Mechanical Behavior and Microstructure Characteristics of Directionally Solidified TWIP Steel

Abstract

The mechanical behavior and microstructure characteristics of three high Mn austenitic steels prepared by directional solidification at withdrawal rates of 60, 120, and 240 μm s⁻¹ were investigated and compared with common TWIP steel with equiaxed grains. For each steel, the Hollomon analysis, differential C–J analysis, and modified C–J analysis as an alternative method to describe the work-hardening behavior were studied. The directionally solidified samples (DS samples) exhibited higher mechanical properties along the axis, five stages (A, B, C, D, and E) divided on the plot of stain hardening rate vs true strain, and a more stable and uniform deformation feature with larger strain-hardening coefficients when the true strain is over 0.25, in comparison with the common TWIP steel. The modified C–J analysis was found to be the best one for revealing the strain-hardening behavior characterized by several different stages with a definite work-hardening exponent n. In the case of DS samples, the dendrite spacings increase but the morphology becomes simple when decreasing the withdrawal rate. The larger volume fraction of twins and prevalent activation of twin systems, together with the fragmentations of the original grains in a sample solidified at a withdrawal rate of 120 μm s⁻¹, lead to the best mechanical behavior in a medium-to-large strain range.