# Improved Mechanical Property Synergy of CoCrNiAlTi Medium-Entropy Alloy Through Boron Microalloying, Thermomechanical Treatment and Aging Treatment

## TL;DR

Adding boron and using specific heat treatments improves the strength and durability of a CoCrNiAlTi alloy.

## Contribution

The study introduces boron microalloying combined with thermomechanical and aging treatments to enhance the mechanical properties of a MEA.

## Key findings

- Boron doping refines grain size from 600 to 200 nm in the alloy.
- Cold rolling followed by annealing and aging achieves optimal mechanical performance with high strength and ductility.
- A nano-sized L12 phase is coherently embedded in the FCC matrix after treatment.

## Abstract

Medium-entropy alloys (MEAs) with a simple phase structure and nanoprecipitates have excellent mechanical properties and considerable potential for advanced structural applications. The current study investigated the effect of boron microalloying and thermomechanical treatment on the microstructure evolution and mechanical properties of Co43Cr15Ni30Al5Ti7 and (Co43Cr15Ni30Al5Ti7)99.7B0.3 MEAs. X-ray diffraction analysis revealed a single phase of face-centered cubic (FCC) structure in all as-cast samples. After cold rolling and recrystallization annealing were completed, a clear ordered FCC (L12) phase was observed concurrently with the FCC matrix. In the alloy doped with 0.3 at.% B, the grain size was refined from 600 to 200 nm. TEM analysis revealed a nano-sized L12 phase coherently embedded in the FCC matrix. Analysis of the mechanical properties of boron-doped MEA samples revealed that cold rolling to 80% thickness followed by annealing at 900 °C for 2 h and aging at 750 °C for 4 h yielded the best mechanical performance. Among all samples, the alloy doped with 0.3 at.% boron achieved an optimal combination of mechanical properties (yield strength: 1817 MPa; ultimate tensile strength: 2313 MPa; ductility: 14.5%).