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 & DEG;C. It has been shown that the long-term aged CoCrNi MEA has held a single gamma phase, a nanoparticulate and discontinuous block gamma ' phase, and eta-Ni-3(Ti,Ta) phase in acicular Widmanstatten, 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 & DEG;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 gamma' phase precipitation. Next, eta phase plays an important role in strengthening, improving or at least harmless plasticizing effect since the eta phase has the preferential orientation relationship of [011](gamma)//[2110](eta), {111}(gamma)// {0001}(eta) with the gamma matrix. It has been confirmed that the strengthening mechanisms are ascribed to the dislocation accumulation and proliferation, formation of stacking faults and nanotwinning in gamma, gamma' and eta phase induced by the precipitation of gamma ' and eta phase.(c) 2022 Published by Elsevier B.V.

Automated summary

This study comprehensively investigated the microstructure and mechanical properties of long-term aged CoCrNi medium entropy alloys (MEAs) at both room and high temperatures. The results showed that the aged MEAs maintained a single gamma phase, nanoparticulate and discontinuous block gamma' phase, and eta-Ni-3(Ti,Ta) phase in various shapes. These aged MEAs exhibited comparable strengths to those treated under standard heat treatment, while surprisingly demonstrating superior plasticity. The strengthening mechanisms were attributed to dislocation accumulation, formation of stacking faults, and nanotwinning induced by precipitation of gamma' and eta phases.