The desirable properties of nanocrystalline metals including high strength and hardness, low friction and wear, and resistance to fatigue crack initiation can be lost if the metal undergoes mechanically-induced grain growth. In prior work, mechanically-induced grain growth has been shown to directly cause fatigue crack initiation and wear-induced delamination in nanocrystalline metals. In-situ TEM and synchrotron x-ray high-cycle fatigue experiments reveal the incipient stages of damage, where grain growth provides the precursor state that precedes the formation of nanometer-scale cracks. Yet grain growth can also be beneficial- in cases such as monotonic tension, the boundary migration process is thought to be an additional deformation mechanism enabling enhanced ductility. For this reason, it is necessary to develop strategies to control boundary stability under mechanical driving forces. Recently there has been a strong interest in the stabilization of nanocrystalline grain boundaries against thermal evolution by lowering the energetic cost of the grain boundary via alloying. Specific binary alloys have been shown to preferentially segregate the solute species to grain boundaries and lower the driving force for boundary migration. The question remains as to whether the resulting thermal stability also gives rise to enhanced stability under mechanical driving forces. Emerging results on the fatigue and wear resistance of several binary nanocrystalline alloys, including the noble metal system Pt-Au, appear to suggest that the thermally stabilized boundaries are indeed resistant to fatigue and wear-induced microstructural evolution, resulting in exceptional performance.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.