Phase Transformation And Mechanical Behaviors Of TiNiNbCo Alloys

The microstructure,phase transformation and mechanical behaviors of Ni_46.5-xTi_44.5Nb₉Co_x(x=0.5,1,1.5,2) wide hysteresis shape memory alloys have been systematically studied by means of optical microscopy observations, SEM, TEM,EDS,EPMA,XRD,DSC and bending tests as well as tensile tests at various temperature in this thesis. The effects of the addition of the fourth element Co and heat treatment on the microstructure, phase transformation and mechanical behaviors are also revealed.The experimental results show that the microstructure of NiTiNbCo alloys consists of TiNi matrix andβ-Nb phase. Co is distributed in TiNi matrix andβ-Nb phase by the form of solid-solution. The addition of Co restrains the appearance of blockly brittle (Ti,Nb)₂Ni phase effectively in NiTiNb alloys. Annealling treatment in the temperature range of 850℃¹000℃or aging treatment in the temperature range of 400℃⁶00℃have no substantial influence on the microstructure of NiTiNbCo alloys, The morphology ofβ-Nb phase in the aged NiTiNbCo alloys is ellipsoid and spherical shape, which is distributed equably in TiNi matrix .The alloys exhibite one-step martensitic transformation during the process of cooling and heating.The addition of Co has no effect on the transformation sequence but plays down the transformation temperature of NiTiNb alloy remarkably. Transformation temperature will descend about 30℃per adding 1at.%Co. when annealling treatment in the temperature range of 850℃¹000℃or aging treatment in the temperature range of 400℃⁶00℃, the transformation temperature of NiTiNbCo alloys shows linear increase with the increment of treatment temperature .Tensile tests show that the elongation of experimental alloys increases obviously with the increment of Co content, while the yield stress increases first and falls down later. The yield stress of Ni₄₆Ti_44.5Nb₉Co_0.5 alloys gets vertex. The strength value change little with the increment of Co content when the Co content exceeds 1.5at%. The results of recovery stress tests show that the recovery stress of Ni46Ti44.5Nb9Co0.5 alloys can reach as high as 490MPa,...