Showing papers in "International Journal of Refractory Metals & Hard Materials in 2012"

Microscopic theory of hardness and design of novel superhard crystals

Abstract: Hardness can be defined microscopically as the combined resistance of chemical bonds in a material to indentation. The current review presents three most popular microscopic models based on distinct scaling schemes of this resistance, namely the bond resistance, bond strength, and electronegativity models, with key points during employing these microscopic models addressed. These models can be used to estimate the hardness of known crystals. More importantly, hardness prediction based on the designed crystal structures becomes feasible with these models. Consequently, a straightforward and powerful criterion for novel superhard materials is provided. The current focuses of research on potential superhard materials are also discussed.

Friction and wear properties of TiN, TiAlN, AlTiN and CrAlN PVD nitride coatings

Abstract: Four nitride coatings, TiN, TiAlN, AlTiN and CrAlN were deposited on YG6 (WC + 6 wt.% Co) cemented carbide by cathodic arc-evaporation technique. The friction and wear properties were investigated and compared using ball-on-disc method at high speed with SiC ball as a counter material. The tests were evaluated by scanning electron microscopy, X-ray diffractometer, energy dispersive X-ray, micro hardness tester and an optical profilometer. The results showed that TiN and TiAlN coatings presented lower friction coefficient and lower wear rate, and that high Al content AlTiN and CrAlN coatings didn't present better anti-wear properties in this test. Oxidation and abrasive wear were the main wear mechanism of TiN coating. In spite of the observation of micro-grooves and partial fractures, TiAlN possessed perfect tribological properties compared with the other coatings. High Al content increased the chemical reactivity and aroused severe adhesive wear of AlTiN coating. CrAlN coating presented better properties of anti-spalling and anti-adhesion, but abundant accumulated debris accelerated wear of the coating under this enclosed wear environment.

Performance of carbide tools with textured rake-face filled with solid lubricants in dry cutting processes

Abstract: Surface texturing with different geometrical characteristics was made on the rake face of the WC/Co carbide tools, molybdenum disulfide (MoS 2 ) solid lubricants were filled into the textured rake-face. Dry cutting tests were carried out with these rake-face textured tools and a conventional tool. The effect of the texture shape on the cutting performance of these rake-face textured tools was investigated. Results show that the cutting forces, cutting temperature, and the friction coefficient at the tool-chip interface of the rake-face textured tools were significantly reduced compared with that of the conventional one. The rake-face textured tool with elliptical grooves on its rake face had the most improved cutting performance. Two mechanisms responsible were found, the first one is explained as the formation of a lubricating film with low shear strength at the tool-chip interface, which was released from the texturing and smeared on the rake face, and served as lubricating additive during dry cutting processes; the other one was explained by the reduced contact length at the tool-chip interface of the rake-face textured tools, which contributes to the decrease of the direct contact area between the chip and rake face.

Machinability investigations on hardened AISI 4340 steel using coated carbide insert

Abstract: The hard turning process with advanced cutting tool materials has several advantages over grinding such as short cycle time, process flexibility, compatible surface roughness, higher material removal rate and less environment problems without the use of cutting fluid. However, the main concerns of hard turning are the cost of expensive tool materials and the effect of the process on machinability characteristics. The poor selection of the process parameters may cause excessive tool wear and increased work surface roughness. Hence, there is a need to study the machinability aspects in high-hardened components. In this work, an attempt has been made to analyze the influence of cutting speed, feed rate, depth of cut and machining time on machinability characteristics such as machining force, surface roughness and tool wear using response surface methodology (RSM) based second order mathematical models during turning of AISI 4340 high strength low alloy steel using coated carbide inserts. The experiments were planned as per full factorial design (FFD). From the parametric analysis, it is revealed that, the combination of low feed rate, low depth of cut and low machining time with high cutting speed is beneficial for minimizing the machining force and surface roughness. On the other hand, the interaction plots suggest that employing lower cutting speed with lower feed rate can reduce tool wear. Chip morphology study indicates the formation of various types of chips operating under several cutting conditions.

