P. o. Box 109600UNTED West Palm Beach. Florida 33410-9600

TECHNOLOGIES (407) 796-2000PRATT&WHITNEY

Government Engines & Space Propulsion

AD-A244 815DTIC

20 December 1991-- EL ECTE 01h~JAN 0 9 1992

Office of Navy Research IScientific Officer DAttn: Dr. A. K. Vasudevan, Code 1216Contract No. N00014-91-C-0124Item No. 0002, Sequence No. AOOI800 N. Quincy StreetArlington, Va 22217-5000

Subject: Submittal of the Progress Report, FR21998-2

Gentlemen:

In accordance with the applicable requirements of the contract, we herewith submit one (I) copy ofthe subject report.

Very truly yours,

UNITED TECIHNOLOGIES CORPORATIONPratt & Whitney

Contract Data Coordinator This document has been approvedIor public release and sale; its

cc: With Enclosures distribution is unlimitod.*

Director, Naval Research, Code 2627DPRODefense Technical Information Center (2 copies)

91 1230 02091-19263

FR2198-02 15 December 1991

FATIGUE IN SINGLE CRYSTAL NICKEL SUPERALLOYSTechnical Progress Report

Charles AnnisProgram Manager

SUNITEDTECHNOLOGIESPRATT&WHITNEY

P.O. Box 109600West Palm Beach, FL 33410-9600(407)796-6565 alGovernment Engines and Space Propulsion

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15 December, 1991

Period of performance16 September 1991 through 15 December 1991

Contract N00014-9 1-C-0124

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Office of Naval Research Statement A per telecon Dr. A K VasudevariDepartment of the Navy ONR/Gode 1222800 NV Quincy Street Arlington, VA 22217- 5000Arlington, Va 22217-5000 NW /89

FR2198-02 15 December 1991

I. Introduction and Program Objective

This program investigates the seemingly unusual behavior of single crystal airfoilmaterials. The fatigue initiation processes in single crystal (SC) materials aresignificantly more complicated and involved than fatigue initiation and subsquentbehavior of a (single) macrocrack in conventional, isotropic, materials. Tounderstand these differences it is helpful to review the evolution of hightemperature airfoils.

Characteristics of Single Crystal Materials

Modern gas turbine flight propulsion systems employ single crystal materials forturbine airfoil applications because of their superior performance in resistingcreep, oxidation, and thermal mechanical fatigue (TMF). These properties havebeen achieved by composition and alloying, of course, but also by appropriatecrystal orientation and associated anisotropy.

Early aeroengine turbine blade and vane materials were conventionally cast.equiaxed alloys, such as IN 100 and Rene'80. This changed in the late 1960s withthe introduction of directionally-solidified (DS) MAR-M200 + Hf airfoils. TheDS process produces a <001 > crystallographic orientation, which in superalloysexhibits excellent strain controlled fatigue resistance due to its low elasticmodulus. The absence of transverse grain boundaries, a 60% reduction inlongitudinal modulus compared with equiaxed grains, and its correspondingimproved resistance to thermal fatigue and creep, permitted significant increasesin allowable metal temperatures and blade stresses. Still further progress wasachieved in the mid-1970s with the development of single crystal airfoils'.

The first such material, PWA 1480, has a considerably simpler composition thanpreceding cast nickel blade alloys because, in the absence of grain boundaries, nograin boundary strengthening elements are required. Deleting these grainboundary strengtheners, which are also melting point depressants, increased theincipient melt temperature. This, in turn, allowed nearly complete y' solutioningduring heat treatment and thus a reduction in dendritic segregation. The absenceof grain boundaries, the opportunity for full solution heat treatment, and theminimal post-heat treat dendritic segregation, result in significantly improvedproperties as compared with conventionally cast or directionally solidified alloys.Single crystal castings also share with DS alloys the <001 > crystal orientation,along with the benefits of the resulting low modulus in the longitudinal direction.

Pratt & Whitney has developed numerous single crystal materials. Like most,PWA 1480 and PWA 1484 are y' strengthened cast mono grain nickel superalloysbased on the Ni-Cr-Al system. The bulk of the microstructure consists ofapproximately 60% by volume of cuboidal y' precipitates in a y matrix. Theprecipitate ranges from 0.35 to 0.5 microns and is an ordered Face CenteredCubic (FCC) nickel aluminide compound. The macrostructure of these materials

I Gell, M., D. N. Duhl, and A. F. Giamei, 1980, 'The Development of Single Crystal Superalloy TurbineBlades,' Superalloys 1980, proceedings of the Fourth International Symposium on Superalloys, AmericanSociety for Metals. Metals Park, Ohio. pp. 205-214.

