Quantifying the Thermo-Mechanical Response and Strain-Rate Effects in Magnesium Microcrystals

Johns Hopkins University, Baltimore MD

Investigators

Abstract

NON-TECHNICAL ABSTRACT: This project will focus on quantifying the deformation mechanisms that dominate at elevated temperatures and different deformation rates in magnesium (Mg) and its alloys. One approach to reduce carbon dioxide emission from fossil-fuel powered vehicles is to increase their fuel efficiency by reducing the vehicle structural weight. To that end, Mg alloys demonstrate a favorable strength-to-weight ratio and provide valid alternatives to aluminum alloys and high strength steels. Thus, there is a growing interest in utilizing Mg alloys in many automotive, aerospace and defense applications. However, at present, one of the limiting factors for the wide production and use of Mg alloys is their low formability at room temperature, which leads to many technological and economical constraints. This is mainly due to the low ductility of Mg alloys at room temperature. As such, most Mg sheets are formed at elevated temperatures below their recrystallization limit (~400ºC). Another challenge is the high strain-rate sensitivity of Mg, which necessitates low speed forming processes. This award will thus supports research to fundamentally identify the deformation mechanisms through a set of state-of-the-art multi-length scale experimental and simulation techniques. The results of this project are expected to assist in the process of developing new Mg alloys that demonstrate improved formability and enhanced ductility and toughness at a wider range of temperatures and strain rates. The integration of research, education and outreach in this project will also: (1) improve STEM achievement in a predominately African American elementary school in Baltimore, currently ranked below 92% of schools in Maryland; (2) involve under-represented students from a local historically black college through internships on research in mechanics and materials; and (3) develop an education portfolio that intensifies the knowledge of undergraduate and graduate students in fundamentals of state-of-the-art multiscale modeling and micro-scale experiments. TECHNICAL ABSTRACT: The primary research objectives of this research are to fundamentally identify the coupled strain rate effect and thermo-mechanical response of Mg and AZ31 microcrystals (fabricated in bulk single crystals) through coupled novel in situ scanning electron microscopy elevated-temperature experiments, and novel large scale three-dimensional discrete dislocation dynamics simulations that account for dislocation-twin boundary interactions. The research hinges around addressing four challenging objectives: (1) Quantify the thermo-mechanical properties and deformation mechanisms in the range of 30 to 400ºC in Mg and AZ31 microcrystals; (2) Identify the origins of the anomalous hardening response observed at 30% of the melting temperature of Mg; (3) Quantify the strain rate effects at different temperatures on the deformation mechanisms in the range of 1e-4 to 0.1 s; and (4) Generate a four dimensional model of the correlation between temperature, strain rate, crystal size, and strength.