Subsequent Liquid Phase Sintering Research Articles

Microstructure and mechanical properties of WC-(Ti,W)C-Co hardmetals fabricated by in-situ carbothermal reduction of oxides and subsequent liquid phase sintering

WC-(Ti,W)C-Co hardmetal was fabricated using carbon black, WC, Co, WO3, TiO2 as raw materials, through an energy-efficient and cost-efficient method that the process of carbothermal reduction of oxides and the liquid phase sintering of hardmetal were combined in one heating cycle. Phase transformation and microstructure characteristics of the hardmetals were examined by XRD, SEM, EDX, and TEM. Moreover, mechanical properties were tested with related mechanical property testing instruments. Results demonstrated that the in-situ carbothermal reactions and outgassing processes had finished below 1200 ℃. Microstructures of hardmetal consisted of WC grains, (Ti,W)C solid solution, and Co binder phase. Moreover, the mean size of WC and (Ti,W)C grains were both smaller than those of hardmetal prepared with TiC powders. A coherent relationship between Co and (Ti,W)C grain was noticed. Moreover, a coherent relationship between two (Ti,W)C grains was observed. Compared with hardmetal prepared with TiC powders, TRS and KIC were improved. The increase of TRS was chiefly due to the smaller grain size and the coherent interface structure. Moreover, the increase of KIC was ascribed to the (Ti,W)C solid solution and the coherent interface structure. Hardmetals prepared through this method showed good mechanical properties with a hardness of 90.7 ± 0.2 HRA, a TRS of 3007 ± 87 MPa, a KIC of 13.1 ± 0.2 MPa·m1/2.

Товщина покриття відновленого контакту електричного апарату та методика її визначення

The regularities of the formation of the main variants of coatings obtained by gas-plasma spraying with subsequent liquid-phase sintering, depending on the conditions of wetting the powder particles with the melt and the porosity of the sprayed layer, have been established. The analysis of methods for controlling the thickness of thermal gas coatings, which indicates the advantages and disadvantages of three methods of measuring the thickness: magnetic, electromagnetic (eddy currents) and metallographic. The research of the dependence of the thermal power of the plasma jet (directly affects the thickness of the sprayed coating) on the thermal equivalent of the electric power of the arc was carried out; dependence is characterized by an effective efficiency. As a result of the research, the thickness of the sprayed coatings of the contacts and the tolerances for their processing were calculated depending on the height of the contact piece. Coating thickness is the shortest distance between the coating surface at a given point and the base metal surface. As a technological characteristic, the thickness affects such an important performance property as the strength of the bond with the base metal. Increasing the thickness of the coating above the optimum leads not only to a deterioration in quality (decrease in bond strength), but also to economic costs. The coating was applied to a given area due to the linear movement of the spray device in combination with either the transverse displacement of the spray device or contact piece. With a single linear movement of the spray device, the sprayed material forms a uniform coating layer, the cross-section of which is described by the Gaussian curve. The requirements for the thickness of the sprayed layer are indicated in the technological documentation, based on the uniformity of the distribution of the coating thickness on the contact surface. The required thickness of the sprayed coating was determined based on the thickness of the working layer, the tolerance for subsequent processing and the tolerance for the dimensions of the contact piece and the thickness of the sprayed layer. The thickness of the sprayed coating on the contacts, in which the lost dimensions are restored, provided for a tolerance for their following processing: turning and grinding. As a result, the thicknesses of the sprayed coatings of the contacts and the tolerances for their machining were calculated depending on the height of the contact piece (the number of switching cycles of the sprayed contact pieces). In thermal spraying of coatings, the build-up of parts occurs at a relatively high rate, that is, approximately 10 - 20 times faster than electrolytic deposition of steel and approximately 100 - 200 times faster than electrolytic chromium plating.

On the use of TiO2 in Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni cermets

In this study, Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni cermets with varying [Mo]/([W] + [Mo]) mole ratios and constant [W] + [Mo] content were prepared by using TiO2 and carbon as substitutes for a part of Ti(C,N). In situ carbothermal reaction took place with subsequent liquid phase sintering. The thermal behavior and outgassing behavior were conducted using differential thermal analysis/thermal gravity analysis (DTA-TG) and mass-spectrometric evolving gas analysis (MS-EGA). The microstructure was investigated by scanning electron microscopy (SEM). After being sintered in N2 atmosphere, cermets without TiO2 additions generally showed rimless microstructures with black grains independently distributed from other phases, whereas the cermets with TiO2 addition showed complete core-rim microstructures. The average grain size generally decreased with increasing Mo content, and increased with increasing TiO2 substitution. Moreover, the average grain size increased with decreased N2 pressure from 50 mbar to 10 mbar at any [Mo]/([Mo] + [W]) ratio. The most interesting result was for the Mo-free cermets with 5 mol% TiO2 and 10 mol% TiO2 addition sintered in 50 mbar N2: both the hardness and fracture toughness increased compared with the cermets without TiO2 addition. The HV-KIC behavior of the majority of the cermets was superior to the industrial grades.

