Phase Transformations and Mechanical Properties of Fe-Mn-Si-Al TRIP-Steels
Résumé
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 γfcc and the free energy ΔGγ→ε for the γ→ε phase transformation were calculated using the regular solution model. It is known that additions of aluminium increase γfcc and therefore strongly suppress the γ→ε transformation while silicon decrease γfcc and sustains the γ→ε transformation. The γ→ε phase transformation takes place in alloys with γfcc≤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.
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