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In order to study the aging hardening characteristics and isothermal aging process of Ti-6Cr-5V-5Mo-4Al-1Nb alloy the time-temperature-transformation (TTT) curve of the alloy was measured by aging hardness method, and the microstructure changes during isothermal process were analyzed by XRD and SEM. It was found that the alloy had a high aging response speed at other aging temperatures except for the slow aging at 300°C and 400°C. When the aging time is prolonged at medium and low temperatures, the alloy can also produce the same aging strengthening effect as that at medium temperature aging, indicating that the temperature window of the alloy aging is wide, the aging strengthening effect is good, and it has the operability of heat treatment strengthening, so it is an ideal structural material. The isothermal aging transformation curve of the alloy is C type, and it can be obtained from the figure that the nose temperature is about 545°C

The reverse transformation temperature, recovery strain and pull-out force of TiNiFe alloy pipe couplings under different diameter expansion ratios (7%, 9%, 11%, 13%, 15%) were studied. The results show that the martensite reorientation occurs and the elastic strain energy released decreases with the increase of the expansion ratio, which leads to the increase of martensite reverse transformation temperature as’ and Af’. The recovery strain increases and the recovery rate decreases with the increase of expansion ratio. The expansion ratio of pipe couplings has a great influence on the pull-out force, which is related to the fit clearance and the diameter of the connected rod. When the fit clearance is the same, the larger the expansion ratio is, the greater the pull-out force is, and when the diameter of the connected rod is the same, the pull-out force decreases with the increase of the expansion ratio.