Precipitation behaviors and properties of micro-beam plasma arc cladded CoCrFeMnNi high-entropy alloy at elevated temperatures

Abstract

In the present research, the effect of precipitates after heat treatment at different temperature on the microhardness, wear and corrosion resistance of the micro-plasma arc cladded CoCrFeMnNi high-entropy alloy layer were investigated in detail. The cladding layer after 500 °C heat treatment for 4 h appeared the Cr-rich BCC phase at grain boundary. Besides, a little Cr-rich δ phase was detected at grain boundary after 700 °C heat treatment. However, the cladding layer after 900 °C and 1160 °C heat treatment still kept a single FCC phase, which was resulted from that the mixed entropy change of the alloy system was higher at high temperature, so the Gibbs free energy changed less, and then the system tended to be thermodynamically more stable. Therefore, entropy played a major role in phase precipitation process. The wear mass loss and wear resistance of layers increased first and then decreased with the increase of heat treatment temperature. The potentiodynamic polarity curves indicated that the corrosion resistance of as cladded and 500 °C heat treated layers was better than that of 304 stainless steel in 3.5% NaCl solution. The 500 °C heat treated cladding layer exhibited a Cr-rich BCC phase between the dendrites, and the Ni-rich/Mn-rich regions between the dendrites were relatively reduced, which weakened the corrosion between the dendrites and improved the corrosion resistance of the cladding layer to some extent. The 700 °C heat treated cladding layer precipitated the δ phase at the grain boundary, which reduced the corrosion resistance of the cladding layer. As the heat treatment temperature increased further, the grain boundary became wider, and the element was homogenized, which also enhanced the dilution effect of the matrix Fe element, and decreased the relative content of Cr element, and finally resulted in the bad corrosion resistance. Mixing entropy of each cladding layer reduced due to the partial element loss and the dilution effect.