Deformation mechanisms of quenching and partitioning steels and magnesium alloys : from quasi-static to high strain rates

Abstract

The desire to boost fuel economy in automotive industry leads to a trend of replacing traditional vehicle chassis and frame with the ones made of lightweight materials, such as quenching and partitioning (Q&P) steels and magnesium (Mg) alloys. Concerning the safety and manufacturing issues, the objective of the industry is to improve the strength and ductility of lightweight materials at normal and extreme conditions. It is therefore of importance to develop a better understanding of the deformation mechanisms of Q&P steels and Mg alloys at quasi-static and high strain rates. An in-house built apparatus for dynamic tensile testing and a line profile analysis method for the calculation of dislocation density were applied to the following studies. First, the transformation-induced plasticity (TRIP) effect in Q&P steels was investigated. Surprisingly, a Q&P 1500 steel deformed in tension at a high strain rate of 1000 s-1 is not work hardened, although most retained austenite grains have transformed to martensite, exhibiting an abnormal TRIP effect. It was found that compared with the coarse austenite grains deformed at quasi-static strain rate, those at high strain rate accommodate more deformation, such that the activity of dislocations in martensite matrix is suppressed. Moreover, the fresh martensite produced at high strain rate deforms plastically, instead of elastically, and thus the lack of composite-like deformation behavior results in a reduced work hardening rate. Second, the densities of dislocations and deformation twins in a Mg alloy subjected to various strain rates were, for the first time, quantitatively studied. It was found that the mechanical twins are saturated at 1 s-1, while the dislocation density considerably increases only at a much higher strain rate of 600 s-1. Besides, the local stress concentration at the strain rate of 1 s-1 or above is likely to induce the coexistence of twinning modes with opposite polarities in the same grains. Owing to the exclusively high flow stress at high strain rate, the dislocations with a 〈112 ̅0〉 (〈a〉) Burgers vector are saturated at a small strain level and subsequently the density of dislocations with a 〈112 ̅3 ̅ 〉 (〈c+a〉) Burgers vector keeps increasing, leading to an increase in work hardening rate. Third, a density functional theory computation based statistical learning strategy was proposed to improve the density of 〈c+a〉 dislocations in pure Mg at quasi-static strain rate by adding appropriate non-rare-earth elements. It was found that the equilibrium volume, the equilibrium bulk modulus and the 1st ionization energy of the alloying elements influence the basal stacking fault energies and thus 〈c+a〉 dislocation density. Two novel rare-earth-elements free ternary Mg alloys, with a high yield strength of ~230 MPa and good tensile elongation of ~9%, were discovered. The ratio of 〈c+a〉 dislocation density to total dislocation density in the present novel Mg alloy is about 30%, which is much higher than the values in the literature.