Conventional Liquid Phase Sintering Research Articles

Effect of High-Energy Ball Milling Time on the Density and Mechanical Properties of W-7%Ni-3%Fe Alloy

The present work was aimed at the investigation of the effect of high-energy ball milling (HEBM) time on the sintering kinetics, structure, and properties of the heavy tungsten alloy (HTA) W-7%Ni-3%Fe. The HTA samples were obtained from nanopowders (20–80 nm) using conventional liquid-phase sintering (LPS) in hydrogen and using spark plasma sintering (SPS) in vacuum. The HTA density was shown to depend non-monotonously on the HEBM time that originates from the formation of nonequilibrium solid solutions in the W-Ni-Fe systems during HEBM. The SPS kinetics of the HTA nanopowders was shown to have a two-stage character, the intensity of which depends on the Coble diffusion creep rate and on the intensity of diffusion of the tungsten atoms in the crystal lattice of the γ-phase. The kinetics of sintering of the initial submicron powders has a single-stage character originating from the intensity of the grain boundary diffusion in the γ-phase. The dependencies of the hardness and of the yield strength on the grain sizes were found to obey the Hall–Petch relation. The hardness, strength, and dynamic strength in the compression tests of the fine-grained tungsten alloys obtained using SPS and LPS were studied.

A theoretical analysis of cold sintering

ABSTRACTCold sintering is an innovative low-temperature processing technique which allows consolidation of several ceramics. Despite recent research activities on the cold sintering of functional and structural ceramics, an analytical study accounting for consolidation and grain growth phenomena is still missing in the literature. In this work, we provide a theoretical analysis of the mechanisms active during cold sintering. The analysis considers two cold sintering approaches, characterised by the application either of isostatic or uniaxial pressure. Physical phenomena and microstructural features are discussed in view of the applied cold sintering approach. The developed pressure-assisted densification models indicate that the processes governing densification during uniaxial cold sintering are more complex than those of conventional liquid phase sintering. A key role is played by the water/material interaction which promotes several effects such as formation of surface defects and secondary phases, dynamic recrystallization and other phenomena still partially unknown.

Microstructure and mechanical properties of a rotary friction welded tungsten heavy alloy

Abstract Tungsten heavy alloys (WHAs) are widely used in many aerospace and defence applications such as radiation shields, kinetic energy penetrators etc. In the current work, attempts were made to join WHA rods (8 mm in diameter) using rotary friction welding techniques. The WHA rods were prepared via conventional liquid phase sintering and had a two phase microstructure containing tungsten particles of spherical shape and surrounded by a solid solution matrix of Fe, Co and Ni. Rotary friction welding process parameters were successfully optimized and a sound weld with a narrow weld interface without any defects was obtained. The weld interface was found to be of a uniform width of around 150 μm throughout the weld interface and showed very fine fragmented tungsten particles with a refined matrix phase. Moreover, the thermomechanically affected zone (TMAZ) elongated tungsten particles were clearly seen to react normally to the compressive force direction. Further, the heat-affected zone width was so narrow that it was not detected in the current study. Microhardness at the interface showed increased hardness when compared with the base material indicating that the microstructural refinement occurred during friction welding. Tensile tests carried out at room temperature as per ASTM-E8, showed lower strength and ductility when compared with the base material (tested along the hot-working direction). The fractography of the fractured surface of the base material and weld sample confirmed that the ductile mode of fracture had occurred.

Effect of Ti(C<sub>0.7</sub>N<sub>0.3</sub>) Content on the Microstructure and Mechanical Properties of Ni Bonded NbC-Ti(C<sub>0.7</sub>N<sub>0.3</sub>) Based Cermets

