Original Research PaperW-Y2O3 composites obtained by mechanical alloying and sintering
Graphical abstract
Introduction
Due to their interesting properties, such as high melting points, good mechanical properties at elevated temperatures, high thermal shock resistance, low coefficient of thermal expansion, and high density, tungsten and its alloys have been applied in various field, such as, the nuclear and military industries [1], [2], [3], [4]. Tungsten heavy alloys (WHA) usually contain tungsten (over 80 wt%), nickel and iron [5], [6], [7]. These alloys are prepared by liquid phase sintering of blended elemental powders. The sintering is carried out above the melting temperature of the matrix phase (Ni, Fe) and below the melting point of tungsten. The typical structure of WHA is characterized by W particles with a size of 10–40 µm immersed in a continuous Ni-Fe-W solid solution [8].
In recent years, there has been an increasing number of studies focusing on improving the mechanical properties of tungsten alloys through strengthening via oxides (ODS) or carbides with a content of up to approximately 3 wt% [9], [10]. Several studies of tungsten composites reinforced with a higher content of a ceramic phase, such as SiC and TiC, have also been reported [11], [12]. The ceramic phases improve the mechanical properties of tungsten and its alloys due to their attractive properties, such as hardness. Due to their very stable physical and chemical properties and high melting points, the exploration of rare earth oxides (La2O3, ThO2, Y2O3) as additives to produce ultra-fine grained tungsten-based materials appears to be a promising research direction [13], [14], [15]. The addition of thermally stable oxide particles dispersed within the tungsten grains causes anchoring and accumulation of the dislocations occurring inside the grains. Additionally, the oxide particles present at the grain boundaries block grain boundary migration and inhibit grain growth, which gives rise to the increased high-temperature strength and creep resistance of the oxide dispersion-strengthened tungsten [13], [16]. The size of the oxide particles and their location appear to be important parameters for the strengthening effect. If the particles are large and distributed at the grain boundaries, they may give rise to stress concentration and low toughness. Therefore, the size distribution of the W oxide particles in the nanometre range [17], [18] is favourable and enhances the mechanical properties of the material. Y2O3 is a candidate rare earth oxide additive. The impact of the addition of Y2O to W on the properties and structure of the obtained materials has been described in previous studies. It was found that the microhardness of the sintered W-(0.3-2)Y2O3 and W-(0.3-2)Y increases with the increasing Y2O3 or Y content [3]. Veleva et. al. [19] reported that the mechanical properties, and particularly the ductility calculated from the bending tests, are improved in hot forged W-2Y2O3 compared to W-2Y and W-1Y2O3 isostatic hot pressed materials.
The mechanical alloying process can be used to obtain a mixture of powders that is then used for further consolidation [20], [21]. It is very important to obtain high density during compaction, so the ODS tungsten alloys needs to be sintered at high temperatures for a long time. Unfortunately, this sintering causes the growth of tungsten grains. Therefore, new methods for consolidation while obtaining good properties of the sintered materials have been investigated. Lee et. al. [21] proposed a two-stage sintering process consisting of primary solid-state sintering followed by secondary liquid phase sintering. The most popular method used for the consolidation of tungsten alloys is hot isostatic pressing (HIP), which has the disadvantages of high energy consumption and grain growth during the HIP process [22], [23]. The spark plasma sintering (SPS) process is an alternative approach for the compaction of fully dense tungsten materials [24] that involves the simultaneous flow of a strong pulsed current through a die equipped with powder and with pressure applied between two opposite punches. Two-step spark plasma sintering was applied to obtain W-5 wt% Y2O3 sintered samples in previous work [25]. The first sample was consolidated at 1200 °C for 2 min and at 1400 °C for 1 min. The consolidation parameters for the second sample were: 1200 °C for 2 min and 1600 °C for 0.5 min. As a result, the microhardness of the second sample was much higher than that of the first sample.
In this work, two sinters with different contents of the strengthening phase, namely, W-5 wt% Y2O3 and W-10 wt% Y2O3, were manufactured by mechanical alloying and subsequent pulse plasma sintering (PPS). For comparison, a reference sample made of pure W was milled and consolidated under the same conditions as the yttria-containing samples.
Section snippets
Experimental
Two kinds of initial powder mixtures, W + 5 wt% of Y2O3 and W + 10 wt% of Y2O3, with different contents of the strengthening phase (referred to as W-5%Y2O3 and W-10%Y2O3) were prepared from elemental powders of W (New Met Koch, purity 99.5%, particle size of approximately 10 µm) and Y2O3 (Alfa Aesar, purity 99.995%, particle size in the range of 50–70 nm). For comparison and to record the changes that may occur in the powders after adding the strengthening phase, pure W was also processed in
Results and discussion
Fig. 1a and b show the sequences of the X-ray diffraction patterns recorded with the increasing milling time of the powder mixtures containing 5 and 10% of Y2O3, respectively. The diffraction lines of yttria disappear after a relatively short milling time. Simultaneously, a significant broadening of the diffraction lines corresponding to W is observed, and the lines shift to higher diffraction angles. The broadening of the diffraction lines can be attributed to the decrease of the crystallite
Conclusions
Investigations performed for W, W-5%Y2O3 and W-10%Y2O3 powder mixtures revealed that the crystallite size of the tungsten-based solid solution decreases with the increase of the milling time for both alloys and reaches approximately 15 nm after 50 h of MA, while the lattice strain increases. Additionally, the samples were contaminated by Fe from vials and balls during the milling processes. The level of this contamination was higher during the milling of the W-10%Y2O3 powder mixture. This may
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 financial support by National Science Centre (NCN) under the project No 2012/07/B/ST8/03583 is highly appreciated.
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