Radioactive isotopes reveal a non sluggish kinetics of grain boundary diffusion in high entropy alloys

M Vaidya,G Wilde,K G Pradeep,B S Murty,S V Divinski

doi:10.1038/s41598-017-12551-9

Abstract

High entropy alloys (HEAs) have emerged as a new class of multicomponent materials, which have potential for high temperature applications. Phase stability and creep deformation, two key selection criteria for high temperature materials, are predominantly influenced by the diffusion of constituent elements along the grain boundaries (GBs). For the first time, GB diffusion of Ni in chemically homogeneous CoCrFeNi and CoCrFeMnNi HEAs is measured by radiotracer analysis using the 63Ni isotope. Atom probe tomography confirmed the absence of elemental segregation at GBs that allowed reliable estimation of the GB width to be about 0.5 nm. Our GB diffusion measurements prove that a mere increase in number of constituent elements does not lower the diffusion rates in HEAs, but the nature of added constituents plays a more decisive role. The GB energies in both HEAs are estimated at about 0.8–0.9 J/m2, they are found to increase significantly with temperature and the effect is more pronounced for the CoCrFeMnNi alloy.

Highlights

In the 21st century, the insistent campaign for enhancing energy efficiency in power generation and aerospace technology renders the development of high performance materials the foremost goal of materials scientists
High-angle grain boundaries (HAGBs) are highlighted in black color, low-angle grain boundaries (LAGBs) are marked by yellow line segments and twins are indicated in red
CoCrFeMnNi shows a increased fraction (~8% more) of LAGBs, it is not expected to affect the present GB diffusion measurements, since the LAGBs reveal typically significantly lower diffusion rates compared to those for the HAGBs2, a corresponding contribution can be observed in dedicated experiments[17]

Summary

Introduction

In the 21st century, the insistent campaign for enhancing energy efficiency in power generation and aerospace technology renders the development of high performance materials the foremost goal of materials scientists. A vital property, involving a specific defect type namely the grain boundary, is the atomic transport or diffusion along the boundary It is the rate-limiting step in several key deformation processes and phase transformations. A new class of multi-component alloys termed high entropy alloys (HEAs) have emerged over the last decade which promises to present attractive mechanical properties including uncompromised strength-ductility combinations[4]. These alloys feature the unique characteristics of possessing multi-component constituent elements in equimolar or near equimolar proportions. The GB diffusion coefficients reported in the present study are essential to guide the selection and design of HEAs as high performance materials

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Conclusion