Molecular dynamics study on the effect of grain size on the plastic deformation of Fe-Cr-Ni alloy nanopolycrystals

Abstract

The mechanical properties and plastic deformation mechanisms of 304 austenitic stainless steel nanopolycrystals (ASSNP) with different grain sizes during uniaxial tension were studied by molecular dynamics method. The results show that the grain size of 304 ASSNP material is not monotonically related to shear modulus and Poisson 's ratio, but there is a critical grain size. The sample with a grain size of 17.6 nm exhibits the best mechanical properties, with the largest shear modulus and the smallest Poisson 's ratio. In the process of plastic deformation, the plastic deformation mechanism of large grain samples is mainly dominated by dislocation slip and supplemented by twin deformation. Although the number of grains in the sample whose grain size is less than the critical grain size is large, and the total area of the grain boundary is also large, it does not play a role in hindering grain deformation. On the contrary, because the grain boundary is thickened, the grain boundary has enough space to slide, which reduces the mechanical properties of the material. The plastic deformation mechanism of small grain samples is the combined effect of dislocation slip, grain boundary slip and grain rotation. In addition, with the increase of strain, the sessile dislocations generated by the intersection of slip planes, the dislocation pile-up at the grain boundary edge, and the total area of the grain boundary increases due to the appearance of more steps. These three factors work together to hinder grain deformation, which makes these samples exhibit hardening phenomenon.