Rare Earth La and Heat Treatment on As-Cast Microstructure and Mechanical Properties of Ti55531 Alloy

The unique combination of high specific strength and excellent corrosion resistance makes titanium alloys attractive for many aerospace and medical applications. Titanium alloy parts are cast using traditional casting，homogenization，cold and hot processing，and heat treatment. The process is complex，expensive to produce and lacks market competitiveness. The lightweight requirements of the aerospace industry put higher demands on the strength and toughness of the cast Ti55531 alloy. Compared with the disadvantages of the high cost and easy cracking of the forging method，the improvement of the properties of the cast alloy is of great importance. In this study，it was proposed to add rare earth element La to as-cast Ti55531 titanium alloy to change the content of O and N，explore the effects of different heat treatment parameters and environmental media on the microstructure and properties of Ti55531-0.1La titanium alloy，establish the corresponding relationship between the microstructure and mechanical properties，and lay a foundation for the preparation of high strength and toughness titanium alloy. Ti55531 alloy added 0.1%La（mass fraction），502 to 727 ℃ high temperature stage was easy to produce oxides in the order of Al2O3-Cr2O3-La2O3-TiO2. At the temperature of 502 ℃，the free energy of Al2O3 increased significantly and turns to an unstable state，and La2O3 was preferently formed after gradual deo. Due to the addition of 0.1%La in this alloy，vacuum annealing was required to prevent the increase in brittleness due to the large amount of oxidation of La and Ti，and slow cooling was also required to facilitate the formation of micro-nano La2O3 in situ. Considering the influence of the superheat of heat treatment and the heat transfer of equipment，the most effective effect was to increase the aging temperature to 580 ℃ and hold for 8 hours based on the theoretical temperature of 502 ℃. Therefore，vacuum heat treatment and long time aging treatment at 580 ℃ for 8 h were very important to improve the properties of Ti55531-0.1%La alloy，which was also the basis for formulating aging heat treatment process. Long time aging treatment at 580 ℃×8 h was very important to improve the properties of Ti55531-0.1%La alloy. La2O3 with the lowest free energy had sufficient formation time at this stage. Slow cooling promoted the formation of micro-nano La2O3 in situ，which could hinder the growth of α phase during heat treatment，refine the matrix structure and improve the strength. A reasonable vacuum solution and an aging procedure were chosen. After vacuum solution at 780 ℃×2 h and furnace cooling at 580 ℃×8 h and furnace cooling at 580 ℃×8 h，the acicular α phase was 0.5 µm wide and 1 µm long，and no coarse β phase was found. This kind of structure had a good effect on the improvement of tensile strength and elongation. The alloy in vacuum solution，furnace cooling to aging temperature，furnace cooling conditions，due to the aging of the secondary α phase effective spacing λ to 1 µm，could greatly improve the precipitation strength ∆ σOr，resulting in higher strength than other heat treatment. In addition，α-phase hexagonal close packed （hcp） structure with micro-nano spacing separated the body-centered cubic （bcc） matrix，inhibited the crack growth，hindered the slip of dislocation，and formed a hard phase which was not easy to slip，and improved the strength. At 580 ℃ × 8 h，due to the vacuum condition and long time aging treatment，La2O3 with the lowest free energy was preferred to form，which could hinder the growth of α phase during heat treatment and fine the structure. At the same time，the matrix β phase was bcc structure，which was conducive to dislocation slip and storage，forming a soft phase" which was easy to slip，and improving the toughness of the alloy. After vacuum solution at 780 ℃×2 h and furnace cooling to aging temperature at 580 ℃×8 h，α phase of hcp structure formed by micro-nano distance refined the matrix β phase of bcc structure. La2O3 formed by adding rare earth La could inhibit the growth of α phase during heat treatment. The improvement in strength and toughness of Ti55531-0.1%La was the main reason. The formation of the biphasic structure was the main reason for the enhanced strength and toughness of Ti55531-0.1%La. This study provided a new structure for enhancing the strength and toughness of the as-cast Ti55531 titanium alloy. © 2025 Editorial Office of Chinese Journal of Rare Metals. All rights reserved."