Full Length ArticleEffect of processing factors on the microstructure and gradual diffusion of tungstenized layers
Introduction
Owing to its high physical sputtering threshold energy, good thermal properties under high temperature condition, and low erosion rate under plasma loading, tungsten is considered as a prime candidate for plasma facing armor material in nuclear fusion reactors [1], [2]. Nonetheless, the drawbacks of tungsten bulk material as the plasma facing material are the high ductile to brittle transition temperature (DBTT), heavy weight, and difficulties in machining and welding [3]. A promising method for industrial applications of tungsten is to coat the designated part with a thin tungsten layer. Some researchers have reported that the W coatings are commonly obtained with the plasma sprayed, magnetron sputtering and chemical vapor deposition (CVD) and electrodeposition [4], [5], [6], [7]. However, the high elastic modulus and thermal expansion mismatch with other metals make the joining of W to heat sink metals really challenging [8]. A high thermal stress would be built between heterogeneous metals after heating them, which will certainly damage the coating-substrate joint. The tungsten coating delaminating from the substrates frequently appears when using conventional methods mentioned above.
The double glow plasma surface alloying (DGPA) can produce two cross-linked glow discharges, which mainly originate from two negative bias of the substrate and the source electrode. One glow discharge heats the work piece and the other one strikes the source electrode. The potential difference between the substrate and the target leads to an unequal electronic potential hollow cathode effect [9]. The effect provides the capability of enhancing the sputtering yield and the activities of substrate surface, which has become one of the most attractive techniques in surface alloying or composite coatings preparation [10], [11], [12]. Therefore, more W ions or atoms sputtered from the source electrode can be deposited on the substrate and diffuse into its inner structures. In addition, plasma surface alloying can achieve a certain thickness of the alloyed layers varied from several microns to 500 μm with the concentration of the alloying elements being from 5% to more than 90%. It is also obtained for desired alloyed layers on the different substrates. However, the role of the diffusion behavior of the alloying elements in the DGPA has not been understood so far. Especially, the forming processing of W alloying element, as a typical low sputtering yield and the refractory material, is still rarely found in the open literature.
In view of this, an effort was made to produce a tungsten coating on Ti-Al-Nb alloy substrate by the DGPA technology. The effects of the processing factors on the microstructure and micro-hardness properties of the alloyed layers are systematically investigated. Meanwhile, a gradual-diffusion model of the W atoms in the plasma surface alloying is proposed and the diffusion mechanisms of other transition metals can be explained.
Section snippets
Material and the process
The substrate was Ti–Al–Nb alloy, including 47 wt.% for titanium, 10.82 wt.% for aluminum and 42.12 wt.% for niobium. The specimens were machined into 10 mm × 10 mm × 5 mm in dimension. The specimens were mechanically polished via a standard metallographic procedure and then were cleaned with acetone before the plasma surface alloying.
The sputtering target of W plate (Ф100 mm × 5 mm) with the purity of 99.9% prepared by powder metallurgy was used as the source electrode to supply alloying elements. The
Effect of the temperature on the tungstenized layer
According to the Ti-W equilibrium phase diagrams, titanium is important as an alloying agent with W and can be stabilized in the β phase. β0 + O binary phase exhibits excellent mechanical properties that the good balance among tensile, creep and fracture toughness can be obtained at the temperature of 900–1000 °C [13], while the brittleness increases as β transforms to β0 when the temperature is above 1050 °C. To study the effect of the temperature on the microstructure of the alloyed layers, a
Conclusions
In the current study, the gradual tungstenized layers were carried out by the DGPA technology. We compared the effects of temperature, the spacing distance between electrodes and the pressure on the surface microstructure and micro-hardness of the alloyed layers. The diffusion modeling of W atoms in the Ti-based alloys is proposed based on the Fick ‘s first law and the diffusion process of the TixW1-x gradual layers is discussed. Major findings of the study are summarized as follows:
- (1)
The optimal
Acknowledgements
This project is supported by National Natural Science Foundation of China (No. 51405242), Natural Science Foundation-Outstanding Youth Foundation of Jiangsu Province of China (BK20160091) and the Six Talent Peaks Project of Jiangsu Province, China (No. GDZB-046).
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