Microstructural characterization of HIP consolidated NiTi–nano Al₂O₃ composites

Highlights: • NiTi–6 wt.% nano α-Al₂O₃ composites have been produced using a HIP method. • Both elemental and prealloyed powders were used for the fabrication of composites. • Generation of mismatch stress and intermetallics affected martensitic transformation. • Nanoparticles partially inhibited thermally induced martensitic transformation. • An interwoven austenite–martensite structure was observed in the composite samples. - Abstract: The microstructure and phase transformational behavior of NiTi-based composites reinforced with 6 wt.% of α-alumina nanoparticles have been investigated. Two kinds of starting materials, elemental Ni–Ti and prealloyed austenitic NiTi, were used to prepare the composites. The samples were consolidated using a hot isostatic pressing method. The X-ray diffraction results showed that while unreinforced NiTi mainly contained B2 phase at room temperature, martensitic B19′ phase appeared in the microstructure after addition of the α-alumina nanoparticles. The differential scanning calorimetry measurements indicated that the martensitic transformation temperatures were elevated in the composite samples, but the transformational enthalpy was reduced in comparison with the NiTi sample. It is believed that the generation of thermal mismatch stress during the sintering and the formation of small contents of NiTi₂/Ni₃Ti intermetallics in the composite samples are responsible for this increment of the martensitic transformation temperatures. Also, due to the nanometric size of α-Al₂O₃, a larger fraction of the matrix is disturbed by the presence of the nanoparticles, which yields the formation of effective barriers to the thermally induced martensitic transformation in the nanocomposite samples. The high-resolution transmission electron microscopy studies of the samples confirmed the higher defect density and partial microplastic deformation in the composite samples