Additive manufacturing of a metastable high entropy alloy: Metastability engineered microstructural control via process variable driven elemental segregation

Compositional paradigm shift in high entropy alloys (HEAs) provided new opportunities for microstructural engineering, whereas process control during laser powder bed fusion (LPBF) additive manufacturing (AM) enable fine microstructure tailoring. Metastability engineering in transformation induced plasticity (TRIP) HEAs by addition of minor alloying elements is an attractive strategy for fine microstructural tuning. This study explored in detail the microstructural evolution during LPBF AM of a metastable Fe40Mn20Co20Cr15Si5 (at.%) dual phase HEA (CS-HEA). LPBF processing window for CS-HEA was established based on quantitative analysis and experiments. Based on melt pool overlap lack of fusion pores were observed at lower energy densities (J) of J & LE; 31.25 J/mm3 and key-hole formation by melt pool destabilization in case of J & GE; 75 J/mm3. The microstructure of CS-HEA consists of metastable FCC-& gamma; and HCP-& epsilon; phases; LPBF process parameters governed the final phase fraction in the alloy which has been correlated to metastability alteration of the high temperature & gamma; phase. Final microstructural engineering was devised by LPBF process control which enabled cooling rate manipulation to guide Mn and Si segregation at the cell boundaries, thereby controlling the matrix metastability and final phase fraction. Additionally, high resolution transmission electron microscopy (TEM) revealed disparity in stacking fault morphology in CS-HEA with LPBF process variable alterations associated with local variations in chemical composition and stacking fault energy. The phase evolution with process parameters also affected the nanomechanical behavior of the alloy.

Automated summary

A paradigm shift in the composition of high entropy alloys (HEAs) has provided new opportunities for microstructural engineering, particularly in laser powder bed fusion (LPBF) additive manufacturing (AM) which allows for fine-tuning of microstructures. This study focused on the microstructural evolution of a metastable dual phase HEA during LPBF AM and established a processing window for the alloy. The microstructure of the alloy was governed by the LPBF process parameters, which influenced the final phase fraction and metastability alteration. Additionally, the study observed variations in stacking fault morphology and nanomechanical behavior of the alloy with changes in process parameters.