Research Article
ω-Assisted refinement of α phase and its effect on the tensile properties of a near β titanium alloy

https://doi.org/10.1016/j.jmst.2019.10.031Get rights and content

Abstract

In this work, the phase transformation sequence during the continuous heating process (3 °C/min) was investigated in a near β titanium alloy. The results show that the staring formation of ω phase is about 267 °C, and the ending precipitation temperature about 386 °C during the heating process. When the heating temperature is greater than 485 °C, there are no ω phase detected within the β matrix. Combined with the microstructural characterization, it is found that ω phase facilitates the nucleation of α phase nearby the ω/β interface and has a great effect on the refinement for α phase. As compared with the specimens directly aged, the specimens with ω-assisted refinement of α phase possess high tensile strength, but there is no yield stage detected on their stress-strain curve. Combined with the analyses of the fracture morphology, the specimens with ω-assisted refinement of α phase present a brittle fracture. This is mainly ascribed to its relatively lager width of grain boundaries and the absence of widmanstätten α precipitates.

Introduction

Near β titanium alloys are widely used in the aircraft and aerospace industries due to their high specific strength, low density, good fatigue and crack resistance for manufacturing land gears and fasteners [[1], [2], [3], [4]]. As compared with traditional (α + β) titanium alloys (e.g. Ti-6Al-4 V, wt%), near β titanium alloys have great capability to obtain the high strength and balanced by a considerable ductility. Their high strengths are mainly ascribe to the precipitation of α phase, including its size, volume faction, morphology and distribution [[4], [5], [6], [7]]. Thus the adjustment for α precipitation is always a hot issue for exploring the optimum balance of the mechanical properties for near β titanium alloys [[8], [9], [10]]. The thermal mechanical process and the heat treatment are usually applied to realize the α phase control. Chen et al. [11] have studied the effect of deformation reduction on the mechanical properties of Ti-15Mo-3Al-2.7Nb-0.2Si (wt%) alloy and found that by 80% rolling reduction the alloy could obtained a higher tensile strength companied by a better ductility as compared with 20% reduction. By related to microstructural characteristics, they ascribe this good combination of mechanical properties to the refined grain size and the fragment of the α precipitation on grain boundaries. Different heat treatments have a great influence on the morphology and size of α precipitation. Dong et al. [4] have studied the correlation between the volume fraction and the size of α precipitates and the mechanical properties in a Ti-7Mo-3Al-3Cr-3Nb (wt%) alloy, and indicated that the high volume fraction and fine size contribute to improve the strength of the alloy. In conclusion, refining α precipitates by thermal mechanical process and heat treatment is an effective way to improve the mechanical properties of the alloys. It is well known that the precipitation of α phase traditionally originates from parent β phase by β→α transformation. Thus all of factors that can affect the phase transformation are the breakthrough points to refine the α precipitates. Owing to the lower stability of β phase in near β titanium alloys, some transient phases could firstly generate under some special conditions. For example, as for near β titanium alloys, there are ω phase firstly precipitated at lower aging temperature or at the early stage of the aging process [[12], [13], [14]]. Previous research has indicated that ω phase formed in the early stage has a great effect on the precipitation of α phase. Although previous work have confirmed that ω phase as an brittle phase deteriorates the mechanical properties, the effect of ω-assisted refinement of α precipitates on the mechanical properties still remains limited.

In this work, a near β titanium alloy, Ti-7333 (Ti-7Mo-3Nb-3Cr-3Al, wt%), was investigated. Firstly, thermal dilatometer was conducted on the β-quenched Ti-7333 alloy with a slow heating rate to investigate the phase transformation sequence during the heating process. According to the results of thermal expansion test, phase transformation sequence during the heating process was investigated, including the formation of ω phase, ω-assisted precipitation of α phase and the refinement of α phase. The mechanical properties of the alloy with the ω-assisted refinement of α phase were investigated, and for the sake of contrast the mechanical properties of the alloys directly aged were investigated as well. Eventually, based on the microstructure characteristics and the fracture morphology, their fracture behaviors are discussed.

Section snippets

Experimental procedure

The alloy studied in this work was the hot-rolled Ti-7333 bar with the nominal composition of Ti-7Mo-3Nb-3Cr-3Al, wt%. The Ti-7333 ingot was firstly prepared by vacuum self-consumable arc melting, and forged in β and (α + β) fields to the bars with the diameter of 27 mm. Then the cross-rolling was conducted on the forged bars. Before the rolling, the forged bars were heated to 820 °C and held for 60 min. The bars were rotated by 90° around the rolling direction (RD) after every pass rolling

Phase transformation sequence during the heating process

Fig. 1(a) gives the microstructure of the as-solution treated specimen. It consists of equiaxed β grains with an average grain size of about 190 μm. Its corresponding XRD pattern, as shown in Fig. 1(b), indicates that only β phase is detected after the β-solution treatment. Differentiated from other near β titanium alloys, such as Ti-5Mo-5Al-5V-3Cr [13,16] and Ti-10V-2Fe-3Al [17], there is no athermal ω phase formed within the β matrix after WQ from the single β phase field owing to its

Conclusions

The phase transformation sequence during continuous heating process and the effect of ω-assisted refinement of α phase on mechanical properties were investigated in a near β titanium alloy. The following conclusions can be drawn:

  • (1)

    The ω phase is firstly formed in β-quenched Ti-7333 at the early stage of the continuously heating process. The temperature for the starting precipitation of ω phase is determined to be about 267 °C, and the ending precipitation temperature about 386 °C. After the

Acknowledgments

This work was supported financially by the National Natural Science Foundation of China (Nos. 51711530151 and 51804279), the Applied Basic Research Foundation of Shanxi Province (Nos. 201901D211255 and 201801D221150) and the Natural Science Research Project of NUC (XJJ201916).

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