Formation of single phase NiTi shape memory alloy via solid state processing in reducing environment

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Date
2016-07-01
Authors
Hafizah Hanim Mohd Zaki
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The fast growing application especially in medical and engineering fields demands the development of novel-structured NiTi which requires an alternative processing technique, other than the conventional melt-casting method. Solid state synthesis is an alternative processing technique that has been attempted for producing novelstructured NiTi such as porous NiTi and other novel forms. Whilst shape memory effect is a unique advantage of NiTi shape memory alloy like, no successful attempt so far to produce single phase NiTi with good transformation. NiTi produced via solid state synthesis have complex microstructures involving multiple phases of Ti2Ni and TiNi3. These phases do not exhibit martensitic transformation and jeopardise the shape memory behaviour of NiTi. Apart from that, the formation of oxides may hinder the formation of single phase NiTi. Therefore, this research examines the processing conditions that produce single phase NiTi with optimum transformation behaviour and also identifies the possible obstacles for forming single phase NiTi. This study synthesised Ni-Ti and Ni-TiH2 powder mixtures in flowing argon and reducing environment using CaH2 and MgH2 powder as the in situ reducing agent. Several effects of parameters such as sintering temperature, time, composition, and post-sintering were studied and a systematic comparative investigation was performed on phase formation, transformation behaviour, shape memory effect, and deformation behaviour of the sintered specimen. Elemental powder sintering of Ni-Ti did not result in single phase NiTi and no distinct peaks of martensitic transformation were observed on the sintered specimens. The use of TiH2 to replace Ti increased the volume fraction of NiTi and displayed martensitic transformation peaks, albeit with low transformation heat (≤13 J/g) revealing that oxidation is an active obstacle in producing single phase NiTi. Synthesis of Ni-TiH2 in reducing the environment, especially the use of CaH2 as an in situ reducing agent, resulted in the formation of single phase NiTi with enthalpy change of ~25-26 J/g, similar to melt-cast NiTi alloys. The surface analysis showed a significant reduction in oxide formation, thus minimising the effects on balanced composition between Ni and Ti. This reflects that an oxygen-free environment allows unhindered Ni-Ti reaction, which leads to the formation of single-phase NiTi. This work established the fact that, other than the thermodynamic considerations often being discussed in similar work as obstacles to single phase formation, oxidation appears to have profound effects on single phase formation in NiTi synthesized in solid state.
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