Novel Processing of High-Temperature Shape Memory Alloys

This proposal is for advanced processing of Ni(Ti, Hf) shape memory alloys to achieve transformation temperatures suitable for use in the 100-200 C range, along with enhanced fatigue life. Binary NiTi alloys are the leading shape memory materials, buttheir transformation temperatures limit them to applications below 100 C. It is now known that partial substitution of Hf for Ti increases the transformation temperatures in a controllable manner. Like NiTi, it appears that Ni(Ti, Hf) experiences fatigueand a marked decrease in recoverable strain with increasing numbers of actuation cycles. The loss of recoverable strain is due to slip, and the key to enhanced fatigue life is to suppress irreversible slip. We propose to suppress irreversible slip inpolycrystalline Ni(Ti, Hf) alloys by controlling three key features of the microstructure: grain orientation, grain size and precipitate formation. The proposed unique processing regimen offers good hope for controlling each of the three keymicrostructure features, and therefore an enhancement of fatigue life of Ni(Ti, Hf) shape memory alloys. This STTR proposal is for developing advanced Ni(Ti, Hf) shape memory alloys for Navy aircraft applications. Successful completion of this STTR willprovide the Navy with the opportunity to take advantage of a new generation of mechanical components in the design of future aircraft. Potentially many of the DOD''s mechanical systems could eventually incorporate SMA devices, and would thereforebenefit from the availability of advanced shape memory alloys. There are also numerous commercial applications of SMA devices that could be impacted by the results of this work including systems in commercial aircraft and automobiles. The methods that weintend to use are not material specific, so our findings might also be applied to ferromagnetic shape memory alloys such as Ni2MnGa.