Erosion wear of CrN, TiN, CrAlN, and TiAlN PVD nitride coatings

Abstract: Four nitride coatings (CrN, ZrN, CrAlN, and TiAlN) were deposited on YT15 cemented carbide by cathode arc-evaporation technique. Microstructural and fundamental properties of these nitride coatings were examined. Erosion wear tests were carried out, the erosion wear of these nitride coatings caused by abrasive particle impact was compared by determining the wear depth and the erosion rates of the coatings. The wear surface features were examined by scanning electron microscopy. Results showed that the coatings with Al (CrAlN and TiAlN) exhibited higher erosion wear resistance over those without Al (CrN and TiN). The H3/E2 of the coating seemed to play an important role with respect to its erosion wear in erosion tests. AlTiN and CrAlN coatings being with high H3/E2 exhibited lower erosion rates, while CrN coating with low H3/E2 showed higher erosion rates under the same test conditions. Analysis of eroded surface of the coatings demonstrated that the TiN and CrN coatings exhibited a typical brittle fracture induced removal process, while AlTiN and CrAlN coatings showed mainly micro cutting and cycle fatigue fracture of material removal mode.

Microstructure, residual stresses and shear strength of diamond–steel-joints brazed with a Cu–Sn-based active filler alloy

Abstract: Brazing of diamonds is important in grinding technology. The brazing parameters can strongly influence the grinding tool's performance. In this work a Cu–Sn-based active filler alloy (73.9 Cu–14.4 Sn–10.2 Ti–1.5 Zr, wt.%) was applied to join monocrystalline block-shaped diamonds onto a stainless steel substrate using three different brazing temperatures (880, 930 and 980 °C) and two different dwell times (10 and 30 min), respectively. The characteristics of the joints were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (microstructure and phase composition), by Raman-spectroscopy (residual stress) as well as by shear testing (bond strength). The microstructural investigations revealed an intermetallic interlayer of type Fe2Ti at the steel-filler alloy interface, which grew with increasing brazing temperatures and longer dwell durations. The brazing parameters strongly affected the residual stresses in the diamond. Compressive residual stresses with a maximum value of − 350 MPa were found in the samples brazed at 880 and 930 °C, whereas tensile stresses of maximum + 150 MPa were determined in samples joined at 980 °C. The effect of the brazing parameters on the shear strength is very pronounced. The shear strength decreased from (321 ± 107) MPa at 880 °C, 10 min to (78 ± 30) MPa at 980 °C, 30 min.

A review of tool wear estimation using theoretical analysis and numerical simulation technologies

Abstract: A thorough understanding of the material removal process in cutting is essential in selecting the tool material and in design, and also in assuring consistent dimensional accuracy and surface integrity of the finished product. Tool wear in cutting process is produced by the contact and relative sliding between the cutting tool and the workpiece, and between the cutting tool and the chip under the extreme conditions of cutting area. This paper presents the information on development of study on theoretical analysis and numerical simulation of tool wear in all over the world. Hidden Markov models (HMMs) were introduced to estimate tool wear in cutting, which are strongly influenced by the cutting temperature, contact stresses, and relative strains at the interface. Finite element method (FEM) is a powerful tool to predict cutting process variables, which are difficult to obtain with experimental methods. The objective of this work focuses on the new development in predicting the tool wear evolution and tool life in orthogonal cutting with FEM simulations.

Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide

Abstract: For the production of hard, high temperature and abrasion resistant parts, like water-jet nozzles or pressing tools for forming glass lenses, binderless cemented carbide is used. In this work, the consolidation of tungsten carbide with additions of VC and Cr3C2 grain growth inhibitors is studied. Tungsten carbide powder dry or wet milled was consolidated by dry pressing, debindering and gas pressurized sintering and, alternatively, by spark plasma sintering. The effect of adding VC and Cr3C2 to binderless tungsten carbide on the grain growth was studied with contents being 0; 0.1; 0.3; 0.5; 0.7 and 1.0 wt.%. Samples with an ultrafine microstructure free of abnormal grain growth, a hardness of 25.5 GPa and a fracture toughness of 7.2 MPa·m1/2 were archived by conventional sintering. Both carbides reduce grain growth, but with Cr3C2 a finer microstructure can be achieved at lower amounts. Compared to the same amount of Cr3C2, the addition of VC results in smaller grains but lower hardness and fracture toughness.

