TC16两相钛合金TRIP效应调控机理研究

TC16 (Ti-3Al-4.5Mo-5V) α+β titanium alloy is designed for manufacturing the fastening piece of aero application by cold forging, while the alloy should show low yield stress and high plasticity for cold forging, meanwhile should show high strength and plasticity under final serving condition. The project proposes to introduce TRIP effect (Transformation Induced Plasticity) into TC16 titanium alloy to optimize the strength and plasticity. Due to the element redistribution during heat-treatment in dual-phase region, i.e. α and β stabilizing elements separate in prior α grain and β phase respectively, the stability of metastable β phase in α+β alloys can be regulated by the design of elements distribution in β phase region. Once the stability of metastable β phase meets a critical condition, the stress induced martensite (SIM) can take place at ambient temperature, inducing the improvement of work hardening rate and plasticity. The proposers have proved that it is effective and feasible to introduce SIM and TRIP effect into α+β titanium alloys by optimizing chemical composition and thermal process. However, some key scientific mechanisms concerning SIM and TRIP effect in α+β titanium alloys remain unknown. The present project intends to investigate the relationship among heat-treatment, element distribution, lattice parameter and triggering stress/strain level of SIM，and plan to uncover the triggering condition of SIM for α+β titanium alloys. Also, the project will investigate the independent and cooperative influences of prior α grain and metastable β phase on SIM behavior. The findings of the present project will make a significant contribution to the regulation of TRIP effect in α+β titanium alloy, and can provide theoretical foundations for future development of dual-phase titanium alloys with TRIP effect.

TC16钛合金是针对航空用紧固件研发的一种冷镦成型α+β两相合金，冷镦成型时合金应具备低屈服强度、高塑性，而紧固件服役时则要求合金具有高强、高塑性。针对该技术需求，申请人基于前期研究提出两相钛合金相变增塑（Transformation Induced Plasticity, TRIP）效应调控的研究思路，即通过热处理调控合金亚稳β相的元素浓度，使其能够发生可控的应力诱发马氏体相变（Stress Induced Martensite, SIM），提升合金塑性并获得不同屈服强度。围绕两相钛合金TRIP调控科学问题，本项目将研究热处理-微区元素浓度-临界诱发SIM应力之间的关系，明确SIM的临界诱发条件；研究初生α对SIM的影响，揭示初生α、亚稳β在提升合金加工硬化率中的独立作用及协同效应。最终明确两相钛合金TRIP效应调控机理，为研发一系列两相TRIP钛合金及其工程化制备奠定理论基础。

围绕α+β双相钛合金中相变增塑(Transformation Induced Plasticity, TRIP)以及β基体中应力诱发马氏体相变(Stress Induced Martensite, SIM)相关的科学问题，以Ti-3Al-5Mo-4.5V（TC16）等双相钛合金为研究对象，研究了钛合金微区元素再分配、微区浓度对微区相组成/显微结构的影响、微区浓度对应力诱导α″马氏体的临界诱发应力的影响，得到以下主要结论：（1）α+β两相钛合金热处理过程中，Al等α稳定元素向α相中偏聚，Mo、V等β稳定元素向β相中偏聚，不同元素在α/β两相界面附近的成分梯度呈现差异。通过热处理可以实现对微区元素浓度的调控。（2）研究了TC16双相钛合金淬火组织随β稳定性的演化。随淬火温度增加，β稳定性逐渐提高，β相的淬火产物由无热ω（ωath）逐渐向α″马氏体转变；探讨了在时效过程中等温ω相（ωiso）演变过程以及ωiso辅助次生α（αs）形核机制。随着时效温度（300℃~450℃）的升高，β基体中首先析出不完全坍塌的胚状ωiso，接着，胚状ωiso在结构上进一步坍塌，形成理想ωiso。（3）研究了TC16钛合金中孪晶型淬火α″马氏体的分解机制。随着时效温度（400℃~600℃）的提高，在孪晶型淬火α″马氏体内部会逐渐发生分解，最终形成木梳状的α和β相。（4）基于对TC16微区浓度、相稳定性的定向控制，通过热处理实现了应力诱导α″马氏体的临界诱发应力的调控。随着低温时效时间的延长或低温时效温度的提高，合金的临界诱发应力逐渐增加。.通过开展以上研究工作，掌握基于热力学及动力学的TC16等双相钛合金微区元素浓度、α/β界面成分梯度的控制方法，实现了双相合金亚稳β相主导塑性变形方式的精准调控；明确了两相钛合金中微区元素浓度分布、α/β界面特征对微区力学行为的作用。在此基础上，发展了基于微区调控的高强高韧钛合金设计及工程化制备的研究思想，建立了基于“微区元素浓度-微区力学行为-宏观力学特征”的双相钛合金材料优化设计方法，实现了由传统“平均成分设计”向“微区浓度调控”的本质转变，为新型高性能钛合金材料及工艺设计提供了理论方法支撑。