Research article
Deformation mechanism of bimodal microstructure in Ti-6Al-4V alloy: The effects of intercritical annealing temperature and constituent hardness

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Abstract

The so-called bimodal microstructure of Ti-6Al-4V alloy, composed of primary α grains (αp) and transformed β areas (βtrans), can be regarded as a “dual-phase” structure to some extent, the mechanical properties of which are closely related to the sizes, volume fractions, distributions as well as nano-hardness of the two constituents. In this study, the volume fractions of primary α grains (vol.%(αp)) were systematically modified in three series of bimodal microstructures with fixed primary α grain sizes (0.8 μm, 2.4 μm and 5.0 μm), by changing the intercritical annealing temperature (Tint). By evaluating the tensile properties at room temperature, it was found that with increasing Tint (decreasing vol.%(αp)), the yield strength of bimodal microstructures monotonically increased, while the uniform elongation firstly increased with Tint until 910 °C and then drastically decreased afterwards, thereby dividing the Tint into two regions, namely region I (830−910 °C) and region II (910−970 °C). The detailed deformation behaviors within the two regions were studied and compared, from the perspectives of strain distribution analysis, slip system analysis as well as dislocation analysis. For bimodal microstructures in region I, due to the much lower nano-hardness of βtrans than αp, there was a clear strain partitioning between the two constituents as well as a strain gradient from the αp/βtrans interface to the grain interior of αp. This activated a large number of geometrically necessary dislocations (GNDs) near the interface, mostly with <c+a> components, which contributed greatly to the extraordinary work-hardening abilities of bimodal microstructures in region I. With increasing Tint, the αp/βtrans interface length density gradually increased and so was the density of GNDs with <c+a> components, which explained the continuous increase of uniform elongation with Tint in this region. For bimodal microstructures in region II, where the nano-hardness of βtrans and αp were comparable, neither a clear strain-partitioning tendency nor a strain gradient across the αp/βtrans interface was observed. Consequently, only statistically stored dislocations (SSDs) with <a> component were activated inside αp. The absence of <c+a> dislocations together with a decreased volume fraction of αp resulted into a dramatic loss of uniform elongation for bimodal microstructures in region II.

Introduction

Ti-6Al-4V alloy, as the workhorse of α+β titanium alloys, has drawn considerable attentions in the last fifty years, particularly in lightweight and corrosion sensitive applications [[1], [2], [3]]. Four typical microstructures, i.e. the lamellar microstructure [[3], [4], [5]], bi-lamellar microstructure [3,[6], [7], [8]], equiaxed microstructure [9,10] and bimodal microstructure [9,10] have been successfully realized in Ti-6Al-4V alloy depending on the thermomechanical-processing route, which provided various combinations of mechanical properties while maintaining an excellent corrosion/oxidation resistance at the same time. The comprehensive processing-microstructure-mechanical properties inter-relationships have been reviewed by Lutjering in detail [2,3]. By β annealing or β processing (in β single phase region), the lamellar microstructure was obtained, which can be further transformed into bi-lamellar microstructure by intercritical annealing (in α+β two phase region) followed by fast cooling. In comparison, the equiaxed and bimodal microstructure can only be realized by α+β processing (in α+β two phase region) followed by different cooling rates. Generally speaking, the lamellar and bi-lamellar microstructures exhibited larger fracture toughness and higher creep strength, whereas the equiaxed and bimodal microstructures possessed higher fatigue strength and good ductility [2,3].

The bimodal microstructure, a composite of equiaxed primary α grains (αp) and transformed β areas (βtrans) is the most attractive one due to a good synergy of strength and ductility. The volume fraction of αp grains (vol.%(αp)) is related to the intercritical annealing temperature (Tint), for which vol.%(αp) continuously decreases with increasing Tint. Moreover, the spatial distribution of αp grains also changes from an interconnected network at lower Tint to an isolated distribution at higher Tint. The size of αp grains (D(αp)), on the other hand, is influenced by several factors, including the initial α lamellae thickness, deformation temperature, strain rate and intercritical annealing time. In general, a finer initial α lamellae thickness, a lower deformation temperature, a higher strain rate and a shorter intercritical annealing time lead to a smaller αp grains size. The size of secondary α lamellae inside the βtrans areas is mainly controlled by the cooling rate after intercritical annealing and a finer lamellae thickness can be expected with a faster cooling rate. In the previous studies of bimodal microstructures [[11], [12], [13], [14], [15]], much attention has been paid to the deformation behaviors of αp grains, while the βtrans areas were generally considered as ‘harder’ and less-deformable areas because of the fine secondary α lamellae inside. In these studies, several different slip systems, including basal, prismatic as well as pyramidal I slip systems have been confirmed inside the αp grains, and Schmid factor has been considered as a relevant parameter to predict which slip system was more likely to activate in the primary α grains [11]. Moreover, a methodology for estimating the critical resolved shear stress (CRSS) ratios of αp grains was also proposed, using in-situ deformation inside a scanning electron microscope and trace analysis [[11], [12], [13], [14]].

