Materials World Network: Nanoscale Studies of Fundamental Mechanisms of Deformation in Amorphous Materials

金属合金的强度、延展性和成形能力都与位错运动引起的塑性变形有关。虽然位错过程被很好地理解，但许多重要的材料（包括氧化物玻璃、金属玻璃、许多聚合物和一些半导体）是非晶体的，并且通过不涉及位错的机制进行塑性变形。更好地了解这些机制将有助于开发具有高强度和提高抗断裂和疲劳性的新材料。这项国际合作研究工作使用先进的实验和计算技术来表征、量化和预测非晶材料塑性变形的基本机制。程序的实验和计算部分由基于非晶材料变形的剪切转变区（STZ）理论的连续统非平衡热力学框架联系起来。为了实现这一目标，这项工作汇集了电子显微镜（W. Chen，日本东北大学）、x射线散射（T. Hufnagel，约翰霍普金斯大学）、微机械测试（L. Greer，剑桥大学，英国）和计算材料科学（M. Falk，约翰霍普金斯大学）方面的国际专家。结构表征工作主要集中在量化变形对非晶材料纳米级结构的影响；这些包括像差校正透射电子显微镜、波动电子显微镜和相干x射线散射。纳米尺度的力学性能是通过仪器纳米压痕和微柱压缩在一定温度范围内测量的。为了将工作置于尽可能广泛的背景下，研究了结构和行为两个极端的材料：金属玻璃，具有很大的非定向粘合和支持塑性变形的实质性能力；非晶硅，具有高度定向共价键和有限的塑性变形。实验成果由原子模拟和连续体模型加以补充。原子模拟可以详细研究变形的基本机制和伴随的结构演化，对解释实验结果至关重要。将适定边值问题的连续统数值解与精心选择的实验进行比较，以测试STZ理论的有效性，并将实验观察结果置于预测本质理论的背景下。通过将研究生、本科生和博士后学者整合到一个包括来自美国、英国和日本的研究人员的国际研究团队中，这项工作促进了研究生、本科生和博士后学者的教育和培训。这项工作的一个中心特点是，团队的初级成员有机会长期访问其他机构，以促进合作，并从长期接触不同文化中受益。整个团队的定期网络会议和年度会议也促进了合作。另一个重点是在本科材料科学和工程教育中使用同伴指导技术。适合二年级《材料结构》课程的内容被开发出来，并且正在评估这种教学方法的有效性。评估结果将以期刊文章和会议发言的形式传播，所产生的教学内容将免费提供给其他教员使用。

The strength, ductility, and forming ability of metallic alloys are all related to plastic deformation, which arises from the motion of dislocations. While dislocation processes are well understood, many important materials (including oxide glasses, metallic glasses, many polymers, and some semiconductors) are non-crystalline and undergo plastic deformation by mechanisms that do not involve dislocations. A better understanding of these mechanisms would aid the development of new materials with high strength and improved resistance to fracture and fatigue. This collaborative international research effort uses advanced experimental and computational techniques to characterize, quantify, and predict fundamental mechanisms of plastic deformation in amorphous materials. The experimental and computational portions of the programs are linked by a continuum non-equilibrium thermodynamic framework based on the shear transformation zone (STZ) theory of deformation of amorphous materials. To accomplish this, the effort brings together international experts in electron microscopy (W. Chen, Tohoku University, Japan), x-ray scattering (T. Hufnagel, Johns Hopkins U.), micromechanical testing (L. Greer, Cambridge U., UK), and computational materials science (M. Falk, Johns Hopkins U.). Structural characterization efforts focus on quantifying the effects of deformation on the nanoscale structure of amorphous materials; these include aberration-corrected transmission electron microscopy, fluctuation electron microscopy, and coherent x-ray scattering. The nanoscale mechanical properties are measured by instrumented nanoindentation and micropillar compression over a range of temperatures. In order to place the work in the broadest possible context, materials at two extremes of structure and behavior are studied: metallic glasses, with largely non-directional bonding and substantial ability to support plastic deformation; and amorphous silicon, with highly directional covalent bonding and limited plastic deformation. The experimental efforts are complemented by atomistic simulations and continuum modeling. The atomistic simulations allow detailed investigation of fundamental mechanisms of deformation and the accompanying structural evolution, and are critical for interpreting the experimental results. Continuum numerical solutions of well-posed boundary value problems are compared to carefully chosen experiments to test the validity of the STZ theory and place the experimental observations in the context of a predictive constitutive theory.The effort advances the education and training of graduate students, undergraduate students, and post-doctoral scholars by integrating them into an international research team that includes researchers from the United States, United Kingdom, andJapan. A central feature of the effort is the opportunity for the junior members of the team to make extended visits to the other institutions to foster the collaboration and benefit from extended exposure to different cultures. The collaboration is also fostered by regular web conferences and annual meetings of the entire team. Another focus is the use of peer instruction techniques in undergraduate materials science and engineering education. Content appropriate for a sophomore-level Structure of Materials course is developed and the effectiveness of this teaching methodology is being evaluated. Results of the assessments will be disseminated via journal articles and conference presentations, and the teaching content produced will be made freely available for other instructors to use.