Optimizing the strength and ductility of materials through control of microstructure

本研究解决了长期存在的金属合金韧性断裂问题。就人身伤害和财产损失而言，过早骨折可能是灾难性的。为了防止故障，系统往往被过度设计，使其成本更高，效率更低。例如，汽车车身面板比需要的厚，从而降低了汽车的燃油效率。深入了解延性断裂的机制以及它们如何受到材料微观结构的影响，将有助于更有效地设计和优化材料。多年来，我在这一领域所取得的进展确实有助于开发更可靠的延性断裂模型，而使用x射线计算机断层扫描（XCT）来详细可视化延性断裂过程（这是世界上没有其他团队做过的）现在已经嵌入教科书中。在弯曲等复杂应力状态下建立可靠的断裂模型时，仍有几个关键问题没有得到解决，而弯曲是控制过程中不可或缺的一部分，例如耐撞性。在接下来的五年里，我将主要关注两个方面。首先是扩大原位方法的使用，这种方法被证明在将局部微观结构与损伤的发展联系起来方面是非常宝贵的，损伤是韧性断裂的前兆。二是将这些方法应用于两类重要材料——先进高强度钢（AHSS）和多相高熵合金（HEAs）的韧性断裂研究。原位方法包括在样品变形时使用显微镜和XCT。这提供了在微观结构尺度上导致断裂的过程的详细历史，包括多相材料中相之间的载荷传递。AHSS代表了一类结合了高强度和延展性的合金，使它们在汽车应用中具有吸引力。然而，对限制延性的机制的研究很少，而且大多数研究都集中在拉伸伸长率上，而不是真正的破坏应变。这一点至关重要，因为前者解释了拉伸成形等某些工艺的局限性，而真正的失效应变与弯曲失效有关。我的团队开发了显微数字图像相关方法来解决这个问题。我们现在有能力将其应用于第三代钢材，这对节能汽车的发展至关重要。多相HEAs是一种新型的微复合材料，它结合了两种高熵合金——一种具有高塑性，另一种具有高强度。这些材料是最近才开发出来的；因此，对控制其延展性的机制还没有了解。我们需要了解变形过程中相尺度和分布对损伤积累的影响。这项研究将使我们能够开发出最佳的微结构，以延迟高应变的断裂。

This research addresses the longstanding issue of ductile fracture in metallic alloys. Premature fracture can be catastrophic in terms of both personal injury and property damage. To prevent failure systems are often over-engineered, making them costlier and less efficient than necessary. For example, auto body panels are thicker than need be, thus impairing vehicle fuel efficiency. An in-depth, fundamentally-based understanding of the mechanisms of ductile fracture and how they are impacted by material microstructure will enable more effective designs that use materials optimally. The progress which I have made on this topic over many years has indeed contributed to the development of more robust models of ductile failure, while the use of x-ray computed tomography (XCT) to visualize the ductile fracture process in detail (something no other group in the world has done) is now embedded in textbooks. Several crucial questions remain unanswered to develop robust models for fracture under complex stress states such as bending, which is integral to processes that control, for example, crash worthiness. Over the next five years I will focus on two main areas. The first is to expand the use of in situ methods that are proving to be invaluable in linking local microstructures to the development of damage, the precursor to ductile fracture. The second is to apply these methods to the study of ductile fracture in two important classes of materials - advanced high strength steels (AHSS) and multi-phase high entropy alloys (HEAs). In situ methods involve the use of microscopy and XCT of samples while they are being deformed. This provides a detailed history of the processes that lead to fracture at a microstructural scale, including load transfer between phases in multi-phase materials. AHSS represent a class of alloys which combine high strength and ductility, making them attractive for automotive applications. However, there have been few investigations of the mechanisms which limit ductility and most of those focus on tensile elongation rather that true failure strain. This is critical since while the former explains limits to some processes such as stretch forming, true failure strain is linked to failure in bending. My group developed microscopic digital image correlation methods to tackle this problem. We now have the capability of applying this to the Generation 3 steels that are emerging as critical to fuel efficient vehicle development. Multi-phase HEAs are a new class of microcomposites that combine two high entropy alloys - one with high ductility, the other with high strength. These materials have only recently been developed; thus there is no understanding of the mechanisms that control their ductility. We need to understand how phase scale and distribution impacts damage accumulation during deformation. This research will enable us to develop optimal microstructures that delay fracture to high strains.