GOALI: Deformation Microscopy of Retained Austenite Transformation in TRIP Steels

Brigham Young University, Provo UT

Investigators

Abstract

NON-TECHNICAL SUMMARY: The efficiency of modern vehicles critically affects individual and national prosperity, national energy independence, city pollution levels (and related health) and climate change. A key element in deciding this efficiency is vehicle weight. While many ongoing efforts aim at introducing lightweight materials (such as aluminum, magnesium and carbon composites) into automotive structures, the dominant constituent, by far, is still relatively heavy steel. Hence this project focuses on the alternative light-weighting strategy of developing advanced high strength steels with improved formability; this will allow less metal to provide the same structural and safety performance. New and emerging microscopy techniques are combined with simulation to reveal the relationships between microstructure and properties of modern high-strength steels. The resulting insights lead to the deeper levels of understanding and improved models that are required for the nascence of so-called third-generation steels and a step-change in vehicle weight and efficiency. Not only does this research project provide an environment for mentoring graduate and undergraduate students in a mixed academic / industrial setting (via a strong collaboration with General Motors), but high school education is being improved via an outreach program. Teachers from local high schools spend time on the project during the summer months while receiving help in developing new and relevant curriculum components for their classes. TECHNICAL SUMMARY: Many studies have been carried out on TRIP (Transformation Induced Plasticity) steels. However, there is still no systematic information concerning the influence of austenite chemistry and microstructure on the resulting transformation to martensite. Such knowledge is critical to the design of texture, morphology and volume fraction of retained austenite in order to enable low-alloy TRIP steels to achieve enhanced strength, formability, and fracture resistance necessary to achieve the desired automotive structure performance. Proposed retained-austenite and martensite identification technologies (based upon emerging electron-backscatter diffraction techniques), along with meso-scale dislocation characterization, are combined with micro-digital image correlation (DIC) to provide a new 'Deformation Microscopy' capability. The resultant framework allows for the assessment of interactions between dislocations and hard phase boundaries, while also precisely capturing the strains at which the various retained austenite islands transform. Knowledge of the fundamental links between microstructure and formability enables the formulation of a meso-scale model for transformation of austenite to martensite, and related deformation phenomena.