Long-term structural stability and excellent mechanical properties of CoCrNi system medium entropy alloys

CoCrNi medium entropy alloys (MEAs) are promising candidates for high- temperature application, but their structural stability and mechanical properties during long-term service are still an open question. This work comprehensively investigated the microstructure and mechanical properties at both room and high temperatures of CoCrNi MEAs after being aged for 500 h at 850 °C. It has been shown that the long-term aged CoCrNi MEA has held a single γ phase, a nanoparticulate and discontinuous block γ′ phase, and η-Ni3(Ti,Ta) phase in acicular Widmanstätten, cellular and rod-like shapes with the addition of Al, Ti, and Ta. Long-term aged CoCrNi MEAs hold same levels (or sometimes slightly weaker levels) of room- and high-temperature strengths as those MEAs under the standard heat treatment. Specifically, CoCrNi MEA with 2%Al, 2%Ti, and 1%Ta presents an ultimate strength of 1012 MPa at room temperature and a yield strength of 695 MPa at 700 °C. Surprisingly, all of the relevant long-term aged MEAs exhibit superior plasticity to that under standard heat treatment. The anomalous mechanical properties are firstly attributed to the γ' phase precipitation. Next, η phase plays an important role in strengthening, improving or at least harmless plasticizing effect since the η phase has the preferential orientation relationship of [011]γ//[2110]η, {111}γ//{0001}η with the γ matrix. It has been confirmed that the strengthening mechanisms are ascribed to the dislocation accumulation and proliferation, formation of stacking faults and nanotwinning in γ, γ' and η phase induced by the precipitation of γ′ and η phase.