Fine Secondary α Phase-Induced Strengthening in a Ti-5.5Al-2Zr-1Mo-2.5V Alloy Pipe with a Widmanstätten Microstructure

In this paper, the Widmanstatten microstructure in an extruded near-α titanium alloy (Ti-5.5Al-2Zr-1Mo-2.5V) pipe was changed effectively by solid solution treatment at 920 °C for 2 h. The treatment produced intermittent lamellar α and discontinuous α grain boundaries (αGB), and aging at 450 °C for 2 h introduced a high density of nanosized secondary α (αs). The microstructure consisted of the lamellar α, αGB and transformed β demonstrated a good combination of yield strength 1064 MPa and elongation 10.5%. Severe plastic deformation occurred inside the lamellar α during the tensile process, resulting in high-density dislocation tangles and dislocation cells. Furthermore, the stretching imposed on the tensile sample before aging generated dislocations, which piled up near the α/β interface. Thus, a coordinated deformation between lamellar α and transformed β, and the resultant strain partition contributed to an improvement in the ductility. Moreover, dislocation motion was effectively obstructed near the α/β interfaces, which dramatically strengthened the alloy. A solid solution at a middle temperature in the α + β region and aging at a low temperature provided an effective way to improve the strength and ductility simultaneously in titanium alloys with Widmanstatten microstructure.