Characterization of Stoichiometric and Aging Effects on NiTiHf High Temperature Shape Memory Alloys

NiTiHf ternary alloys exhibit attractive high temperature shape memory alloy (HTSMA) properties. Material properties include moderate transformation strain, relatively high transformation temperature, stable pseudoelastic (PE) behavior, and very small irrecoverable strain during load biased tests. The addition of Hf is more attractive than Pt and Pd due to the fact that Hf has a lower cost. Four NiTiHf alloys with different chemical compositions spanning stoichiometry were used to analyze microstructure and mechanical behavior. Research will mainly be focused on the Ni-rich alloys based on that the Ni-lean alloys do not exhibit the previously mentioned HTSMA material properties. The alloys analyzed all had 20(at.)% Hf with varying Ti concentrations (29(at.)%, 29.7(at.)%, 30(at.)%, and 30.5(at.)%). All of the alloys were formed by induction melting, homogenized at 1050C for 72hrs, and then hot extruded with a diameter reduction of 7:1 at a temperature of 900C. In addition to the homogenized and extruded condition, several short-term aging cycles above 400C were completed on each of the alloys.;Certain microstructure and mechanical properties are obtained when Hf is added to NiTi and a proper aging cycle is used. Results from isothermal compression tests that were executed above the austenite finish temperature determined that the Ni-rich NiTiHf alloys exhibited all the necessary properties to be considered for HTSMA applications. The test temperature range of the isothermal compression tests were chosen to characterize the transition from PE behavior to austenite plasticity and to examine the deformation behavior of the B2 phase at high temperatures.;Results from compression tests showed a strengthening effect when the test temperature was increased through the PE test region. The strengthening effect is due to the yield stress and plasticity being related to the stress induced martensite that is created during the compression test executed above the austenite finish temperature. This is commonly known as the LeChatellier's principal where stressed induced martensite becomes more difficult to create at higher temperatures. At low temperatures the alloys are B19' and transform to B2 at elevated temperatures. As the test temperature is increased it eventually reaches a high enough temperature where stress induced martensite can no longer be created and the sample deforms plastically in the austenite state. Results also revealed that the strength level for the alloy at a given temperature and the temperature range over which pseudoelasticity versus austenite plasticity was dominant was significantly affected by aging. In addition, the aging effects were analyzed by several transmission electron microscopy techniques. Detailed STEM characterization has revealed that the nanoscale precipitates that form upon aging have a novel ordered structure relative to the B2 austenite phase. Characterization of the nanoscale precipitates have shown that the precipitates are Hf rich and are different then the commonly seen Ni3Ti4 or P phase precipitates in other NiTi based shape memory alloys. Based on the isothermal compression tests the nanoscale precipitates strengthen the alloys.