Microstructure evolution and mechanical properties in a gas tungsten arc welded Fe₄₂Mn₂₈Co₁₀Cr₁₅Si₅ metastable high entropy alloy

Weldability studies on high entropy alloys are still relatively scarce, delaying the deployment of these materials into real-life applications. Thus, there is an urgent need for in-depth studies of the weldability of these novel advanced engineering alloys. In the current work, an as-cast Fe₄₂Mn₂₈Co₁₀Cr₁₅Si₅ metastable high entropy alloy was welded for the first time using gas tungsten arc welding. The weld thermal cycle effect on the microstructure evolution over the welded joint was examined using electron microscopy in combination with electron backscatter diffraction, synchrotron X-ray diffraction analysis, and thermodynamic calculations. Furthermore, tensile testing and hardness mapping were correlated with the microstructure evolution. The microstructure evolution across the joint is unveiled, including the origin of the ε-h.c.p. phase at different locations of the material. Different strengthening effects measured throughout the joint are associated with the weld thermal cycle and resulting microstructure. A synergistic effect of smaller grain size of the ε-h.c.p. phase in the fusion zone, overturns the reduced volume fraction of this phase, increasing the local strength of the material. Moreover, the brittle nanosized σ phase was also found to play a critical role in the joints’ premature failure during mechanical testing.