Tensile behavior of hybrid tungsten composites with zirconium carbide nanoparticles and tungsten fibers

Highlights

• Hybrid tungsten composites reinforced with ZrC_p and W_f were developed by the conventional powder metallurgy process

• The fracture energy was maximized by co-introducing 0.2 wt% ZrC_p with 20 wt% W_f

• In addition to extrinsic toughening, the fibers promote the fine-grain toughening mechanism by refining the matrix grains

• ZrC exerts contradictory effects on W–ZrC_p–W_f composite, increasing fracture energy of the matrix and interfacial strength

Abstract

Hybrid tungsten composites reinforced with zirconium carbide (ZrC) nanoparticles and tungsten fibers were developed by the conventional powder metallurgy process (ball-mill mixing of powders and fibers followed by spark plasma sintering). The synergistic and mutual influences of the fibers and nanoparticles on tungsten were investigated in tensile behavior and fracture-energy tests. Aided by the ZrC nanoparticles, up to 30% of the fibers could be embedded in the tungsten matrix. The fracture energy was maximized by co-introducing 0.2 wt% ZrC particles with 20 wt% short tungsten fibers. The fracture-energy enhancement of the short fibers is contributed by pseudo-toughness from the fiber–matrix interface, inherent toughness from the fibers themselves, and grain refinement (by 50%) of the tungsten matrix. The fracture energy of the composite is very sensitive to the ZrC content, because the two-way action of ZrC weakens the pseudo-toughness of the interface energy.

Introduction

Owing to its high thermal conductivity, sputtering threshold and melting point, tungsten is the leading candidate material for future fusion reactors and spallation targets in accelerator driven systems (ADSs) [1,2]. However, tungsten is brittle and is degraded by recrystallization embrittlement and heat load cracking [3,4]. Consequently, it cannot withstand continuous heat shock and is unsuitable for fusion reactor and ADS applications, where large heat load and intensive irradiation production are normal.

Tungsten-based materials with high strength and low-temperature toughness are a promising solution to these problems [5,6]. High strength protects against crack formation, and low-temperature toughness enables the stress release via plastic deformation rather than cracking. Currently, tungsten-based materials are modified by alloy/dispersion strengthening, minimizing grain sizes and fiber reinforcement [[7], [8], [9], [10], [11], [12]]. Carbides such as ZrC appear to stabilize tungsten material and elevate its mechanical properties [9,10], and fiber-reinforced tungsten composites exert a pseudo-toughening effect that promises to strengthen tungsten [11,12]. The enhancing mechanisms of these two prospective solutions are very different. In carbide-dispersion strengthened tungsten-based materials (CDSW), nano-carbides react with oxygen to form thermally stable nano-oxide particles, which purify and strengthen the tungsten grain boundaries, establishing stabilized microstructures with good toughness [9]. By comparison, fiber-reinforced (W_f/W) composites have aroused attention for their pseudo-toughness capability. By utilizing the energy dissipation from interfacial debonding and sliding and the plastic deformation of fibers, W_f/W composites show ductile fracture behaviors [11].

Although both of these approaches have improved the properties of tungsten, the performances of tungsten reinforced by either method have not met application requirements. As pointed out by Zhang and Xie [9], very small carbides (of size ≤10 nm) are favored as the strengthening second-phase in future applications of CDS-W alloy. However, the formation of stably ultrafine second-phase particles is very challenging. Meanwhile, the fibers of W_f/W composites are usually pre-coated to maximize their profitable interfacial toughness. W_f/W composites are suitable for small and medium samples but not for large-scale production through the powder metallurgy (PM) route. In W_f/W composite bulks formed by PM and reinforced with bare short fibers, the fracture energies are non-ideal at low temperatures [12].

Reports on other material systems have revealed that combining fibers with a few particles improves the mechanical properties over those of the individual components. For example, combined WO₃ particles and Al₁₈B₄O₃₃ whiskers achieved higher ultimate tensile strength and elongation than aluminum [13]. The improvement increased with decreasing particle/whisker ratio, indicating that the fibers supplemented with a small amount of particles achieved high performance. Carbon nanoparticles significantly increase the fatigue life of carbon fiber-reinforced epoxy, because they induce enormous plastic deformations of the matrix [14]. Reinforcement with Al₂O₃ fibers and particles significantly improves the yield and ultimate strengths of magnesium matrix composites, but reduces their elongation [15]. Nanoparticles and fibers have also improved the properties of concrete [16].

Inspired by the above researches, we considered that a combination of CDS-W and W_f/W might yield a hybrid tungsten-based material with strong mechanical properties. Zirconium carbides have proven their excellence as matrix reinforcements [10], and zirconia interfaces of tungsten fibers are stable under heat loads [17,18]. However, the synergistic and mutual influences of both enhancements are unknown. Understanding these influences is essential for developing advanced tungsten materials by multiple methods. In the present article, we therefore introduce nano ZrC and W fibers into a tungsten matrix, and fabricate small samples by PM and spark plasma sintering (SPS). Focusing on tensile behavior, we reveal the pros and cons of the particle and fiber combination on the tungsten materials under loading.