Influence of Mo Segregation at Grain Boundaries on the High Temperature Creep Behavior of Ni-Mo Alloys: An Atomistic Study.

Qian Li,Huayuan Tang,Yonggang Zheng,Hongfei Ye,Hongwu Zhang,Jiayong Zhang

doi:10.3390/ma14226966

Abstract

Based on molecular dynamics simulations, the creep behaviors of nanocrystalline Ni before and after the segregation of Mo atoms at grain boundaries are comparatively investigated with the influences of external stress, grain size, temperature, and the concentration of Mo atoms taken into consideration. The results show that the creep strain rate of nanocrystalline Ni decreases significantly after the segregation of Mo atoms at grain boundaries due to the increase of the activation energy. The creep mechanisms corresponding to low, medium, and high stress states are respectively diffusion, grain boundary slip and dislocation activities based on the analysis of stress exponent and grain size exponent for both pure Ni and segregated Ni-Mo samples. Importantly, the influence of external stress and grain size on the creep strain rate of segregated Ni-Mo samples agrees well with the classical Bird-Dorn-Mukherjee model. The results also show that segregation has little effect on the creep process dominated by lattice diffusion. However, it can effectively reduce the strain rate of the creep deformation dominated by grain boundary behaviors and dislocation activities, where the creep rate decreases when increasing the concentration of Mo atoms at grain boundaries within a certain range.

Highlights

In the past few decades, nanocrystalline (NC) materials have attracted more and more attention because of their excellent properties such as ultra-high strength at room temperature [1,2,3,4,5,6,7]
The research based on molecular dynamics simulations that was conducted by Keblinski et al indicated the Coble creep is one of the main creep mechanisms of ultrafine grained materials [32,33]
Three types of models are constructed: the first one is the NC Ni sample without impurity atoms, the second type is the NC Ni sample with a certain proportion of Mo atoms randomly distributed in grains, and the third type is the NC Ni sample with a certain proportion of Mo atoms segregated at grain boundary (GB), examining the effect of segregation on the creep behaviors of NC materials

Summary

Introduction

In the past few decades, nanocrystalline (NC) materials have attracted more and more attention because of their excellent properties such as ultra-high strength at room temperature [1,2,3,4,5,6,7]. When functioned at high temperatures and under continuous stresses, creep deformation, which may cause accidental deformation or even failure of materials and structures [8,9,10], is an inevitable important issue. Extensive researches have been performed to study the creep mechanism of nanometallic materials, through both experiments and simulations [9,10,11,12,13,14,15,16,17,18]. Previous simulation results have clarified that the main creep mechanisms in NC materials are the lattice diffusion, grain boundary (GB) diffusion, GB sliding and dislocation activity. The well-known Brid-Dorn-Mukherjee classical equation [21], which comprehensively describes the effects of external stress, temperature and grain size, is widely used to analyze the creep behavior of NC materials. Its expression is as follows [21]: ε

A D0 Gb kBT b d p σ G n exp

Simulation Methods and Parameters

Effect of Segregation on Creep Behavior

Creep Mechanism Analysis

Influences of Grain Size and Temperature