Microstructural stability, phase evolution and mechanical properties of a forged W-modified high-Mn β-γ-TiAl alloy
Introduction
Due to lightweight (4 g/cm3) and attractive specific strength, γ-TiAl alloys have been regarded as a competitive advanced structural material in the field of aviation, aerospace and automotive engine [[1], [2], [3], [4]]. It is believed that this material, as a new generation high temperature structural materials, has great potential to replace the traditional superalloys in the temperature range of 600∼900 °C to realize a great increment on the thrust-weight ratio and fuel efficiency [5]. In TiAl alloys, the deformable TiAl alloys (β-γ-TiAl alloys [4]), which usually have fewer metallurgical defects, no peritectic segregation, fine uniform microstructure and higher strength at both room temperature and high temperature, are becoming a research focus in recent years. Meanwhile, good hot-working ability ensures these alloys be used to produce various shapes of work pieces at a lower manufacturing cost [6].
Researchers showed that Mn was a strong β phase-stabilized element [7], which can significantly improve the hot working ability of TiAl alloy. Besides, Mn can also greatly inhibit βo phase from transforming into brittle ωo phase in Nb contained TiAl alloy [8]. In addition, the cost of Mn is significantly lower than that of Mo and Nb [9]. Therefore, besides the Ti–Al–Nb alloy systems, Ti–Al–Mn based β-γ-TiAl alloys have attracted attention worldwide in recent years. One of the most typical β-γ-TiAl alloy is Ti–42Al–5Mn (at.%) which was developed by NIMS, Japan in 2002 [10]. The hot workability of this alloy was proved to be much better than the famous Ti–43Al–4Nb–1Mo-0.1B (TNM) alloy [11,12]. It can be deformed even under conventional forging condition, which greatly reduces the components manufacturing costs. Reports confirmed that valves made of this alloy have already been used in a certain type of racing engines in Japan [13].
However, the previous research has pointed that even small amount of Mn addition, such as 1.0∼2.0 at.% Mn, is detrimental to the oxidation behavior of Ti-48 Al alloy [14,15]. In the case of high Mn content Ti–42Al–5Mn alloy, we recently found [16] Mn participated in oxidation reaction and formed Mn2O3. This oxide usually mixed formed in the outermost layer TiO2, which is easily to peel off and cause a great amount of voids in the oxidation layer, thereby degrading its high temperature oxidation resistance property. The oxidation problem induced by Mn cannot be any doubt as the key bottleneck for long-term stable service at high temperature in the future. Fortunately, our previous research also proved [16] alloying with only small amount of W (0.5∼1.0 at.%) could significantly improve this alloy's high-temperature anti-oxidation property, thus providing an effective way to facilitate the application of Ti–42Al–5Mn alloy at higher temperature.
It is recognized that the microstructure and mechanical properties of TiAl are strongly influenced by alloying, for example W. Traditionally, W is believed to be a solid-solution strengthening element in TiAl [7], and W atoms are assumed to reduce the dislocation mobility because of their low diffusivity thereby increasing the creep strength of Ti–48Al–2W-0.1B [17]. But it is also proved that the excessive addition of W, such as 2.0 at.%, can result in the formation of ordered β (βo) phase in microstructure, which produces an negative effect on the creep strength [18]. Moreover, the amount and morphology of β phase would change with the increasing content of W. For instance, in Ti–47Al-0.5Si [19], it was found that when alloying with 2.1 at.% W, β phase was observed in three types of morphology: i) blocky β phase containing needle-like γ phase and fine silicide particles inside; ii) rod-like β phase and iii) fine needle-like β phase. When the content of W reduced to 1.5 at.%, only a very small amount of rod-like β phase appeared. As analyzed above, the available literatures were generally concentrated on the effect of relatively high W addition, for example, 1.5∼2.1 at.%, on the microstructure and creep resistance in some traditional γ-TiAl alloys. Whereas the change on microstructure evolution behaviors and mechanical properties, caused by a small amount of W addition, e.g. 0.8 at. % in this study, in the β-γ-TiAl alloys are still unclear.
Hence, in this paper, Ti–42Al–5Mn alloy was used as the base alloy, and W was added to prepare Ti–42Al–5Mn-0.8 W alloy. The forging for these two alloys was carried out under conventional conditions, and then long-term ageing treatment was performed at 800 °C. The influence of W on the microstructure (as cast, forged and aged) and long-term ageing tensile properties of these alloys were systematically investigated. This study aimed to clarify the effect of W on the microstructure and mechanical properties of this easily deformed high Mn-containing β-γ-TiAl alloy, and to provide a theoretical support for its future industrial application.
Section snippets
Materials and methods
Ti–42Al–5Mn and Ti–42Al–5Mn-0.8 W alloy ingots (weight 10 kg) were prepared by single-step vacuum induction melting (VIM). Ti and Al were industrial pure raw materials, and Mn was the purified manganese. In order to reduce the segregation of W during melting, Al–W master alloy was used. The chemical compositions of the alloy samples are listed in Table 1, in which, Alloy-1 is the base alloy and Alloy-2 is the alloy containing W.
After cut off top and bottom, the ingots were formed into bars with
As-cast microstructure
Fig. 3 shows EPMA-BSE images of as-cast microstructure corresponding to Ti–42Al–5Mn and Ti–42Al–5Mn-0.8 W alloys. It can be seen from the figure that the alloy without W addition are mainly composed of coarse columnar α2/γ colonies (as shown in Fig. 3(a)). The elongated βo and γ phases (named γ-platelets (γp)) can be observed at the colony boundaries. These precipitated βo and γ phases appeared in strip shape (as seen Fig. 3(b)), and the reason to this type morphology has been elaborated in our
Conclusions
- (1)
W has positive effect on reducing the size of lamellar colony in as-cast samples. After forging, the microstructures in Ti–42Al–5Mn and Ti–42Al–5Mn-0.8 W were all nearly lamellar. However, with W alloying, the volume percentage of lamellar colonies was slightly decreased from 76.5% to 69.1%, and the lamellae size is refined.
- (2)
W is a typical βo stabilization element. Alloying with W can increase the content of the βo phase in the forged Ti–42Al–5Mn-0.8 W alloy from 10.7% to 16.0%.
- (3)
The contents of W
Funding
This work was supported by National Natural Science Foundation of China [Grant No. 51971215], Natural Science Foundation of Liaoning Province of China [Grant No. 2019-MS-330], and China postdoctoral science foundation [Grant No. 2019M661152].
Author statement
Xiaobing Li, Hongjian Tang and Pengxiang Zhao: Sample preparation, Methodology, Data curation, Investigation.
Xiaobing Li and Weiwei Xing: Original draft preparation.
Weiwei Xing and Kui Liu: Supervision.
Yingche Ma and Bo Chen: Discussion and Revision.
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.
Acknowledgements
The authors thank the National Natural Science Foundation of China under grant No. 51971215, the National Natural Science Foundation of China of Liaoning province under grant No.2019-MS-330, the China Postdoctoral Science Foundation under grant No. 2019M661152 for their financial supports.
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