简单退火和多重热处理对激光熔覆成形Ti-5321合金断裂韧性的影响

Effect of Single Annealing and Multiple Heat Treatment on Fracture Toughness of Ti-5321 Alloy Prepared by Laser Cladding Forming

摘要:

为满足航空航天领域对快速制备高强韧复杂钛合金零件的需求，采用激光熔覆成形新型高强韧Ti-5321（Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe）合金，具有成形快、效率高、成形构件性能良好等优势。通过对Ti-5321合金进行简单退火和多重热处理（β相区固熔缓冷加时效，BASCA），揭示了其显微组织的演变规律，并探究不同组织对断裂韧性的影响。结果表明：简单退火后，合金呈细长针片状α交织成的网篮组织，其抗拉伸强度为1102 MPa，断裂韧性为68.1 MPa·m^1/2。经BASCA热处理后，细长针片状α相全部转变为粗大的片层α相和超细针状α相，且合金的抗拉伸强度提高至1309 MPa，而断裂韧性降低至45.5 MPa·m^1/2。BASCA热处理能够提高合金强度，但会降低合金韧性。这是由于网篮组织中细长的针片状α相能够大大增加裂纹扩展阻力，增大裂纹扩展路径曲折程度，从而提高合金的韧性。而BASCA热处理组织中大片层α相具有一定的方向性，裂纹只有在穿过不同β晶粒的大片层α相时才会发生偏转；且裂纹扩展主要发生在超细针状α相中，而超细针状α相尺寸过于细小，不能阻碍裂纹的发展或使裂纹偏转，因此经BASCA热处理后合金的韧性变差。

Abstract:

In order to satisfy the requirements of aerospace field for rapid preparation of high strength and toughness complex titanium alloy parts, new Ti-5321 (Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe) alloy with high strength and high toughness was prepared by laser cladding forming, which possesses the superiority of rapid prototyping, high efficiency, and good formability. Through single annealing and multiple heat treatment (β-annealing with slow cooling and aging, BASCA) on Ti-5321 alloy, the microstructure evolution was revealed, and the influence of different microstructures on the fracture toughness was explored. Results show that after single annealing, the alloy morphology presents the basket-weave structure consisting of elongated lamellar α phases. Its ultimate tensile strength is 1102 MPa, and fracture toughness is 68.1 MPa·m^1/2. After BASCA heat treatment, the elongated lamellar α phase changes to coarse lamellar α phase and ultrafine needle-like α phase. Thus, the ultimate tensile strength increases to 1309 MPa, whereas the fracture toughness reduces to 45.5 MPa·m^1/2. BASCA heat treatment can enhance the strength but degrade the toughness of alloys. This is because the elongated lamellar α phase in basket-weave structure can greatly increase the crack growth resistance and aggravate the tortuous degree of crack growth path, thus improving the alloy toughness. Coarse lamellar α phase after BASCA heat treatment has a certain degree of directionality, and the cracks only deflect when passing through the coarse lamellar α phase of different β grains. Crack propagation mainly occurs in the ultrafine needle-like α phase. However, due to the extremely small size of ultrafine needle-like α phase, it cannot hinder the development of crack path or deflect the cracks. Thus, the toughness of coarse lamellar structure becomes more inferior after BASCA heat treatment.