基于孪生随机形核的镁合金细观塑性本构与损伤行为研究

This project mainly focuses on the twinning mechanism and the micro-structural heterogeneous evolution of Mg alloy polycrystalline material due to short of independent slip system and occurring a large number of twin deformation, which results in poor plasticity and hard deformation at room temperature. In consideration of the randomness and heterogeneity of twinning nucleation and growth, the distribution and evolution of twin dislocation type relating to twinning nucleation are represented adopting the random theory or WEIBULL theory. The new description of Meso-plastic constitutive relation is attempted to develop based on the crystalline plasticity theory and the numerical simulation study is developed with polycrystalline aggregate. The main researches are that (1) An experimental study is carried out on the twin variety and morphology and crystallographic orientation with the matrix of Mg alloy samples at the different strain. (2) The mechanism of twinning nucleation is described with the random theory or WEIBULL theory and the new twinning deformation model is developed using the crystalline plasticity theory. On this basis, the new description of crystal meso-constitutive equation is established. (3) The law of twinning nucleation and growth is investigated by establishing representative volume element of polycrystalline aggregate with the new description of crystal meso constitutive equation at all kinds of loading path so as to further revealing the micro-structural heterogeneous evolution and ascertaining the damage mechanism of twin boundaries.

本项目针对镁合金在室温变形条件下独立滑移系不足，出现大量孪生变形，导致室温塑性低及变形加工困难的问题，着重研究孪生变形机制及其对镁合金多晶材料微结构非均匀演化的影响，考虑孪生形核与长大的随机性、非均匀性的特点，采用随机理论或WEIBULL理论描述与孪生形核相联系的孪生位错类型分布与演化，结合晶体塑性理论尝试发展一种细观塑性本构关系的新的方程描述，采用多晶集合体开展数值模拟研究。主要研究：（1）针对各种应变量的镁合金样品中孪晶的种类、形貌以及与基体之间的晶体学关系开展试验研究。（2）采用随机理论或WEIBULL理论对孪晶形核机制进行描述，并结合晶体塑性理论发展新的孪生变形模型，在此基础上，建立晶体细观本构方程的新的力学描述。（3）采用新的本构方程描述，建立多晶集合体代表性单元研究在各种路径加载情形下孪晶形核与长大规律，以及对微结构不均匀演化的影响，探明循环应变下孪晶界损伤机制。

室温下镁合金变形出现大量孪生变形，导致塑性低及变形加工困难等问题。本项目研究关注了镁及其合金为密排六方的晶体结构，孪生形核具有随机性、非均匀性的特点，力图采用WEIBULL随机分布描述孪生变形与微结构之间的关联特性，结合滑移与孪生两种变形机制建立新的镁合金晶体塑性本构关系，开展了一系列试验和数值模拟研究。本研究及重要结果包括：.1.采用EBSD对应变为4.9%的镁合金微结构进行统计，表明晶粒大小、晶界倾角等微结构分布基本符合概率函数WEIBULL分布特征；大晶粒、高Schmid因子、小角度晶界有利于孪生形核，而孪晶长大对微结构的敏感度较弱。.2.对镁合金在不同加载方式下的断裂形貌分析。发现断裂面形貌存在阶梯形态、解理形态和韧窝形态三种，与之相对应的细观变形机制分别为孪生-去孪生交替变形机制，孪生变形产生孪晶界裂纹及扩展机制和位错滑移与夹杂颗粒或微空洞相互作用机制。断裂面主要由多种细观变形机制及其对应破坏模式竞争形成。.3.基于滑移、孪生及去孪生变形，考虑了背应力演化，构建了镁合金晶体塑性本构关系。a.成功预测了2种平移应变下的扁长状屈服面，表现出镁合金单晶塑性各向异性性质。b.模拟了循环变形行为，表明多晶体内的孪晶体分数、应力应变及晶粒的应力三轴度在空间分布上的不均匀特性。模拟结果验证了模型的合理性。.4.采用伪滑移模式及含晶粒尺寸、晶向和晶界倾角等微结构参数的多晶三维微结构有限元模型，a.对挤压镁合金的单轴拉伸和压缩变形行为进行模拟，统计分析了孪晶体分数分布与晶粒尺寸、晶界位向角及晶界倾角等之间关联性；b.对轧制镁合金开展了不同路径下单轴压缩加载试验，表明镁合金宏观塑性各向异性行为与滑移、孪生变形机制的启动组合紧密相关。.5.采用WEIBULL概率函数描述孪生随机形核强度分布，对几种不同分布参数的镁合金压缩变形行为进行数值模拟。结果表明在强度均值相近情况下，镁合金多晶体的硬化行为趋于相同。但不同强度分布参数对多晶体内的应力、孪晶体分数有较大影响。