Microstructure evolution and tensile properties of as-rolled TiB/TA15 composites with network microstructure

https://doi.org/10.1016/j.msea.2021.140783Get rights and content

Highlights

  • As-sintered and as-rolled TiBw/TA15 composites with excellent metallurgical bonding were achieved.

  • Room temperature strength and ductility of the composites were simultaneously improved a lot after rolling.

  • Flat network microstructure continued to exert the reinforcement and strain bearing of bi-connected structure after rolling.

  • TiB rotation, larger silicides and refined grains during rolling deformation contribute to the strength enhancement.

Abstract

To investigate the microstructure evolution and mechanical properties of DRTMCs with network microstructure during the rolling deformation, the TiB whiskers reinforced Ti-6.5Al–2Zr–1Mo–1V (TA15) composites (TiBw/TA15) with the addition of silicon element and different TiB volume fractions were successfully fabricated by hot pressing sintering and rolled in the α+β region. Compared with the as-sintered composites, the as-rolled composites presented the lower local TiBw content, higher matrix connectivity, remarkable grain refining and larger silicides. After rolling deformation, the room-temperature ultimate strength and fracture elongation were dramatically improved. The as-rolled 3.5 vol.% TiBw/TA15 composite obtained better combination properties of 1248 MPa and 10.5% at room temperature. The increase in strength can be attributed to grain refining of the matrix, directional distribution of TiBw, and stronger hinder of silicides. The enhancement of ductility was due to increasing matrix connectivity, grain refining as well as DRX behavior of bimodal microstructure. As for high-temperature properties, the tensile strengths increased by 26–32% at 600 °C and 20–32% at 650 °C after rolling, which is also attributed to directional distribution of TiBw, grain refining of matrix and larger silicides. The highest strengths at 600 °C and 650 °C are 820 MPa and 698 MPa, respectively. The fracture behavior of the composites was discussed in detail.

Introduction

Titanium matrix composites (TMCs), combining excellent mechanical properties and high corrosion resistance, have attracted extensive attention and have been widely used in various fields such as aerospace, military, automotive and sports equipment, etc [[1], [2], [3], [4]]. In particular, in-situ synthesized discontinuously reinforced TMCs (DRTMCs) have been studied by more researchers owing to better combination properties and simpler fabrication [5]. Nevertheless, the conventional DRTMCs with even distribution easily led to the limited reinforcing effect and room-temperature embrittlement [6,7]. To improve the reinforcing effect of ceramic phase and room-temperature ductility of the composites, Huang et al. [[8], [9], [10], [11]] successfully fabricated a series of DRTMCs with network structure, two-scale laminate-network structure, and two-scale network structure by mainly adjusting the distribution of ceramic phase in the titanium/titanium alloy matrix, and obtained excellent mechanical properties. Herein, compared with the conventional fabrication method of high-energy ball milling + reaction hot pressing (RHP), the combination of low-energy ball milling (LEM) and RHP served as a critical step, which can obtain the network distribution of TiBw, reduce the contamination of Ti matrix powder and save much time [8].

However, subsequent mechanical processing is required for DRTMCs to close defects retained in the first fabrication and further enhance their mechanical properties. Usually, thermal-mechanical processing (TMP) is used for DRTMCs due to excessive deformation resistance and rebound deformation during the cold deformation process [12]. The TMP methods mainly include conventional ways (such as hot forging, hot rolling, hot extrusion [[13], [14], [15]]) and severe plastic deformation ways (such as accumulative roll bonding, hot-pressure torsion, equal channel angular pressing, multi-axial forging, and rotary swaging [[16], [17], [18], [19], [20]]). Due to the advantages of mature practical application and cost saving, the hot rolling method was selected in this work for TMP of DRTMCs with network microstructure. In recent studies, Zhang et al. [21] found that 2.5 vol.% (TiBw + TiCp)/Ti–6Al-2.5Sn–4Zr-0.7Mo-0.3Si composite can obtain a simultaneous improvement in room-temperature tensile strength and ductility after direct rolling in the α+β region, then discussed the dynamic recovery (DRV) and dynamic recrystallization (DRX) behavior during the rolling deformation. Tabrizi et al. [22] reported that the bending strength of (TiB + TiC)/TC4 composite increased after α+β hot rolling, which is attributed to complex strengthening mechanisms of thermal fluctuation, Orowan and geometrically necessary dislocations. Guo et al. [23] reported that the as-rolled (TiB + La2O3)/Ti composite presented superior room-temperature tensile properties, which is mainly due to matrix grain refining, TiB rotation and intense texture. Based on the above, the positive roles of hot rolling in improving mechanical properties of DRTMCs have been sufficiently verified, especially for rolling in the α+β region. Meanwhile, in our previous work [24], for as-rolled 8 vol.% and 12 vol.% TiBw/pure Ti composites with network structure, the large rolling reduction ratio can lead to high strength and ductility. This is attributed to that micro-cracks from fractured TiBw can be made up by matrix flow under relatively large deformation. However, there are few studies concerning the effect of α+β hot rolling on the microstructure evolution and mechanical properties for the network-structure DRTMCs with the addition of silicon element.

