Size effect on the deformation mechanisms of nanocrystalline platinum thin films

Xinyu Shu,Yan Lu,Yanhui Chen,Haibo Long,Xuechao Sha,Hao Zhou,Deli Kong,Yinong Liu,Shengcheng Mao,Shiduo Sun

doi:10.1038/s41598-017-13615-6

Abstract

This paper reports a study of time-resolved deformation process at the atomic scale of a nanocrystalline Pt thin film captured in situ under a transmission electron microscope. The main mechanism of plastic deformation was found to evolve from full dislocation activity-enabled plasticity in large grains (with grain size d > 10 nm), to partial dislocation plasticity in smaller grains (with grain size 10 nm < d < 6 nm), and grain boundary-mediated plasticity in the matrix with grain sizes d < 6 nm. The critical grain size for the transition from full dislocation activity to partial dislocation activity was estimated based on consideration of stacking fault energy. For grain boundary-mediated plasticity, the possible contributions to strain rate of grain creep, grain sliding and grain rotation to plastic deformation were estimated using established models. The contribution of grain creep is found to be negligible, the contribution of grain rotation is effective but limited in magnitude, and grain sliding is suggested to be the dominant deformation mechanism in nanocrystalline Pt thin films. This study provided the direct evidence of these deformation processes at the atomic scale.

Highlights

Metallic materials may deform plastically in a number of different mechanisms
Many molecular dynamics (MD) simulations have been conducted in recent years to investigate the deformation mechanisms in nanocrystalline metallic materials
Some MD simulation investigations suggest that for nanocrystalline metallic materials dislocations emitted from grain boundaries are only partial dislocations[5,6,7,8], and full dislocations[5,9] are formed by merging the partial dislocations emitted from the same site at grain boundaries

Summary

Introduction

Metallic materials may deform plastically in a number of different mechanisms. A general knowledge base has been established of the main plastic deformation mechanisms[1,2,3], with the most prevalent being dislocation activities. It is understood that at below a certain critical grain size, it is difficult to retain dislocations and to allow normal dislocation activities to operate[2,4], due to the different mechanics conditions of the matrix at this extremely small scale[5,6,7,8]. In this regard, how plastic deformation may occur and how it can be accommodated in nanocrystalline metallic materials need to be understood. In this work, using a home-made device for tensile deformation[27,28,29,30] inside TEM with double-tilt capability to perform in situ high-resolution TEM (HRTEM) analysis, we studied the deformation process of a nanocrystalline platinum (Pt) thin films with grain sizes ranging from 20 nm to 3 nm, crossing the regions for both the “Hall-Petch relationship” and the “inverse Hall-Petch relationship”

Methods

Results

Conclusion