NbC-xTi (C0.7N0.3)-10Ni-7.5VC (vol%) based cermets with 0, 5, 10, 15 or 25 vol% Ti (C0.7N0.3) were prepared by conventional pressureles liquid phase sintering at 1420°C in vacuum. Detailed microstructural investigation was performed by SEM, EPMA and XRD analysis. Sintering results indicated that the partial replacement of NbC by Ti (C0.7N0.3) had a significant effect on the carbide grain growth, microstructure, hardness as well as fracture toughness of the fully densified NbC-based cermets. The Ti (C0.7N0.3)-free NbC cermet was composed of homogeneous cubic (Nb,V)C solid solution grains, whereas core-rim structured NbC grains were observed in cermets with Ti (C0.7N0.3) addition. All sintered cermets with  15 vol% Ti (C0.7N0.3) were composed of a fcc solid solution Ni binder and a cubic core-rim solid solution (Nb,V,Ti)C phase with a Nb-rich core and a Ti-rich rim. 3.8 vol% of residual pristine Ti (C0.7N0.3) was present in the cermets with 25 vol% Ti (C0.7N0.3) addition. The 15 vol% Ti (C0.7N0.3) starting powder based cermet exhibited the finest average NbC grain size of 1.48 μm, with a core-rim structure and an interesting combination of hardness (1486 kg/mm2) and fracture toughness (8.7 MPa.m1/2).

Effect of Carbon Content on the Microstructure and Mechanical Properties of NbC-Ni Based Cermets

The aim of this work was to correlate the overall carbon content in NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo starting powders with the resulting microstructure, hardness, and fracture toughness of Ni-bonded NbC cermets. A series of NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo cermets with different carbon content were prepared by conventional liquid phase sintering for 1 h at 1420 °C in vacuum. Microstructural analysis of the fully densified cermets was performed by electron probe microanalysis (EPMA) to assess the effect of carbon and VC or Mo additions on the NbC grain growth and morphology. A decreased carbon content in the starting powder mixtures resulted in increased dissolution of Nb, V, and Mo in the Ni binder and a decreased C/Nb ratio in the NbC based carbide phase. The Vickers hardness (HV30) and Palmqvist indentation toughness were found to decrease significantly with an increasing carbon content in the Mo-free cermets, whereas an antagonistic correlation between hardness and toughness was obtained as a function of the Mo-content in Mo-modified NbC cermets. To obtain optimized mechanical properties, methods to control the total carbon content of NbC-Ni mixtures were proposed and the prepared cermets were investigated in detail.

The Influence of Processing Parameters on Mechanical Properties of Spark Plasma Sintered Chromium Carbide Based Cermets

Present paper discusses the influence of spark plasma sintering (SPS) on the microstructure and perfomances of chromium carbide based cermets. The effect of SPS parameters (temperature, pressure) is discussed. It is shown that SPS enables to produce more fine grained chromium carbide based cermets compared to conventional liquid phase sintering. Hardness and fracture toughness are exhibited.

Microstructure and mechanical properties of WC-40Co composite obtained by impact sintering in solid state

The influence of the densification temperature on the density, microstructure, mechanical properties, resistivity and coercivity of WC- 40 wt%Co composite produced by impact sintering has been investigated. The studied material possessed a 54% volume content of cobalt and tungsten carbide particles of initial size 1–3 μm. Impact sintering was carried out in a vacuum of 0.013 Pa in a temperature range covering both the solid and the liquid state: 1150, 1200, 1250 and 1290 °C.The samples were held isothermally for 20 min before consolidation and the time of the impact consolidation was 0.003–0.006 s. For comparison conventional liquid-phase sintering of samples was carried out at 1340 °C for a period of 30 min. The results showed that the impact sintering in the solid state temperature range enabled almost pore-free samples with high mechanical properties to be obtained. In particular, the samples that were impact sintered at 1200 °C have the following mechanical properties: transverse rupture strength (TRS) 3116 ± 156 MPa, fracture toughness 30.3 ± 1.2 MPam1/2, compression strength 3095 ± 106 MPa, proof stress at compression 1682 ± 82 MPa, plastic deformation at compression 11.9 ± 1.3%, strain energy density 302 ± 35 J/cm3 and Vickers hardness 7.7 GPa. The maximum TRS = 3143 ± 63 MPa and the maximum plastic deformation at compression 12.3 ± 1.1% have the samples obtained at the impact sintering temperature of 1250 °C. The impact sintering of samples at 1290 °C leads to decrease in TRS, hardness, compressive strength and proof stress at compression. The second halves of the samples were subjected to annealing at 1250 °C within 180 min after sintering. Additional annealing of the impact sintered samples led to significant changes in specific resistivity and coercivity, increased the fracture toughness, but had negligible influence on the TRS. The compressive strength and plastic deformation decreased after annealing.

