Microstructural evolution and mechanical properties of hot-rolled Ti-30Zr-5Al-2.5Sn alloy with mixed α and α′ phases

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Abstract

Ti-30Zr-5Al-2.5Sn (TZAS, wt. %) is a novel α-type Ti-Zr based alloy. Herein, the effect of phase and microstructure evolution on mechanical properties of hot-rolled TZAS alloy with mixed α and α′ martensite phase were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and statistical analysis. The content of primary α phase decreased as the hot-rolling temperature increased from α to α+β region. When the rolling temperature increased to 875 °C, the predominant phase transformed from α phase to α′ martensite phase. The spheroidization degree of TZAS alloys increased with rolling temperature until 850 °C. As the rolling temperature increased from 775 °C to 825 °C, the yield strength gradually enhanced, attributing to the synergistic effect of grain-boundary strengthening, dislocation strengthening and second-phase strengthening from martensite. While, as further increase of rolling temperature, the coarsen primary α grains led to the decrease of yield strength. A peak yield strength of 1038 ± 26 MPa and total elongation of 14 ± 0.6% were obtained after rolling at 825 °C. One should note that work hardening was also a key factor for the alloys strengthening. Furthermore, the fracture mechanisms of studied alloys were also discussed in depth.

Introduction

Titanium (Ti) and Ti alloys are widely employed in aerospace, medical and industrial applications due to their high strength-to-weight ratio, outstanding biocompatibility and excellent corrosion resistance [[1], [2], [3], [4]]. To expand the application horizon of Ti alloys, special attention has been paid to Ti-Zr based alloys by many researchers [[5], [6], [7]]. Ti and zirconium (Zr), belonging to the same group (IVB) in the periodic table, exhibit many similar physicochemical properties, such as configuration of extra-nuclear electron and valence electron. Based on the equilibrium Ti-Zr binary phase diagram, these two elements can form infinite solid solutions with a body-centered cubic (bcc) β phase and hexagonal close-packed (hcp) α phase in high- and low-temperature regimes, respectively [8]. Moreover, it has been demonstrated that Zr plays an effective role in solution strengthening of Ti alloys, such as Ti-Zr [9], Ti-Zr-Nb [10], Ti-Zr-Ta [11], Ti-Zr-Mo [12] and Ti-Zr-Ta-Nb [13]. However, the yield strength of these alloys, ranging from 500 to 800 MPa, is not sufficient for applications of high-strength structural materials. Therefore, further studies are required to develop Ti-Zr based alloys with higher strength and excellent ductility at room temperature.

Phase composition is usually considered as a significant factor to influence the mechanical properties of Ti alloys, therefore, a large number of studies have focused on phase transformations during heat treatment processes [[14], [15], [16]]. Under equilibrium conditions, solid Ti alloys exhibits either α or β phase, and α phase transforms from β phase. The defining characteristic of β → α transformation is Burgers orientation relationship (BOR) between two phases: {0002}α∥{110}β and <11–20>α∥<111>β. The given orientation relationship results in 12 possible crystallographic variants of α in a parent β grain [17]. In addition, the martensitic transformations of β phase are of considerable significance in Ti alloys. According to the increasing β-phase stabilizing content, three types of martensite phases are obtained from β phase during quenching, i.e., α′ phase (hexagonal close-packed structure), α" phase (orthorhombic structure) and ω phase (simple hexagonal structure) [18]. In general, martensite in Ti and Zr alloys contains more closely spaced interfaces separating adjacent laths, twins and higher density of dislocations than α phase. Therefore, fully martensitic Ti alloys exhibit higher strength at the expense of ductility. To keep comprehensive properties of Ti alloys, martensitic transformations are systematically avoided or the formed martensite phases are intentionally decomposed.