Additive manufacturing of magnesium–zinc–zirconium (ZK) alloys via capillary-mediated binderless three-dimensional printing

Several challenges are involved in employing fusion-based additive manufacturing (AM) technologies for magnesium (Mg) alloys. In this study, a novel 3D printing (3DP) technique was used to additively manufacture a green component made of Mg-5.9Zn-0.13Zr powder at ambient temperature that can be densified during subsequent liquid phase sintering, obtaining a functional part with zero process contaminants. Density measurement, microstructural, and mechanical investigations of the green components sintered for 5 h at different temperatures ranging from 535 °C to 610 °C indicated an improvement in properties with an increase in sintering temperature. Swelling during sintering was observed at temperatures ≥ 580 °C. Sintering at 573 °C with an extended holding time up to 60 h resulted in comparable density, compressive properties, and elastic modulus similar to that of human cortical bone. Furthermore, mercury porosimetry analysis revealed that additively manufactured Mg components could provide pores characteristics matching with those found in human bone.

Microstructure and mechanical properties of Ti(C,N)-based cermets fabricated by in situ carbothermal reduction of TiO2 and subsequent liquid phase sintering

Ti(C,N)-based cermets were prepared by in situ carbothermal reduction of TiO2 and subsequent liquid phase sintering in one single process in vacuum. The densification behavior, phase transformation, and microstructure evolution of the cermets were investigated by DSC, XRD, SEM, and EDX. The results showed that the carbothermal reduction of TiO2 was completed below 1250°C, and Ti(C,N)-based cermets with refined grains were obtained after sintered at 1400°C for 1h by this method. The hard phase of the cermets mainly exhibited white core/gray rim structure, in great contrast to the typical black core/gray rim structure of hard phase in traditional cermets. Ti(C,N)-based cermets prepared by this novel method showed excellent mechanical properties with a transverse rupture strength of 2516±55MPa, a Rockwell hardness of 88.6±0.1 HRA, and a fracture toughness of 18.4±0.7MPam1/2, respectively.

Microstructure and mechanical properties of a new group of nanocrystalline medical-grade stainless steels prepared by powder metallurgy

This paper focuses on the structure and mechanical properties of powder metallurgy stainless steels (Fe–Cr–Mn–Mo–Si–N–C) developed for biomedical applications. The samples were prepared by mechanical alloying and subsequent liquid-phase sintering with a eutectic Mn–Si alloy additive. By changing the sintering aid content, the pore configuration, compressive strengths, and impact properties of the samples were assessed. The Rietveld X-ray diffraction analysis showed after sintering at 1050°C for 60min followed by water-quenching, a nanocrystalline austenitic structure was formed in the material. According to the mechanical experiments, by increasing the additive content from 0wt% to 6wt%, sintering densification, yield stress, compression strength, and absorbed impact energy were improved, where spoiling occurred when adding 8wt% additive. Also, as realized from the impact fracture surface features, despite the presence of some unmelted additive particles, the role of the pore elimination in toughness prevailed over that of these particles.

Fabrication of Nanostructured Medical-Grade Stainless Steel by Mechanical Alloying and Subsequent Liquid-Phase Sintering

This article focuses on the microstructure of medical-grade P558 (ASTM F2581) stainless steel produced by mechanical alloying and liquid-phase sintering. Rietveld X-ray diffraction and transmission electron microscopy reflect that the mechanically alloyed stainless steel powder is a nanocrystal dispersed amorphous matrix composite. Mn-11.5 wt pct Si eutectic alloy as additive improves densification of the synthesized P558 alloy via liquid-phase sintering mechanism. X-ray mapping shows that after sintering at 1323 K (1050 °C) for 1 hour, a uniform distribution of dissolved Mn and Si is achieved. Moreover, the development of a nanostructured, fully austenitic stainless steel after sintering at the same temperature is realized by X-ray diffraction and transmission electron microscopy.

Thermodynamics of TiC- and Ti(C,N)-based cermet processing prior to liquid phase sintering stage

The thermodynamics of the various events occurring during heating to liquid phase sintering temperatures of powder compacts of TiC- and Ti(C,N)-based cermets has been investigated. The results show that via a series of dissolution, solute repartitioning and mixed rim formation reactions, Mo 2C can not only help keep the systems free of oxides and residual oxygen but also moderate the Ti content in the Ni binder phase during the heating stage, thus prevent the formation of intermetallic compound and ensure good wetting in the subsequent liquid phase sintering stage.

