Aluminium matrix tungsten aluminide and tungsten reinforced composites by solid-state diffusion mechanism

In-situ processing of tungsten aluminide and tungsten reinforced aluminium matrix composites from elemental tungsten (W) and aluminium (Al) was investigated by thermal analysis and pulsed current processing (PCP). The formation mechanism of tungsten aluminides in 80 at.% Al-20 at.% W system was controlled by atomic diffusion. The particle size of W and Al in the starting powder mixture regulated the phase formation and microstructure. PCP of micron sized elemental Al and W resulted in formation of particulate reinforcements, W, Al4W and Al12W, dispersed in Al matrix. W particles were surrounded by a ~3 μm thick dual-layer structure of Al12W and Al4W. The hardness of Al matrix, containing Al12W reinforcements, was increased by 50% compared to pure Al, from 0.3 GPa to 0.45 GPa and W reinforcements showed a hardness of 4.35 GPa. On PCP of 80 at.% Al-20 at.% W mixture with particle size of W and Al ~70 nm, resulted in formation of Al4W as major phase along with small fractions of Al5W and unreacted W phase. This suggested strongly that the particle size of the starting elemental Al and W could be the controlling parameter in processing and tailoring of phase evolution, microstructure of particulate reinforced Al matrix composite.

particulate W and W-Al intermetallics in Al matrix can improve the mechanical performance such as specific strength, oxidation resistance and thermal stability over the elemental Al and W. However, the low solubility of W in Al and high reactivity of W with Al raises the difficulty to fabricate Al-W composites by equilibrium synthesis routes. Y.C. Feng et al. have synthesized Al 12 W reinforced Al-MMC from Al99W1 (vol.%) system through reaction sintering followed by hot extrusion with small volume fraction of reinforcements 25 . The final composite consists of Al 12 W particles with a size of <1 µm distributed in the Al matrix after the complex sintering process. Mechanical alloying has been used to form Al-W composites 26,27 , but it involves long processing time (normally >10 h) of high-energy ball milling and has poor control of the alloying process. High sintering temperature and prolonged sintering time lead to heterogeneous composite structure 28,29 and unexpected grain growth. In this regard, pulsed current processing (PCP), as a novel and non-equilibrium sintering technique, has shown its advantage in fabricating Al-MMCs from consolidating powdered metals at relative low temperature and within a short time 30,31 . PCP combines uniaxial force, pulsed direct electrical current and low atmosphere pressure during sintering, which gives fully dense materials 32 . Here, we report that the atomic diffusion kinetics in Al-W system in combination with non-equilibrium processing resulted in fabrication of a W and W-Al intermetallics reinforced Al matrix composite. The composite showed remarkable densification during processing and uniform distribution of particulate reinforcement in the Al matrix. Two significant factors, heating rate and sintering time that control the phase formation and microstructure evolution of Al-W composites were investigated by thermal analysis. The effect of particle size, i.e. diffusion length scale would be discussed.

Results and Discussion
The pulsed current processing (PCP) of micron-sized Al (80 at.%) and W (20 at.%) powder mixture (denoted as m-80Al20W) resulted in formation of particulate reinforced Al matrix composite (Al-MMC). The density of the composite is 6.3 g/cm 3 , which is higher than its theoretical density of 5.9 g/cm 3 , calculated using the rule of mixture. The fact that the density of the composite has exceeded theoretical density could be attributed to the loss of Al from the die during PCP under combined effect of temperature, pressure and vacuum. The X-ray diffraction (XRD) data of Al-MMC in Fig. 1a indicates the formation of Al 4 W and Al 12 W intermetallic phases in addition to Al matrix and W reinforcements. This infers that Al and W reacted during PCP to form equilibrium intermetallic phases following the Al-W equilibrium phase diagram. The microstructure of Al-MMC in Fig. 1b shows the homogenous distribution of bimodal particulate reinforcements of 2-5 µm (Al 12 W) and 20-50 µm (multi-phase structure) in Al matrix. The XRD data and microstructure in Fig. 1 suggest that the Al-MMC consist of Al and W as major phases and small fraction of Al 4 W and Al 12 W intermetallics. The high magnification inset in Fig. 1b reveals the formation of intermetallic dual-layer structure at the Al/W interface. The energy dispersive analysis (EDS) confirms that layers formed around W particles are Al 4 W and Al 12 W intermetallics (Supplementary information, Fig. S1). The intermetallic phases grow in layers as suggested by multilayer growth mechanism in diffusion couples 19,33,34 . Initial nucleation of intermetallic compounds starts at the boundary of Al/W interface. The subsequent grain growth is controlled by solid-solid diffusion. Because the Al 12 W phase is relatively rich in Al, it expands predominately towards Al phase, while Al 4 W grows towards W. Meanwhile, the outmost Al-Al 12 W interface shows evidence of disintegration and migration of particles from Al 12 W layers resulting in observed presence of Al 12 W particle reinforcements of 2-5 μm in Al matrix (Fig. 1b). The W reinforcements showed a hardness of elemental W, 4.35 GPa, and Al matrix reinforced with Al 12 W particles showed a hardness of 0.45 GPa, which was increased 50% of pure Al due to the incorporation of intermetallic phases in Al matrix.