Nevertheless, it has been found in our recent study [10] that the nano-hardness of βtrans areas turned out to be lower than that of αp grains in general, which was totally opposite to the previous perceptions. In addition, the nano-hardness difference was found to be dependent on the intercritical annealing temperature. This rendered a plastic strain partitioning between the two constituents, which in turn altered the mechanical behaviors, particularly the work-hardening abilities of bimodal microstructures. Therefore, in our opinion, the mechanical behavior of bimodal microstructures in Ti-6Al-4V alloy is somehow analogous to that of dual-phase steel composed of soft ferrite and hard martensite/bainite [[16], [17], [18]], in which the strain partitioning behavior largely affected the yield strength and uniform elongation of the material. However, what makes the bimodal microstructure of α+β titanium alloys more unique is the changeable nano-hardness difference between the two constituents depending on the intercritical annealing temperature, which further complicates the microstructure-mechanical property relationship. All these factors necessitate a thorough study of the influence of vol.%(αp) on the mechanical behaviors of bimodal microstructures in Ti-6Al-4V alloy, which to the best of our knowledge, is lacking so far. The outline of this study is constructed as follows. Firstly, by designed thermomechanical-processing routes, three series of bimodal microstructures having the αp grain sizes of 0.8 μm, 2.4 μm and 5.0 μm were obtained, and in each serial the vol.%(αp) was systematically modified by changing the intercritical annealing temperature. Secondly, these bimodal microstructures were tensile tested at room temperature and the correlations between the tensile properties and microstructural parameters were analyzed. Finally, the detailed deformation mechanisms of the bimodal microstructures were discussed from the perspectives of strain distribution analysis, slip system determination as well as dislocation observation.

Section snippets

Experimental procedure

Ti-6Al-4V ingot (Al: 6.29 wt.%, V: 4.35 wt.%, Fe: 0.225 wt.%, O: 0.155 wt.%, Ti: Bal.) provided by Kobelco was used in this study. Cylinder samples (Φ8 mm × 12 mm) were prepared from the ingot and β annealed at 1100 °C for 1 h followed by water quench. A fully martensite microstructure (Fig. 1(a)) can be obtained in the water-quenched samples. It was comprised of α lamellae with a fine lamellae thickness (∼1.0 μm). Uniaxial compression experiments were conducted using a

Results

During the hot deformation in α+β two-phase region (Fig. 1(b)), α lamellae in the martensite initial microstructure recrystallized/globularized into equiaxed α grains, leading to the formation of equiaxed α+β dual-phase microstructures, the grain size of which was related to the deformation temperature and strain rate [[30], [31], [32], [33]]. During subsequent intercritical annealing (Fig. 1(b)), the volume fraction of α phase (or β phase) was further modified according to the annealing

Discussion

In the preceding section, three series of bimodal microstructures (with different grain sizes) have been successfully fabricated and their mechanical properties were also evaluated at room temperature. It is interesting to see a two-stage evolution tendency of the uniform elongation with respect to the annealing temperature, based on which the microstructures were divided into two groups (group I: annealed at 830−910 °C and group II: annealed at 910−970 °C). In the following part of this study,

Conclusions

In this study, we discovered a unique intercritical annealing temperature dependence of the mechanical properties of the bimodal microstructure in Ti-6Al-4V alloy, particularly for the uniform elongation and work-hardening ability. This dependence originated from the nano-hardness difference between primary α grains and transformed β areas in the microstructure, due to aluminum partitioning between the two constituents. Specifically speaking, at relatively lower annealing temperatures (830−910

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

The authors are grateful to the financial support from Cross-ministerial Strategic Innovation Promotion Program (SIP) supported by the Cabinet Office of Japanese government and the Elements Strategy Initiative for Structural Materials (ESISM) in Kyoto University supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Y.Z. Tian would like to acknowledge the support by the Fundamental Research Funds for the Central Universities under grant No. N180204015.

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