Research ArticleMulti-dimensional morphology of hydride diffusion layer and associated sequential twinning in commercial pure titanium
Introduction
The combination of high corrosion resistance, strength-to-weight ratio, and exceptional biocompatibility of titanium and its alloys allows widespread applications especially in aerospace and biomedical industries [1]. When subjected to hydrogen-rich service conditions, nevertheless, the formation and transformation of hydride precipitations in Ti would induce high internal stress in the alloys, which is regarded as the origin of several important alloy properties decrease, such as ductility and toughness [2]. Indeed, the solubility of H in titanium is low, 0.15 wt.% in α-Ti at 300 °C [3], the consensus is that, even a small quantity of hydrogen will cause the formation of TiHx hydrides in Ti and produce a considerable drop in alloy properties [4]. Accordingly, hydride precipitation has been considered a major barrier for reliable applications of Ti alloys under hydrogen-rich conditions.
The phase structures of hydrides formed in Ti at room temperature are determined by hydrogen concentration: γ-TiH with face-centered tetragonal (FCT) structure forms at relative low hydrogen concentration and transforms into the face-centered cubic (FCC) δ-TiHx (with x ranging between 1.5 and 2). The FCC phase is the most stable and prevalent hydride in titanium, which changes its lattice structure and transforms into the FCT ɛ-TiH2 with a further increase of hydrogen concentration [5], [6], [7], [8], [9]. Another key factor that dominates the formation of hydride precipitations in titanium is crystallographic orientation. Four types of orientation relationships between the hexagonal close packed (HCP) Ti matrix and hydrides have been reported in Refs. [5], [6], [7], [8], as shown in Table 1. OR1 and OR2 were the most common orientation relationships in pure titanium [10]. While OR3 and OR4 were rarely observed, which were firstly reported in Ref. [5].
In our previous work [10], the orientation dependence of hydride precipitation in commercially pure titanium has been investigated in detail. The formation of hydrides is significantly dependent on the matrix orientation for the reason of strain accommodation. The most favorable grain orientations for OR1 and OR2 hydride transformation are with and interface planes parallel to the diffusion surface, as the largest misfit strains along their interface plane normal can be accommodated by free surface relaxation. Besides, the interaction between hydride variants can also play a supportive role in accommodating the phase transformation-induced strain. OR2 hydride packets were observed under electron backscattered diffraction (EBSD) characterization in Ref. [11]. The hydride grains inside the packet are in {111}<11> twin relationships to relax the anisotropic misfit strain of hydride transformation. Conforto and Caillard [8] proposed that the hydride precipitates in different ORs prefer to form clusters to realize a more isotropic distribution of misfit strain produced by hydride transformation and decrease the total stored elastic energy. Indeed, hydride clusters formed by OR1 and OR2 hydrides were observed in Ref. [10], which can be energy favorable to accommodate the nearly perpendicular misfit strain of these two ORs transformations. However, the research efforts of Ref. [10] on orientation dependence and the interactions of hydride variants formed by electrolytic hydrogen charging were only carried out on 2D sample surface. Technically, a certain time of electropolishing on the post-charged sample surface is always necessary to achieve EBSD phase identification, which will inevitably, erase some valuable information of the hydride diffusion layer. It is therefore essential to conduct further investigations on the section microstructure of the hydride layer.
Owing to the large volume expansion of hydride transformation, dislocations are necessary to accommodate the dilatation misfit between hydride and hexagonal matrix. The formation of OR1 hydrides is related to a ledge mechanism [6], including a shear component in and the normal dilatation component in caused by volume variation. Conforto and Caillard [5] proposed that the transition of OR1 produces two opposite prismatic loops along the interface, contributing to the relaxation of a misfit in the direction. Carpenter [12] suggested that OR2 hydrides are formed by the dissociation of the normal dislocations. Around the top and bottom ends of the hydride precipitate, shear loop dislocations on the basal plane with B = are punched out and extending in the direction of their Burgers vectors [7]. Up to now, a detailed study on the formation mechanism and misfit dislocations related to the OR3 and OR4 hydrides has not been reported. In addition, twins were also observed together with OR2 hydride precipitates [10,11,13], but the hydride-twin accommodation mechanism is still not well understood.
In the present work, multi-dimensional characterization of hydride layer in commercial pure Ti (CP-Ti) formed by electrolytic hydrogen charging was performed. The anisotropic section microstructure of the hydride diffusion layer was examined in detail for the first time via EBSD measurements, driving a further in-depth study on the orientation dependence of hydride interaction. The associated plastic behaviors induced by hydride transformation and corresponding accommodation mechanism were also investigated.
Section snippets
Methodology
Commercially pure Ti rolled sheets (T40, ASTM grade 2) with a thickness of 1.5 mm were used in the present work. The as-received sheets were firstly annealed in vacuum condition at 800 °C for 3 h to obtain a fully recrystallized microstructure with an average grain size of ∼50 μm. The texture is a typical split basal texture with basal pole oriented ± 20° away from normal direction (ND) of the sheet towards transverse direction (TD). Hydrogen charging was performed for the specimens by
Section morphology of hydride diffusion layer
The secondary electron image (SEI) showing the section microstructure of the hydride layer is presented in Fig. 1(a). It can be seen that the sample and the resin are bonded tightly, with a stress-free area at the junction (the area between red dotted lines), which is an ion polishing area obtainable under EBSD measurement. The higher-magnification SEI for the black box area in Fig. 1(a) is shown in Fig. 1(b), in which, the interface between Ti matrix and hydride layer in a light gray curve can
HCP-FCC structure transformation of OR1 and OR2 hydrides
In addition to the anisotropic lattice misfit strain, the mechanisms of HCP-FCC structure transformation for OR1 and OR2 hydrides are also different [17], as illustrated in Fig. 6, which leads to the multiple plastic accommodation modes: prismatic dislocations are found in OR1 favorable grains (Fig. 4) and twins are commonly induced by OR2 hydrides (Fig. 5)
The HCP-FCC transformation of OR1 hydride is shown in Fig. 6(a). There exists a shear of 30° along [110] on a prismatic plane in Fig. 6(a)
Conclusions
The interactions of different hydride variants are for the first time investigated by section characterization of hydride diffusion layer through EBSD measurement. The section microstructure of the hydrogen diffusion layer is dependent on Ti matrix orientation. The hydride layers in the grains with the orientations of and aligned to the diffusion direction mainly encourage OR1 hydrides. The grains with and basal plane parallel to the diffusion surface are favorable for
Acknowledgements
The authors would like to thank Prof. Eric Fleury for the hydrogen charging device. The first author Qian Wang is grateful to the China Scholarship Council for the financial support of her Ph.D. study in France.
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