Dynamic deformation behavior of 93W-5.6Ni-1.4Fe heavy alloy prepared by spark plasma sintering

In this study, split Hopkinson pressure bar was used to evaluate the dynamic deformation behavior of the 93W–5.6Ni–1.4Fe heavy alloy (93WHA) prepared by spark plasma sintering (SPS) and conventional liquid-phase sintering (CLS). The influence of the microstructural characteristics (such as W grain size, W–W contiguity and volume fraction of the matrix) on the dynamic deformation behavior was investigated. In contrast to the conventional liquid-phase sintered 93WHA, the spark plasma sintered 93WHAs exhibit high yield strength and flow stress during high strain rate compression, due to the decreased mean matrix thickness (the mean matrix thickness is related to the W grain size, W–W contiguity and volume fraction of the matrix). The decreased matrix mean thickness and increased number of grain boundaries in the spark plasma sintered 93WHAs result in an increase of aspect ratio of W grains in the core of the deformed specimen and a decreased width of shear band along the direction of maximum shear stress.

Influential factors on hardness uniformity of Vickers hardness blocks for high hardness range

Reference Vickers hardness blocks for high hardness range were developed from WC 9–Co and WC 6–Co mixtures by two different processes of powder metallurgy, conventional liquid phase sintering in hydrogen and vacuum-sintering with subsequent hot isostatic pressing in one cycle, sinter-HIP, in order to analyse the appropriateness of the manufacturing process and the influence of starting characteristics of the mixtures on hardness uniformity and overall metrological characteristics. WC powder of 150nm grain size and 2.5m2/g specific surface area with the addition of grain grow inhibitors was used as starting material. The emphasis of the research was placed on hardness uniformity of the test surface as it is the most important property placed on hardness blocks. For that purpose the surface of the blocks was divided into radial and circumferential divisions, forming different sections on the block surface. Hardness measurements were performed in each section with HV1 measuring method. A total of 40 indentations were performed, based on which the conclusions about hardness uniformity of Vickers hardness blocks were drawn. Hardness measurements and uniformity were tested by analysis of variance, ANOVA, for single factor in order to determine if significant hardness variations across the block surface were present. From the research conducted it was concluded that hardness distribution across test surfaces of Vickers hardness blocks had a trend according to a process of manufacturing and the amount of binder in the WC–Co mixture. The best material and metrological characteristics were obtained on the block manufactured by sinter-HIP with 9wt.% Co.

Mechanical properties of (Ti,V)C–Ni composite prepared using ultrafine solid-solution (Ti,V)C phase

(Ti,V)C–Ni composite was sintered using ultrafine (Ti,V)C solid-solution powders prepared by mechanically alloying a Ti–V alloy/graphite powder mixture. The (Ti,V)C–Ni powders were sintered to full density at 1400°C, which is significantly lower than that required for the conventional liquid-phase sintering of TiC–Ni or Ti(CN)–Ni powders. The examination of linear shrinkage curve during sintering revealed that the consolidation of the (Ti,V)C–Ni composite proceeds in three distinct steps. Firstly, it shrinks by the solid-state sintering. Afterwards, it expands by CO gas evolution. Finally, Ni melt penetrates into the pores between carbide particles during the liquid-phase sintering. The fracture toughness values of the (Ti,V)C–Ni composite were notably higher than those of TiC–Ni or even Ti(CN)–WC–Ni composites. The improved fracture toughness of the (Ti,V)C–Ni composites is believed to originate from the solid-solution effect of TiC and VC in addition to the lack of core/rim structure, which is common in Ti(CN)–WC–Ni composite.

Comparative study of room temperature and low temperature magnetization and magnetoelectric coupling behavior of Ti and Pr doped BiFeO3

Four different compositions of Ti and Pr doped BiFeO3 samples were synthesized by conventional rapid liquid phase sintering method and their magnetic and magnetoelectric properties were studied. XRD pattern confirms the single phase formation of BiFeO3 for all samples without any impurity. Room temperature magnetic (M–H) curve shows slight increment in the value of magnetization from 0.070 to 0.078emu/gm for pristine BiFeO3 to Pr and Ti co-doped (Bi0.9Pr0.1Fe0.9Ti0.1O3), respectively. The M–H curve at 80K also shows that the value of magnetization increases from 0.08 to 0.10emu/g for pure BiFeO3 to Pr and Ti co-doped (Bi0.9Pr0.1Fe0.9Ti0.1O3). Ferroelectric behavior observed through P–E loop shows enhancement in the value of maximum polarization Pmax from 0.185 to 0.859μC/cm2 for pure BiFeO3 to Pr doped (Bi0.9Pr0.1FeO3), respectively. Room temperature magnetoelectric effect was studied through dynamic method and second order quadratic magneto-electric coefficient (β) was calculated for all the samples. The value of β is found to be maximum 34.72×10−5mV/cmOe2 for Pr, Ti co-doped BiFeO3 (Bi0.9Pr0.1Fe0.9Ti0.1O3) and minimum 12.25×10−5mV/cmOe2 for pristine BiFeO3 (BiFe0.9Ti0.1O3).

