Deformation-Induced Twinning Kinetics and Mechanical Properties of Fe-18Mn-0.6C TWIP steel Depending on Temperature and Al content

Abstract

High manganese steels have recently drawn much interest in steel and automobile industries due to their excellent room temperature mechanical properties such as strength, ductility and formability within the context of twinning induced plasticity (TWIP). Metastable austenite in these high manganese steels was observed to generate three types of deformation products in terms of deformation-induced ε-martensite, deformation-induced twins, and glide dislocations depending on stacking fault energy (SFE). The SFE has been reported to change with alloying elements and also affects deformation structure and mechanism of these steel grades. The effects of various alloying elements such as C, Mn, Si, Cr and Al on SFE have thus been well documented now. Temperature effect on mechanical properties, SFE and deformation mechanism of high manganese steels has not, however, been well clarified to date, even though SFE is known to depend strongly on temperature change. In addition, high temperature properties are also important even during a room temperature forming process, due to temperature rise up to 400oC caused by adiabatic heating. However, only a few studies were reported to date concerning high temperature properties of these steel grades. It is, therefore, attempted in this study to investigate temperature dependence of mechanical properties in Fe-18Mn-0.6C-(0~3)Al TWIP steels focusing on the formation kinetics of deformation twins as well as high temperature deformation mechanisms utilizing an internal variable theory for inelastic deformation. Higher SFE, caused either by raising test temperature or Al addition for the present alloy, was found to make formation of twins difficult. The stability parameter (β), used in a kinetics relation developed within the framework of an internal variable theory was also observed to decrease with the increase in test temperature and Al content. This stability parameter was in fact prescribed to characterize the stability of meta-stable austenite phase during inelastic deformation. Total elongation was found to depend strongly by this stability parameter (β) in the temperature range between RT and 400oC, where pronounced deformation twinning was observed. Deformation twinning was also observed to operate at higher temperatures above 400oC, but only in a very few amount. The Md temperature, above which deformation induced twins cannot be formed, was estimated as Md ≅ 1,147 oC by plotting the β parameter in terms of test temperature. Deformation structure was, on the other hand, mainly consist of dislocation forest and network structure resulting from easy cross-slip of glide dislocations due to higher SFE at elevated temperatures above 400oC. As a consequence, the changes in tensile elongation in the two temperature regimes could successfully be explained by considering the SFE, stability parameter (β), deformation mechanisms and structures. In other word, degradation of ductility with increasing temperature and Al content appeared due to easier twin formation in austenite phase at relatively low temperature between RT and 400oC, while the usual temperature dependence of tensile elongation observed above 400 oC was due to the usual dislocation glides rather than deformation twinning.