Synergistically achieving high strength and impact toughness in Ti–6Al–4V-0.5Mo-0.5Zr alloy pipe with bimodal microstructure

Abstract

To achieve high strength and high impact toughness, three kinds of bimodal microstructure of Ti–6Al–4V-0.5Mo-0.5Zr alloy pipe were tailored through solution treatment at the α+β phase region (860 °C, 890 °C, and 920 °C) and followed by aging at 500 °C. The bimodal microstructure consists of the primary α (αp) and β-transformed domain (βt), in which the secondary α phase (αs) precipitated in the β matrix, with different volume fraction. With the increase in solution temperature, the grain size of αp remained unchanged while the volume fraction decreased. At the same time, the volume fraction of βt and the width of αs phase increased. Furthermore, the content of low-angle grain boundaries inside the αp phase decreased. Subsequent aging resulted in finer precipitation of αs within the βt domain. Solution treatment at 890 °C and ageing at 500 °C (denoted as the STA890 sample) achieved an excellent combination of yield strength 903 MPa, tensile strength 1023 MPa, elongation 12.9%, and impact energy 31 J. With the increase of αs width, the strength of the alloy decreased, and the ductility increased slightly. Higher impact energy originated from {10 1‾ 2} tension twins activated in the STA890 sample during the impact loading process, αp and βt underwent severe bending and plastic deformation. The present study provides a feasible industrial heat treatment strategy for improving impact energy with high strength of the α+β titanium alloy pipe used for oil drilling.