Effect of SPS process sintering on the microstructure and mechanical properties of nanocrystalline TiC for tools application

Abstract: The most important spark plasma sintering (SPS) parameters (Temperature, holding time and pressure), have been reviewed to assess their effect on the densification, grain growth kinetics and mechanical properties of nanocrystalline TiC synthesized by mechanical alloying. Experiments were performed in the 1350–1800 °C temperature range with holding time from 1 to 10 min under various pressure values of 50, 80 and 100 MPa. The results of experiments revealed that the mechanical properties of the material were improved with raising the sintering temperature and extending the sintering time. However, a hardness decrease was observed as a result of abnormal grain growth under higher sintering temperatures. The optimized process parameters for SPS process are identified as a sintering temperature of 1650 °C, a pressure 100 MPa and a sintering time of 5 min. The resulting mechanical properties are: a relative density of 97.9%, a micro-hardness of 2570 Hv, a nano-hardness of 28 GPa, a fracture toughness of 4.9 MPa·m 1/2 and a compressive strength of about 2210 MPa.

Spark plasma sintering of B4C ceramics: The effects of milling medium and TiB2 addition

Abstract: Boron carbide (B 4 C) ceramics, with a relative density up to 98.4% and limited grain growth, were prepared at 1600–1800 °C by spark plasma sintering (SPS) technique. The effects of powder milling medium (water and 2-propanol) on the powders' surface characteristics and TiB 2 addition on the sintering densification were investigated. The ball milling processing of B 4 C powders in water can promote the sintering of B 4 C ceramics. A B 2 O 3 layer on B 4 C particle surface is concluded to promote the densification of the B 4 C ceramics at an early sintering stage. This B 2 O 3 layer, which normally inhibits the densification process at the final stage of the sintering, can be reduced through reaction with TiB 2 particles, resulting in further densification of the B 4 C ceramics.

Microstructure and cavitation erosion behavior of WC–Co–Cr coating on 1Cr18Ni9Ti stainless steel by HVOF thermal spraying

Abstract: A WC–Co–Cr coating was deposited by a high velocity oxy-fuel thermal spray (HVOF) onto a 1Cr18Ni9Ti stainless steel substrate to increase its cavitation erosion resistance. After the HVOF process, it was revealed that the amorphous phase, nanocrystalline grains (Co–Cr) and several kinds of carbides, including Co3W3C, Co6W6C, WC, Cr23C6, and Cr3C2 were present in the coating. The hardness of the coating was improved to be 11.3 GPa, about 6 times higher than that of the stainless steel substrate, 1.8 GPa. Due to the presence of those new phases in the as-sprayed coating and its higher hardness, the cavitation erosion mass loss eroded for 30 h was only 64% that of the stainless steel substrate. The microstructural analysis of the coating after the cavitation erosion tests indicated that most of the corruptions took place at the interface between the un-melted or half-melted particles and the matrix (Co–Cr), the edge of the pores in the coating, and the boundary of the twin and the grain in the stainless steel 1Cr18Ni9Ti.