High temperature dielectric and magnetic response of Ti and Pr doped BiFeO3 ceramics

Abstract In the present study, four different compositions of Ti and Pr doped BiFeO3 samples were synthesized by conventional rapid liquid phase sintering and their structural, dielectric, ferroelectric and magnetic properties were investigated. High temperature dielectric study shows different anomalous response for all compositions. A major peak in dielectric constant versus temperature curve is observed at 640 K for pristine BiFeO3 which shifts to 545 K for Ti doped (BiFe0.9Ti0.1O3) sample. Moreover, multiple peaks are observed for Pr doped (Bi0.9Pr0.1FeO3) sample in temperature range of 400–800 K. The room temperature P–E loop study shows that the value of maximum polarization increases from 0.185 to 0.859 µC/cm2 for Pr doped (Bi0.9Pr0.1FeO3) sample and also increases the ability of BiFeO3 to withstand higher electric field. Room temperature magnetic (M–H) curve shows a slight increment in the value of magnetization from 0.071 to 0.078 emu/gm for pristine BiFeO3 to Pr and Ti co-doped Bi0.9Pr0.1Fe0.9Ti0.1O3, respectively.

Densification of nano-SiC by ultra-high pressure effects of time, temperature and pressure

High density SiC ceramics with 4 wt.% (in mass) and without Al 2O 3 as sintering additive were fabricated by nano-SiC as raw materials under different technical condition (1000–1300 °C, 4.0–4.5 GPa, 15–35 min) using China-type twain face anvils apparatus. The effect of sintering parameters on the mechanical property of SiC ceramics was studied. The sintered SiC was characterized by XRD, SEM, EDS and micro-hardness instruments. The results indicated that pure SiC could be sintered to high density (SiC sintered at the condition of 4.5 GPa/1250 °C/35 min could reach 96% of its theory density) by ultra-high pressure technique, and Al 2O 3 is a kind of effective sintering additive, SiC doping with only 4% Al 2O 3 could be sintered to full density by ultra-high pressure technique. The grain size was kept in nano-scale, its crystal lattice shrinkage was about 0.45%, while β-SiC has not changed. The micro-hardness and density were increased with increasing sintering temperature, pressure and holding time. The ultra-high pressure method could effectively lower the sintering temperature, shorten the sintering time and reducing the usage of sintering additive. Under 4.5 GPa the lowest sintering temperature was 1300 °C, which was above 600 °C lower than that for conventional liquid-phase sintering and its density is much higher.

Suppression of abnormal grain growth in WC–Co via pre-sintering treatment

When 92WC–8Co (wt%) samples with WC powder of 0.4 μm size were sintered by conventional liquid phase sintering at 1400 °C, densification to more than 99% of the relative density was obtained after only 1 h of sintering and abnormal grain growth occurred. However, when similar samples were pre-sintered in a solid state before liquid phase sintering, the samples were densified to more than 99% of the relative density without the formation of any abnormal grains within 30 min of liquid phase sintering. As the grain growth behavior during liquid phase sintering was similar for the two kinds of samples, these findings show that pre-sintering in the solid state is an effective means of avoiding abnormal grain growth in submicron-sized WC–Co cemented carbides. As a result of the suppression of abnormal grain growth with full densification, the Vickers hardness of the samples pre-sintered and sintered for 30 min was higher than that of conventionally sintered samples.

