Abstract
Sn effect on the phase transformation behavior, microstructure, and micro hardness of equiatomic Ni-Ti shape memory alloy was studied. NiTi and NiTiSn alloys were produced using vacuum induction melting process with alloys composition (50% at. Ni, 50% at.Ti) and (Ni 48% at., Ti 50% at., Sn 2% at.). The characteristics of both alloys were investigated by utilizing Differential Scanning Calorimetry, X- ray Diffraction Analysis, Scanning Electron Microscope, optical microscope and vicker's micro hardness test. The results showed that adding Sn element leads to decrease the phase transformation temperatures evidently. Both alloy samples contain NiTi matrix phase and Ti2Ni secondary phase, but the Ti2Ni phase content decreases with Sn addition and this is one of the reasons that leads to decrease the micro hardness of alloy with adding Sn element in a noticeable manner. The micro hardness decreases from 238.74 for NiTi equiatomic alloy to 202 for NiTiSn alloy after heat treatment.
Highlights
Ni-Ti shape memory alloys (SMAs) are interesting materials in engineering applications because they have shaped memory effect and superelasticity
The results show that equi-atomic NiTi SMA consists of NiTi matrix phase presented by dashed pointer and precipitates of Ti2Ni phase presented by the solid pointer
Optical photographs show that the dendritic structure becomes greater after heat treatment, the X-ray analysis and by using scherrer equation reveal that the NiTiSn SMA has a grain size greater than equiatomic Ni-Ti SMA
Summary
Ni-Ti shape memory alloys (SMAs) are interesting materials in engineering applications because they have shaped memory effect and superelasticity. The two factors that effect on Ni-Ti SMAs application in engineering are the phase transformation temperatures and mechanical characteristics [1, 2]. The third alloying element addition to the binary Ni-Ti shape memory alloy plays a significant role in changing phase transformation temperatures and mechanical characteristics [3, 4]. The phase diagram of Ni-Ti alloy system is important for heat-treatments of the alloys and improvement of the shape memory characteristics. The research interests are restricted in the central region bounded by Ti2Ni and TiNi3 phases (Fig.). Since the single phase Ni-Ti B2 near the equiatomic composition transforms into a monoclinic phase martensitically B19', as shown in Fig., it is clear that the boundary on Ti-rich side is close to 50%Ni, and is nearly vertical, while the boundary on Ni-rich side decreases with low temperature and the solubility decreases greatly [3]
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