The Effect of Aluminum Additions on the Microstructure and Thermomechanical Behavior of NiTiZr Shape-Memory Alloys
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The microstructure, thermal cycling, and mechanical behavior of Ni48.5Ti31.5−x Zr20Al x (x = 0, 1, 2, 3) alloys were studied in the solution-treated and aged condition using microscopy techniques, differential scanning calorimetry, and compression tests. The microscopy techniques used include optical, scanning, and transmission electron microscopy, and three-dimensional, atom–probe microscopy. The results of this study indicated a strong dependence of the transformation behavior on alloy chemistry and thermal cycling. The aluminum additions served to decrease transformation behaviors from 351 K to 596 K (78 °C to 323 °C) and reduce thermal stability. Additionally, aluminum was shown to increase the plateau stress in the aged condition, whereas the formation of coarse-grained intermetallic phases caused the embrittlement of the microstructure, reducing its ductility. The addition of Al resulted in the refinement of the coarse, lenticular precipitates identified as Ni4(Ti,Zr)3.
KeywordsMartensite Differential Scanning Calorimetry Transformation Temperature Lave Phase Aged Alloy
M.V. Manuel and D.H.D. Hsu acknowledge financial support from National Science Foundation award number CMMI-0824352. G.B. Thompson and T.T. Sasaki recognize NASA grant NNX09AO61A from the NASA FAP Supersonics project, Dale Hopkins, API, for support. The authors would also like to thank Dr. Ron Noebe of NASA Glenn Research Center for the fruitful discussions and his scientific insight.
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