Unveiling the Puzzle of Brittleness in Single Crystal Iridium

TL;DR

This study uncovers that high-density sessile dislocation loops cause brittleness in single-crystal iridium, revealing a new embrittlement mechanism in FCC metals through combined experimental and computational analysis.

Contribution

It provides the first direct experimental evidence linking dislocation loops to brittleness in iridium and introduces a novel embrittlement mechanism unique to FCC metals.

Findings

Sessile dislocation loops are the primary cause of brittleness.

Dislocation loops form via transformation from perfect dislocations under stress.

The mechanism may be applicable to other FCC metals.

Abstract

Materials for extreme environments require high strength yet ductile to tolerate catastrophic damage. Face-centered cubic (FCC) metals are typically ductile under stress, but single-crystal FCC iridium exhibits intrinsically brittle, limiting its wider applications. Great efforts on theoretical studies have attributed this to non-planar dislocation cores or impurities, while direct experimental evidence has remained elusive. Here we report that high-density, sessile Frank dislocation loops with zero-net Burgers vectors are the primary cause of the brittleness, identified through atomic-resolution scanning transmission electron microscopy. Through first-principles calculations, supported by discrete dislocation dynamics simulations, we reveal that these loops form via an energetically favorable transformation from mixed perfect dislocations under stress, a process unique to iridium among other FCC metals. The immobile loops act as potent barriers, drastically increasing yield strength and work hardening by impeding dislocation glide and consuming mobile dislocations. These decisive results not only deepen the understanding of the iridium brittleness, but also describe the existence of a new embrittlement mechanism inherent to the FCC lattice and not previously described in the literature. The latter may enable novel routes for property tuning across a broad class of materials, which is of paramount importance to metallurgical technology