Significantly strengthening the Ti70Nb10Mo10Zr10 alloy via architecting two-scale silicide reinforcements

https://doi.org/10.1016/j.jallcom.2020.155255Get rights and content

Highlights

  • Two-scale silicide reinforcements were architected in a Ti70Nb10Mo10Zr10 alloy.

  • The silicides distributed at grain boundaries formed a novel network structure.

  • Remarkable strength enhancement was achieved at both room and high temperatures.

  • The microstructure evolution mechanism and strengthening mechanism were discussed.

Abstract

To further strengthen the Ti70Nb10Mo10Zr10 alloy (denoted as TI70 alloy) with a single BCC solid solution microstructure, two-scale silicide reinforcements were successfully tailored by adding 1 at.%, 2 at.% and 4 at.% Si element during powder metallurgy process (denoted as TI70-Six alloys). The nano-scale S1-type (Ti,Zr)5Si3 silicides were precipitated and distributed dispersively at grain interior to provide dispersion strengthening effect, while the nano-scale and micro-scale S2-type (Ti,Zr)6Si3 silicides were distributed at the grain boundaries and formed a novel network microstructure which could prohibit grain growth and enhance grain boundary strengthening effect. The TI70-Six alloys strengthened by the two-scale silicides exhibited remarkable strength enhancement not only at ambient temperature but also at high temperatures. The compressive yield and fracture strengths of the TI70-Si4 alloy were increased to 1550.4 MPa and 2002.5 MPa at ambient temperature. Moreover, the TI70-Si4 alloy exhibited ultrahigh yield strengths of 1048 MPa and 437 MPa at 600 °C and 800 °C, which were increased by 61.1% and 111.2% compared with TI70 alloy, respectively. The superior strengths can be mainly attributed to the coordinated strengthening mechanisms of two-scale silicide reinforcements and novel network microstructure.

Introduction

Titanium has been honored as “the future metal” due to its low density, high strength, excellent corrosion resistance as well as many other desirable properties, and has been widely used in modern society, such as the aerospace, automotive and medical industries [1,2]. Among all the titanium-based materials, β−Ti alloys have shown desirable mechanical properties and oxidation resistance and been applied in many aircraft components, such as airframes and engine nozzles [3,4]. Under such an application background, it is essential to further enhance the strength, especially high-temperature strength. Recently, many achievements have been made to improve the room-temperature strength and ductility, such as modifying the grain morphology by plastic deformation and heat treatment [5], metastability-engineering [[6], [7], [8], [9]], and thermal-induced and deformation-induced dispersive ω-phase [[10], [11], [12], [13]]. These strengthening methods, however, would no longer be effective at high-temperatures due to the metastable characteristic. It is reported that the ω-phase precipitated from the metastable β-Ti alloys would decompose and transform into HCP-structured α-phase and BCC-structured β-phase and then would not serve as strengthening phase. Multiple studies have shown that the ω-phase can only be stable at temperatures below 300 °C–500 °C depending on the chemical composition of titanium alloys [12,[14], [15], [16], [17]]. To improve the high-temperature strength, thermally stable second phases are necessary to provide strengthening effect. It has been widely reported that introducing ceramic phases as reinforcements is an effective method in enhancing the room- and high-temperature strengths of titanium and other alloys [[18], [19], [20], [21], [22], [23]]. Through this method, a vast amount of achievement has been made in strengthening titanium alloys, mainly the α−Ti alloys [[24], [25], [26], [27], [28]], α+β−Ti alloys [18,19,29,30] and only a few attempts in further strengthening the β-Ti alloys [[31], [32], [33]]. In these researches on β-Ti alloys, the primary goal was to enhance the room-temperature mechanical properties, the strength and strengthening mechanism at high-temperatures are still lacking.

Recently more and more studies indicate that silicide particles such as Ti5Si3, (Ti,M)5Si3 (M = Zr, Hf, etc) and (Ti,Zr)6Si3 [18,20,[34], [35], [36], [37]], can also serve as excellent reinforcement to improve the mechanical properties at room- and high-temperature. Considering the desirable features of the novel β-Ti alloys and the improvement that the silicide particles could bring, it is of great significance that the silicide particles strengthening β-alloys being studied systemically to be a novel method to further enhance the β-Ti alloys.

