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

Kip O. Findley, Colorado School of Mines, DMR 0955236

Advanced Steel Products and Processing Research Center

Project Overview:“Smart” metal alloys that take advantage of stress or strain induced phase transformations are being implemented more widely in transportation and medical industries. For example, TRIP (transformation induced plasticity) steels exhibit promise for increased strength, ductility, and crashworthiness over alloys traditionally used in the automotive industry. However, the fatigue behavior, which is important for automotive structural applications, is not well understood.

Objectives:• Characterize and quantify the microstructural

influences, specifically changes in microstructure during loading, that affect fatigue performance.

• Develop models to predict the fatigue performance based on microscale changes in the structure.

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, and location of retained austenite, which can vary significantly as shown in these images.

The plot shows fatigue stress versus strain (deformation) hysteresis loops that were predicted by models that account for the transformation of austenite to martensite during fatigue. The predicted fatigue behavior matches closely to experimental results.

Partially transformed retained austenite in TRIP steel after deformation. The red arrow points to austenite. The orange arrow points to martensite.

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

Advanced Steel Products and Processing Research Center

A undergraduate laboratory module that explores stress and strain induced phase transformations for “smart” materials used in biomedical and automotive applications was improved through the insight gained from the research. Mechanical testing grips that improve fatigue testing efficiency and accuracy were purchased through the NSF grant. The grips will enable improvement in an undergraduate laboratory devoted to fatigue testing.

A middle school science teacher was hired to develop a miniature mechanical test frame and associated module for his class. The module will address key physical science concepts in strength, ductility, and density. It will also be used in an urban after school program in Denver called Colorado Uplift. He also interacted with graduate and undergraduate researchers and participated in workshops such as the one shown in the picture where he is forging.

The participants in the program 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. Interactions such as these have provided the opportunity for the work to be validated and disseminated in a steel research community that includes users and producers of steel. The picture shows industrial and Colorado School of Mines participants at one of the semi-annual center meetings.