Micro-Deformation Mechanism Of Ti-Ni-Cu Alloy Ribbons

The martensitic transformation, mechanical behavior and microstructural evolution during deformation of Ti-Ni-Cu alloy ribbons have been investigated by means of TEM, XRD, DSC and tensile tests. The micro-deformation mechanisms of B19 martensite have been revealed.The results show that the Ti-Ni-Cu alloys and ribbons undergo B2-B19 one-step martensitic transformation in the cooling and heating process. When the Cu content are between 15 and 25at.%, the transformation order has no change, but the transformation hysteresis decreased with the increase of Cu content.The experiment results show that, After annealed at 500℃for 1 hour, the microstructure in the Ti_50.2Ni_29.8Cu₂₀ alloy ribbons are B19 martensitic. In addition, there are a few Ti2Ni precipitates. With the increase of annealing temperature, the transformation temperature increased, but the transformation hysteresis decreased gradually with the increase of the annealing temperature. With the increase of annealing time, the transformation temperature of Ti_50.2Ni_29.8Cu₂₀ alloy ribbons increased, but transformation hysteresis are not sensitive to the annealing time.The results show that the microstructure and phases on different surface of the Ti_50.2Ni_29.8Cu₂₀ alloy ribbons are different. The phases on wheel side are B19 martensite, however, there are both B19 martensites and B2 phase on the free side. After annealed at 500℃for 1h, B19 martensites are (011) compound twins in the Ti_50.2Ni_29.8Cu₂₀ alloy ribbons. In most grains, the (011) martensite plates are in two approximately perpendicular orientations, and the (011) martensite plates show a so-called single-pair morphology in few grains. There are GP zone and Ti2Ni precipitates in the martensite plates, they Separated the plate into a cellular structure. When the temperature is up to 700℃,the GP zones disappeared, and there are only Ti2Ni precipitates.Tensile test results show that the Yield strength and Young's modulus of Ti_50.2Ni_29.8Cu₂₀ alloy ribbons increased with the increase of annealing temperature, and the Yield strength and Young's modulus are both decreased with the increase of annealing time.In the Ti_50.2Ni_29.8Cu₂₀ alloy ribbons, (011)B19 compound twins martensites are in the process of reorientation at a deformation strain of 2%. In most grains, the martensite variants almost in one orientation. On the stage of deformation strain of 5%, the B19 martensites which are one orientation take place further reorientation, and the morphology of B19 martensites transform to single variant. In the ribbons, a lot of B19′martensites which are ( 1(?)1(?)1 )B19′type I twins appear, which represents that a stress-induced transformation from B19 to B19′has occurred.