In this study, silicide strengthened β-Ti alloys were designed and fabricated by ball milling and hot-pressing sintering using cp-Ti powders, β-stabilizer element powders and MoSi2 powders as raw materials. The composition of the alloying elements was designed with the reference of the Bo-Md diagram which has been a useful tool for modifying the alloy composition to achieve the microstructure as desired [38]. Bo is the bond order that represents the strength of the covalent bond between Ti and an alloying element. Md is the metal d-orbital energy level of an alloying element. In this case, the chemical composition was chosen to be Ti70Nb10Mo10Zr10 (denoted as TI70) of which the Bo-Md value lies in the beta region. According to the available Ti–Si binary phase diagram and Ti–Zr–Si, Ti–Nb–Si, Ti–Mo–Si ternary phase diagrams [[39], [40], [41], [42]], the solubility of Si in the TI70 alloy was reasonably deduced to be between 1 at.% and 2 at.% at 1300 °C, thus the Si additions were chosen as 1 at.% 2 at.% and 4 at.% to attain different silicide reinforcements in the TI70 alloy for comparative studies. By adding different amounts of Si element in the TI70 alloy, two-scale silicide reinforcement microstructures were successfully architected in the silicides strengthened TI70 alloys (denoted as TI70-Six): (1) precipitated nano-scale S2-type (Ti,Zr)6Si3 silicides distributed at the grain boundaries, S1-type (Ti,Zr)5Si3 silicides distributed dispersively inside the grains, and (2) in-situ formed micro-scale S2-type (Ti,Zr)6Si3 silicides distributed at the grain boundaries. Significant strength enhancement has been achieved at room- and high-temperatures by the coordinated strengthening effect of the two-scale silicide reinforcements. Moreover, the microstructure evolution mechanism and strengthening mechanism are discussed.

Section snippets

Experimental procedures

The chemical composition of the matrix alloy was designed to be Ti70Nb10Mo10Zr10 (atomic percentage). High purity (>99.9%) spherical titanium powder with a mean diameter of 100 μm, high purity (>99.9%) angular niobium powder with a mean diameter of 5 μm, high purity (>99.9%) angular molybdenum powder with a mean diameter of 5 μm, and high purity (>99.9%) angular zirconium powder with a mean diameter of 45 μm were used as raw materials. The Si element was added by MoSi2 powder with a mean

Microstructure characterization

Fig. 1 shows the XRD patterns evolution through the TI70-Six alloys manufacturing process, which showed that the diffraction intensity of elemental powders decreased and the FWHM (full width at half maximum) increased as the ball milling prolonged. That is to say, the grain size of the raw material powders decreased due to the severe deformation induced by ball milling. However, mechanical alloying was not fully achieved since diffraction peaks of all the elemental powders could still be

Microstructure evolution mechanism of the TI70-Six alloys

The TI70 alloy and TI70-Six alloys were designed and fabricated by ball milling the elemental and MoSi2 powders and hot-pressing sintering. As a new material synthesizing method, powder metallurgy has many advantages compared to other traditional methods. The schematic diagram of the preparation process of the TI70-Six alloys is presented in Fig. 7. Instead of long-time homogenizing treatment at high temperatures, the uniform distribution of the elements was achieved by the ball milling process

Conclusions

  • (1)

    Two-scale silicide reinforcements were successfully architected via introducing 1 at.%, 2 at.% and 4 at.% Si element into the TI70 alloy during powder metallurgy process. The nano-scale S1-type (Ti,Zr)5Si3 silicides were precipitated and distributed dispersively at grain interior, while the nano-scale and micro-scale S2-type (Ti,Zr)6Si3 silicides were distributed at the grain boundaries and formed a novel network microstructure.

  • (2)

    The TI70-Six alloys with two-scale silicide reinforcements and the

CRediT authorship contribution statement

Shan Jiang: Methodology, Investigation, Data curation, Writing - original draft. Lujun Huang: Conceptualization, Supervision, Writing - review & editing, Project administration. Qi An: Validation, Writing - review & editing. Xiang Gao: Formal analysis. Shuai Wang: Writing - review & editing. Lin Geng: Project administration. Rui Zhang: Visualization. Fengbo Sun: Writing - review & editing. Yang Jiao: Formal analysis.

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.

Acknowledgment

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 [grant numbers 51822103, 51731009 and 51901056], the Fundamental Research Funds for the Central Universities [grant number HIT. BRETIV.201902 and 2020002].

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