The Role of the Ω Phase on the Microstructure Evolution of Precipitation and Mechanical Property in Ti-3al-5mo-4v-2zr-1fe

Abstract

This paper presented the influence of ω phase on precipitation and intragranular ultrafine α precipitates on the mechanical properties of a near β titanium alloy Ti-3Al-5Mo-4V-2Zr-1Fe(Ti-35421) after different aging treatments. The results illuminated that the size of the precipitated microstructure undergoing the double aging (DA) was uniform and the orientation of acicular α precipitation was about 60°. Compared with isothermal annealing (IA), the microstructure of DA had more fine grains and less precipitation free zones (PFZ). The grain morphology of the two kinds of heat treatment changed from short-rod shape to acicular shape or plate shape with the increase of aging temperature. The ultimate tensile strength of the IA alloy could achieve a better combination of the ultimate tensile strength (1272MPa) and ductility (8.12%) and the ultimate tensile strength of the DA alloy even reached 1376MPa but its plasticity was poor. The mechanical properties of the two kinds of aging presented the same phenomenon with the increasing aging temperature, that is, the strength was decreasing and the plasticity was rising. The fracture mode of DA alloy was mainly cleavage fracture and IA alloy was dimple fracture. The precipitation nucleation mechanism of DA was very different from that of IA due to the existence of ω phase, which affected the mechanical properties of the microstructure under the condition of DA. The α-stable element Al diffused in the microstructure aggregated in ω phase, which provided a favorable nucleation site for the precipitation of secondary a phase. It reduced the PFZ in the microstructure and the dispersion distribution strengthened the β matrix structure. Whereas, it was difficult to form ω phase in microstructure due to the high aging temperature during IA heat treatment, the fine grains in the microstructure were coarsened and the PFZ increased gradually with the increase of temperature.

Keywords: Near β titanium alloy, Phase transformation, Double aging, mechanical properties, ω phase