Transformation Strains of Nitihf High-Temperature Shape-Memory Alloys

Although high-temperature shape-memory alloys (HTSMA) with useful transformation temperatures above 150ºC are highly desirable, there is a near-term need for alloys with transformation temperatures even slightly in excess of what conventional binary NiTi alloys can provide. Such materials would find immediate application in the aeronautics sector for both on-wing (gurney flaps, variable geometry chevrons) and in cabin (locks and latches) applications, and would be equally utilized for applications in the automotive and other commercial sectors.  Slightly Ni-rich, polycrystalline and single crystal ([001], [–340], and [678] orientations) Ni_50.3Ti_29.7Hf₂₀ with an M_F ~130ºC was investigated using isothermal compression tests in the martensite and austenite states and load-biased thermal cycling to determine its viability in the aforementioned applications. Unlike conventional SMA actuator materials, the NiTiHf alloy investigated here exhibited high work output and good dimensional stability, without the need for “training” or complicated thermomechanical processing cycles. Results of the polycrystalline and single crystal tests will be compared with each other and with theoretical calculations of maximum transformation strains. In closing, the theoretical potential of this material to exhibit nearly 20% transformation strain in single crystal form will be tempered with relastic expectations in polycrystalline compononents by contrasting our early findings of this alloy with established knowledge of binary NiTi.