Fabrication of NiTi and NiTi-nano Al2O3 composites by powder metallurgy methods: Comparison of hot isostatic pressing and spark plasma sintering techniques

Abstract

To escape the detrimental effect of NiTi matrix decomposition on hot-isostatic-pressing (HIP)-processed NiTi-nano alumina composites, which leads to the formation of NiTi2/Ni3Ti intermetallics; in this study, spark plasma sintering (SPS) was utilized to restrict this phenomenon in NiTi and NiTi-6 wt% nano alumina composites. After optimization of the SPS processing conditions, the microstructural aspects of the SPS and HIP-consolidated samples were compared. According to X-ray diffraction analysis, the SPS-processed composites contained more B2-NiTi phase than the HIP-processed samples did. This is because of lower NiTi matrix decomposition due to formation of NiTi2/Ni3Ti phases. Scanning electron microscopy studies revealed that at the interface of alumina and the NiTi matrix, Ti-rich phases had evolved, while in the grain interiors, matrix decomposition mostly led to the formation of Ni-rich intermetallics. High-resolution transmission electron microscopy investigations confirmed an increased Ni/Ti ratio in the austenitic NiTi matrix in the vicinity of the Ti-rich phases, and the formation of martensitic NiTi near the Ni-rich phases. The results from differential scanning calorimetry indicated that the latent heat of martensitic transformation in the SPS composite is higher than that of HIP-processed composite samples due to lower matrix decomposition and higher NiTi phase, which can participate in martensitic transformation.