Single crystal elastic constants of high-manganese transformation- and twinning-induced plasticity steels determined by a new method utilizing nanoindentation

A better understanding of single crystal elastic anisotropy in high-manganese austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels is necessary primarily to improve the accuracy of stacking-fault energy measurements and validate elastic constants determined by ab initio simulations. In the present work, a method utilizing nanoindentation in combination with orientation imaging microscopy is developed for the purpose of calculating single crystal elastic constants from cubic polycrystalline specimens. Applying the method on two Fe-(22/25)Mn-3Al-3Siwt% alloys yielded single crystal elastic constants C₁₁, C₁₂ and C₄₄ of 175/83/97 and 174/85/99GPa respectively. Indentation moduli found in the literature for a third TWIP steel, with composition Fe-18Mn-1.5Al-0.6C, produced elastic constants of 169, 82 and 96GPa via the proposed method. Anisotropy ratios of the three alloys ranged from 2.11 to 2.22, considerably lower than values of ~3.5-3.9 measured for binary austenitic Fe-Mn alloys. The discrepancy is attributed to alloying additions of Al, Si and C, which cause the Néel transition to occur below room temperature in the TRIP/TWIP alloys, thereby reducing suppression of the tetragonal shear modulus (C₁₁-C₁₂)/2 typically associated with antiferromagnetic ordering. In addition, decreases in Mn content reduced the normal shear modulus C₄₄. In a test, the model calculated single crystal elastic constants to be within ~4% of established values for a series of materials with a wide range of elastic anisotropy.