Advanced Steel Products and Processing Research Center

1. Advanced Steel Products and Processing Research Center CAREER: The Stability and Influence of Metastable Retained Austenite During Fatigue of Advanced Steel Alloys Kip O. Findley, Colorado School of Mines, DMR- 0955236 Project Overview: Metastable metal alloys that take advantage of stress or strain induced microstructural changes are being implemented more widely in transportation and medical industries. However, there are several factors associated with fatigue loading of alloys that demonstrate stress-induced microstructural change that need to be explored and could have major implications on using these alloys in automotive structural applications. • Objectives: • Characterize changes in microstructure during loading that affect fatigue performance of transformation induced plasticity (TRIP) steels. • Develop models to predict the fatigue performance based on microscale changes in the structure. Left: Atomic force microscope image of retained austenite (RA) and two types of microstructures that surround it, polygonal ferrite (PF) and bainitic ferrite (BF). Right: Graph of hardness, obtained with nanoindentation, of BF and PF as a function of applied strain. Despite being very different initially, the hardness of PF and BF is comparable at larger amounts of deformation, so their mechanical influence on RA transformation may be similar. Microstructures of TRIP steels. The green phase, retained austenite, transforms to a harder phase, martensite, during deformation such as fatigue, which changes the material mechanical properties. The amount of the transformation depends on size, morphology, location, and composition of retained austenite, which can vary significantly as shown in these images. Left: Stress-strain plot of metastable alloys that contain different amounts of aluminum and silicon. Right: Plot of amount of austenite to martensite transformation (Feritscope Magnetic Fraction) as a function of strain. The plot shows that alloying greatly affects the amount of transformation, which affects the stress-strain behavior.

2. Advanced Steel Products and Processing Research Center CAREER: The Stability and Influence of Metastable Retained Austenite During Fatigue of Advanced Steel Alloys – Broader Impacts Kip O. Findley, Colorado School of Mines, DMR- 0955236 • Broader Impacts • The project team participated in the Colorado Uplift Global After School Program for urban middle and high school students (see photos at right).They performed experiments with the students to teach basic mechanical properties. One of the activities utilized the tensile frame developed by the middle school teacher in the previous project year. Members of the undergraduate materials club at CSM also participated. • The project team presented their work to sponsors of the Advanced Steel Processing and Products Research Center (ASPPRC), who is also supporting this work. AK Steel has enabled portions of this work by producing experimental heats of material. These interactions have provided the opportunity for the work to be validated and disseminated in a steel research community that includes users and producers of steel. • An undergraduate laboratory module that explores stress and strain induced phase transformations for materials used in biomedical and automotive applications was improved through the insight gained from the research. Hydraulic grips, purchased through the grant, were implemented to improve efficiency and data of a fatigue laboratory module. • A middle school science teacher was hired to perform summer research on the grain size effects on retained austenite stability. He learned experimental techniques including heat treating, tensile testing, and metallography, and he obtained promising results that will be further developed with an undergraduate research this academic year. • The project team, including the middle school science teacher, developed modules for a workshop in the Bechtel K-5 Education Excellence Initiative program for teachers. They introduced the teachers to basic mechanical properties and performed interactive demonstrations with them. Tensile testing with Colorado Uplift students “Toughness” testing