Compared with near equiatomic Ni-Ti alloys, Ti50Ni50-xCux alloys have some special properties and show great value of application. But their hot work abilities have not been well investigated.
Hot work abilities of Ti50Ni50-xCux alloys were investigated by means of Gleeble thermal simulation test. Effect of the Cu content, tempeature and deformation speed on deformation resistance was abtained according to the compression curves. The true stress-true strain curves were modified because of the existence of friction. The constitutive equation of Ti50Ni45Cu5 alloy was established by taking advantage of Jonas hyperbolic sine model and the modified datas. The calculation result based on the constitutive equation agrees with the experimental result well, which proves the validity of the equation.
Most of previous researches have been done with Ti50Ni50-xCux alloys, in which Cu atoms substitute for Ni atoms and occupy Ni sites. In Ti50-x/2Ni50-x/2Cux alloys, the possibility of Cu atoms occupying Ni sites is 0.52±0.18. The main research was made on the microstructure, transformation behavior and shape memory effect of Ti50-x/2Ni50-x/2Cux alloys.
Microstructures of Ti50Ni50-xCux alloys are composed of NiTi phase and a little of Ti2Ni phase. When the Cu content is low, microstructures of Ti50-x/2Ni50-x/2Cux alloys are also composed of NiTi phase and a little of Ti2Ni phase and heat treatment has no effect on them. As the Cu content adds up, Cu atoms in NiTi phase will be over-saturated and CuNiTi phase will appear.
When Cu atoms in NiTi phase is over-saturated, a lot of needle-like CuNiTi phase will precipitate after heat treatment. The precipitation is essentially a course of diffusion and temperature of heat treatment has a great effect on it.
Ti50-x/2Ni50-x/2Cux alloys (x=2, 5, 10, 15) show complicated transformation behavior. They perform a two-stage transformation or a one-stage transformation during the course of cooling and the transformations are all from cubic(B2) to monoclinic(B19′). Heat treatment has no effect on the transformation behavior of Ti49Ni49Cu2 alloy, but influences that of Ti47.5Ni47.5Cu5、Ti45Ni45Cu10 and Ti42.5Ni42.5Cu15. Heat treaemnet results in CuNiTi phase precipitating in NiTi phase and the precipitation lowers the transformation temperatue.
Ti49Ni49Cu2 alloy has good hot work ability. Ti49Ni49Cu2 wire shows good superelasticity and shape memory effect. The maximum recovery strain can reach up to 6.2%. It was also observed that the stress-induced martensite can get stablized easily in some tensile tests.
Hot work ability of Ti47.5Ni47.5Cu5 alloy is bad and prone to fracture during the course of forging because CuNiTi phase greatly deteriorates hot work ability. But Ti47.5Ni47.5Cu5 alloy also shows good shape memory effect.
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