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FR2198-02 15 December 1991

is characterized by parallel continuous primary dendrites spanning the castingwithout interruption in the direction of solidification. Secondary dendrite arms(perpendicular to solidification) define the interdendritic spacing. Solidificationfor both primary and secondary dendrite arms proceeds in < 001 > typecrystallographic directions. Undissolved eutectic pools and associatedmicroporosity reside throughout the interdendritic areas. These features act asmicrostuctural discontinuities, and often exert a controlling influence on thefatigue initiation behavior of the alloy. Also, since the eutectics are structurallydissimilar from the surrounding matrix their fracture characteristics will differ.

Single Crystal Fatigue

The fatigue process in single crystal airfoil materials is a remarkably complex andinteresting process. In cast single crystal nickel alloys, two basic fracture modes,crystallographic and non-crystallographic, are seen in combination. They occurin varying proportions depending upon temperature and stress state.Crystallographic orientation with respect to applied load also affects theproportion of each and influences the specific crystallographic planes and slipdirections involved. Mixed mode fracture is observed under monotonic as wellas cyclic conditions.

Single crystal turbine blades are cast such that the radial axis of the componentis essentially coincident with the < 001 > crystallographic direction which is thedirection of solidification. Crystallographic fracture is usually seen as eitheroctahedral along multiple (I11) planes or under certain circumstances as (001)cleavage along cubic planes.

Non-crystallographic fracture is also observed. Low temperatures favorcrystallographic fracture. At higher temperatures, in the 427C range, smallamounts of non-crystallographic propagation have the appearance oftransgranular fatigue in a related fine grain equiaxed alloy. Under someconditions, this propagation changes almost immediately to the highlycrystallographic mode along (11l) shear planes, frequently exhibiting prominentstriations emanating from the fatigue origin and continuing to failure inoverstress. Under other conditions the non-crystallographic behavior cancontinue until tensile failure occurs. At intermediate temperatures (around 760C)non-crystallographic propagation is more pronounced and may continue untiltensile overload along (I 1) planes occurs, or may transition to subcriticalcrystallographic propagation. At 982C, propagation is almost entirelynoncrystallographic, similar to transgranular propagation in a polycrystal.

Damage Catalogue

This program will identify and compile descriptions of the fracture morphologiesobserved in SC airfoil materials under various combinations of temperature andstress associated with advanced Navy aeropropulsion systems. We will suggestfatigue mechanisms for these morphologies and catalogue them as unique damagestates. Most testing will be accomplished under ancillary funding, and thereforebe available to this effort at not cost. The work is oranized into four tasks, whichare described in the following paragraphs.

3

FR2198-02 15 December 1991

If. Program Organization

The program is structured into four tasks, three technical and one reporting. Theindividual tasks are outlined here.

Task 100 - Micromechanical Characterization

This task will define the mechanisms of damage accumulation for the varioustypes of fracture observed in single crystal alloys. These fracture characteristicswill be used to establish a series of Damage States which represent the fatiguedamage process. The basis for this investigation will be detailed fractographicassessment of failed laboratory specimens generated in concurrent programs.Emphasis will be on specifically identifying the micromechanical damagemechanisms, relating them to a damage state, and determining the conditionsrequired to transition to an alternate state.

Task 200 - Analytical Parameter Development

This task will extend current methods of fatigue and fracture mechanics analysisto account for microstructural complexities inherent in single crystal alloys. Thiswill be accomplished through the development of flexible correlative parameterswhich can be used to evaluate the crack growth characteristics of a particulardamage state. The proposed analyses will consider the finite element and thehybrid Surface-Integral and Finite Element (SAFE) methods to describe themicromechanics of crack propagation.

Task 300 - Probabilistic Modeling

This task will model the accumulation of fatigue damage in single crystal alloysas a Markov process. The probabilities of damage progressing between thedamage states defined in Task 100 will be evaluated for input into the Markovmodel. The relationship between these transition probabilities and fatigue lifewill then be exploited to establish a model with comprehensive life predictivecapabilities.