In the present work, TA15, a kind of near-α high-temperature titanium alloy with a mature application, was selected as the metallic matrix. The LEM, RHP and multi-pass hot rolling processes were adopted to fabricate as-sintered and as-rolled TiBw/TA15 composites with network structure. Considering the addition of silicon element, the matching compositions and deformation conditions were designed to obtain high-formability bimodal microstructure and maintain network microstructure during the rolling deformation. The microstructure evolution and mechanical properties of the as-sintered and as-rolled TiBw/TA15 composites were detailedly investigated, which can help understand the relationship for deformation parameter, microstructure evolution and mechanical properties. This can also provide a positive guide for the microstructure design and properties optimization in the TMCs and MMCs research.

Section snippets

Raw materials and fabrication of composites

Fig. 1 shows the fabrication processes of the as-sintered and as-rolled TiBw/TA15 composites. The large-sized spherical TA15 alloy powders (Ti-6.5Al–2Zr–1Mo–1V, size of 90–150 μm, purity of 99.9%) were taken as the metallic matrix as shown in Fig. 1a). Then the fine TiB2 powders (size of 1–8 μm, purity of 99.8%) and Si powders (size of 2–3 μm, purity of 99%) were taken as additives. Convenient for the comparison, the weight fractions of TiB2 were selected as 1 wt%, 2 wt%, 2.5 wt% to obtain a

Phase identification

The X-ray diffraction patterns of the as-sintered TiBw/TA15 composites with different volume fractions are shown in Fig. 2a). There only exist α-Ti, β-Ti and TiB phases without TiB2 and Si, indicating that in-situ chemical reactions have proceeded completely during the sintering process. Fig. 2b) presents the relative densities of the as-sintered and as-rolled composites. It can be observed that all the relative densities can reach above 99.0%, which reveals an excellent metallurgical bonding.

Conclusions

In the present work, the as-sintered and as-rolled composites with different TiBw volume fractions were successfully fabricated. The main conclusions are as follows:

  • (1)

    Compared with the as-sintered composite, the as-rolled composite presented higher matrix connectivity and lower local TiBw content. The flat network structure can still play a role in biconnected strengthening and toughening. The composite with a higher TiBw content presented lower matrix connectivity, occurrence of TiBw cluster,

Credit author statement

Shuai Wang: Data Curation, Writing - Original Draft.

Lujun Huang: Conceptualization.

Shan Jiang: Experimental assistance.

Rui Zhang: Experimental assistance.

Baoxi Liu: Experimental assistance.

Fengbo Sun: Experimental assistance.

Qi An: Supervision.

Yang Jiao: Supervision.

Lin Geng: Supervision.

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.

Acknowledgments

This work was financially supported by the National Key R&D Program of China [grant number 2017YFB0703100], Key-Area Research and Development Program of GuangDong Province [grant number 2019B010942001], the National Natural Science Foundation of China (NSFC) [grant numbers 51731009, 51822103, and 51901056], the Fundamental Research Funds for the Central Universities [grant numbers HIT. BRETIV.201902 and HIT.NSRIF.2020002].

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