Microstructure evolution and densification during spark plasma sintering of nanocrystalline W-5wt.%Ta alloy

The present work reports the effect of Ta on densification and microstructure evolution during non-isothermal and spark plasma sintering of nanocrystalline W. Nanocrystalline W-5wt.%Ta alloy powder was synthesized using mechanical alloying. The nanocrystalline powder was characterized thoroughly using X-ray diffraction line profile analysis. Furthermore, the shrinkage behavior of nanocrystalline powder was investigated during non-isothermal sintering using dilatometry. Subsequently, the alloy powder was consolidated using spark plasma sintering up to 1600 {\deg}C. The role of Ta on stabilizing the microstructure during spark plasma sintering of nanocrystalline W was investigated in detail using electron backscatter diffraction. The average grain size of spark plasma sintered W-5wt.%Ta alloy was observed as 1.73 micron.Comment: 14 pages, 3 figure

Dilatometric analysis on shrinkage behavior during non-isothermal sintering of nanocrystalline tungsten mechanically alloyed with molybdenum

The paper attempts to study the shrinkage behavior of nanocrystalline tungsten mechanically alloyed with molybdenum (5, 10, 15 and 20 wt.%). The dilatometric analysis was performed by Setsys Evolution TMA (ambient to 1600°C) using Constant Heating Rate (CHR) method. The significant improvement in shrinkage with alloying of molybdenum is attributed to reduced grain size, lowered tungsten carbide contamination and enhanced diffusion kinetics. The initial stage sintering kinetics of W–20Mo alloy has been investigated. The densification starts with Mo diffusion (calculated activation energy = 128 kJ/mol) and proceeds with the diffusion of both along the grain boundaries (calculated activation energy = 307 ± 1 kJ/mol)

Grain-size-dependent non-monotonic lattice parameter variation in nanocrystalline W: the role of non-equilibrium grain boundary structure

The grain size dependency of lattice parameter during high-energy ball milling of W has been investigated. The lattice parameter varies non-monotonically with grain size during milling with a lattice contraction initially followed by an expansion. The lattice parameters were calculated in view of the non-equilibrium grain boundary structure that evolved during milling using excess free volume and the interfacial stresses at the grain boundaries. The calculated lattice parameters closely match the experimentally observed values

Initial-stage sintering kinetics of nanocrystalline tungsten

Initial-stage sintering kinetics of nanocrystalline tungsten has been studied in the temperature range of 1273-1473 K (1000-1200 °C). Nanocrystalline tungsten sinters initially through a grain boundary diffusion mechanism. The calculated activation energy was 388 ± 11 kJ/mol at low temperatures (1273-1373 K (1000-1100 °C)) and 409 ± 7 kJ/mol at high temperatures (1373-1473 K (1100-1200 °C)), which are close to the experimentally measured activation energy for grain boundary diffusion (385 kJ/mol)

XRD characterization of microstructural evolution during mechanical alloying of W-20 wt%Mo

The present paper reports a study on the microstructural evolution during mechanical alloying of W-20wt%Mo using XRD line profile analysis. The W-20wt%Mo powder blends were ball milled using Fritsch Pulverisette P-5 high energy ball mill for varying milling time (0, 5, 10, 15, 20 h) at 300 rpm with 10:1 and 2:1 ball to powder weight ratio. The crystallite size and lattice strain were calculated using Langford method (‘average’ Williamson-Hall Plot). The precision lattice parameter calculation was done using Nelson–Riley method. The effect of milling on alloying behaviour has been investigated using change in relative integrated intensity ratios (IMo/IW/W(Mo)) of (211) XRD peak. There was complete alloying in case of 10:1 ball to powder weight ratio, whereas half of the Mo (∼10% Mo) was alloyed with 2:1 ball to powder weight ratio. The paper also discusses the applicability of the criteria based on lattice parameter change (peak shift) for alloying behaviour during mechanical milling

Crystallographic-shear-phase-driven W₁₈O₄₉ nanowires growth on nanocrystalline W surfaces

The present paper reports the growth and branching mechanism of W18O49 nanowires grown on nanocrystalline tungsten (W) surfaces. Nanocrystalline W powder was prepared using high energy ball milling. The nanopowder was non-isothermally heat-treated up to 1400°C in argon atmosphere. The nanowires of 30–50 nm diameter were observed on nanopowder surfaces after the heat-treatment. A model based on crystallographic shear phase formation to accommodate the oxygen deficiency during the reduction of WO3 is discussed to explain the growth and branching mechanism of these nanowires which correlates well with the observed angle for the branching

On Joule heating during spark plasma sintering of metal powders

Joule heating as a primary heating source mechanism was probed during Spark Plasma Sintering (SPS) of pure metal powders (Fe, Ni and Cu). Resistance to electric path was estimated from voltage–current measurements obtained online during these experiments. Resistance was observed to saturate at the same value irrespective of the type of metal powder, after attaining a sintering temperature of ∼0.3T_m. This saturation in resistance is attributed primarily to the Joule heating that occurs at the graphite-foil and punch in an SPS system