Development of B4C–HfB2 composites by reaction hot pressing

Abstract: This paper reports on the development of HfB2 reinforced B4C composite prepared through reaction hot pressing of B4C and HfO2. These composites were characterized for phase composition, microstructure, mechanical and physical properties. Full dense composites were obtained by hot pressing at 1900 °C and a pressure of 40 MPa. HfB2 phase was identified as a reaction product in the composites by XRD analysis. Hardness of all the composites was measured to be in the range of 28–35 GPa. High elastic modulus (525 GPa) and shear modulus (196 GPa) were measured for composite corresponding to 10 wt.% HfO2. Fractography of the composites indicates transgranular mode of fracture. Fracture toughness of the composites was in the range of 4–7 MPa.m1/2 which is higher than that of monolithic B4C (2.43 MPa.m1/2). Microstructural observation of crack propagation patterns indicates the major toughening mechanism as crack deflection, crack arrest and crack bridging.

Effect of removing internal residual metallic phases on wear resistance of polycrystalline diamond compacts

Abstract: Internal residual metal phases (mainly cobalt) were effectively removed from polycrystalline diamond compacts (PDCs) by electrolysis to improve their high temperature wear resistance. Through turning granite (dry cutting), we have checked the wear resistance of PDCs with different residual metal removal depth (RMRD). The relationship between the measured wear rate and RMRD was obtained, and the results showed that PDCs treated by electrolysis have a significant improvement in wear resistance. X-ray diffraction (XRD) and scanning electron microscopy (SEM) observation of PDCs' wear surface indicated that diamond–graphite phase transformation occurs for the samples with residual metal phase. The wear mechanism is discussed, and our study suggests that the graphitization is the main wear process of PDCs with internal residual metal phases.

Microstructure and properties of WC–Co/3Cr13 joints brazed using Ni electroplated interlayer

Abstract: The brazed joints of WC–Co cemented carbide and 3Cr13 stainless steel using Ni electroplated on Cu–Zn alloy as interlayer were investigated. The shear strength of the WC–Co/interlayer/3Cr13 joints increased firstly and then decreased with the increase of brazing temperature or brazing time. The maximum shear strength value of the brazed joints was 154 MPa at 1100 °C for 10 min. The characterizations of the WC–Co/interlayer/3Cr13 joints were studied by SEM, EDS and XRD. The results showed that the brazed joints fractured in the bulk WC–Co substrates near the interlayer. The added Ni promoted the formation of interdiffusion zone, which possessed positive effects on the bond strength of the WC–Co/interlayer/3Cr13 joints. Austenite solid solution was formed in the WC–Co/interlayer/3Cr13 joint, and the majority of austenite solid solution presented as columnar crystal. The number of austenite crystals on the WC–Co/interlayer interface was tremendously more than that on the interlayer/3Cr13 interface.

Study on the formation of core–rim structure in Ti(CN)-based cermets

Abstract: Ti(CN)-based cermets were synthesized from Ti(CN) WC Mo 2 C TaC Ni Co composite powders by vacuum-low pressure sintering. The phase evolution and the formation of core–rim structure in Ti(CN)-based cermets were systemically investigated during difference reaction stages at 950–1450 °C. The results show that the secondary carbides such as Mo 2 C and TaC are begun to dissolve at 950 °C, finished at 1150 °C, and the solution temperature of WC phase is range from 1150 to 1300 °C, which are result in increase of the cermets lattice constant. At the same time, the inner rim is also formed, and Ti(CN)-based cermets are composed of (Ti, W, Mo, Ta)(CN) and Ni/Co solid solution phase. While at 1350 °C, it was found that the outer rim began to precipitate from the liquid phase with the metal binder. With increase of sintering temperature, mechanical properties of cermets improved obviously were related intimately to the increase of outer rim thickness.

Effect of Mo and Co additions on the microstructure and properties of WC-TiC-Ni cemented carbides

Abstract: In this work, the effects of 1.0 wt.% additions of Mo and Co on the microstructure and properties of WC-TiC-Ni cemented carbides were investigated using scanning electron microscope, mechanical properties tests, corrosion resistance and abrasion resistance tests. The results show that 1.0 wt.% Mo addition can refine the WC grains and increase the hardness. Moreover, with the addition of minor Mo, the corrosion resistance and abrasion resistance of alloys improved significantly. The addition of 1.0 wt.% Co can inhibit the growth of WC grains, improve the density and hardness slightly, and enhance the abrasion resistance of cemented carbides. However, the minor Co has negative effect for the corrosion resistance.

