Materials World Network: Annealing Twin Formation for Grain Boundary Engineering

来自卡内基梅隆大学(CMU)材料科学与工程系(CMU)和法国索菲亚·安提波利斯巴黎矿业学院材料成形中心(CEMEF)的一组研究人员研究了使材料实现晶界工程(GBE)的退火孪晶形成机制。GBE作为一种寻求通过控制材料的晶界含量来提高材料的耐腐蚀性和疲劳寿命等广泛性能的工艺，正得到越来越多的应用。GBE通常包括反复的变形和退火循环，但关于这些步骤如何导致优化的微结构的研究很少。本研究重点研究再结晶过程的早期阶段随初始晶粒度、退火温度、颗粒含量和先前应变水平的变化，以确定产生高比例理想晶界的机制，尤其是通过退火孪晶破坏一般的高角度晶界网络的机制。也有很好的证据表明，在低到中等应变时，孪晶部分最容易获得所需的增加，这与应变诱导边界迁移(SIBM)机制密切相关。这项研究提供了检验以下假设的机会：GBE过程取决于SIBM和孪生的组合，以及发生地点的空间和纹理分布是理解该过程的关键。CMU具有放大特定感兴趣区域的能力，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结构演变。CEMEF小组由法国国家研究机构(ANR)支持，拥有热舞台显微镜和先进的模拟工具，可用于整个变形和退火过程。将获得的关键新信息包括关于退火过程中孪晶形成的数据和晶界网络的拓扑结构，而不是只测量部分“特殊边界”。基于假设的方法解决了使GBE能够实现的孪晶形成的潜在机制。到目前为止，大多数工作都集中在前后表征上，这优化了性能，但没有提供对GBE过程的了解或控制。如果这一假说得到证实，也将解释为什么大变形不会促进GBE，因为出现了其他再结晶机制，例如亚晶网络的异常粗化。成功地描述了GBE的关键机制，有可能扩大这一过程的应用范围。美国工业部门的几个部门都在使用高温合金，因此可能会对这项工作的结果感兴趣，如Integran、普拉特&惠特尼和通用电气。

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.