核压力容器用钢高温大变形所致织构大晶粒的形成条件与演化机理

The grains with obvious texture tendency are easy to merge into textured large grains because of the large deformation and following with long holding time at high temperature. Textured large grains have strong impact on the anisotropy of the materials. And they can severely reduce the plasticity, toughness and fatigue properties of forgings. The formation of textured large grains is due to the comprehensive effect of the initial grain, the distribution of the strain during deformation and the preferred orientation during nucleation and growth of the newly recrystallized grains. However, it is difficult to reveal the formation and evolution mechanism only using experimental observation. At present, there is also a lack of effective calculation method. The project aims to investigate the refinement condition and evolution mechanism of the textured large grains during heavy forging process for nuclear island, though experimental observation, forging process and multiscale simulation, taking the evolution of the dislocation density and the texture as the main line, combining the characterization of grain boundary with the preferred orientation by using molecular dynamics and building the transformation rules, and establishing the multi-level mesoscale model, to reveal the formation condition and evolution mechanisms of the textured large grains. The research of this project will form an effective computational method for the prediction of grain and texture evolution, and the achievement will provide theoretical basis for the control of heavy forging for nuclear island.

因大变形而产生明显织构倾向的晶粒长时间处于高温环境易发生合并形成织构大晶粒，引起材料性能各向异性，严重降低锻件的塑性、韧性和疲劳性能。织构大晶粒的形成与初始晶粒状态、变形过程中的应变分布和再结晶形核与长大的择优取向等因素有关，然而仅通过实验观测很难揭示其形成条件与演化机理，目前尚缺乏有效的研究手段。本项目以厘清核岛大锻件锻造过程织构大晶粒的形成条件和演化机理为目的，采用微观组织表征、锻造工艺实验和多尺度模拟相结合的方法，以非连续热变形过程位错密度与织构演变为线索，结合分子动力学计算表征与晶界特征相关的择优取向并转化为织构模拟中的取向转变规则，建立考虑各向异性晶粒三维晶体学信息的多级元胞模型，揭示织构大晶粒形成条件与演化机理，提出避免出现织构大晶粒的极限参数范围确定方法。通过对上述问题的深入研究和解决，形成一套行之有效的晶粒与织构演化计算方法，为核岛大锻件织构大晶粒的预控提供理论分析依据。

核电大锻件材料在热变形过程中由于局部大变形和高温长时间停留易导致形成织构大晶粒。目前对核电用钢在复杂热变形过程中晶粒形核长大的择优取向和织构大晶粒的形成与演化机理之间的关联机制尚不明确。本项目以典型能源大锻件材料为研究对象，开发了能描述再结晶有效形核和晶粒拓扑变形机制的多级元胞自动机模型，并将其与晶体塑性模型相结合，构建并验证了能够考虑初始晶体学信息和晶粒内位错密度分布梯度的晶体塑性—多级元胞自动机模型。在此基础上，按照基于“择优长大”理论制定的有形成再结晶织构趋势的再结晶织构演变元胞状态的转变规则，进行了再结晶织构演变的模拟研究。为深入研究再结晶织构及与之相关的织构大晶粒的演化机理服务。本项目的主要结论如下：（1）通过将多级元胞空间的概念引入到传统的元胞自动机模拟框架内，制定了再结晶中的元胞状态转变规则及不同级元胞空间之间的数据传递规则，首次建立了用于模拟动态再结晶的多级元胞自动机模型。（2）厘清了多级元胞自动机模型中晶界映射应变增量及全场多级晶粒拓扑变形模块中的级数对模拟结果的影响规律。结果表明，多级元胞自动机模型中的离散元胞空间面积（即晶粒拓扑映射精度）随级数的增加而增大，而随离散应变增量的增加而减小。（3）构建了能够考虑初始晶体学信息和晶粒内位错密度分布梯度的晶体塑性—多级元胞自动机模型，并通过微观观测实验验证了该模型的有效性。基于“择优长大”理论制定的有形成再结晶织构趋势的再结晶织构演变元胞状态的转变规则，建立了晶体塑性—多级元胞自动机模型再结晶织构演变模拟方法。通过对上述问题的深入研究和解决，形成了一套行之有效的晶粒与织构演化计算方法，为核能大锻件织构大晶粒的预控提供理论分析依据。项目执行期间共发表中英文学术论文9篇，申请技术发明专利1项，开发的计算软件已经为我国第三代核电大锻件自主研制提供了科学的计算手段和分析方法。