【作者】 罗宇；

【导师】 张静；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2021， 硕士

【摘要】 作为目前最轻的结构金属材料,镁合金在电子通讯、航空航天、汽车制造等行业都有广阔的应用前景。但是,镁合金的强度低、塑性成形性能差,制约了镁合金的应用。镁合金具有密排六方的结构,在室温下,非基面位错由于较大的临界剪切应力难开启,而{10-12}孪生由于较小的临界剪切应力易开启,因此在镁合金塑性变形中扮演着重要的角色。{10-12}孪晶拥有6个等价的变体,在复杂应力加载条件的作用下,晶粒内部不同区域可能会同时或先后发生不同的孪生过程,不同孪晶变体彼此不可避免地会相遇、发生相互作用并对后续的孪生产生影响,同时对组织结构的演变以及材料性能也有重要的影响。{10-12}孪晶的交互作用分为共轴孪晶交互作用和非共轴孪晶的交互作用,共轴孪晶交互作用的两个孪晶之间具有较小的取向差(约7.4°),非共轴孪晶交互作用的两个孪晶之间有较大取向差(约60°),导致两种孪晶交互作用后对微结构产生不同的影响,从而导致力学性能的差异。因此,澄清不同类型{10-12}孪晶交互作用对后续微观结构的影响,可为调控孪生、改善材料性能提供理论依据,具有重要的科学意义和实际应用价值。本文以轧制态AZ31镁合金板材作为研究材料,沿着轧向进行2%的预压缩,然后分别沿着轧向和横向再压缩,使材料产生不同类型的{10-12}孪晶变体的交互作用。采用金相显微和准原位EBSD观察不同类型孪晶交互作用后微结构的变化情况。结合分子动力学模拟不同类型孪晶交互作用后微结构的演化以及应力场变化。最后通过力学性能测试,探索不同类型孪晶交互作用对力学性能的影响。主要结论如下:(1)当预压缩方向与加载方向相同时,交互作用类型以共轴孪晶变体的交互作用为主,晶粒内部孪晶面积的增长主要依靠孪晶长大,共轴孪晶之间会发生相互兼并,很少会形核新的孪晶变体。当预压缩方向和再次加载的方向不同时,交互作用类型以非共轴孪晶交互作用为主,原有的孪晶基本停止生长,孪晶面积增长主要依靠新孪晶的形核和长大。(2)共轴孪晶变体交互作用后将联合在一起,非共轴孪晶变体交互作用后,形成的孪晶-孪晶界面(TTB)将阻碍新孪晶进一步传播,导致其侧向长大。此外,还发现非共轴孪晶交互作用会导致原有孪晶发生退孪生,并促进孪晶在孪晶界面处形核。(3)非共轴孪晶变体交互作用相对于共轴孪晶变体交互作用将产生更明显的加工硬化效应。并且,如果该交互作用应力集中区域有新的孪晶形核点,将更有利于后续孪晶形成。(4)共轴孪晶变体交互作用后,交互作用附近区域内应力的增量并不明显,而非共轴孪晶交互作用后,交互作用附近基体区域将产生明显的内应力增加,并且距离交互作用区域越近内应力的增加越明显。(5)非共轴孪晶的交互作用使得材料的强度和塑性均有所提高,且会导致材料在压缩后期阶段的应变硬化速率显著提高。更多还原

【Abstract】 As the lightest structural metal material at present,magnesium alloy has a broad application prospect in electronic communication,aerospace,automobile manufacturing and other industries.However,the application of magnesium alloys is limited due to their low strength,poor plasticity and poor forming properties.Magnesium alloys have a dense hexagonal structure,at room temperature,non-basal dislocations are difficult to open due to the large critical shear stress,while {10-12} twinning is easy to open due to the smaller critical shear stress,so deformation twinning plays an important role in the plastic deformation of magnesium alloys.The {10-12} twin has six equivalent variants,and under complex stress loading conditions,different twinning processes may occur simultaneously or sequentially in different regions of the grain,where the different twin variants inevitably meet,interact with each other and influence subsequent twinning,and have important implications for the evolution of the structure and material properties.The interaction of twins is divided into co-zone and non-co-zone twin interactions,with a small misorientation(～7.4°)between the two twins for co-zone twin interactions and a large misorientation(～60°)between the two twins for non-co-zone twin interactions,resulting in different effects on the microstructure of the two types of twin interactions,leading to differences in mechanical properties and mechanical properties.Therefore,clarifying the effect of different types of {10-12} twin interactions on the subsequent microstructure can provide a theoretical basis for regulating twinning and thus improving material properties,which is of great scientific significance and valuable for practical application.In this paper,the as-rolled AZ31 magnesium alloy sheet was used as the study material.It was pre-compressed by 2% along the RD direction,and then recompressed along the RD and TD directions respectively to produce different types of {10-12}twinning variants in the material.The changes in twinning in different compression directions were then observed by metallographic microscopy and quasi-in-situ EBSD combined with molecular.And molecular dynamics was used to study evolution of the microstructure and changes of stress field after the interaction of twins.Finally,the effect of different types of twin interaction on the mechanical properties was explored through mechanical property tests.The main conclusions are as follows.(1)When the pre-compression direction is the same as the re-loading direction,the interaction type is dominated by the interaction of co-zone twin variants,and the growth of the twin area within the grain mainly relies on twin growth.Twins mergers occurs between co-zone twins,rarely nucleating new twin variants.When the pre-compression direction and the re-loading direction are different,the interaction type is mainly non-cozone twin interaction,and the original twin basically stops growing,and the growth of twin area mainly relies on the nucleation and growth of new twins.(2)After co-zone twin interaction,the twin-twin interface(TTB)is formed,which prevents further twin propagation and leads to lateral growth of the twin.When non-cozone twin variants interact,twin de-twinning occurs and the multiple TTB interfaces forms.When non-co-zone twin variants interact,they can lead to twin de-twinning and promote twin nucleation at the matrix and the twin boundary.(3)Non-co-zone twin variants interaction will have a more pronounced workhardening effect than co-zone twin variants interaction and will be more conducive to subsequent twin formation if there are new twin nucleation sites in the stress concentration region of the interaction.(4)After co-zone twin variants interacting,the increase in internal stress in the region near the interaction is not significant,whereas after non-co-zone twin interaction,the increase in internal stress in the matrix region near the interaction will be significant,and the closer to the interaction region the more significant the increase in internal stress.(5)The interaction of non-co-zone twins increases the strength and plasticity of the material,and increases the strain hardening rate in the later stages of compression.更多还原