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–3Si wt% alloys yielded single crystal elastic constants C11, C12 and C44 of 175/83/97 and 174/85/99 GPa 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 96 GPa 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.