CAREER: Controlling Austenite Stability and Response During Deformation of Advanced High Strength Steels

钢作为结构材料无处不在，几乎在运输，航运，建筑或采矿设备的每一件中都可以找到;在建筑物，道路，管道，电器和许多其他应用中。在过去的15年里，在日益具有挑战性的燃油经济性标准的推动下，用于汽车车身的钢板的强度是以前合金的五倍。通常，增加强度会导致延展性（材料拉伸或弯曲而不断裂的能力）的降低，但这些新的高级高强度钢（AHSS）板材将这种降低的严重程度降至最低。实现这一目标的主要方法是通过设计钢的微观结构来抵抗断裂，这是基于对在制造钢时如何控制这种结构的新理解。该学院早期职业发展计划（CAREER）奖支持基础科学研究，以揭示这些钢的加工，微观结构及其特性和性能之间的关系。将研究广泛的变形条件，以更好地了解材料响应，并允许设计具有更好性能的钢。这将进一步改善汽车结构，并有助于满足严格的燃油经济性标准，这项工作的目的是关闭设计循环，以发展对先进高强度钢（AHSS）合金热机械加工（TMP）过程中奥氏体稳定性的基本理解，该合金使用相变诱导塑性（TRIP）机制，以产生强度和延展性的特殊组合。应变诱导的马氏体-马氏体转变抑制应变局部化并由此产生优异性能的能力取决于奥氏体的热稳定性和机械稳定性;如果奥氏体太稳定，则其在变形期间不会转变，并且如果奥氏体不稳定，则其在室温下不会保留。目前稳定奥氏体的方法集中于通过进行精确的热循环来增加碳含量，所述热循环允许碳从其它相扩散到奥氏体。在这里，研究重点是使用变形条件，包括应变速率，应变状态，温度和应力状态/压力，以了解如何通过对专门设计的合金进行集中的机械测试来进一步控制奥氏体稳定性。在这项工作中发展的理解将导致对AHSS中奥氏体稳定性的基于科学的理解，可以将其纳入热力学和机械材料模型，并且这将使得能够使用集成计算材料工程（ICME）和材料基因组计划（MGI）原理来实现先进制造，并进一步改善可以在这些低合金中实现的特殊性能，低成本材料是我们在交通运输领域及其他领域实现轻质结构的基础。该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估的支持。

Steels are ubiquitous as structural materials, and are found in nearly every piece of transportation, shipping, construction, or mining equipment; in buildings, roads, pipelines, appliances, and in many other applications. In the past 15 years, driven by increasingly challenging fuel economy standards, sheet steels used in automotive bodies have achieved strengths that are five times those of previous alloys. Normally, increasing strength results in a compromise in ductility - the ability for a material to stretch or bend without fracture - but these new Advanced High Strength Steel (AHSS) sheets minimize the severity of this compromise. The primary way this has been achieved is by designing the microscopic structure of the steels to resist fracture, based on new understanding of how to control this structure when manufacturing steels. This Faculty Early Career Development Program (CAREER) Award supports fundamental scientific research to uncover the relationships between the processing of these steels, the microscopic structure, and their properties and performance. A broad range of deformation conditions will be studied to better understand the material response and allow the design of steels with even better properties. This will further improve the automobile structures and help meet stringent fuel economy standards, and can also be translated to other sectors to improve the efficiency and performance in many other steel applications.This work will aim to close the design loop to develop a fundamental understanding of austenite stability during thermomechanical processing (TMP) of advanced high strength steel (AHSS) alloys that use the Transformation Induced Plasticity (TRIP) mechanism to yield exceptional combinations of strength and ductility. The ability for strain-induced austenite-to-martensite transformation to suppress strain localization, and thereby produce excellent properties, is dependent on the thermal and mechanical stability of the austenite; if austenite is too stable, it will not transform during deformation, and if austenite is unstable, it will not be retained at room temperature. Current approaches to stabilize austenite focus on increasing carbon content by performing precise thermal cycles that allow diffusion of carbon from other phases to austenite. Here the research focuses on the use of deformation conditions, including strain rate, strain state, temperature, and stress state/pressure, to understand how to further control austenite stability by performing focused mechanical testing of specifically designed alloys. The understanding developed in this work will result in a science-based understanding of austenite stability in AHSS that can be incorporated into thermodynamic and mechanical materials models, and which will enable the use of Integrated Computation Materials Engineering (ICME) and Materials Genome Initiative (MGI) principles to realize advanced manufacturing and further improve the exceptional properties that can be realized in these low-alloy, low-cost materials that underpin our efforts for lightweight structures in the transportation sector and beyond. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.