Materials World Network: Developing a New Mg Alloy with Optimized Texture for Enhanced Formability

该项目的目标是由弗吉尼亚大学（UVA）和汉堡哈堡技术大学（TUHH）以及附近德国Geestacht的国家实验室GKSS共同努力，开发一种具有优化纹理的新型锻造镁合金，以提高成形性。尽管镁合金具有许多吸引人的性能，但其应用受到限制，部分原因是低温成形性差。由于六方密排晶体结构固有的塑性各向异性，其形成行为受到晶体织构的强烈影响。传统镁合金板材具有较强的织构（基底面与板材平面平行），不同合金间的织构变化不大。最近才有报道称，含有大量稀土元素和钇元素的合金在挤压过程中形成的织构比常规合金弱得多，初步结果表明，随机化镁合金板的织构可以改善成形性。稀土/Y的加入对织构演变的影响机制目前尚不清楚，因此，合适的合金设计策略也是未知的。有人认为，再结晶的粒子激发成核（PSN）（无论是静态的还是动态的）是原因。然而，未发表的结果表明，在第二相较少的稀稀土/Y合金中也可能发生织构改性。在提出的研究中，关键的一步将是区分固溶体合金化效应和粒子相关效应。一旦确定了纹理修饰的机制，计算机建模将用于集中搜索i)控制机制和ii)最佳成分。例如，RE/Y溶质原子可能形成纳米级团簇，其与位错相互作用的方式目前没有被假设理想的随机解的理论所解释。如果模拟结果显示了同样的结果，那么将使用同步加速器的漫射散射或其他实验技术来寻求验证。无论如何，相稳定性和计算相图（CALPHAD）分析将用于确定有前途的成分。此外，很少有经验可以推荐最佳纹理。成形极限图（FLDs）提供了在各种成形条件下板料响应的评估。利用多晶塑性模型预测FLD的织构依赖性。这种方法允许探索实验上不可能的概念（例如，改变纹理独立于晶粒尺寸，合金含量等），这些预测将使用来自TUHH/GKSS的实验FLDs进行验证。学生和年轻科学家将前往合作机构参与相关研究，例如，在UVA开发可成形性模型的学生将从接触TUHH/GKSS的可成形性测试设备和专业知识中受益匪浅。拟议的研究将服务于一个更广泛的目标，使轻型镁合金在汽车上的应用增加，从而促进提高燃油效率和减少有害温室气体的排放。此外，还有一系列便携式电子产品（手机、相机、笔记本电脑等），它们将受益于易于制造的材料，这种材料具有典型的聚合物材料的密度和典型的金属的耐用性、导热性和电子干扰屏蔽性。所探索的基本概念和建立的模型，例如，再结晶，原子聚类和织构效应，对许多材料系统具有广泛的影响。

The goal of this project, a joint effort between the University of Virginia (UVA) and the Technical University Hamburg Harburg (TUHH) and the nearby national laboratory GKSS at Geestacht, Germany, is to develop a new wrought Mg alloy with optimized texture for promoting enhanced formability. Despite many attractive properties, the application of Mg alloy sheet is limited, in part, due to poor low temperature formability. Their forming behavior is strongly affected by crystallographic texture because of the intrinsic plastic anisotropy of the hexagonal close packed crystal structure. Conventional Mg alloy sheets exhibit strong textures (with basal planes parallel to the sheet plane) with little variation between alloys. It has only recently been reported that alloys containing significant additions of rare earth (RE) elements and yttrium (Y) develop much weaker textures during extrusion than conventional alloys, and preliminary results show that randomizing the texture of Mg alloy sheets results in improved formability. The mechanism by which RE/Y additions impart a change in the texture evolution is presently unknown, thus, an appropriate alloy design strategy is also unknown. It has been suggested that particle stimulated nucleation (PSN) of recrystallization (either static or dynamic) is responsible. However, unpublished results show that texture modification is also possible in dilute RE/Y alloys which are expected to have very little second phase. A critical step in the proposed research will be to discriminate between solid solution alloying effects and particle related effects. Computer modeling will be used to focus both the search for i) controlling mechanisms and ii) optimal compositions once the mechanism(s) of texture modification are identified. For example, it is possible that RE/Y solute atoms form nano-scale clusters which interact with dislocations in ways not presently accounted for by theories which assume ideal, random solutions. If the modeling results suggest as much, validation will be sought using diffuse scattering at a synchrotron or other experimental techniques. Regardless, phase stability and calculated phase diagram (CALPHAD) analyses will be used to identify promising compositions. Additionally, there is little experience upon which to recommend optimal textures. Forming limit diagrams (FLDs) provide an assessment of sheet metal response under various forming conditions. The texture dependence of the FLD will be predicted using a polycrystal plasticity model. This approach allows exploration of concepts which are not possible experimentally (e.g., to alter texture independent of grain size, alloy content, etc.) The predictions will be validated using experimental FLDs from TUHH/GKSS. Students and young scientists will travel to the partner institutions to become involved in relevant aspects of the research, for instance students developing formability models at UVA will benefit strongly from exposure to formability testing facilities and expertise at TUHH/GKSS. The proposed research will serve a broader goal of enabling increased application of lightweight Mg alloys in automobiles, thereby promoting improved fuel efficiency and reduced emissions of harmful greenhouse gases. Further, there are a range of portable electronics goods (cell phones, cameras, laptops, etc.) which would benefit from a readily manufactured material with a density which is typical of polymeric materials and the durability, thermal conductivity and electronic interference shielding typical of a metal. The fundamental concepts explored and models developed, e.g., recrystallization, atomic clustering, and texture effects, have broad implications for many materials systems.