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

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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.