A molecular dynamics study of atomic diffusion effects on thermomechanical properties applying laser additive alloying process for the Cantor high entropy alloy

Abstract

High entropy alloys (HEAs) can be manufactured in many conventional ways, but it becomes difficult to fabricate heterogeneous materials and structures. The Selective Laser Melting (SLM) method generally melts pure elemental powders or prefabricated alloy powders without an alloying process. In-situ alloying in SLM, which is also called Laser Additive Alloying (LAA), using pure elemental powders becomes a promising method for creating HEA with heterogeneous structures. However, the effect of the diffusion of elements in the molten pool on the formation of HEA remains unclear. In this paper, the well-discussed Cantor HEA was studied in an in-situ alloying situation, where pure elemental powders (Co, Cr, Mn, Ni, Fe) distributed on a powder bed were irradiated by laser and were subsequently allowed to cool back to room temperature. The diffusion of specific elements, with respect to their original clusters, was tracked via Mean Square Displacement (MSD) as well as the final composition of key locations. Our model was verified by showing a good agreement with the overall average diffusion rates of each element in the Cantor HEA qualitatively in other works from literature. Results initially showed that as the energy density increases, better diffusion was observed through a pixel overlay analysis focusing on the mixing of different elements. The best-case scenario of diffusion from the pixel overlay map indicated a strong presence of 3 to 4 elements after the laser scanning. Given the conditions in the MD simulation, there was no apparent segregation of elements during the alloying process. The solidified powder bed was sectioned into a 5 by 4 grid to analyze the composition of the entire powder bed. In addition, we also conducted a simulation by implementing a 0.03 nm/ps laser scanning in a meander 2-track scan to completely melt the powder bed. After cooling and equilibration, Polyhedral Template Analysis was applied to analyze the crystal structure of the solidified powder bed in the presence of increasing components. When alloying the Cantor alloy all elements experience a complex diffusion behavior, elements like Cr experience fairly rapid diffusion rates compared to other elements, despite this, Cr only diffuses for a short period and diffuses minimally during the process. The analysis of element-specific behavior, such as diffusion, can provide a framework for the LAA production of HEA. This MD study provides a detailed analysis about the effect of diffusion on the formation of HEA system if in-situ alloying is adopted, this study is further extended to analyze the effect of diffusion on the thermomechanical properties of HEAs. Moreover, Young's Modulus, Shear Modulus, and lattice thermal conductivity coefficient were simulated and compared with published experimental works. Lastly, the diffusion coefficients were calculated in 3 near-equimolar alloys and 3 alloys with various Fe atomic percentages to study the effects of entropy on the tracer diffusion rates. The findings in this study in terms of diffusion behavior can assist the design and manufacturing of other novel HEAs for applications in extreme environments.