Oxidation induced amorphicity and subsequent delayed crystallization in binary transition metal alloy NbMo

Multi-Principle Element Alloys (MPEAs) are a new material that shows high promise for new applications but lacks a fundamental understanding of where its change compared to mono-metals comes from. Studying simplified cases via binary alloys allows us to demonstrate such changes in relative isolation compared to the complicated high entropy alloy case. Through temperature-ramped in-air annealing, thin (100nm) NbMo films are shown to form quasi-amorphous oxide, as is also observed by TEM (Transmission Electron Microscopy). Our observations suggest that the act of mixing in oxygen causes the alloy to change phases, which can be reasoned by considering oxygen as a hypothetical third metallic particle whose inclusion breaks the Hume-Rothery rules. Consequently, the alloy, including oxygen, is unable to form a single solution, which is observed with the phase turning amorphous rather than mixed crystalline. As such, this work makes a first step in trying to find fundamental characteristics that allow us to predict thin film MPEA behavior in order to show how alloys behave in oxidizing and reducing environments.