Stabilizing nanocrystals via interface co-segregation and clustering.

Abstract

Nanocrystalline (NC) metals are generally metastable and prone to undergo grain coarsening, which severely impedes their engineering application. A strategy to counteract this phenomenon lies in decreasing the energetic driving force for grain growth, through grain boundary segregation. The natural thought of ‘more segregation is better’, which was exercised over decades to solve this problem, however, does not work because interfaces have only limited solute decoration tolerance. Herein, we propose to solve this long-standing problem by doping with multiple solutes, which creates a synergistic combination of thermodynamic and kinetic stabilization mechanisms without inducing formation of new phases. This is enabled by introducing chemical compositional complexity to GBs, which possibly allows more efficient GB stabilization through segregation, thus drastically reducing alloying costs while retain the original properties of the NCs. We applied this new design approach to NC-Nb and show that we can increase its instability temperature from 873 to 1173 K with as little alloying as only 2 at.% of Co, Ni, Ti and Hf with equal fractions. The material maintains its nanocrystalline structure after annealing at 1173 K for 50 h. Therefore, this work presents a promising design strategy for developing stable dilute NC materials for advanced engineering application.