Tensile property and microstructure assessment of hydrogen treated Ti-6Al-4V alloy

Abstract

Temporary hydrogen alloying (THA) treatment was used to refine the coarse-grained microstructure of a Ti-6Al-4V alloy with the intention of modifying tensile strength and ductility. Microstructure evolution was characterised using X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that THA treatment does not refine the coarse network of prior beta grains, but it alters the coarse Widmanstätten microstructure by nucleating submicron beta grains within the individual lamellae. The modified microstructure increases ultimate tensile strength (UTS) in the order of 100 MPa. The dehydrogenation temperature affected tensile ductility. The lowest dehydrogenation temperature (675 °C) had the highest ductility loss, and increasing the dehydrogenation temperature recovered ductility until full ductility was recovered at 750 °C. The ductility loss was attributed to the embrittling effect of the hydrogen induced phase Ti3Al which remains in the microstructure even after hydrogen removal and only completely dissolves at 750 °C. The results demonstrate that THA treatment significantly alters the initial coarse-grained microstructure with concomitant improvement in strength, but that the promotion of Ti3Al precipitation can cause embrittlement unless the dehydrogenation treatment is controlled to optimise Ti3Al dissolution.