Microwave sintering effects on the microstructure and mechanical properties of Ti−51at%Ni shape memory alloys

Abstract

Ti−51at%Ni shape memory alloys (SMAs) were successfully produced via a powder metallurgy and microwave sintering technique. The influence of sintering parameters on porosity reduction, microstructure, phase transformation temperatures, and mechanical properties were investigated by optical microscopy, field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), compression tests, and microhardness tests. Varying the microwave temperature and holding time was found to strongly affect the density of porosity, presence of precipitates, transformation temperatures, and mechanical properties. The lowest density and smallest pore size were observed in the Ti−51at%Ni samples sintered at 900°C for 5 min or at 900°C for 30 min. The predominant martensite phases of β2 and β19′ were observed in the microstructure of Ti−51at%Ni, and their existence varied in accordance with the sintering temperature and the holding time. In the DSC thermograms, multi-transformation peaks were observed during heating, whereas a single peak was observed during cooling; these peaks correspond to the presence of the β2, R, and β19′ phases. The maximum strength and strain among the Ti−51at%Ni SMAs were 1376 MPa and 29%, respectively, for the sample sintered at 900°C for 30 min because of this sample’s minimal porosity.