Stabilization of mechanical strength in a nanocrystalline CoCrNi concentrated alloy by nitrogen alloying

TL;DR

This study demonstrates that nitrogen alloying in a CoCrNi medium-entropy alloy enhances grain refinement, increases hardness and strength, and improves thermal stability after high pressure torsion and annealing, though ductility decreases at high temperatures.

Contribution

It introduces nitrogen alloying as a method to stabilize nanocrystalline structure and improve mechanical properties of CoCrNi alloys after severe plastic deformation.

Findings

Nitrogen reduces grain size by 40%.

Nitrogen alloyed alloy shows 10-15% higher hardness and strength after HPT.

Nitrogen improves thermal stability and hardness after annealing.

Abstract

The mechanical performance and microstructures of a CoCrNi medium-entropy alloy (MEA) and NCoCrNi, alloyed with 0.5at% N, after high pressure torsion (HPT) and subsequent annealing treatments in a temperature range of 150-1000 {\deg}C were investigated. The introduction of N results in a reduction of grain size by 40% and ~10-15% increased hardness and bending strength after HPT. Both materials showed strain hardening and plasticity after HPT even at these strength levels over 1500 MPa which is induced by the saturated nanocrystalline state. Annealing at intermediate temperatures of 300-500 {\deg}C resulted in an additional increase of hardness and strength by ~20-40%, compared to the HPT deformed state. This effect was ~10 % more pronounced in the nitrogen alloyed CoCrNi that also showed better thermal stability. However, the increase in strength after 500 {\deg}C annealing was accompanied by a drastic loss of ductility in both materials.