Effect of hot isostatic pressing nitrogen on the microstructure and properties of a Ti（C, N）-based cermet

The high-temperature, high-pressure hot isostatic pressing technology was used for depositing hard coatings on Ti（C, N）-based cermets. The rnicrostructure and properties of the sample were investigated using optical microscopy, scan- ning electron microscopy, X-ray diffraction, electron probe microanalysis, and microhardness tester. The results showed that the rich titanium and nitrogen in surface zone were induced by the heat treatment. The high nitrogen activity of the surface region was the driving force for outward transport of titanium and inward transport of tungsten in the cobalt binder. The toughness and hardness were improved and a hardness gradient was formed. It is the high-temperature, high-pressure N2 that enables closure of holes, thereby alleviating defects and prolonging tool life.     

Microstructure and Wear Resistance Evolution of HVOFSprayed WC-（W,Cr）2C-Ni Coating with Post Heat Treatment In Air Atmosphere

WC-(W,Cr)2C-Ni coating was prepared on 1Cr18Ni9Ti stainless steel and C-276 Ni-base Hastelloy by high velocity oxy-fuel(HVOF)spraying.The effect of post heat treatment in air atmosphere on the microstructure,phase composition,microhardness,fracture toughness,and wear resistance of HVOF-sprayed WC-(W,Cr)2C-Ni coating was investigated.The microstructure and phase composition of the coatings were analyzed by means of field emission scanning electron microscopy(FESEM)and X-ray diffraction(XRD).The microhardness and fracture toughness of the coatings were measured using a microhardness tester and a Vickers hardness tester.Moreover,dry friction and wear behavior of the coatings sliding against Si3N4 ball was investigated using an oscillating friction and wear tester;and the worn surfaces of the coatings were analyzed by means of scanning electron microscopy(SEM).It was found that heat treatment within 500-800°C resulted in crystallization of amorphous phase in as-sprayed coating,generating nanoscale new phases such as NiWO4,CrWO4 and Cr2WO6.Besides,heat treatment led to increase of the microhardness of as-sprayed coating,and the highest microhardness was obtained after heat treatment at 800°C.The fracture toughness and wear resistance of the as-sprayed coating increased with increasing heat treatment temperature up to 700°C but tended to decrease with further elevating temperature.In other words,the mechanical properties and wear resistance of the as-sprayed coatings were worsened owing to excessive growth of oxidation grains and depletion of ductile Ni binder after heat treatment above 700°C.Thus it was suggested that as-sprayed ceramic composite coating should be post heat treated in air at a moderate temperature of 700°C so as to achieve the optimized mechanical properties and wear resistance.     

Analysis of Oxides Formed on the Surface of the Alloy 690 in Hydrogenated Supercritical Water

The oxides formed on the surface of the alloy 690 in hydrogenated supercritical water at 400℃ for 1000 h were investigated using scanning electron microscopy,transmission electron microscopy,scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy.The oxides on me surface of the alloy 690 exhibited multi-layer structure:an outer layer consisted of granular crystallites(NiO and NiFe_2O_4) and a compact inner layer(spinel and Cr_2O_3).Chemical analysis indicated that the outer layer was enriched in nickel but depleted in chromium,whereas the inner layer was enriched in chromium and iron but depleted in nickel.The inner layer was also characterized as layered structure by Fe-rich spinel on top of continuous Cr_2O_3 layer.Besides,Cr_2O_3 nodules were readily observed at the oxides/alloy interface.     

Microstructure evolution of Nb3Sn superconductors during diffusion treatment by bronze route

The element diffusion process of Nb3Sn superconductors by bronze route was studied using X-ray diffraction,scanning electron microscopy and energy dispersive spectroscopy.The critical current of superconductors was measured by four-point method.The results show that a diffusion layer has formed around the boundaries between the filaments and bronze matrix after 15 h heat treatment.The diffusion layer thickness keeps stable after heat treatment duration of 50～75 h.The stable and solid Nb3Sn layer is obtained in the sample after 100 h heat treatment.Excessive heat treatment would induce superconductivity degeneration because of superconductor grain coarsening.The characteristics of the element diffusion process were discussed.The diffusion of tin atom is the governing factor in diffusion.In this study,Nb3Sn superconductors with good superconducting property were fabricated successfully at 670 ℃ after 100 h heat treatment.     