Task 400 - Reporting

Running concurrently with the analytical portions of the program, this task willinform the Navy Program Manager and Contracting Officer of the technical andfiscal status of the program through R&D status reports.

111. Technical Progress

During this reporting period, our efforts have been concentrated in identifyingand characterizing numerous damage states in both PWA 1480 and PWA 1484+ HIP. Our approach builds on accepted micro mechanisms and dislocationdynamics for the deformation process in two phase FCC systems and at timeswill propose new hypotheses on the sequential nature of the accumulation anddissipation of energy. Sources for this studv include specimen fracturcs and testdata produced under past and present NASA, Air Force and P&W IR&Dprograms.

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FR2198-02 15 December 1991

This effort pays particular attention to energy dependant fracture modetransition indicators. The transitions may be considered as bridging sequentialdamage states (or sub states), which may under a particular set of circumstancesor conditions lead to an absorbing state such as failure. These indicators are inthe form of condition (temperature or stress intensity for example) dependantfracture details on the y' precipitate level. Further, these fracture modes are phasespecific in some cases.

The eutectic phase exhibits an energy dependant sequence of fracturecharacteristics identical to that of the bulk microstructure but at a lower energyinput level. This causes it to become a preferential fatigue crack initiation siteunder certain circumstances.

To date we have documented the stress intensity / temperature dependency offracture mode transitions at room temperature, 800F and I IOOF in K-gradienttests. These transitions repeat in reverse order upon subsequent constant loadcycling. In addition, we have identified environment (hydrogen) dependant modechanges similar to those seen in air threshold behavior at room temperature.

We are also developing and employing advanced techniques for the analysis andcharacterization of fracture surfaces. This work is being carried out from themacro level to 10,000 X via optical and conventional SEM.

By the adaptation of the Philips CM-20 scanning/transmission electronmicroscope (STEM) for use as a fracture analyses tool, images in excess of 50,000X are being produced. Magnifications in this range are necessary in resolving themicro-mechanisms of fracture on the y-y' interface level.

Graphic representations of particular fracture modes are being assembled viaUnigraphics Shaded-Solid Modeling. The models are three dimensional and canbe rotated to provide needed views of a particular fracture. They aid in theprocess of understanding and describing crack tip / microstructure interactions.

A further development has identified an automated method of obtaining bothgraphic and statistical assessments of micro mechanical transition indicators. TheRodenstock Laser Imaging Profilometer is intended to assess optically surfaceroughness via a laser stylus. For the purposes of this program, the instrumentrepresents a scanning laser micro analyzer with statistical analysis capabilities.This provides us with a way to obtain quickly a mathematical "signature of aparticular fracture mode or transition and of assessing the stocastic componentof a K-dependant transition.

Representative examples of one such fracture mode transition and the methodsof analysis we have described are provided in the following figures.

A PWA 1480 CT specimen was recently examined via the Laser Stylus ImagingProfilometer. The device was employed to characterize the transition regionbetween local octahedral (Paris region) and <001 > planar fracture (thresholdbehavior). The PWA 1480 room temperature threshold crack growth evaluationwas conducted in a decreasing-K mode (negative K-gradient). A fracture modetransition occurred at approximately 5 ksi rootin, accompanied by a markedchange in growth behavior. A plot of the test results is shown in Figure 1. The

5

FR2198-02 15 December 1991

mode change is shown schematically in Figures 2 and 3. The transition was fromlocal octahedral, (11) crystallographic fracture on the level of the cuboidal y'precipitate, to decohesion at the y-y' interface upon entering the threshold region.The transition is from local octahedral to decohesion and has also been identifiedas the the mode transition responsible for more than a 10 X increase in crackgrowth rate under conditions of hydrogen embrittlement. Comparative fracturesurfaces for the two modes are shown in Figure 4. Details of a decohesion failurein PWA 1484 are shown in Figure 5. The transition zone for this type of fracturesequence is displayed in a 3 axis profile plot in Figure 6. The data from theprofile plot has been converted to a topographical contour plot in Figure 7.

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FR2198-02 15 December 1991

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FR2 198-02 15 December 1991

IRODENSTOCK METROLOGYRM0

Displayr..................Points.....................7158Scanls........................50Display range............ ..5000 milMeasurement range.........30 JimTable speed................0.079 in/minY Length...................0.020 inX Length...................0.004 inName of data set..........DANO

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FR2 198-02 15 December 1991

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