The role of the metal-oxide Interface’s terminating layer on the selective cold cracking of a commercial Niobium–Hafnium–Titanium (C-103) alloy

Abstract

A commercial Niobium–Hafnium–Titanium (Nb-Hf-Ti) alloy (C-103), produced by vacuum arc remelting (VAR), showed clear signatures of internal oxidation viz. presence of monoclinic-HfO2 at all the grain boundaries. Minor cold rolling, to ∼10% reduction in thickness, initiated cold cracking. This was observed at some, but not all, of the high angle Nb–Nb grain boundaries containing coarse HfO2 2nd phase oxide precipitates. Closer microscopic examination revealed cold cracking at selective Nb-HfO2 interfaces. There appeared no clear mesoscopic experimental rationale behind this selective interfacial cold cracking. In order to understand this, density functional theory (DFT) calculations were performed on the metal-oxide interface models. They showed significant separation or decohesion energy difference (∼76%) with different atomistic bonding at the terminating layer of the Nb-HfO2 interface. In particular, the interface with one atomic layer of Nb bonded with oxygen of HfO2 had very low work of separation, which was about ∼8% lower than the Nb–Nb interface of the bulk Nb. In brief, the DFT simulations brought out the defining role of metal-oxide interface’s terminating layer on the interface decohesion. This is in agreement with the past theoretical research performed on several metal||non-metal interfaces. The present study extends this hypothesis to a practical example of selective cold cracking of a commercial Nb-Hf-Ti alloy.