Bulk WC–Al2O3 composites prepared by spark plasma sintering

Abstract: WC and WC–Al 2 O 3 materials without metallic binder addition were densified by spark plasma sintering in the range of 1800–1900 °C. The densification behavior, phase constitution, microstructure and mechanical properties of pure WC and WC–Al 2 O 3 composite were investigated. The addition of Al 2 O 3 facilitates sintering and increases the fracture toughness of the composites to a certain extent. An interesting phenomenon is found that a proper content of Al 2 O 3 additive helps to limit the formation of W 2 C phase in sintered WC materials. The pure WC specimen possesses a hardness (HV 10 ) of 25.71 GPa, fracture toughness of 4.54 MPa·m 1/2 , and transverse fracture strength of 862 MPa, while those of WC-6.8 vol.% Al 2 O 3 composites are 24.48 GPa, 6.01 MPa·m 1/2 , and 1245 MPa respectively. The higher fracture toughness and transverse fracture strength of WC-6.8 vol.% Al 2 O 3 are thought to result from the reduction of W 2 C phase, the crack-bridging by Al 2 O 3 particles and the local change in fracture mode from intergranular to transgranular.

Thermal and thermo-mechanical properties of Ti–Al–N and Cr–Al–N coatings

Abstract: Metastable Ti–Al–N and Cr–Al–N coatings have been proven to be an effective wear protection due to their outstanding mechanical and thermal properties. Here, a comparative investigation of mechanical and thermal properties, for Ti–Al–N and Cr–Al–N coatings deposited by cathodic arc evaporation with the compositions (c-Ti0.52Al0.48N, c/w-Ti0.34Al0.66N and c-Cr0.32Al0.68N) widely used in industry, has been performed in detail. The hardness of Ti0.52Al0.48N and Ti0.34Al0.66N coatings during thermal annealing, after initially increasing to the maximum value of ~ 34.1 and 38.7 GPa with Ta up to 900 °C due to the precipitation of cubic Al-rich and Ti-rich domains, decreases with further elevated Ta, as the formation of w-AlN and coarsening of precipitated phases. A transformation to Cr2N and finally Cr via N-loss in addition to w-AlN formation during annealing of the Cr0.32Al0.68N coating occurs, and thus results in a continuous decrease in hardness. Among our coatings, the mixed cubic-wurtzite Ti0.34Al0.66N coating exhibits the highest thermal hardness, but the worst oxidation resistance. The Cr0.32Al0.68N coating shows the best oxidation resistance due to the formation of dense protective α-Al2O3-rich and Cr2O3-rich layers, with only ~ 1.4 μm oxide scale thickness, after thermal exposure for 10 h at 1050 °C in ambient air, whereas Ti–Al–N coatings are already completely oxidized at 950 °C.

Mechanical properties and microstructure of TiB2–TiC composite ceramic cutting tool material

Abstract: article i nfo Article history: Received 5 December 2011 Accepted 23 February 2012 TiB2-TiC composite ceramic cutting tool material was prepared by sintering during hot-pressing in vacuum. The effects of nano-scale Ni and Mo additives and sintering heating rate on mechanical properties and grain characteristics were investigated. TiB2 and TiC grains exhibited prismatic and equiaxed shapes respectively. The diameter and aspect ratio of prismatic TiB2 grains were influenced by nano-scale Ni/Mo additives. A higher heating rate could cause a higher aspect ratio of prismatic TiB2 grains. The good mechanical properties of TN1((TiB2-TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min) were ascribed to a relatively fine and homogenous microstructure. And a brittle B4MoTi solid solution phase and wider distribution of grain size induced the lower flexural strength of TNM2((TiB2-TiC)/(Ni,Mo) composite ceramic sintered at heating rate of 100 °C/min), but the higher aspect ratio of TiB2 grains could prevent cracks from propagating and ameliorated the fracture toughness. The optimum resultant mechanical properties were obtained by (TiB2-TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min.

