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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Abstract
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.