Plastic Forming of Metals at the Nanoscale: Interdiffusion-Induced Bending of Bimetallic Nanowhiskers.

Yuanshen Qi,Eugen Rabkin,Michael Kalina,Gunther Richter,Eylül Suadiye

doi:10.1021/acsnano.0c04327

Abstract

Controlled plastic forming of nanoscale metallic objects by applying mechanical load is a challenge, since defect-free nanocrystals usually yield at near theoretical shear strength, followed by stochastic dislocation avalanches that lead to catastrophic failure or irregular, uncontrolled shapes. Herein, instead of mechanical load, we utilize chemical stress from imbalanced interdiffusion to manipulate the shape of nanowhiskers. Bimetallic Au–Fe nanowhiskers with an ultrahigh bending strength were synthesized employing the molecular beam epitaxy technique. The one-sided Fe coating on the defect-free, single-crystalline Au nanowhisker exhibited both single- and polycrystalline regions. Annealing the bimetallic nanowhiskers at elevated temperatures led to gradual change of curvature and irreversible bending. At low homological temperatures at which grain boundary diffusion is a dominant mode of mass transport this irreversible bending was attributed to the grain boundary Kirkendall effect during the diffusion of Au along the grain boundaries in the Fe layer. At higher temperatures and longer annealing times, the bending was dominated by intensive bulk diffusion of Fe into the Au nanowhisker, accompanied by a significant migration of the Au–Fe interphase boundary toward the Fe layers. The irreversible bending was caused by the concentration dependence of the lattice parameter of the Au(Fe) alloy and by the volume effect associated with the interphase boundary migration. The results of this study demonstrate a high potential of chemical interdiffusion in the controlled plastic forming of ultrastrong metal nanostructures. By design of the thickness, microstructure, and composition of the coating as well as the parameters of heat treatment, bimetallic nanowhiskers can be bent in a controlled manner.

Highlights

Controlled plastic forming of nanoscale metallic objects by applying mechanical load is a challenge, since defectfree nanocrystals usually yield at near theoretical shear strength, followed by stochastic dislocation avalanches that lead to catastrophic failure or irregular, uncontrolled shapes
We used the easy-lift technique in a focused ion beam−scanning electron microscope (FIB-SEM) dual beam instrument to harvest and transfer individual NWs from the W substrate to Mo foil substrates, see Figure 1c,d
We have successfully demonstrated the feasibility of controlled plastic deformation of the ultrastrong metallic nanostructures through the chemical interdiffusion route, employing either the grain boundaries (GBs) Kirkendall effect or full diffusion intermixing

Summary

Introduction

Controlled plastic forming of nanoscale metallic objects by applying mechanical load is a challenge, since defectfree nanocrystals usually yield at near theoretical shear strength, followed by stochastic dislocation avalanches that lead to catastrophic failure or irregular, uncontrolled shapes. ACS Nano www.acsnano.org stochastic distribution of plastic strain.[12] For this reason, the mechanical loading of defect-free metal nanocrystals cannot be employed for their controlled plastic forming into a desired shape. Such forming may be necessary because the variety of shapes of the as-synthesized defect-free nanocrystals is severely limited by the relative specific surface energies of the crystal facets.[15,16]. Studies of mechanical behavior of NWs employing three-point bending and cantilever beam bending reveal high elastic strains at the onset of plastic yielding, often followed by a strain burst and abrupt fracture.[20−23] Currently, the curvature or morphology of metal NWs cannot be precisely manipulated by plastic deformation via mechanical load.[24]

Methods

Results

Conclusion