Influence of Austenitizing Temperature on the Microstructure and Corrosion Resistance of 55Cr18Mo1VN High-Nitrogen Plastic Mould Steel

The influence of austenitizing temperature on the microstructure and corrosion resistance of 55Cr18 MolVN high-nitrogen plastic mould steel was investigated.The microstructure,elemental distribution and Cr-depleted zone of different heat-treated samples were investigated by X-ray diffraction,electron probe microanalyzer analysis,and transmission electron microscopy.The corrosion resistance was evaluated using electrochemical measurements,and the analysis of passive film was carried out by X-ray photoelectron spectroscopy.The results indicated that the volume fraction of precipitates decreased,and the homogeneity of elements was improved with increasing austenitizing temperature.The degree of Cr-depleted zone around coarse M_(23)C_6 was severer than that around M_2N,and pitting corrosion initiated preferentially around M_(23)C_6.The corrosion resistance of the samples increased with the austenitizing temperature.With the increase in austenitizing temperature,the passive film was thickened and Cr(Ⅲ)_(Cr_2O_3) in the inner layer of passive film was enriched,which enhanced the corrosion resistance of the steel.The higher content of nitrogen in solid solution at higher austenitizing temperature contributed to the increased intensity of CrN and NH_3,leading to the increase in pH value in the pit,and promoting the repassivation of 55Cr18 MolVN steel.     

Preparation of N-Doped Bi2WO6 Microspheres for Efficient Visible Light-Induced Photocatalysis

The N-doped bismuth tungstate （BizWOt） photocatalysts with high visible light activity were prepared by the hydrothermal method using urea as a nitrogen source. The as-prepared N-doped Bi2WO6 samoles were characterized by X-ray diffraction, scanning electron microscopy, specific surface area, photocurrent analysis, and UV-Vis diffuse re- flectrance spectroscopy. The photocatalytic activity was evaluated by photocatalytic degradation of rhodamine B （RhB） solution under visible light irradiation. The photocatalytic mechanisms were analyzed by active species trapping experi- ments which revealed that the holes were the main active species of N-doped BizWO6 products in aqueous solution under visible light irradiation, rather than .OH and O-. With the assistance of H202, the photocatalytic activity for degradation of RhB could be further improved because H202 reacted with conduction band electrons to generate more hydroxyl radicals. KEY WORDS：     

Electrical Discharge Machining of Al2024-65 vol% SiC Composites

In the present work, the wire electrical discharge machining(WEDM) process of the 65 vol% SiCp/2024 Al composite prepared by pressure infiltration methods has been investigated. The microstructure of the machined composite was characterized by scanning electron microscope, the average surface roughness(Ra), X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy(TEM) techniques. Three zones from the surface to the interior(melting zone, heat affected zone and un-affected zone) were found in the machined composites, while the face of SiC particles on the surface toward the outside was ‘‘cut' to be flat. Increase in Al and Si but decrease in C and O were observed in the core areas of the removed particles. Si phase, which was generated due to the decomposition of SiC, was detected after the WEDM process. The irregular and spherical particles were further observed by TEM. Based on the microstructure observation, it is suggested that the machining mechanism of 65 vol% SiCp/2024 Al composite was the combination of the melting of Al matrix and the decomposition of SiC particles.     

Enhanced photocatalytic properties of ultra-long nanofiber synthesized from pure titanium powders

Using Ti powder as reagent,ultra-long TiO2 nanofibers were prepared via hydrothermal method in NaOH solution.The samples were char-acterized respectively by means of field emission scanning electron microscopy (FESEM),transmission electron microscopy (TEM) with selected area electron diffraction (SAED),and X-ray diffraction (XRD).The diameter and the length of the ultra-long TiO2 nanofiber were ～100 nm and >200μm,respectively.The ultra-long TiO2 nanofibers were anatase after heat treatment at 450 ?C for 1 h.Moreover,the optical properties of the products were investigated by UV-visible light absorption spectrum.Furthermore,methyl orange was used as a target molecule to estimate the photocatalytic activity of the specimens.Under the same testing conditions,the photocatalytic activity of the ultra-long TiO2 nanofibers was higher than that of P25.Direct electrical pathway and improved light-harvesting efficiency were crucial for the superior photocatalytic activity of the ultra-long TiO2 nanofibers.     

