shown in Table 1. There appears

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Volume 4, Number B APPLIED PHYSICS LETTERS 15 April 1964

Table I. The Relative Effect of Strain Rate on the Fracture Strain of Mo Tested Under Ultrahigh

Vacuum and Atmospheric Pressure of Dry Nitrogen or Air.

A verage fracture strain

(E) 'V 1 0 - 1 0 Torr F vac'

(EP )atm,760 Torr

Relative increase

4.2 X 10- 5

0.49

0.37

32.6%

Strain Rate E (sec ~ 1)

2

4.2 X 10-4

0.40

0.31

31.1%

3

0.36

0.27

32.0%

4

4.2 X 10- 3

0.35

0.26

34.0%

I (EP)atm -------------------------------------------------------~

to be no significant contribution to the percentage increases in ductility regardless of strain rate for the two orders of magnitude investigated.

During each vacuum test, pressure was monitored continually. A surface desorption or outgassing effect was noted during the early stages of de­formation, as depicted in Fig. 2. A rapid rise in pressure followed by a drop-off corresponding to the yield point was observed with the pressure even­tually returning to its original value. This phe­nomenon was subsequently determined to arise solely from the Mo specimens rather than from other system components. Further, that the outgassing continues to occur during the entire stress-strain curve is illustrated in Fig. 3, which shows the effect of strain rate changes on the pressure.

At present it is felt that a correlation exists between the desorption and the extended ductility under ultrahigh vacuum. Presumably, there occurs

a change in surface energy that affects the disloca­tion and fracture processes at or near the specimen surface. Further tests are planned to define the mechanisms that are involved.

II. R. Kramer and S. Podlaseck, Acta Met. 11 (1), 70 (1963).

2F . L. Torney, Jr., and F. Feakes, Rev. Sci. IT/str. 34 (9),1041 (1963)·

70

60

Ne 50 e "-'" " 40

:if '" ~ 30 UJ

'" " ~ 20

10

o

'" x e e ..,.. 10-9

a:

" :ll II! Q.

STRESS-STRAIN CURVE UNDER VACUUM

.. :&42---4.2 X 10-5 sec-I t INCREASE IN RATE

I DECREASE IN RATE

TEST CHAMBER PRESSURE

TRUE STRAIN

Fig. 3. Effect of strain rate change on test chamber

pressure during the room_temperature tensi Ie deformation

of a molybdenum polycrystalline specimen.

155

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