Effects of Cold Rolling or Precipitation Hardening Treatment on the Microstructure, Mechanical Properties, and Corrosion Resistance of Ti-Rich Metastable Medium-Entropy Alloys

Abstract

Titanium-rich metastable medium-entropy alloys, designed for low elastic moduli, sacrifice strength. However, enhancing their mechanical strength is crucial for bio-implant applications. This study aims to enhance the mechanical properties and corrosion resistance of a metastable Ti₈₀-Nb₁₀-Mo₅-Sn₅ medium-entropy alloy using various treatments, including cold rolling (at 50% and 75% reduction) and precipitation hardening (at room temperature, 150 °C, 350 °C, 550 °C, and 750 °C). The results showed that the alloy underwent a stress-induced martensitic transformation during the rolling process. Notably, the α phase was precipitated in the β grain boundaries after 30 days of precipitation hardening at room temperature. The yield strengths of the alloy increased by 51% and 281.9% after room-temperature precipitation and 75% cold rolling, respectively. In potentiodynamic corrosion tests conducted in phosphate-buffered saline solution, the pitting potentials of the alloy treated using various conditions were higher than 1.8 V, and no pitting holes were observed on the surface of the alloys. The surface oxide layer of the alloy was primarily composed of TiO₂, Nb₂O₅, MoO₃, and SnO₂, contributing to the alloy's exceptional corrosion and pitting resistance. The 75% rolled Ti₈₀-Nb₁₀-Mo₅-Sn₅ demonstrates exceptional mechanical properties and high corrosion resistance, positioning it as a promising bio-implant candidate.

Keywords: cold rolling; corrosion behavior; mechanical properties; medium-entropy alloys; metastable; precipitation hardening.

Figure 1

XRD patterns of Ti₈₀–Nb₁₀–Mo₅–Sn₅ alloy at various conditions.

Figure 2

IPF-EBSD images of Ti₈₀–Nb₁₀–Mo₅–Sn₅ at various conditions with single β phase.

Figure 3

EBSD images (IPF and phase map) of Ti₈₀–Nb₁₀–Mo₅–Sn₅ at various conditions with multiphase.

Figure 4

Mechanical properties of Ti₈₀–Nb₁₀–Mo₅–Sn₅ obtained using three-point bending tests. (a) Stress–deflection curves, and (b) bending strengths, yield strengths, and moduli.

Figure 5

Yield strength/elastic modulus ratios (×1000) of Ti₈₀–Nb₁₀–Mo₅–Sn₅ at various conditions, a biomedical HEA [20], a biomedical MEA [47], and conventional Ti alloys [46].

Figure 6

The polarization curves of Ti₈₀–Nb₁₀–Mo₅–Sn₅ at various conditions after potentiodynamic polarization tests in phosphate-buffered saline at 37 °C.

Figure 7

SEM photos of Ti₈₀–Nb₁₀–Mo₅–Sn₅ at various conditions after potentiodynamic polarization tests.

Figure 8

(a) Nyquist diagrams and (b) Bode plots of Ti₈₀–Nb₁₀–Mo₅–Sn₅ at various conditions in PBS solution at 37 °C.

Figure 9

Equivalent electrical circuit (EEC) model of three Ti₈₀–Nb₁₀–Mo₅–Sn₅ samples.

Figure 10

The chemical characterization of the surface of CR75 after potentiodynamic polarization test. (a) Full spectrum and (b–f) narrow scans.