On the influence of Ti-rich precipitations on the properties of NiTiHf High Temperature Shape Memory Alloys

In the last decades, High-Temperature Shape Memory Alloys (HT-SMAs) have been studied in many research activities in different scientific fields. In recent years, a large variety of alloy systems have been introduced, exhibiting high phase transformation temperatures (PTTs) and suitable functional properties. HT-SMAs based on the NiTiHf alloy system are promising candidates for applications that require PTTs exceeding those of conventional binary NiTi SMAs. By alloying Hf to the NiTi-system, the PTTs are increased. But also, the functional properties can meet the requirements for HT actuator applications in terms of stroke, activation stress and cyclic stability. However, processing of NiTiHf SMA is still challenging due to brittle behavior during processing with conventional hot-rolling, swaging or wire drawing techniques. In the present work, we show how the martensitic transformation of NiTiHf alloys is affected by Ti-rich precipitations, comparing a Ti-rich Ni_49.5Ti_35.5Hf₁₅ SMA with a stoichiometric Ni₅₀Ti₃₅Hf₁₅ SMA. The samples for the characterization are taken from thin sheets produced with a special hot-rolling and heat-treatment procedure being capable to handle the brittle as-cast microstructure. The samples are characterized in an annealed state and we show our results of the functional and microstructural analysis, differential scanning calorimetry, internal friction measurements and also the cyclic behavior of actuator components. We discuss our findings with regard to their influence on actuator applications and possible alloy optimization parameters for an improved product development for HT-SMA applications.