Nitriding-Assisted Surface Enhancement of Refractory Multi-Principal Element Alloys for Materials Design

Abstract

There is a variety of transportation and energy conversion systems that require materials with improved temperature capabilities and mechanical durability to better their performances and efficiencies. Surface enhancement and alloy design are common strategies to achieve the goals. However, a longstanding challenge for the materials created using these methods is to have desired surface performances and bulk mechanical properties simultaneously. Multi-principal element alloys (MPEAs) with significant fractions of multiple elements provide exciting opportunities to control surface reactions by tuning the alloy chemistry while maintaining favorable bulk properties. Refractory MPEAs are of particular interest due to their ability to retain high yield strength to elevated temperatures. This work focuses on studying the complex interplay of nitrogen dissolution, element diffusion, and nitride formation in refractory MPEA systems during gas nitriding to provide general rules for future alloy design. The nitride-forming behavior of several pure refractory metals was first studied to serve as a reference for MPEA component selection. The tendency to form nitride compound layers and diffusion zones as a function of reaction time, temperature, and nitriding potential was identified based on mass gain measurements, microstructure observations, and quantitative microchemical analyses. The hardness increase in the nitrided zone was then correlated with the microstructures and the local composition. Building on these understandings and thermodynamic calculations, the development of phases and microstructures of NbTaTiZr and NbTaTiMo during gas nitriding was investigated. Parallel studies with the addition of Al were also performed to manipulate the formation of single BCC solid solution or dual-phase BCC and ordered B2 phase, and in hopes of enhancing the bulk properties as well as surface performances. Characterization of the microstructure evolution with depth, nitride layer growth, internal nitridation, and intermetallic compound formation by TEM and EDS suggested that the elements tend to de-mix according to the nitrogen affinity of the elements. The results showed that metals with higher nitrogen affinities usually have higher contents in outer surface nitride layers and are more likely to form internal nitride precipitates. This leads to higher nitrogen intake in MPEAs with larger fractions of high nitrogen affinity elements. The addition of Al increases the complexity of the phases in the alloy, and reduces the thicknesses of the surface nitride layers and the nitrided zone. This work has taken the first step to provide insight to help refractory MPEA composition design and development of surface enhancement strategies in the future.