Ultrahigh transverse rupture strength in tungsten-based nanocomposites with minimal lattice misfit and dual microstructure
Graphical abstract
Introduction
Tungsten is one of the most promising candidate materials for use in plasma-facing components in fusion reactors [1]. Moreover, tungsten is used for a wide variety of high-temperature applications in the strategic sector due to its high melting temperature (3673 K), high hot strength and hardness (300 MPa and 250 Hv at ~2773 K), good erosion resistance at temperatures above 2273 K (~ 0.05 mm/s) and good thermal conductivity (0.25 Cal/cm.s.K), all of which are critical for structural applications in extreme environments [[1], [2], [3], [4]]. However, the intrinsic plasticity issue in tungsten and their poor fracture resistance affects their performance [5]. The production of a fine and uniform grain size after consolidation is one of the main metallurgical approaches to achieving reasonable ductility and strength in tungsten and other metals and alloys [6,7]. To this end, the addition of second phase oxide or carbide particles has been widely studied with improvements in sinterability and mechanical properties reported for tungsten and its alloys [[8], [9], [10], [11], [12], [13], [14]]. Oxides such as yttria, zirconia, hafnia and lanthanum oxide exhibit finer grain sizes with excellent mechanical properties [8,9]. Unfortunately, agglomeration of the oxides can occur during processing, causing a drastic fall in the toughness. Alternatively, transition metal carbides such as ZrC, TiC, HfC and TaC have been observed to improve the strength and hardness of tungsten [[10], [11], [12], [13], [14], [15]] and also “clean” the grain boundaries by acting as scavengers for oxygen that might otherwise segregate there [16]. However, to our knowledge, no microstructural evidence to substantiate these claims has yet been presented.
Consolidation of tungsten and its alloys and composites have conventionally been carried out by pre-sintering followed by hot rolling [17] at temperatures above 2573 K. The resultant elongated grains leads to anisotropic properties along different directions which can be partially overcome by using a high-temperature cross-rolling process [18]. However, the rolling practices are energy-intensive, time-consuming and expensive for industrial scale operations. Simpler and more convenient powder metallurgy (PM) routes such as hot pressing, hot isostatic pressing, microwave sintering, spark plasma sintering (SPS) and even pressure-less sintering have been adopted to overcome the disadvantages of the conventional ingot processing [[19], [20], [21], [22], [23]]. It is observed that the key focus of most if not all of these studies are the materials processing details. Much less investigation into the finer microstructural aspects is available. Details on the micro-deformation mechanisms of the tungsten matrix and the intra- or inter-granular microanalysis around grain and interface boundaries, which is so critical in determining the strengthening mechanism of tungsten, has been largely overlooked. Moreover, there is a paucity of information available on the issues around the scalability of the processes in an industrial context and the plausibility of actual components for use in real applications.
The goal of this work was to develop a novel process involving milling-reduction-SPS for the production of a W-TaC sintered composite having a density ~ 19.00 gcc−1, a fine and uniform grain structure, a room temperature transverse rupture strength (TRS) or fracture strength ≥750 MPa and Vickers hardness ≥500 HV10. Achieving these room temperature mechanical properties qualifies the composite as a potential material for actual high-temperature structural applications [24]. Plausible as it might be to fabricate a tungsten alloy with these properties, we sought, critically, to manufacture medium-sized engineering components such as a disc of diameter ~ 100 mm and thickness ~ 10 mm using the SPS process. Such samples can be machined into near-net shaped components for real-time applications, which help reduce the material and energy consumption and reduce the overall cost of the components. TaC was selected as the additive for this study because it possesses a melting point of 4073 K [25], higher than the other refractory carbides such as HfC, ZrC, Cr2C3 and WC, providing thermal stability of the composite in excess of 3273 K whilst also improving the high-temperature erosion resistance [25]. Attempt was made to utilize the intrinsic advantages of the SPS process viz. shorter sintering time and lower sintering temperatures along with the role of the additive to retain a fine grain size and high density, both of which significantly affect the mechanical properties of the composites. Advanced microscopy approaches and molecular dynamics simulations were applied to understand the relationships between the process, microstructure and properties.
Section snippets
Processing of tungsten sheets
Commercial W powder (Swastik Tungsten Pvt. Ltd., Ahmednagar, India) with purity 99.9% and particle size ~4.5 μm was used for the current investigation. The TaC powder (Sigma Aldrich, Bangalore, India) had purity 99.5% and particle size ~1.0 μm. Chemical analysis of the commercial W powder by ICP-OES, C-S and O-N analyzer reveal the following impurities (in ppm): Ca = 31, Na + K = 18, Mg + Si = 8, Al + Sn = 6, Cu + Mn = 7, C = 12, S = 6, and O = 850. The compositions selected for the current
Process optimization and mechanical properties
The density, grain size, hardness and TRS along with the standard deviations for the pure tungsten WT0 samples sintered under different SPS temperatures and applied stresses are summarized in Table 2. As the density obtained for samples sintered at 1773 are only between 16 and 17 gcc−1 (83–88% of theoretical) they were not investigated further. From the table it is clear that temperatures exceeding 1923 K lead to large grain sizes along with a reduction in the hardness and strength, following
Conclusions
A novel W-TaC-Ta2O5 nanocomposite is fabricated on a large scale (100 mm diameter and 10 mm thick) by a simple powder metallurgy process comprising milling-reduction-sintering, alleviating the drawbacks of the conventional hot-rolling process. Experimental evidence of the well-known ‘O-scavenging’ effect of carbide particles in a W-MC (M = Zr, Hf, Ta, Ti) composite is provided by the atom probe tomography investigations. The cluster analysis shows no segregation or clustering of O-atoms at the
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
DC would like to acknowledge V. Lokesh for helping to prepare the tungsten samples. He would also acknowledge the Department of Science and Technology, Govt. of India for funding the SPS facility at ARCI. The authors gratefully acknowledge the National facility for Atom Probe Tomography (NFAPT) for carrying out the APT studies. This research did not receive any specific fund from funding agencies in the public, commercial, or not-for-profit sectors.
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