Materials World Network: Annealing Twin Formation for Grain Boundary Engineering

来自卡内基梅隆大学（CMU）材料科学工程系和法国索菲亚安提波利斯巴黎矿业学院材料成型中心（CEMEF）的一组研究人员研究了退火孪晶形成的机制，这些机制使材料的晶界工程（GBE）得以完成。GBE作为一种寻求通过控制材料的晶界含量来改善诸如耐腐蚀性和疲劳寿命等广泛性能的工艺而被越来越多地应用。GBE通常涉及重复的变形和退火循环，但很少有人研究这些步骤如何导致优化的微观结构。本研究的重点是详细表征的再结晶过程的早期阶段作为初始晶粒尺寸，退火温度，颗粒含量和先前的应变水平的函数，以确定的机制，引起高分数的理想的边界，但最特别的是破碎的网络一般高角度边界退火孪晶。也有很好的证据表明，在孪晶分数的期望的增加是最容易获得在低到中等应变，这是强烈相关的应变诱导边界迁移（SIBM）机制。这项研究提供了一个机会来测试的假设，GBE过程取决于SIBM与孪生的组合，它发生的位置的空间和纹理分布是理解这个过程的关键。CMU能够放大特定的感兴趣区域，并通过使用双光束FIB-SEM设备进行原位连续切片来表征它们。CMU还将与Integran USA合作，研究纳米晶镍和纯镍的微观结构演变。CEMEF小组在法国国家研究机构（ANR）的支持下，拥有用于整个变形和退火过程的热台显微镜和先进的模拟工具。获得的关键新信息包括退火过程中孪晶形成的数据和晶界网络的拓扑结构，而不是仅仅测量“特殊边界”的分数。基于假设的方法解决了孪晶形成的潜在机制，使GBE得以实现。到目前为止，大多数工作都集中在前后表征上，这具有优化的性能，但没有提供对GBE过程的理解或控制。这一假设如果得到证实，也将解释为什么大变形不会促进GBE，因为其他再结晶机制的出现，如异常粗化的亚晶粒网络。GBE的关键机制的成功描绘有可能扩大该过程的应用范围。美国工业部门的几个部门使用高温合金，因此可能会对这项工作的成果感兴趣，例如Integran，Pratt Whitney和General Electric。

A team of investigators from the Materials Science & Engineering Department at Carnegie Mellon University (CMU) and the Centre for Material Forming (CEMEF) at the Paris School of Mines in Sophia Antipolis, France, examines the mechanisms of annealing twin formation that enable Grain Boundary Engineering (GBE) of materials to be accomplished. GBE is being applied increasingly as a process that seeks to improve a wide range of properties such as corrosion resistance and fatigue life through control of the grain boundary content of the material. GBE typically involves repeated cycles of deformation and annealing but little research has been done on how these steps result in optimized microstructures. This research focuses on detailed characterization of the early stages of the recrystallization process as a function of initial grain size, annealing temperature, particle content and prior strain level in order to determine the mechanisms that give rise to high fractions of desirable boundaries but most especially the break-up of the network of general high angle boundaries by annealing twins. There is also good evidence that the desired increases in the twin fraction are easiest to obtain at low to moderate strains, which is strongly associated with the Strain Induced Boundary Migration (SIBM) mechanism. This research offers the opportunity to test the hypothesis that the GBE process depends on a combination of SIBM with twinning, and that the spatial and textural distributions of the locations where it takes place are key to understanding the process. CMU has the capability to zoom in on particular regions of interest and characterize them with in-situ serial sectioning by using a dual-beam FIB-SEM equipment. CMU will also partner with Integran USA to investigate microstructural evolution in both nanocrystalline nickel and pure Ni. The CEMEF group, supported by the French National Research Agency (ANR), has hot stage microscopy and advanced simulation tools for the entire deformation plus annealing process. Key new information to be gained includes data on twin formation during annealing and the topology of grain boundary networks, as opposed to only measuring fractions of "special boundaries".The hypothesis-based approach addresses the underlying mechanisms of twin formation that enable GBE to be accomplished. Most work so far has focused on before-and-after characterization, which has optimized properties but offers no understanding or control of the GBE process. The hypothesis, if verified, will also explain why large deformations do not promote GBE because other recrystallization mechanisms arise, such as abnormal coarsening of sub-grain networks. Successful delineation of the key mechanisms for GBE has the potential to expand the range of application of the process. Several parts of the industrial sector in the US make use of superalloys and will therefore be likely to be interested in the outcome of the work, such as Integran, Pratt & Whitney, and General Electric.