The influence of heating rate on inter atomic diffusion process in m-80Al20W system and formation of intermetallic compounds in the composite was studied by Differential Scanning Calorimeter (DSC) in the temperature range 30-1000 °C. DSC curves of 5 °C/min, 10 °C/min and 20 °C/min in Fig. 2 show distinguished thermal events, indicating higher data resolution at raised heating rate 35,36 . The onset of first endothermic event that starts at around 660 °C is attributed to the melting of Al, as Al has a melting point of 660 °C. The melting of Al is followed immediately by the formation of intermetallics which is related to the exothermic peaks. In the case of heating rate of 20 °C/min, the two exotherms appear at 720 °C and 738 °C respectively, which refer to the formation of two  Fig. 2(c) show good consistency with Al 4 W and Al 5 W reference diffraction patterns indicating a high fraction of equilibrium intermetallic phase Al 4 W was formed and low fraction of Al 5 W was also detected ( Supplementary Information, Fig. S2), which is consistent with the appearance of two exothermic peaks in DSC data ( Fig. 2(a)). The complete phase transformation is attributed to lower heating rate compared to PCP.
In order to elucidate the phase evolution in Al-W system in solid state, m-80Al20W compacts were heated to temperatures below the melting point of Al and held for 1 hour. The artifacts of DSC curves caused at step change on the onset and offset of temperature holding platforms were removed by adjusting baselines in the software Origin. As shown in Fig. 3A-C, the first obvious exothermic event showed up when Al melts at around 660 °C in all three DSC curves. However, the intensity of endothermic melting peaks and following exothermic peaks become less pronounced as the holding temperature increases from 550 to 650 °C. The tendency of reduction in reaction enthalpy implies that the Al and W diffuse to form intermetallics before Al melts. The amount of reacted elemental Al increases with the increasing holding temperature as higher temperature promotes atomic diffusion 37 , leading to the formation of higher contents of intermetallic compounds. Similar diffusion-controlled mechanism of intermetallic formation in Au-Al system was reported by Elliott Philofsky 19 . In their study, dif-   Based on the experiments with the holding temperature platforms, the diffusion-controlled mechanism is further verified by terminating the heat treatment at different steps: the onset and the offset of the 1 hour holding platform (Al-W compacts at a 1 , a 2 , b 1 , b 2 , c 1 and c 2 in Fig. 3A-C) and determining the phase evolution by XRD. According to the analysis results of XRD patterns in Fig. 3a-c, Al-W compact at a 1 consists of Al and W, which implies that no reaction has taken place at 550 °C. At a 2 , XRD peak of intermetallic compound Al 12 W is detected in Al-W compact (Fig. 3a), which is treated at 550 °C and held for 1 hour. Compare with the PCPed m-80Al20W, where the diffusion process was accelerated by the combined effect of rapid heating rate, current and uniaxial force resulting in formation Al 4 W and Al 12 W intermetallic compounds, the XRD data at a 2 suggests that Al 12 W forms first in Al-W system. On increasing the holding temperature to 600 °C, the formation of intermetallic is promoted in Al-W compact b 2 . The XRD data of Al-W compact at b 2 shows increase in diffraction intensities of Al 12 W at b 1 at 2θ = 28.8° in Fig. 3b. On increasing the temperature to 650 °C, the XRD data of Al-W compact at c 2 shows the high intensity Al 12 W peaks compared to a 2 and b 2 (Fig. 3c). In addition to Al 12 W peaks, the Al-W compact at c 2 shows the presence of Al 4 W (See Fig. S3 in Supplementary Information). It indicates that higher temperature and longer holding times are required to obtain equilibrium intermetallic phase Al 4 W in m-80Al20Wsystem.