Preparation of Functionally Graded W/Cu Interlayers and Brazing to CuCrZr and CFC for Actively Cooled Flat Tile Divertor Mock-Ups

The designed plasma facing materials for the divertor-components in ITER are up to now carbon (graphite or CFC) and tungsten. The heat flux loading can result in temperatures of up to 550°C at the interface between the plasma facing material and the CuCrZr heat sink, an operation temperature which is too high for CuCrZr. Additionally the temperature gradient and the mismatch in thermal expansion (CTE) of both parts of the divertor result in high stresses at the interface. A tungsten-copper composite material with a gradation from pure copper on the one side to pure tungsten on the opposite side is supposed to support the stress reduction. The tungsten contributes to the strength of the composite, whereas the copper provides the required thermal conductivity (TC) of at least 200 W/mK. The graded W/Cu layers were produced by the “chemical mixing” method and subsequent liquid phase sintering. Here the use of sub-µm W-particle sizes offers an opportunity to gain well dispersed W-particles with high Cu-containing composites resulting in lower CTE values and higher thermal conductivities compared to coarser W-powders. Brazing of the functionally graded W/Cu interlayers to CuCrZr and CFC materials resulted in adjoined mock-ups.

High-porous materials made from alloy steel fibers: Production, structure, and mechanical properties

New high-porous corrosion-resistant materials are produced from steel fibers and a base porous material made by pressing and sintering of St.10 steel fibers. The materials obtained by chromizing (4.8–7.1 wt.%) and nickelizing (2.1–2.4 wt.%) a 60–90%-porosity base material have yield strength higher by a factor of 2 to 6 than that of the base material. The materials obtained by electrochemical coating of a base fibrous material with eutectic composite Ni-Ni3B and subsequent liquid-phase sintering at 1120 °C contain 3.5–14.0 wt.% Ni. The yield strength of these materials at a porosity of 60–90% is higher by a factor of 2 to 3 than that of the base material.

Structuring on sintering in the presence of liquid phase in Cr-Cu-iron group metal systems. I. Cr-Cu system

Structuring has been examined for Cr-Cu composites under conditions of impregnation and subsequent liquid-phase sintering at 1200°C in a vacuum of (2–4) · 10−3 Pa with reduced and electrolytic chromium powders. The size distribution for the particles of the refractory component in the microstructure containing the reduced chromium on liquid-phase sintering for 60 min corresponds to a logarithmic normal distribution; the distribution parameters are sensitive to the volume fraction of refractory particles. The calculated values for the dihedral angle are close to one of the modes of the distribution for the dihedral angles in the microstructure for specimens made of electrolytic chromium (115°). At 1200°C, the equilibrium Crs-Cul system obeys the condition \(\frac{{\sigma _{sl} }}{{\sigma _{ss} }} \geqslant 0.5\). This indicates the probability of formation or preservation of framework structure elements during the liquid-phase sintering, which are observed by experiment in specimens containing reduced chromium.

Presintering effects on ground-based and microgravity liquid phase sintering

Tungsten heavy alloys are an ideal system for liquid phase sintering studies. The sintering behavior and postsintering microstructure depend to a large degree on the processing conditions. While grain growth, final microstructures, and mechanical properties are fairly well understood, the role of presintering conditions is poorly documented. This article discusses presintering condition effects on the subsequent liquid phase sintering densification and distortion during both ground-based and microgravity sintering. It provides unique insight into the events accompanying densification and shape distortion, such as solid-liquid segregation and pore stability. Solid-liquid segregation is closely connected to densification. Presintering conditions play an important role in the mobility and the location of the pores in the sample, which can be explained in terms of the interfacial energy between different phases. In ground-based sintering, the cold-isostatically pressed sample has the most uniform solid-liquid distribution with the largest shrinkage. In microgravity sintering, the vacuum-presintered sample shows the largest degree of densification and elimination of pores.

Melt-infiltration processing of TiC/Ni3Al composites

A simple melt-infiltration processing route has been developed for the fabrication of TiC/Ni3Al ceramic/intermetallic composites, which involves a combination of infiltration and subsequent liquid phase sintering. For Ni3Al contents from 8 to 25 vol. %, densities in excess of 98% of theoretical are readily obtained when processing at 1450 °C. TiC and Ni3Al are the only phases detected in the densified materials. Ni3Al ductility is retained after processing, leading to the possibility of ductile phase toughened TiC composites for elevated temperature applications (up to ∼1100 °C).

Fabrication of nanostructured ceramics by hybrid processing

Different organometallic precursors and processing routes were used to investigate the crystallization behaviour and the microstructural development of SiN and SiCN materials. Micro-nano-structured Si 3N 4/SiC-composites were processed by liquid phase sintering of either amorphous polyhydridochlorosilazane (VB33, Hoechst AG) derived SiCN powder or amorphous SiC 1+x (pyrolysis product of polymethylvinylsilane, PVS) coated α-Si 3N 4 powder. In addition, a Si 3N 4 material was produced by pyrolysis of VB33 under NH 3 and subsequent liquid phase sintering. The obtained microstructures were characterized by scanning and transmission electron microscopy (SEM, TEM). The crystallization behaviour has been investigated by X-ray measurements.

Change in the fine structure of titanium diboride occurring during the sintering of A TiB2-Fe(Mo) cermet

Sintered composite materials base~ on titanium diboride can be employed instead of tungsten-containing hard metals for the manufacture of cutting tools [i]. During the processing of such composite materials the composition and structure of each of their components undergo pronounced changes. It was therefore considered of interest to study the changes taking place in the fine structure of TiBa during vibratory milling and subsequent liquid-phase sintering.