The present work reports the role of different atomic mobility induced localized pore evolution on densification during spark plasma sintering of nanocrystalline W-Mo alloy powder. The shrinkage (or expansion) behavior of cold compacted milled powders was studied using dilatometry during non-isothermal sintering up to 1600 °C. Subsequently, the milled powders were densified to ~95% relative density using spark plasma sintering up to 1600 °C. The enhanced localized Joule heating due to dynamically evolved porous structure could be attributed for the densification during spark plasma sintering. © 2018 Elsevier Ltd

Budaraju Srinivasa Murty

Department of Metallurgical and Materials Engineering

N. S. Karthiselva

Binary alloys

Densification

Molybdenum alloys

Nanocrystalline powders

Nanocrystals

nanoindentation

Porosity

Powders

Spark plasma sintering

Tungsten alloys

Atomic mobility

KIRKENDALL POROSITY

MILLED POWDERS

Nanocrystallines

Non-isothermal sintering

PORE EVOLUTION

Porous structures

Relative density

Nanocrystalline alloys

Fulltext

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IIT Madras

Scripta Materialia

In the present investigation, nanocrystalline W and W-xRe (x = 3, 5 wt.%) alloy powders were produced by mechanical milling/alloying using high energy ball milling. The nanocrystalline nature (∼50 nm) of these powders was validated by the Rietveld refinement of their respective X-Ray diffraction patterns. Subsequently, spark plasma sintering of the ball milled powders was carried out. It was observed that pure W was not able to densify completely (relative density of 93%) at a temperature of 1500 °C. However, the addition of 5 wt.% Re resulted in near complete densification (relative density of 97%) at the same sintering temperature. The enhanced densification of W-Re powders is mainly attributed to the ductilising effect of Re assisted by the nanocrystallinity of powders, and the application of pressure during sintering. © 2019 The Society of Powder Technology Japan

Advanced Powder Technology

Effect of Re on microstructural evolution and densification kinetics during spark plasma sintering of nanocrystalline W

Simultaneous spark plasma sintering (SPS) of metal powders (Fe and Ni) with their respective aluminide powders (FeAl and NiAl) was attempted to produce a cylindrical aluminide coating on a cylindrical metal core. The interdiffusion zone (IDZ) formed between the metal and the aluminide was analyzed to evaluate the diffusion coefficient. The composition variation and the actual temperature estimated at the interface were taken into consideration while evaluating the diffusion coefficient. The diffusion coefficients estimated based on the IDZ are found to be higher than those obtained by conventional methods but are in agreement with those obtained from previous SPS studies. © 2019 Elsevier Ltd

An investigation on diffusivity while achieving a cylindrical aluminide coating on metals using simultaneous spark plasma sintering of powders

A novel oriented attachment growth mechanism of nanograins has been suggested for spark plasma sintering and annealing of NiO nanopowder. A hierarchical microstructure built by cube-shaped nanograins was observed during pressure-less spark plasma sintering at temperatures above 1000 °C and when the powders were annealed in ambient atmosphere at and above 700 °C. Irrespective of the processing method used, a rotation assisted grain attachment of irregularly shaped nanocrystals results in such a microstructure with parallel epitaxy, twinning or line defects at the grain interface. The unique microstructural features and the governing factors for the attachment process have been discussed. © 2017

Journal of the European Ceramic Society

Novel coalescence-driven grain-growth mechanism during annealing/spark plasma sintering of NiO nanocrystals

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. © 2016 Elsevier B.V.

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

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. © 2014 Acta Materialia Inc.

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

The densification kinetics during spark plasma sintering (SPS) of FeAl and NiAl powders were analyzed using a model proposed by Bernard-Granger and Guizard [10]. Creep parameters obtained through densification data are in good agreement with those obtained from conventional creep experiments. Validity of the model was illustrated for aluminides in the form of deformation mechanism maps. This validation assures plausible confidence to predict creep behavior using densification data obtained during pressure assisted sintering of metallic alloys. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

On correlation between densification kinetics during spark plasma sintering and compressive creep of B2 aluminides

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 ∼0.3Tm. This saturation in resistance is attributed primarily to the Joule heating that occurs at the graphite-foil and punch in an SPS system. © 2014 Acta Materialia Inc. All rights reserved.

Journal	Scripta Materialia
Publisher	Acta Materialia Inc
ISSN	13596462
Open Access	No