The microstructure of Al-W compacts at b 1 , b 2 and c 2 are presented in Fig. 4. A thin layer (approximately 500 nm) is found between Al and W phase at b 1 ( Fig. 4a and b) indicating the formation of initial diffusion zone at the boundaries of W particles at 600 °C. The composition of the 500 nm layer is identified as intermediate phase Al 12 W according to the XRD data. The formation of Al 12 W is controlled by diffusion of Al and W atoms and subsequent nucleation and growth from this diffusion zone. According to Kirkendall effect, the atomic diffusion involves the exchange between atoms and vacancies. Therefore, the directional mass flow from one phase towards another that is caused by diffusion process should be balanced by opposite directional vacancy flow 39 . The increase in porosity in Al matrix (Fig. 4a,c and e) may result from the directional mass flow from Al to W, i.e. Al diffuses faster than W in Al-W system, with increase in the volume fraction of intermetallic phases. Massive diffusion of Al atoms into the diffusion zone results in formation of intermetallic compound with high atomic fraction of Al, i.e. Al 12 W in Fig. 4b. The growth of intermetallic phases (≈10 µm thickness) results during 1 hour of extended holding time at 600 °C as shown in Fig. 4c and d. EDS spectra of m-80Al20W compact at b 2 ( Supplementary Information, Fig. S4) reveals that the formation of Al 4 W between W particle and Al 12 W. As the initial solid state reaction between Al and W formed Al 12 W at Al/W interfaces, the following reaction is controlled by the diffusion of atoms at the W/Al 12 W interface, so Al 4 W is formed in between Al 12 W and W, and expected to continue until Al 12 W is consumed. Similar microstructure evolution and mechanism was reported in Al-Ti system 39,40 . Moreover, Fig. 4f shows that for the m-80Al20W compact at c 2 , the W particulates are surrounded by 10-20 µm Al 4 W layers ( Supplementary Information, Fig. S5) by increasing the holding temperature from 600 °C to 650 °C. The microstructure evolution shown by the comparison among Fig. 4b,d and f is in accordance with XRD results in Fig. 3 confirming that the formation of W-Al intermetallics is controlled by solid-solid diffusion. Furthermore, it can be concluded that long holding time at high temperature are required to form equilibrium intermetallic phases in diffusion controlled phase transformation in m-80Al20W system. Therefore, the Al-W composites could be fabricated by tailoring the temperature, holding time and partial diffusion process.
It is suggested that in Al based binary system such as Fe/Al and Ni/Al, low solid solubility limits the diffusivity. . According to Fick's first law 43 , the diffusion flux is proportional to diffusivity while inversely proportional to diffusion distance. The maximum diffusion distance in Al-W system is defined by particle size. Therefore, reduced particle size should theoretically improve the diffusion controlled phase transformation. In this regard, the starting material was replaced by nano-sized 80Al20W powder (n-80Al20W) and treated with the same PCP route as m-80Al20W. The PCPed n-80Al20W resulted in formation of intermetallic compounds Al 4 W as the dominating phase and Al 5 W, along with trace amount of unreacted W in the sample (Fig. 5). The sample showed a hardness of 1.55 GPa. M.G. Golkovski et al. 44 reported the nanoharness of Al 4 W to be 9-11 GPa. The reason of PCPed n-80Al20W exhibits lower hardness than pure Al 4 W is attributed to the high porosity of the sample 45 . The calculated crystallite size of Al 4 W is 65.9 nm, calculated by Sherrer's equation. High porosity in the PCP compact is revealed in Fig. 5a. The lack of densification arises from the reduced diffusion distance as well as increased contact area between Al and W particles, which allowed massive diffusion of the atoms and vacancies during the diffusion process. Voids are therefore created in the original position of Al and W atoms according to Kirkendall effect 46,47 . As described in Fig. 6, in order to achieve complete phase transformation, the distance that Al and W atoms need to transport and form Al-W intermetallics equals to the diameter of the starting particle diameter. The usage of nano sized particles reduced the diffusion path from >1 µm down to 70 nm, thus leading to equilibrium phase transformation of the raw materials. The thorough phase transformation indicates that, despite of the barrier of phase transformation that is posed by limited diffusivity, the diffusion process in n-80Al20W system is greatly enhanced compare to m-80Al20W with reduced diffusion distance, i.e. particle size in PCP.
