Shape recovery and phase
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I.
INTRODUCTION
THE /3-NiA1 alloys undergo a CsCl-tetragonal martensitic transformation during cooling, tu The martensitic transformation occurs over a wide temperature range, depending upon the alloy composition, with Ms temperature ranging from 100 K at 61 at. pct Ni to 1200 K at 69 at. pct Ni. t2'31 Alloys containing from 62.5 to 66 at. pct Ni have shown shape memory behavior and, when deformed above the Ms temperature, have shown superelasticity associated with the formation of stress-induced martensite, t41 Deformation behavior of the L10(3R)-type Ni-A1 martensite has been observed by tensile testing, optical and electron microscopy, and in situ X-ray analysis. tS] The substructure of the martensite was found to be internal twinning on the {111} (112)fct system. I6,71The maximum strain by detwinning is related to the magnitude of the twinning shear, orientation factor for the twinning system, and volume ratio of the internal twins, ts~ This indicates that deformation of the martensite below M i in NiAI alloys occurs by means of detwinning of internal twins and that the process of detwinning can be studied as a potential means of ductilizing polycrystalline NiA1 alloys. The present work concerns the deformation behavior of martensite in Ni-AI alloys by means of compression and the morphology of NiA1 martensite studied by transmission electron microscopy. II.
EXPERIMENTAL PROCEDURE
Alloys were prepared from 99.99 pct A1 and 99.9 pct Ni by arc-melting under an argon atmosphere in a watercooled copper crucible. Each alloy was remelted at least six times to insure homogeneity. Titanium was also melted for chamber gettering before melting the alloys. After Y.D. KIM, formerly with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, is with the Korean Institute of Scientific Research, Seoul, Korea. C.M. WAYMAN, Professor, is with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted April 17, 1991.
METALLURGICAL TRANSACTIONS A
melting, the density was checked to determine any gross errors in sample preparation. Homogenizing heat treatments (1250 ~ for 3 days) were carried out in quartz capsules evacuated to 10 -5 mm Hg and then backfilled to 0.2 atmosphere with pure argon. The alloys were water-quenched to room temperature for martensitic transformation. Compression testing was performed to observe the deformation behavior of martensite in NiAI alloys. For compression testing, the sample dimensions were 5 x 5 • 12.5 mm. Quantitative tests to determine the shape recovery rate were performed on the compressed sample by a dilatometer. Discs of 3-mm diameter were prepared by electrical discharge machining. The discs were then double-jet electropolished at - 3 0 ~ in a solution of 33 pct nitric acid in methanol at a voltage of 12 to 15 V. All foils were examined in a Hitachi H-800 transmission electron microscope operating at 200 kV with rotation and double tilt specimen holders. III.
EXPERI
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