Effect of Isothermal Temperature on Microstructure and Mechanical Properties of High AI-Low Si TRIP Steel

In this study, the effect of isothermal temperature on microstructure and mechanical properties of a high Al-low Si TRIP steel was investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron back scattered diffraction, and tensile test. The results show that typical microstructure containing ferrite, bainite, and retained austenite can be obtained when two-stage heat treatment process was utilized. When annealing temperature is 840 ℃ and austempering temperature is 400 ℃, the tensile strength is 542 MPa and the product of strength and elongation is 17,685 MPa%. The morphologies and stability of the retained austenite in low silicon/high aluminum TRIP steel were finally discussed.     

Effect of Isothermal Temperature on Microstructure and Mechanical Properties of High Al–Low Si TRIP Steel

In this study, the effect of isothermal temperature on microstructure and mechanical properties of a high Al–low Si TRIP steel was investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron back scattered diffraction, and tensile test. The results show that typical microstructure containing ferrite, bainite, and retained austenite can be obtained when two-stage heat treatment process was utilized. When annealing temperature is 840 °C and austempering temperature is 400 °C, the tensile strength is 542 MPa and the product of strength and elongation is 17,685 MPa%. The morphologies and stability of the retained austenite in low silicon/high aluminum TRIP steel were finally discussed.     

Microstructure of the surface zone in a heat-treated cermet material

The microstructure of a Ti(C, N)-(Ti, W)(C, N)-(Ti, Ta)(C, N)-WC-Mo₂C-Co cermet with a heat-treatment induced surface gradient has been studied. The post-sintering heat-treatment was performed at 1200 °C in a nitrogen atmosphere and the resulting microstructure was compared to that of the as-sintered material. The microstructures were characterised with optical microscopy, X-ray diffraction, electron microprobe analysis, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, electron energy-loss spectroscopy and energy-filtered transmission electron microscopy. It was found that a 50-μm deep nitrogen-rich surface zone was introduced by the heat treatment. η-phase was observed in the surface zone. The nitrogen gradient caused diffusion of titanium towards the surface, thereby forcing the cobalt-rich binder to be transported inwards. The composition of the binder phase and outer rim of the carbonitride grains showed only minor changes with depth. The variations in binder composition may be a direct consequence of the heat treatment, but are also likely to be influenced by the formation of η-phase. A nitrogen-rich phase was present in the surface zone of the heat-treated material both as thin layers (≈ 70 nm) surrounding the carbonitride grains and as small parts of some carbonitride grains. In addition, the Ti(C, N) cores in the surface region were found to have different N/(C + N) ratios.     

Microstructure and performance of functionally graded Ti(C,N)-based cermets prepared by double-glow plasma carburization

Ti(C,N)-based cermets were subject to double-glow plasma carburization which substantially improved the surface hardness. The microstructures were studied using scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). It was found that the double-glow plasma carburization introduced a graded surface zone enriched in carbonitride phase and deficient in binder phase. The microstructure of the surface zone of the carburized cermet was different from that of the as-sintered cermets. Besides the carbonitride grains with typical core–rim structure, the amount of smaller grains without obvious rim phase increased greatly, and some carbonitride grains adjoined and congregated with each other. The formation of the graded layer was due to the high carbon activity in the surface region during carburization, which caused the titanium, molybdenum and tungsten to transport outwards and forced the nickel to transfer inwards. After double-glow plasma carburization, the surface hardness was increased greatly and the hardness distribution along the depth was in accord with the binder distribution.     

Effect of VC addition on sinterability and microstructure of ultrafine Ti(C, N)-based cermets in spark plasma sintering

The effect of vanadium carbide (VC) addition on the sinterability and the microstructure of ultrafine Ti(C, N)-based cermets consolidated through spark plasma sintering (SPS) was systematically investigated using optical microscope, scanning electron microscope (SEM) with X-ray energy dispersive spectrometer (EDS), X-ray diffractometer (XRD) and transmission electron microscope (TEM). Our results reveal that the addition of VC increases the porosity of sintering body and depresses the sinterability of Ti(C, N)-based cermets. It is also found that the VC addition has a significant influence on the microstructure of ultrafine Ti(C, N)-based cermets, which inhibits the dissolution of titanium-containing compounds and the formation of inner rim phase and outer rim phase, thus preventing from grain growth. Owing to the depressed dissolution and precipitation, nitrogen liberation is mitigated, therefore resulting in less amount of graphite phase in the samples. In substance, VC changes the solubility of metallic elements in the binder, which makes more elements of Mo and W to be reserved in the binder and thus greatly decreases the content of titanium dissolved into the binder. The re-building solubility rule determines the development of phases and microstructure.     

