Stabilizing nanocrystals via higher entropy interfaces

Abstract

Nanocrystalline (NC) metals are attractive due to their outstanding mechanical, physical and chemical properties. However, they are generally metastable and prone to undergo grain coarsening, which severely impedes their engineering application, particularly at elevated temperatures. A strategy to counteract this phenomenon lies in decreasing the energetic driving force for grain growth, through grain boundary segregation according to the Gibbs adsorption isotherm. The natural thought of ‘more segregation is better’, which was exercised over decades to solve this problem, however, does not work because interfaces themselves are thermodynamic entities, with only limited solute decoration tolerance. Herein, we propose to solve this long-standing problem via introducing multi-component co-segregated interfaces. This concept allows us to maximize the solute decoration of interfaces and minimize their energy, without triggering formation of new phases. This is enabled by extending the high entropy alloy (HEA) concept from the bulk to interfaces, a strategy we refer to as interfaces with higher entropy. This design concept is not transferred one-to-one but constrained by a few rules that reflect the nature of interfaces: use of several co-segregating elements with high mixing entropy; segregation remains in the dilute limit; and use of elements with high segregation coefficient. The latter criterion marks an important difference to the HEA concept as a high segregation coefficient guarantees that alloying elements are deposited only to those locations where they are needed, namely, to the interfaces, thus drastically reducing alloying costs. We applied this new design approach to NC-Nb and show that we can increase its stability from 873 to 1023 K with as little alloying as only 1 at.% of Ti, Ni, Co and Hf with equal fractions. The material maintains its nanocrystalline structure even after 2200 h at 973 K. This work thus offers a new paradigm for designing stable dilute NC materials for advanced engineering application.