5-20. High-strength titanium alloy design through pseudo-spinodal mechanism and high-throughput experiments

Wu Di，Liu Bin，Zhang Xiaoyong，Liu Libin，Zhao Jicheng，Zhou Kechao

Central South University

Abstract: Titanium alloys are widely used in the aerospace industry due to their high strength, high toughness, low density, high temperature resistance and corrosion resistance. In recent years, near-β and metastable β-type titanium alloys have obtained high strength, good plasticity and fracture toughness matching due to the remarkable age-strengthening effect. The dispersed α phase precipitated in the β matrix is the main reason for obtaining high strength. Recently, a new pseudo-spinodal mechanism in titanium alloys has been proposed. The phase field simulation results show that the precipitation efficiency of the α phase is 100 times that of the nucleation-growth mechanism, and the super-dispersed α phase can be obtained.

Based on the theory of pseudo-spinodal, this study combines CALPHAD and high-throughput experiments to study the effects of the contents of Mo, Cr, Fe and Zr on the microstructure and properties of Ti–6Al–4V alloy.

Studies have shown that Ti–6Al–4V has coarse α lath after aging at 600°C. As the content of Fe, Cr, Mo, and Zr increases, the α phase becomes thinner. When the contents of Fe, Cr, Mo and Zr reach 5%, 6%, 6% and 30% (mass percentage), respectively, the alloy obtains ultra-fine α phase, and then the α phase gradually coarsens as the content of Fe, Cr, Mo and Zr increases. The Gibbs free energy curves of the α and β phases of the alloys were calculated by the CALPHAD. It was found that the alloy compositions obtained by the ultrafine α phase were all within the narrow range of pseudo-spinodal. The strengths of Ti6Al4V5Fe, Ti6Al4V6Cr1Fe, Ti6Al4V6Mo and Ti6Al4V30Zr, which are designed by pseudo-spinodal and high-throughput experiments, have reached or exceeded the Ti-5553 alloy.

In addition, in the high-throughput experiments of material processing, the Ti–3Al–5Mo–4.5V alloy was hot extruded and finite element simulated by making samples of double truncated cone shape. The results show that during the extrusion process, the surface of the sample has less strain and the core strain is larger, and a continuous strain gradient is obtained in one sample. When the strain of the alloy exceeds 1.8, the alloy will obtain ultrafine grains. According to the high-throughput experimental results, the alloy was subjected to rolling. The results show that when the rolling strain is 1.8, the alloy has an ultra-fine structure with strength of 1430 MPa and elongation of 6.8%.

In order to quickly obtain the optimum heat treatment temperature, a tapered hollow graphite rod was produced. When the current passed through the graphite rod, a continuous change in temperature was achieved due to the continuous change of the conductive cross-sectional area. At the same time, multiple thermocouples can be used to test the temperature at different locations, and a temperature gradient can be obtained in one sample. Through the obtained temperature gradient, the microstructure of Ti-5553 alloy annealed at different temperatures was quickly obtained, and the most suitable heat treatment temperature of the alloy was quickly found.

Keywords: High-throughput experiment; high-strength titanium alloy; pseudo-spinodal mechanism; CALPHAD; material design

基于伪调幅分解和高通量实验的高强钛合金设计

吴迪，刘彬，张晓泳，刘立斌，赵继成，周科朝

中南大学

摘要：钛合金由于其具有高强度、高韧性、低密度、耐高温和耐腐蚀等优良特性而广泛应用于航天航空领域。近年来，近β和亚稳β型钛合金由于显著的时效强化效应，可获得极高的强度以及良好的塑性和断裂韧性匹配，成为高强度钛合金的重要发展方向。钛合金β基体中析出的弥散α相是其获得高强度的主要原因。近来，钛合金中一种新的伪调幅分解机制被提出，相场模拟结果显示，该机制作用下α相的析出效率是形核–长大机制的100倍，可获得超弥散α组织。

本研究以伪调幅分解理论为指导，结合CALPHAD计算与高通量实验，研究Mo、Cr、Fe、Zr四种元素含量变化对Ti–6Al–4V合金组织性能的影响。

研究发现，在600℃时效状态下Ti–6Al–4V基体具有较粗的α相片层组织，随着Fe、Cr、Mo、Zr含量增加α相逐渐变薄，当Fe、Cr、Mo、Zr含量分别达到5%、6%、6%、30%时，合金获得超细的α组织，之后随着Fe、Cr、Mo、Zr含量增加，α相逐渐粗化。利用CALPHAD方法计算相关体系合金α和β相的吉布斯自由能曲线，发现获得超细α相的合金成分，均处于可发生伪调幅分解的狭小成分范围内。利用伪调幅分解和高通量实验设计的Ti6Al4V5Fe、Ti6Al4V6Cr1Fe、Ti6Al4V6Mo、Ti6Al4V30Zr四种新型钛合金强度均达到或超过了Ti5553合金。

关键词：高通量制备；高强钛合金；伪调幅分解；CALPHAD；材料设计