Microstructure and performance of a cermet material heat-treated in nitrogen

A Ti(C,N)–TiN–WC–Co cermet material was subjected to a heat-treatment in nitrogen which substantially improved the plastic deformation resistance during cutting. The microstructure was characterised using analytical electron microscopy. It was found that a surface zone rich in titanium and nitrogen was induced by the heat-treatment. The high nitrogen activity in the surface region was the driving force for outward transport of titanium and inward transport of tungsten in the cobalt binder. An irregularly shaped nitrogen-rich titanium carbonitride reprecipitated from the binder phase during the heat-treatment, often as surrounding layers on already existing carbonitride grains. The morphological change of the hard phase might explain the dramatically improved plastic deformation resistance by obstructing grain boundary sliding. The experimental findings were in accordance with thermodynamic calculations performed using the Thermo-Calc software.     

Effect of carbon content and cooling mode on the microstructure and properties of Ti(C,N)-based cermets

Ti(C,N)-based cermets were prepared by vacuum liquid sintering. The effects of carbon content as well as cooling mode on the microstructure, magnetic and mechanical properties of the cermets were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and vibrating sample magnetometer (VSM). Hardness and transverse rupture strength (TRS) were also measured. The grains of Ti(C,N)-based cermets became finer and solid solubility of titanium, molybdenum, tungsten in binder phases decreased with increasing carbon content. The thickness of the rim phases increased when the cermet was annealed at 1360 °C for 30 min during cooling, which resulted in the decrease of the hardness and the transverse rupture strength (TRS). On the other hand, the magnetic saturation of Ti(C,N)-based cermets increased with increasing carbon content, which was due to the decrease of the solid solution of alloy elements in binder phases.     

Effect of carbon content on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo₂FeB₂ based cermets with different carbon contents were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured. The free carbon present in the green compact significantly decreased the grain size; however, a high carbon content resulted in the formation of graphite phase and Fe₃C phase. An increasing carbon content promoted the dissolution of Mo in the binder phase. In addition, the binder phase varied from ferrite to martensite with increasing carbon content. The highest hardness was found for the cermets with 0.5 wt.% carbon addition, whereas the cermets without carbon addition exhibited the maximum TRS and fracture toughness.     

The effects of C:N ratio on the aqueous corrosion response of TiC and Ti(C,N) cermets with a Ni3Al-based binder

The aqueous corrosion behaviour of TiC and Ti(C,N) based cermets, sintered with 30 vol.% Ni₃Al binder, has been investigated. For comparison purposes, the single-phase constituents were prepared using spark plasma sintering (SPS). The potentiodynamic and cyclic polarisation responses of the cermets and single-phase materials have been determined. Post-test corrosion solutions were analysed using inductively coupled plasma optical emission spectrometry, while samples were evaluated using scanning electron microscopy and associated energy dispersive X-ray analysis. Selective attack of the binder occurs, while the extent of corrosion and primary operative mechanism depends on the C:N ratio in the Ti(C,N).     

Effect of carbon content on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo₂FeB₂ based cermets with different carbon contents were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured. The free carbon present in the green compact significantly decreased the grain size; however, a high carbon content resulted in the formation of graphite phase and Fe₃C phase. An increasing carbon content promoted the dissolution of Mo in the binder phase. In addition, the binder phase varied from ferrite to martensite with increasing carbon content. The highest hardness was found for the cermets with 0.5 wt.% carbon addition, whereas the cermets without carbon addition exhibited the maximum TRS and fracture toughness.     

Effects of NiTi additions on the microstructure and mechanical properties of Ti(C,N)-based cermets

X-ray diffractometer (XRD) and scanning electron microscope (SEM) were used to observe and investigate the microstructure and fracture morphology of Ti(C,N)-based cermets added with NiTi alloy powder. A new ceramic phase is found with the structure of gray core, black inner rim, noncontinuous white inner rim and continuous gray outer rim. The fracture toughness and the transverse rupture strength have a distinct trend to increase with the increase of NiTi powder content in Ti(C,N)-based cermets, while the hardness has the opposite trend. Grain refinement and the increase of metallic phase are the dominant strengthening and toughening mechanisms. Additionally, the crack deflection and bridging may play an active role in improving the properties, as well as the special structure of large metallic binder containing many small ceramic particles. In cermets with a higher content of NiTi powder, the microcrack and the crack closure effect induced by martensitic transformation are advantageous to the mechanical properties.