Tungsten carbide platelet-containing cemented carbide with yttrium containing dispersed phase

A fine and platelet tungsten carbide patterned structure with fine yttrium containing dispersed phase was observed in liquid phase sintered WC-20%Co-1%Y 2O 3 cemented carbide with ultrafine tungsten carbide and nano yttrium oxide as starting materials. By comparing the microstructures of the alloy prepared by hot-press at the temperature below the eutectic melting temperature and by conventional liquid phase sintering, it is shown that hexagonal and truncated trigonal plate-like WC grains are formed through the mechanism of dissolution-precipitation (recrystallization) at the stage of liquid phase sintering. Yttrium in the addition form of oxide exhibits good ability in inhibiting the discontinuous or inhomogeneous WC grain growth in the alloy at the stage of solid phase sintering.

VC-doped WC–NbC–Co hardmetals

The influence of the addition of 0.9, 1.8, 3.6 and 5.4 wt.% VC on the microstructure and mechanical properties of WC–NbC–12 wt.% Co with 10, 20, 30 or 40 wt.% NbC hardmetals prepared by conventional liquid phase sintering in vacuum for 1 h at 1420 °C was investigated. Microstructural analysis revealed that the WC grain size as well as the amount of residual WC–Co in the sintered hardmetals was strongly influenced by the VC and NbC content. The WC grain growth could be strongly limited and even completely inhibited by the combined addition of NbC and VC, whereas the NbC phase converted into a (Nb,W,V)C solid solution during sintering. Microstructural investigation was supported by thermodynamic equilibrium calculations to predict the composition of the NbC phase as a function of the NbC and VC content in the starting powder. The fracture toughness and hardness of the hardmetals were evaluated and explained in terms of the final WC grain size and the residual WC–Co fraction in the hardmetal.

VC- and Cr3C2-doped WC–NbC–Co hardmetals

This study compares the microstructure and mechanical properties of plain and 0.9 or 3.6 wt% VC- or Cr3C2-doped WC–12 wt% Co hardmetals with 40 wt% NbC, prepared by pulsed electric current sintering (PECS) in the solid state for 4 min at 1240 °C and conventional pressureless liquid phase sintering (CS) for 1 h at 1420 °C. The addition of VC or Cr3C2 was found to inhibit grain growth of the residual WC grains, whereas the size of the solid solution (Nb,W,V/Cr)C grains was hardly influenced. The type of grain growth inhibitor and densification temperature however, strongly influenced the composition of the NbC solid solution formed, which was thermodynamically and experimentally assessed.

AlN ceramics prepared by high-pressure sintering with La2O3 as a sintering aid

Abstract Aluminum nitride ceramics with La 2 O 3 as a sintering aid were obtained under high pressure (5.0 GPa) in a cubic anvil high-pressure and high-temperature apparatus. The results showed that the method could effectively lower the sintering temperature and shorten the sintering time. Under 5.0 GPa, the lowest sintering temperature was 1300 °C, which was above 300 °C lower than that for conventional liquid-phase sintering. The lattice parameters of the specimens sintered at high pressure were found to decrease by about 0.09% than that of the AlN powder, and residual compressive stress was found, while hexagonal lattice of AlN was not changed.

Effects of Cr3C2 additions on the densification, grain growth and properties of ultrafine WC–11Co composites by spark plasma sintering

Abstract Cemented carbide powders based on WC–11Co with a grain size of about 250 nm added various extents of Cr 3 C 2 were consolidated to full density by spark plasma sintering (SPS). The density, hardness and fracture toughness of the SPS consolidated samples were measured and the microstructure, fracture surfaces and grain size were observed by SEM. XRD investigation of the spark plasma sintered WC–Co samples revealed that no new phase formed. The WC–11Co composite powder were fully densified within 5 min by SPS at the temperature of 1200 °C, which is much lower temperature than that of conventional liquid phase sintering, under a pressure of 40 MPa. Addition of grain growth inhibitors Cr 3 C 2 reduced the grain size to 435 nm, but led to the lower density. The adding amount of Cr 3 C 2 should be controlled within a certain range, and the samples adding the appropriate proportion of Cr 3 C 2 can obtain higher hardness and toughness.

Low-temperature sintering of high-density aluminium nitride ceramics without additives at high pressure

High-density AlN ceramics were fabricated without additives under high pressure (3.15–4.95 GPa) at a low temperature (1300 °C), which is 300–500 °C lower than that for conventional liquid-phase sintering, in a cubic anvil high-pressure and high-temperature apparatus. The relative density of pure AlN ceramics is 94.92% at 1300 °C and 4.75 GPa for 50 min. Grain boundary phases were found in the neck area between AlN grains, indicating that grain growth by solution–precipitation must play some role.