Deformation Behavior of Two-Phase Gradient Nanograined Fe95Ni5 Alloys under Different Types of Loading

In this paper, we used molecular dynamics simulations to study the atomic mechanisms of phase transformations, plasticity features, and mechanical properties of two-phase Fe95Ni5 (at. %) samples with a gradient nanograined structure under uniaxial deformation and shear. The simulated samples with a uniform distribution of Ni atoms are composed of fcc grains from 10 to 30 nm in size, which in turn contain bcc interlayers in the form of lamellae of various distribution and size. It was shown that uniaxial loading or shear causes the bcc-fcc phase transformation in the lamellae. In the vast majority of cases, phase transformations are initiated at the junction of lamellae and grain boundaries. Deformation-induced phase transformations in lamellae occur at the front of bands propagating from grain boundaries. Grains larger than similar to 15 nm can have several bands or regions with differently orientated fcc lattices, whose meeting results in grain fragmentation. It was found that the atomic volume increases abruptly during the bcc-fcc structural phase transformation. The Kurdyumov-Sachs orientation relation is valid between the initial bcc and formed fcc structures. It was shown that the volume fraction and spatial distribution of the bcc phase significantly affect the yield stress of the sample. The yield stress can be increased by forming the bcc phase only in large-grained layers. This behavior is associated with the fragmentation of large grains, and consequently with grain refinement, which, in accordance with the Hall-Petch relation, improves the strength of the material.

Automated summary

This study investigates the atomic mechanisms of phase transformations, plasticity features, and mechanical properties of two-phase Fe95Ni5 samples with a gradient nanograined structure under uniaxial deformation and shear using molecular dynamics simulations. It is found that uniaxial loading or shear causes the bcc-fcc phase transformation in the lamellae, which is initiated at the junction of lamellae and grain boundaries. Deformation-induced phase transformations occur at the front of bands propagating from grain boundaries. Grains larger than 15 nm exhibit fragmentation due to the collision of different orientated fcc lattices. The study also reveals that the atomic volume increases abruptly during the bcc-fcc phase transformation, and the yield stress of the sample is significantly affected by the volume fraction and spatial distribution of the bcc phase.