Compositions and Processing for Improved High Temperature Shape Memory Alloys

The Phase I program demonstrated the feasibility of equal channel angular extrusion (ECAE) as a processing method for high temperature shape memory alloys. Ni(TiHf) was processed at temperatures that provided sufficient ductility, but for which recrystallization was not significant. Favorable changes in microstructure were observed as a result of ECAE, in comparison with homogenized samples. Transformation characteristics, such as recoverable strain and creep, were also favorably impacted by ECAE. Now in Phase II we will conduct more extensive ECAE to process alloys based on (NiPd)Ti, which is selected because it possesses much lower hysteresis than Ni(TiHf) and offers the possibility of an austenite finish temperature around 110 C. New alloy compositions will be identified as candidates by a method that predicts minimal hysteresis and other associated virtues, such as good fatigue behavior. Samples cut from ECAE bars will be used for thermomechanical testing in tension, microscopic examination and differential scanning calorimetry. ECAE and post-ECAE machining will be used to produce tubes for torque testing, as called for by the application.