High temperature strengthening via nanoscale precipitation in wrought CoCrNi-based medium-entropy alloys

摘要

In order to make CoCrNi medium-entropy alloys (MEAs) applied for high temperature fields, we hereby demonstrate an approach of manipulating the mechanical properties of fcc structured MEA systems by the precipitation of nanoscale L1_2-(Ni, Co, Cr)_3(Ti, Al, Ta) phase. A new kind of wrought CoCrNi MEAs with superior high temperature mechanical properties has been developed by composition design and hot processing. It is confirmed that the composite addition of Al, Ti and Ta element into CoCrNi MEA not only promotes the precipitation but also improves the thermal stability of nanoscale γ' phase, resulting in significant enhancement at high temperature. Especially, the (CoCrNi)_(95)Al_2Ti_2Ta_1 MEA exhibits the yield and tensile strength at 700 °C up to ～620 MPa and ～939 MPa, respectively, which are higher than those of IN718 superalloys, and holds an acceptable engineering strain of 28.3%. The high temperatures strengthening mechanisms can be ascribed to the annealing twinning induced grain boundary strengthening, precipitation strengthening by the suppression of the local deformation around nanoscale γ' grain boundaries, and Orowan bypass mechanism of dislocations. The excellent mechanical properties and processability makes the MEAs promising for high temperature components.