In conclusion, a particulate tungsten aluminides/W reinforced Al-MMC has been successfully fabricated by in-situ PCP. The Al-MMC contains uniformly distributed W particulates with Al 4 W and Al 12 W dual-layer structure in Al matrix. The mechanical property of the composite fabricated from m-80Al20W is improved with a hardness of 0.45 GPa in Al matrix and 4.35 GPa for W reinforcements. We have shown that the formation mechanism of W-Al intermetallic compounds is controlled primarily by atomic diffusion process between solid state Al and W. Shorter diffusion path, i.e. smaller starting particle size showed strong effect on the microstructure evolution and phase formation with phase transformation toward equilibrium intermetallic phase Al 4 W. The temperature and time barrier that arise from limited solid state diffusion rate was overcome. The results presented in this work provide a guideline for designing fabrication routes for intermetallic reinforced Al-MMCs.

Methods
Starting materials. Elemental aluminium (APS 1-15 µm, 99.5% purity, Alfa Aesar) and tungsten (<1 µm, 99.95% purity, Alfa Aesar) powders were used as reactant micron-sized powders. Meanwhile, aluminium (70 nm, 99.9% purity, US Research Nanomaterials, Inc.) and tungsten (70 nm, 99.9% purity, US Research Nanomaterials, Inc.) were selected to prepare nano-sized powder precursors. Both micron-and nano-sized powders (denoted as m-80Al20W and n-80Al20W) were prepared by mixing 80 at.% Al − 20 at.% W powders. The elemental powder mixtures were thoroughly mixed in a ball mill for 2 h using 4 mm stainless steel balls at ambient temperature. The weight ratio of ball to powders was 5:1. The environment for ball milling was air and argon for m-80Al20W and n-80Al20W, respectively.
Processing. Pulsed current processing (PCP) for both m-80Al20W and n-80Al20W were conducted on Dr.
Sinter 2050 (Sumitomo Coal Mining Co., Ltd., Japan). The samples were sintered in vacuum by following the same PCP thermal program: the powder was filled into 12 mm graphite die and heated to 550 °C with a heating rate of 100 °C/min, and held for 2 minutes. The pressure employed during sintering was 20 MPa.
The m-80Al20W powder mixture were consolidated into green compacts by a laboratory hydraulic press (Manual Hydraulic Press -15Ton, Specac, England) at a pressure of 5 Ton before performing thermal analysis in Differential Scanning Calorimeter (STA 449 C Jupiter ® , NETZSCH, Germany). The effect of different heating rates was done by performing the thermal program from 30 °C to 1000 °C with different heating rates (0.5, 1, 5, 10 and 20 °C/min, respectively). In order to study the effect of prolonged holding time, the m-80Al20W compacts were heated from 30 °C to 1000 °C with a heating rate of 20 °C/min, but with different holding temperature stages (550 °C, 600 °C, and 650 °C) for 1 hour during the process.

Characterization.
A Matsuzawa MXT-CX microhardness tester (Matsuzawa Co., Japan) was used to measure the hardness of Al-MMC sample fabricated from m-80Al20W in PCP. The applied load for microhardness testing was 50 g. Each hardness value was calculated from the average of 9 indentations. The sintered density was Figure 6. Schematic of the diffusion controlled mechanism in Al-W system as a function of particle size, i.e. diffusion path. By using nano sized particles, the diffusion path is reduced down to 70 nm, which consequently resulted in formation of more equilibrium phase Al 4 W in the composite (minor amount of Al 5 W is not shown here).
Scientific RePoRTS | 7: 12391 | DOI:10.1038/s41598-017-12302-w measured by Archimedes water immersion method. The theoretical density was calculated based on the mixture rule at the assumption of no reaction taking place.
Both the PCP samples and the samples from thermal analysis were characterized by an X-ray diffractometer (Empyrean, PANalytical, United Kingdom) and Cu-Kα radiation with a wavelength of 0.154 nm. The samples was cleaned and polished on the analysed surface before sent to XRD characterization. The XRD patterns were analysed by the software PANalytical X'Pert HighScore Plus to determine the compositions. Thermal analysis was performed using Netzsch TA software.
All the samples were prepared by following standard metallographic procedures for microstructure characterization. Because of different requirement of the image resolution, Scanning Electron Microscopy JSM-IT300 (JEOL, Tokyo, Japan) mounted with an energy dispersive spectrometer (EDS) that is calibrated with Cobalt standard, and Magellan 400 XHR-SEM (FEI Company, Eindhoven, the Netherlands) were used for microstructure observation for samples after thermal analysis and PCP samples.