Study On Microstructures, Tensile Properties And Shape Memory Performance Of The Cold-drawn Ti₅₀ni₄₇Fe₃Shape Memory Alloy Rods

In order to further improve mechanical properties and shape memory performance of the Ti50Ni47Fe3shape memory alloys (SMAs), the Ti50Ni47Fe3SMA rod was prepared by the cold drawing in the paper. At the same time, the microstructures, mechanical properties and shape memory performance of the cold drawn rod and the influence of heat treatment on them were investigated by the optical microscope (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), electrical transport testing system (ETTS), Rockwell hardness tester (HRC) and tensile testing machine.The aim is to provide reference data for its application in engineering.The microstructures are similar at the center and half radius areas of the cold drawn rod. However, the grains are finer and size distribution is uniform at the edge area, and the density of deformation twins is higher. The recrystallization first occurs at the edge part of the rod at500℃. With raising heat treatment temperature, the recrystallization extent enhances and the first formed recrystallized grains occur growth, especially in the edge and half radius areas. When heat treatment temperature is750℃, the microstructures of the whole cross-section become homogeneous and the grains become equiaxed, the twin density markedly decreases.The deformation twins in the cold drawn rod are {112} type, and the dislocation configuration represents tangled and cellular structures, at the same time, the Ti4(Ni, Fe)2O oxide particles were observed in some grains, around which there existed the wider elastic distortion area and therefore, inducing the high density dislocations. The twin density, shape and distribution are near in the cold drawn rods heat treated at temperatures below or equal to500℃those in the as cold drawn, the density is still higher, and the size is bigger, the boundaries are curved, and the twin distribution orientation is close in every grain. When heat treatment temperature is500℃, the density of dislocations slightly reduces inside grains and twins, and dislocation wall and therefore, producing sub-grains, can be observed. The twins become fine and density decreases, and more twin orientations can be observed inside a grain at600℃. When heat treatment temperature reaches700℃, the twin density obviously declines and number of grains, inside which there are no twins, increases. With increasing temperature, the dislocation density in both grains and twins continues to decrease, and at750℃, the fine annealing twins can be observed inside the grains.With the increase of temperature at the range of below or equal to500℃, the yield strength slowly decreases, and they are higher that of the as cold drawn and remain over585MPa. At550℃, the yield strength sharply decreases, the extent in the decrease reaches to13MPa. Subsequently, with continuously increasing temperature, the yield strength slowly decreased again. When the temperature is850℃, the yield strength is still as high as535MPa. For the cold drawn rod cooled in the air after holding for30min at500℃, with raising the pre-strains, the shape recovery strain ssrs and recovery stress both increase, the starting temperature of reverse martensitic transformation As’first increases, and reaches the maximum at9%pre-strain, and then drops. Therefore, the highest recovery strain and recovery stress are obtained, meanwhile, the wider thermal hysteresis (As’-Ms) is kept for the cold drawn rod pre-strained to10%at the pre-strain ranges from6%to10%.