Influences of grain size on the deformation behavior of a twinning-induced plasticity metastable β titanium alloy

https://doi.org/10.1016/j.jallcom.2022.168274Get rights and content

Highlights

  • TWIP effect and pronounced work hardening behavior occur for large β grain sizes.

  • The introduction of TWIP effect enhances the Hall-Petch coefficient for pure slip.

  • The decrease of β grain size suppresses the TWIP effect.

Abstract

The influences of grain size on the deformation behavior of a metastable β-type Ti–12Mo (wt%) alloy has been investigated by using uniaxial tensile testing, scanning electron microscopy and electron backscatter diffraction. The results reveal that the twinning-induced plasticity (TWIP) effect mediated by {332}<113>β twins occurs in this alloy when its mean grain size (d0) ranges from 49 to 104 µm, leading to pronounced work hardening behavior and a large uniform elongation (> 25%). Furthermore, there is a Hall-Petch relationship between the yield strength and d0 for this d0 range, with a Hall-Petch coefficient higher than that expected for pure slip deformation. However, when this alloy is in a state with heterogeneous grain structures and smaller grain sizes (d0 ∼ 30 µm), only a very limited volume fraction (< 1.8%) of {332}<113>β twins occurs and these twins prefer to nucleate in large grains (> ∼40 µm), but they can propagate into rather small grains (< ∼25 µm). In such a state, the Ti−12Mo alloy has a high yield strength (1025 MPa) but does not exhibit any sign of work hardening, i.e., the TWIP effect is absent. Based on the α"-assisted {332}<113>β twinning mechanism, the effects of grain size on the {332}<113>β twinning and yield strength are discussed in terms of its role in affecting the stress-induced β→α" martensitic transformation.

Introduction

Metastable β titanium alloys have found important applications in aviation industry owing to their high specific strength and high corrosion resistance [1], [2]. After solution treatment in the β phase region and subsequent water quenching, most metastable β titanium alloys consist of low-stability β phase and a minor amount of athermal ω phase [3]. In such alloys, stress-induced β→α" martensitic transformation, {332}<113>β twinning or {112}<111>β twinning may be activated during their plastic deformation depending on the stability of their β matrix [4], [5], [6], [7], [8]. It was found that the progressive formation of {332}<113>β twins in metastable β titanium alloys upon straining led to pronounced work hardening behavior and excellent ductility (total elongation up to >50%), due to the twinning-induced plasticity (TWIP) effect [4], [9], [10]. In recent years, many metastable β titanium alloys with TWIP effect have been reported, e.g., Ti–12Mo [4], Ti–9Mo–6W [11] and Ti–8.5Cr–1.5Sn [12]. However, most of these alloys suffer from a low yield strength (< 600 MPa), in spite of their excellent ductility.

Due to their relatively low thermal stability, the β-solution-treated and subsequently water-quenched metastable β titanium alloys can be remarkably strengthened by precipitation of α phase via aging in their α + β phase region [2], [13], [14]. For instance, the yield strength of the β-solution-treated and water-quenched Ti-15V-3Cr-3Sn-3Al alloy can be increased from 770 MPa to 1270 MPa via aging at 450 °C for 16 h [14]. However, it should be noted that the precipitation of α phase results in the enrichment of β-stabilizing elements in the β matrix, which stabilizes the β matrix and hence suppresses the {332}<113>β twinning or TWIP effect [15], [16], [17]. As a result, the plastic deformation of the α-hardened metastable β titanium alloys is commonly mediated only by dislocation slip, so such alloys normally exhibit relatively low ductility (total elongation < 13%) [14].

In addition to the precipitation hardening, grain refinement can also effectively increase the yield strength of metastable β titanium alloys [18], [19], [20]. Since the composition of the β matrix will not be changed after grain refinement, its low stability can be maintained. Thus, if the deformation mechanisms of metastable β titanium alloys only depend on their β phase stability, the {332}<113>β twinning or TWIP effect in such alloys would not be suppressed after grain refinement, i.e., grain refinement would enhance the yield strength of such alloys without sacrificing their ductility. However, recent work by Im et al. [21] demonstrated that the {332}<113>β deformation twins formed in coarse β grains (average size ∼ 154 µm) are denser than those in fine β grains (average size ∼ 25 µm) in a Ti–15Mo alloy. Gao et al. [22] observed a similar phenomenon in a Ti–7Mo–3Cr alloy, where the {332}<113>β deformation twins only occurred in the largest β grains (∼70 µm). Thus, it seems that the refinement of β grains also suppresses the {332}<113>β twinning. Gao et al. [22] proposed that there was a Hall-Petch relationship between the β grain size and the critical resolved shear stress (CRSS) of {332}<113>β twinning, i.e., the {332}<113>β twinning might be suppressed by the increase of its CRSS with the reduction of β grain size. Nonetheless, no direct experimental evidence is available to validate this proposal.

The aim of the present study is to clarify the role of β grain size in affecting the deformation behavior of an extensively studied metastable β titanium alloy, Ti–12Mo (wt%), with special focus on the grain size dependence of {332}<113>β twinning. Samples with various β grain sizes are obtained by annealing of cold-rolled Ti–12Mo alloy sheets at 850 °C for different times. The mechanical response of these samples is determined by uniaxial tensile tests. To examine the evolution of deformation-induced microstructures, pre-polished tensile specimens are subjected to uniaxial tensile deformation and interrupted at various strain levels for microstructural characterization.

Section snippets

Experimental

A 5 kg ingot with a nominal composition of Ti–12Mo (wt%) was produced by vacuum arc remelting. The chemical composition of this ingot measured by inductively coupled plasma optical emission spectroscopy is listed in Table 1. The β→α transition temperature of this ingot was measured to be ∼821 °C using dilatometry, which is in line with that reported in the work by Sun. et al. [23], [24]. To break the cast microstructures and heal the internal pores, the ingot was subjected to a series of

Initial microstructures

Fig. 1 shows the initial microstructures of the Ti–12Mo alloy after cold rolling and recrystallization at 850 ℃ for 5–60 min. For each recrystallized specimen, the surface perpendicular to the normal direction (ND) of the original cold-rolled sheets was examined. Fig. 1(a, d, g, j) are BSE images, illustrating that relatively homogeneous equiaxed grain structures are already formed when the holding time at 850 ℃ reaches 10 min. However, when the holding time is 5 min, there are unrecrystallized

Discussion

The results of the present study reveal that the grain size influences the deformation behavior of the metastable β-type Ti–12Mo alloy. The σy of this alloy enhances with decreasing d0 and {332}<113>β twins occur during plastic deformation for all of the four grain sizes (30, 49, 70 and 104 µm) studied here. There is a Hall-Petch relationship between σy and d0 when d0 ranges from 49 to 104 µm, with a Hall-Petch coefficient k higher than that expected for pure slip deformation. For this d0

Conclusions

By solution treating the cold-rolled metastable β-type Ti−12Mo alloy at 850 °C for different periods, four different β grain structures with d0 ranging from 30 to 104 µm were produced. The initial microstructures, stress-strain response and deformation-induced microstructures of this alloy in these four states (GS30, GS49, GS70 and GS104) were studied by using uniaxial tensile testing, TEM, SEM and EBSD. Based on the experimental results, the following conclusions can be drawn.

  • (1)

    A weak texture

CRediT authorship contribution statement

X. Huang: Investigation, Formal analysis, Visualization, Writing – original draft. J.S. Li: Supervision, Project administration. M.J. Lai: Conceptualization, Validation, Writing – review & editing, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 52071266) and the State Key Laboratory of Solidification Processing (NPU), China (Grant No. 2019-QZ-05).

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