Effect of phase transformation on the ductile to brittle transition behavior of Ti–V–Nb–Zrx body-centered cubic high-entropy alloys at elevated temperatures

Abstract

Single-phase body-centered cubic (BCC) lightweight Ti–V–Nb–Zrx solid-solution alloys were fabricated by reducing the Zr concentration within the range of 0–5 at% to improve the mechanical properties of equimolar TiVNbZr alloy at elevated temperatures. The fabricated Zrx alloys indeed exhibited greater strength above 800 °C than TiVNbZr, but some special mechanical responses occurred at 800 °C: (ⅰ) superficial cracks with different lengths and amounts along the high-angle grain boundaries (HAGBs) on the compressed sample surface, and (ⅱ) near-brittle fracture of the tensile stress-strain curves. Based on the analysis of the deformed and undeformed surface, local stress concentration and phase transformation were responsible for the cracks and difference in compression behavior, and atmospheric oxygen, deformation localization and phase transformation were mainly responsible for the tensile fracture difference at 800 °C. Phase transformation promote stress dispersion, combined with better oxidation resistance, which make the Zr-containing alloys fractured via the transgranular mode in tensile test. In particular, the Zr3 alloy exhibited work hardening and tensile ductility. Based on the tensile and compression properties of the Zrx alloys, it was found that at RT and 600 °C, atomic size difference (δ) governed alloy strength; from 800 to 1000 °C, δ and melting temperature (Tm) co-operatively governed alloy strength; at 1200 °C, alloy strength was independent of δ, but dependent on Tm. The fabricated Ti–V–Nb–Zrx alloys have medium mechanical properties but a small strength loss at high temperatures, and are suitable for various RT deformation processing methods.

Keywords

• Phase transformation
• Ductile to brittle transition
• Atomic size difference
• Transgranular mode