The microstructure of Ti alloys after thermomechanical processing (TMP) also renders an important influence on mechanical properties of Ti alloys. In general, Ti alloys experience a series of TMP steps from ingot to final product, altering the phase composition as well as microstructure. For instance, hot forging and hot rolling, which are typical high-temperature deformation processes, are utilized to convert α lath-like microstructure into globular or spheroidal microstructure by using the stored deformation energy. Compared to lamellar microstructure, globular or equiaxed grains usually lead to higher ductility [19]. Bi-modal microstructure and fully lamellar microstructure are typical microstructures in α+β Ti alloys. Lütjering et al. [16] have reported that bi-modal microstructure, which contains equiaxed primary α grains, usually exhibited higher ductility than fully lamellar microstructure. Ji et al. [15] have investigated the effect of deformation of constituent phases on mechanical properties of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloys. The elongation of the alloys increased due to the higher deformation amount of both primary α and secondary α in the microstructure with higher volume fraction of primary α grains. In addition, textural evolution also has a significant impact on the mechanical properties of Ti alloys. Prakash et al. [20] have studied the microstructural and textural evolution of Ti-6Al-4V alloy during hot rolling process. Strong texture variations have been observed after rolling in terms of volume fraction of β phase. It has been shown that crystallographic texture impacts on the mechanical performance as crack initiation is dominated by the development of cleavage facets in the primary α grains having their c-axis under tension. However, the influence of both primary α phase and α′ martensite of Ti-Zr based alloys after TMP in two-phase region on mechanical properties has not been studied in detail yet.

Therefore, the present work aims to investigate the influence of microstructural evolution and mixed α and α′ martensite phases on the mechanical properties of TZAS alloys. For this purpose, hot rolling and subsequent water quenching processes were carried out ranging from 775 to 875 °C. The mechanisms of fragmentation and spheroidization of lamellar α phase, which significantly influence the microstructure of TZAS alloys, were also discussed.

Section snippets

Materials and methods

Sponge Ti (99.9 wt %), sponge Zr (Zr + Hf ≥ 99.5 wt %), industrially pure Al (99.9 wt %) and Sn (99.9 wt %) were utilized as raw materials to prepare TZAS alloy ingots. The nominal composition of as-prepared alloy was Ti-30Zr-5Al-2.5Sn (wt. %). The studied alloys were melted in a vacuum non-consumable electro-arc furnace under argon atmosphere with Ti as the oxygen getter. The ingots were turned and re-melted at least six times to ensure a homogeneous composition. The weight of each alloy ingot

Microstructural and phase analysis

OM and SEM micrographs of as-cast and hot-rolled TZAS alloys are shown in Fig. 3. Average width of lamellar α grains, size of equiaxed α and prior-β grains, and corresponding fraction of α′ martensite are shown in Table 1. As illustrated in Fig. 3a, the as-cast specimen exhibits typical basketweave and part widmanstätten microstructures. The 775-WQ specimen shows coarse α grains with no α′ martensite (Fig. 3b). When the hot-rolling temperature increases to 800 °C (Fig. 3c), the 800-WQ sample

Conclusions

In summary, the microstructure and mechanical properties of hot-rolled Ti-30Zr-5Al-2.5Sn alloys, with different mixtures of α and α′ phases, have been systematically investigated. The main conclusions are as follow:

  • (1)

    The hot-rolled TZAS alloys contained only α/α′ phases. The content of primary α phase decreased as the hot-rolling temperature increased from 775 °C to 875 °C, while the predominant phase transformed from α phase to α′ martensite phase.

  • (2)

    The spheroidization degree of TZAS alloys

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Wei Ma: Methodology, Investigation, Writing - original draft. Shuguang Liu: Writing - original draft, Data curation, Visualization. Xing Zhang: Formal analysis. Bohan Chen: Validation. Fei Wang: Validation. Xinyu Zhang: Data curation. Mingzhen Ma: Supervision. Riping Liu: Validation, Writing - review & editing.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51531005/51671166/51827801).

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