Synergistic strengthening and anti-erosion mechanism of laser melting deposited CoCrNiTix medium-entropy alloys

In order to develop a highly anti-erosion MEA composition suitable for strengthening and repairing 904L steel blades in strong acid reactors, a cluster formula of [Cr-Ni₆Co₆]Cr₅ with varying Ti atoms was designed to investigate the effect of Ti atoms on the strengthening and erosion resistance mechanism. The results reveal that the moderate Ti atoms (x≤0.15 in CoCrNiTi_x) remain entirely dissolved into the FCC solid solution. Excessive Ti contents facilitate the formation of L1₂ nano-precipitates and inter-dendritic coarse D0₂₄ intermetallic phases. Adding Ti atoms effectively improves the strength yet deteriorates the ductility. Notably, the Ti_0.30 MEA exhibits the highest σ_0.2 ~ 806.01 MPa and σ_b ~ 1059.52 MPa, along with a prominent work hardening rate, primarily due to coherent L1₂ precipitation strengthening combined with grain refinement. As x = 0.15, the Ti_0.15 MEA shows the best solid-particle erosion resistance since the Ti atoms completely dissolved in the dendrites markedly improve the yield strength and strain hardening capacity by mutual interactions among high-density dislocations and twins. Moreover, the single FCC phase structure ensures excellent formation ability of uniform and stable passive films against plastic distortion damage. These findings provide a paradigm for developing LAM-ed super-corrosion-resistant MEA alloys.