| Among all the shape memory alloys, performance of the near-equiatomic Ti-Ni alloys is the best, life is the longest and use is the most stable. So far, the alloy rods are mainly processed by the hot rolling. In this paper, the hot-rolled rods are swaged to different deformation amounts in order to increase mechanical properties of the rods and get ideal reverse martensite transformation temperature. The purpose is to provide theoretical and experimental basis for its practical application in engineering.In this article, microstructures and phase transitions of Ti-Ni alloys with different Ni content, the influence of heat treatment on microstructures of the deformed rods, and of deformation amounts on microstructures, phase transitions and tensile properties of Ti-49.7at.%Ni and Ti-50.7at.%Ni alloys are studied by DSC, electronic universal testing machine with a program temperature controlling condition room, optical microscope and XRD. The following main results are obtained.Microstructure of the hot-rolled Ti-50.7at.%Ni alloy rods consists of parent phase, Ti4Ni2O oxide and few martensite at room-temperature. For the Ti-49.7at.%Ni alloy, microstructure consists of martensite and Ti4Ni2O oxide. The hot-rolled Ti-49.7at.%Ni and Ti-50.7at.%Ni alloy rods have occurred recrystallization and formed fine grains after heat treated at400℃. With the increase of heat treatment temperature to600℃, the recrystallized grains have occurred growth.The swaging deformation improves the homogeneity of the microstructures of Ti-49.7at.%Ni alloy rods, but the size of grains does not take place significant change. The phase transition peak of Ti-49.7at.%Ni alloys moves to low temperature zone during heating. The As point decreases from70.67℃of the hot-rolled rod to56.38℃of the swaged rod deformed to13.5%deformation amount. With increasing deformation amount, the surface relief on Ti-50.7at.%Ni alloy sample surface can be observed due to the occurrence of stress-induced martensite transformation. The phase transition peak of Ti-50.7at.%Ni alloys first moves to high temperature zone during heating, and then moves to low temperature zone when deformation amount reaches10.3%. The phase transition peak becomes more and more gentle. The As point raises from27.77℃of the hot-rolled rod to34.98℃of the swaged rod deformed to3.1%deformation amount, and the As and Ms drops down to temperatures less than room temperature at10.3%deformation amount.The hardness value does not change when the hot-rolled Ti-49.7at.%Ni and Ti-50.7at.%Ni alloy rods are heat treated at temperatures below to300℃. The hardness value dramatically decreases at400℃and then becomes stable after600℃. The hardness value of Ti-50.7at.%Ni alloys is higher than that of Ti-49.7at.%Ni alloys at the same heat treatment temperature. With the increase of the deformation amount, the hardness of the two alloy rods increases.Compared with the hot-rolled rods, yield strength as of the swaged Ti-49.7at.%Ni alloy rods after loaded in tension to rupture at room temperature decreases and fracture strength Ob changes insignificantly at3.2%deformation amount. The as increases and is lower than that of the as hot-rolled and σb increases obviously at13.5%deformation amount. The as of the swaged alloy rods heat treated at400℃after loaded in tension to rupture at room temperature decreases at3.2%deformation amount, and then increases and is significantly higher than that of the as hot-rolled at13.5%deformation amount, the σb increases with raising deformation amount. With the increase of deformation amounts, both σs and σb of the alloy rod after loaded at120℃enhance.The yield strength σs of the swaged Ti-50.7at.%Ni alloy rods after loaded in tension to rupture at room temperature decreases and fracture strength σb changes insignificantly at3.1%deformation amount in comparison with the hot-rolled rods. The σs increases and is higher than that of the as hot-rolled and σb increases obviously at10.3%deformation amount. The σs and σb of the swaged alloy rods heat treated at400℃after loaded in tension to rupture at room temperature decrease at3.1%deformation amount, the as increases and is lower than that of the as hot-rolled and the ab changes insignificantly at10.3%deformation amount. With raising deformation amount, both as and σb of the alloy rod after loaded at120℃increase. |
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