Processing and Scalability of NiTiHf High-Temperature Shape Memory Alloys

Development of melting and processing techniques for NiTiHf high-temperature shape memory alloys at the laboratory scale has resulted in pronounced success and repeatability for actuation purposes. Even the Ni-rich NiTiHf formulations, which are more challenging from a compositional control standpoint since small changes in chemistry can result in large transformation temperature variations, are reproducibly processed at the laboratory scale. Since properties of the slightly Ni-rich NiTiHf alloys have proved promising, large-scale production of such alloys now requires renewed attention. In this work, several melting techniques were used to process NiTi-20Hf (at.%), ranging from vacuum induction melting to plasma arc melting, with heats ranging in size from 0.4 to 250 kg with a target composition of Ni50.3Ti29.7Hf20 (at.%). All cast ingots were subsequently hot extruded into bar. The resulting chemistries, microstructures, and inclusion types and sizes were evaluated as a function of melting technique. Finally, the thermophysical, mechanical and functional properties were measured for a number of material heats that varied in size and primary processing technique. The results indicated that various melting techniques could result in alloys with slightly different end compositions that can affect the mechanical and functional properties. Some of the compositional changes are inherent to the melting process, such as formation of carbides, Ni loss, and other attributes that can be adjusted or minimized by optimizing melting practices. Finally, alloy properties were correlated to the actual compositions of each heat, through corrections based on differential scanning calorimetry measurements, indicating that most scatter in properties can be explained by slight chemistry variations.