Microstructural Characterization and Shape Memory Response of Ni-Rich NiTiHf and NiTiZr High Temperature Shape Memory Alloys
NiTiHf and NiTiZr high temperature shape memory alloys (HTSMAs) have drawn a great deal of attention as cheaper alternatives to Pt, Pd and Au alloyed NiTi-based HTSMAs while NiTiZr alloys also providing at least 20% weight reduction then its NiTiHf counterparts with the same stoichiometry. (Ti + Hf/Zr)-rich compositions were already reported to have high thermal hysteresis, poor dimensional and thermal stability due to their low matrix strength hampering their practical applications. However, Ni-rich compositions of NiTiHf alloys were shown to have very promising shape memory responses recently due to generation of fine Ni-rich particles after proper heat treatments not only strengthening the matrix but also leading to relatively high transformation temperatures. Comparable studies have not been performed on Ni-rich NiTiZr compositions. Furthermore, very few published work are present on these new Ni-rich NiTiHf and NiTiZr systems. Hence many critical characteristics still remains unknown and further investigation is necessary to reveal the effect of precipitation on the microstructures and its subsequent effect on the transformation characteristics and shape memory responses.
The present study focuses on the extensive microstructural and thermo-mechanical property characterizations of the Ni-rich NiTiHf and NiTiZr HTSMAs in order to develop the fundamental knowledge necessary for the optimization and development of reliable, cheap, lightweight HTSMAs operating up to 300 ?C with improved thermal and dimensional stability. Several different compositions of Ni-rich NiTiHf and NiTiZr HTSMAs are systematically precipitation heat treated for the microstructural control and then subjected to multi-scale microstructural and thermo-mechanical characterizations to achieve this goal. Differential scanning calorimetry measurements are conducted on the aged samples to reveal the transformation characteristics and furthermore generate the time-temperature-transformation temperature (TTT) diagrams of the individual alloy systems. The shape memory response and characteristics of the alloys are investigated through load-biased thermal cycling and superelasticity tests. The microstructures of the aged samples are extensively characterized using transmission electron microscopy (TEM) to build up microstructure-property relationships as well as providing deeper understanding of precipitate crystal structure, composition and morphology.
Such an experimental approach is crucial for the development of new ternary alloy compositions and for the careful control of the microstructure to obtain desired properties. The outcomes of the present study is expected to help to reveal the potential of these alloys to be utilized in a wide range of applications at elevated temperatures in aerospace, automotive and oil-gas industries.