CAREER: The Stability and Influence of Metastable Retained Austenite During Fatigue of Advanced Steel Alloys

Colorado School Of Mines, Golden CO

Investigators

Abstract

TECHNICAL SUMMARY: Recently, advanced high strength steels have been developed with significant amounts of retained austenite in the microstructure. Stress and strain induced transformation of retained austenite to martensite results in excellent combinations of strength and ductility in these alloys. The stability of retained austenite during fatigue cycling and its effect on cyclic stress-strain behavior has not been comprehensively investigated and is the focus of this investigation. Two steel alloy systems that contain retained austenite will be used for the investigation, transformation induced plasticity (TRIP) alloys and superbainite. TRIP alloys have superior strength and formability, while superbainite has vast potential for structural purposes because of excellent combinations of strength and toughness. Four key variables will be assessed: retained austenite stacking fault energy, dislocation density, morphology, and the surrounding microstructure. Experimental and computational tools will be used to quantify the impact of retained austenite on fatigue behavior. The tools include servohydraulic load frames for fatigue testing, advanced electron microscopy and x-ray diffraction for characterization, the Los Alamos Neutron Science Center for simultaneous mechanical testing and characterization, and object-oriented finite element analysis. Fundamental and quantitative understanding will be obtained for the relationship between the above variables, retained austenite stability, and fatigue properties. It is anticipated that this understanding can be applied to other steel alloys and material systems such as nitinol that exhibit similar phenomena. NON-TECHNICAL SUMMARY: The objective of the proposed work is to determine the effect of deformation-induced changes on fatigue properties of advanced steel alloys, and also use this understanding to promote engineering and advanced materials applications to high school students and teachers, undergraduates, and graduate students. Smart metal alloys that take advantage of changes in the material during deformation are being implemented more widely in medical, infrastructure, and transportation industries. Enhanced properties can be obtained in steel alloys through deformation-induced changes. Two alloy systems that take advantage of this phenomenon will be studied in this investigation. TRIP (transformation induced plasticity) steels exhibit promise for increased strength, ductility, and crashworthiness over alloys traditionally used in the automotive industry. Superbainite, recently developed at Cambridge University, is processed to provide outstanding combinations of strength and toughness. Since both of these alloys are intended for structural purposes, they may be subject to fatigue loads, but their fatigue behavior is not well understood. Using the proposed research as a basis, programs will also be developed to educate high school teachers and urban high school students in the Colorado Uplift program about materials engineering and more broadly, about the impact of materials development on their everyday lives. The objective is to inspire students to pursue science and engineering degrees and become educated future voters and policymakers. The research will also be incorporated into the undergraduate laboratory curriculum in the mechanical behavior of materials course at the Colorado School of Mines as an example of smart material systems. Undergraduate and graduate researchers will be educated through the research process and will participate in the high school outreach activities and thus gain an understanding of how science and community service can be intertwined. Finally, the project will facilitate future domestic and international collaborations with partners such as Los Alamos National Laboratory and Cambridge University.