Phase Transformations and Mechanical Properties of Fe-Mn-Si-Al TRIP-Steels

Abstract
Deformation twinning, martensitic phase transformation and mechanical properties of austenitic Fe-(15-30)wt%Mn alloys with additions of aluminium and silicon have been investigated. Tensile tests were carried out at different strain rates and temperatures. The formation of twins, α'- and ε-martensite during plastic deformation was analysed by optical microscopy, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The stacking fault energy γ_fcc and the free energy ΔG^γ→ε for the γ→ε phase transformation were calculated using the regular solution model. It is known that additions of aluminium increase γ_fcc and therefore strongly suppress the γ→ε transformation while silicon decrease γ_fcc and sustains the γ→ε transformation. The γ→ε phase transformation takes place in alloys with γ_fcc≤20mJ/m². The stacking fault energy of the Fe-25Mn-3Si-3Al alloy was calculated as a function of temperature and related with microstructural changes of the strained sample at different temperatures. These steels with reduced density of about 7,3 g/cm^-3 combine high tensile ductility up to 80 % at high strain rates with true tensile strength of about 1000 MPa. The excellent plasticity induced by twinning and additional phase transformation up to extremely high strain rates of about ε=10³s^-1 results in an extraordinary shock resistence and enables deep drawing and backward extrusion operations of parts with complex shapes and high production rates.