Evolution of deformation twins with strain rate in a medium-manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V

    
    Deformation twins are often found in medium manganese wear-resistant steels Fe–8Mn–1C–1.2Cr–0.2V (Mn-steel), and their evolution with strain rate has been an intensively studied research topic. Up to now, multiple studies have been conducted to investigate the effect of strain rate on the evolution of deformation twins in Mn-steel. In this article, a comprehensive overview will be provided of these studies, along with the corresponding results, followed by a discussion of their implications.
    
    The term “deformation twin” refers to the rotation of the majority of the atoms within the crystal substructure of a material. In the case of Mn-steel, it is believed that the high content of manganese influences the overall crystallography to create this specific type of twinning. It is also believed that the development of deformation twins is strongly dependent on the strain rate of the material, which is why their evolution with strain rate has been under such scrutiny.
    
    Multiple studies have used different types of specimens, including round- and flat-end tensile bars, to analyze the evolution of deformation twins with strain rate. These studies have concluded that there is non-uniform propagation of the deformation twins when varying depths of the specimen are taken into consideration, and they have found that the overall fraction of deformation twins decreases with increasing strain rate in all specimen depths.
    
    In addition, it has been observed that when the strain rate of the specimen exceeds a certain value, no further propagation of the deformation twins is observed. This phenomenon has been attributed to the formation of another type of twin – the dislocation twins – which prevent further propagation of the deformation twins.
    
    Further research has also suggested that a relationship exists between the fraction of deformation twins and the grain size of the specimen. Smaller grain size specimens have been found to have greater fractions of deformation twins than those with larger grain sizes, even at increased strain rates. This finding has been attributed to the fact that smaller grain sizes allow for enhanced dislocation slip and thus lesser strain relaxation which in turn, results in higher fractions of deformation twins.
    
    A series of research has been dedicated to finding the optimal strain rate for deformation twin formation in Mn-steel. Different results have been reported from different researchers, some suggesting that the optimal strain rate for twin formation is lower than the strain rate at which the onset of plastic flow occurs, whereas others suggest that it is equivalent to the yielding strain rate. It is also believed that the size of the grain boundaries play a role in the strain rate at which the maximum formation of deformation twins is achieved.
    
    Lastly, various numerical studies have been conducted to analyze the relationship between deformation twins and strain rate. Such studies have focused on the deformation sequence of the material when subjected to various strain rates. It has been found that the deformation twins incrementally increase in the beginning, followed by a rapid decrease at higher strain rates and then a plateau, indicating that the maximum fraction of deformation twins has been reached and further propagation is no longer possible.
    
    To conclude, the evolution of deformation twins with strain rate in medium manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V (Mn-steel) is a highly studied research topic. Multiple tests, observations, and numerical studies have been conducted to analyze it, and they have concluded that the fraction of deformation twins decreases with increasing strain rate and that a relationship also exists between the grain size and fraction of deformation twins. However, more research is needed to understand the exact relationship between these two parameters.

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