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