Study On The Microstructure, Phase Transformation And Texture Of Ti₄₄ni₄₇nb₉ Shape Memory Alloy Tubes

Ti44Ni47Nb9 shape memory alloy has wide thermal hysteresis and excellent processing performance. The shape memory connecting and fastening parts made by the alloy can be stored and transported at room temperature, so their application in engineering is very convenience. Therefore, they will be widely applied to the aviation, spaceflight, petrochemistry fields. The jointing of them with the connected parts is carried out by the shape memory effects in a given direction, and the texture formation results in anisotropy of shape memory behaviors. At present, the study on Ti44Ni47Nb9 alloy texture is very few, and the study on microstructure, phase transformation and texture of Ti44Ni47Nb9 shape memory alloy tubes has not been reported in public yet.In this thesis, the microstructures, textures and phase transformations in the forged rod, hot-extruded tube and cold-rolled tube of Ti44Ni47Nb9 alloy and influence of heat treatment on them were investigated by optical microscope (OM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) in details. The aim is to provide experimental data, optimize processing parameters for its application in engineering, at the same time, and to lay the foundation for revealing the mechanism that preferential orientation effects mechanics and memory behaviors of shape memory alloys.Microstructural studies show that B2 phase in the forged rod has broad fibred shape, and B2 phase fibre in the hot-extruded tube becomes fine and has been broken, and the microstructure of the cold-rolled tubes consists of B2,β-Nb and (Ti,Nb)4Ni2O phase, B2 phase fibre has heavily been broken and its grain size becomes more finer. Recrystallization starting temperature of the cold-rolled tube is about 400℃, with the increase of quenching temperature, the grain size of B2 phase decreases, after quenching temperature reaches 600℃, some recrystallized grains have grown up. When the quenching temperature is 850℃, the grain growth is very obvious.Phase transformation results indicate that phase transformation points of the hot-extruded tubes increase with the increase of quenching temperature, thermal hysteresis is basically constant. With the increase of annealing temperature, Ms point declines, As point change is not obvious, thermal hysteresis increases. There is no phase transformation during thermal cycle for the cold-rolled tubes due to fine grains and a lot of defects inside grains. When the quenching temperature is more than 400℃, phase transformations occur because recrystallization reduces defects in B2 phase, at the same time, with the increase of the quenching and annealing temperatures, transformation points go up and thermal hysteresis drops.Texture analyses show that main texture components of the forged rod are near{112} <111>and{123}<111>. Orientations of many grains are close to{111}<U V W>in the hot-extruded tube, the textures of the hot-extruded tube quenched at 600℃are still close to {111}, but intensity markedly enhances. And plane orientations of some grains of the annealed hot-extruded tube are close to ont only{221} and{211}, but also{111}. When the quenching temperature is 850℃, the main texture components are close to{112} <110>, but for annealed hot-extruded tube, the texture component enhances, and{112} <120>component appears also. In the cold-rolled tube, the major texture components are {111}<110>and{112}<110>, the above texture components in the cold-tube treated at 600℃do not change, but intensity enhances, especially in the annealed cold-rolled tube. When quenching temperature is 850℃, the intensity of the main texture components weakens, and plane orientations of many grains are close to{112} and{221}, and the main texture components obviously depart from{111}<110>and{112}<110>for the annealed cold-rolled tube, which is associated with grain growth during heat treatment.Ms of the cold-rolled tube annealed at 600℃is-111.70℃, thermal hysteresis is 50.36℃, <111>orientations of many grains are parallel to the radial direction of the tubes, which are beneficial to the application of the alloy in engineering.