高强钛合金伪调幅分解均匀析出行为及其强韧化效应

Homogeneous precipitate with high nucleation rate governed by pseudospinodal decomposition was found recently in titanium alloy and this new precipitation mechanism plays an important role in improvement of desirable mechanical properties of titanium alloy. Titanium alloys with high strength are selected as experimental materials in present project. The critical composition is a key parameter for pseudospinodal decomposition and it is calculated by the thermodynamic database of titanium alloy. The dependence of nucleation modes of alpha phase on aging temperature and chemical composition of beta phase will be investigated in detail. SEM、EDS、TEM and 3DAP will be used to identify the crystal structures and chemical composition of alpha and beta phase, respectively, and the variance of crystal structures and chemical composition of two phases during aging will be characterized. Nucleation and distribution of alpha plates are calculated and studied via the phase-field method, which could show the key parameters for controlling nucleation rates of alpha plates in beta phase. The results also shed light on the competition between homogeneous precipitation of alpha phase inside beta grains and along beta grain boundaries. The strength of micron-scale beta transformation with different chemical composition will be measured by nano-mechanic method, which can build a relationship between beta phase transformation and mechanical properties. Based on above results, fine homogenous precipitation of alpha plates without beta grain boundary alpha will be obtained in both metastable beta and alpha/beta titanium alloys, which will bring about good combination of high strength, ductility and toughness. The research of this project will supply a new theory for designing microstructures and mechanical properties and will also present experimental data for developing theory of strengthening and toughening of alloys.

最近在钛合金中发现了形核率极高的伪调幅分解均匀析出现象，有望对合金组织与性能调控发挥重要作用。本项目以高强钛合金为研究材料，利用热力学数据计算伪调幅分解均匀析出的关键参数—临界成分；研究beta相时效中alpha相形核方式随成分和温度的变化规律，揭示不同时效阶段alpha、beta相晶体结构与化学成分演化特点，确定伪调幅分解均匀析出alpha相形核率的调控参数；澄清alpha相在beta晶内均匀析出与沿beta晶界非均匀析出方式的竞争和制约关系。采用微纳技术测试不同成分微米尺度单一beta相转变组织的强度等性能，将相变形核方式与组织、性能相关联。用beta相伪调幅分解高形核率的特点抑制晶界alpha的形核与生长，调控合金微观组织结构，实现钛合金高强度与高塑性、韧性的良好匹配。本项目研究将为高强韧钛合金组织与性能的设计提供新的理论依据，为金属材料强韧化理论的发展提供重要的实验支持。

航空航天工业期望能够提供更高的强度、塑性与韧性的高性能钛合金。本项目研究了高强高韧钛合金的伪调幅相变机制与规律及其在组织设计中作用，期望对钛合金的强度塑性匹配进行改进；同时对高强高韧钛合金的塑性变形过程、本质以及与组织结构的关联开展了系统深入的研究，对合金的强化、增塑和韧化提供重要的参考。对Ti1023合金的伪调幅分解机制展开研究，用Padant数据库进行计算550-700℃下alpha、beta两相自由能并确定了临界成分C0，根据伪调幅发生条件确定Ti1023合金的伪调幅分解发生于550℃。Ti1023合金伪调幅分解机制除了高的形核率，还使合金硬度随时间表现出二次硬化效果，并且有反常的膨胀行为和合金体积的异常变化。通过第一性原理计算发现二次硬化和异常膨胀行为源于Al,V,Fe在alpha、beta两相中的扩散结果。基于伪调幅分解和常规形核机制设计了高温/低温时效工艺获得了多尺度的层片组织，相较于传统单一温度时效，高温/低温时效获得多尺度组织的强度和塑性匹配更好。用纳米力学测试技术获得了Ti55531合金beta晶粒析出纳米尺度alpha相的组织的本征强度可达到2.2GPa。在压缩变形中滑移在纳米尺度alpha和beta相之间传递类似于“接力赛”模式，使微尺度试样具备稳定的塑性。在组织结构优化设计研究中发现微米尺度初生alpha相对于高强高韧钛合金的塑性具有关键作用，引入微米尺度alpha相提高合金的塑性变形能力，获得良好的强度与塑性的匹配。在TC21高强高韧钛合金中发现滑移在层片状alpha和beta相运动方式，滑移起始于alpha相，运动到alpha-beta相界面，能否穿过界面取决于beta 相的结构，beta相内无析出alpha相则滑移穿过界面，如果有析出alpha相则滑移阻止于界面，层片alpha则开启多滑移。基于层片组织的滑移与界面的作用特点建立了层片组织钛合金的强度预测模型。在新型高强高韧钛合金微观组织与强度、韧性的本征关系的研究中发现，在相同强度水平下，层片组织的断裂韧性高于双态组织，主要是两种组织中背应力的不同。