Synthesis of ultrafine/nanocrystalline W–(30–50)Cu composite powders and microstructure characteristics of the sintered alloys

Abstract: Ultrafine/Nanocrystalline W–Cu composite powders with various copper contents (30, 40 and 50 wt.%) have been synthesized by sol-spray drying and a subsequent hydrogen reduction process. The powders were consolidated by direct sintering at temperatures between 1150 and 1260 °C for 90 min. The powder characteristics and sintering behavior, as well as thermal conductivity of the sintered alloys were investigated. The results show that the synthesized powders exist in ultrafine composite particles containing numerous nanosized particles, and the composition distributed very homogeneously. As the copper contents increase, the grain size of the powders decreases. The subsequent sintered parts show nearly full density with the relative density more than 99% at the temperature of 1250 °C. The sintered parts have very fine tungsten grains embedded in a bulk matrix. With increased copper contents, the tungsten grain size decreases and the microstructural homogeneity of the sintered alloys improves further. The thermal conductivity properties, while a little lower than that of the theoretical value, depend on the copper contents.

Friction and wear behaviors of WC/Co cemented carbide tool materials with different WC grain sizes at temperatures up to 600 °C

Abstract: The friction and wear behaviors of WC/Co cemented carbide tool materials with average WC grain sizes ranging from 0.6 to 2.2 μm were evaluated in ambient air at temperatures up to 600 °C using a ball-on-disk high temperature tribometer. The friction coefficient and wear rate were measured. The microstructural changes and the wear surface features of the WC/Co cemented carbides were examined by scanning electron microscopy. Results showed that the friction coefficient of WC/Co cemented carbides decreased with the increase of test temperature. All the tested samples showed the highest friction coefficient when sliding at 200 °C, and exhibited the lowest friction coefficient in the case of 600 °C. The wear rate of WC/Co cemented carbides increased with the increase of test temperature. The cemented carbide with the smallest WC grain size showed improved wear resistance at temperature up to 600 °C, which corresponds to its higher value of hardness. The difference of the worn surface features of the WC/Co cemented carbide after sliding at different temperature is related to the chemical transformation during sliding wear tests. Abrasion and grain cracking seemed to be the main wear types at temperature less than 200 °C, the wear owing to binder removal by plastic deformation and grain pull out were suggested to be the main wear mechanism at intermediate temperature, while the mechanism of oxidative wear dominated at 600 °C.

Investigation on the characteristics of micro- and nano-structured W-15 wt.%Cu composites prepared by powder metallurgy route

Abstract: The properties of W-15 wt.%Cu composites were investigated by preparing two distinct composites of micrometer and nanoscale structures. Micrometer composite was produced by mixing elemental W and Cu powders and nanometer one was synthesized through a mechanochemical reaction between WO3 and CuO powders. Subsequent compaction and sintering process was performed to ensure maximum possible densification at 1000–1200 °C temperatures. Finally, the behavior of produced samples including relative density, hardness, compressive strength, electrical conductivity, coefficient of thermal expansion (CTE) and room temperature corrosion resistance were examined. Among the composites, nano-structured sample sintered at 1200 °C exhibited better homogeneity, the highest relative density (94%) and mechanical properties. Furthermore, this composite showed superior electrical conductivity (31.58 IACS) and CTE (9.95384 × 10– 6) in comparison with micrometer type. This appropriate properties may be mainly attributed to liquid phase sintering with particle rearrangement which induced by higher capillary forces of finer structures.

Effects of Cr3C2 addition on the corrosion behavior of Ti(C, N)-based cermets

Abstract: The Ti(C, N)-based cermets with different Cr3C2 addition were prepared and the effects of Cr3C2 addition on the microstructure and properties of cermets were discussed. The corrosion behavior of the cermets with different Cr3C2 addition was investigated emphatically in 2 mol/L HNO3 solution. The results indicate that there is no obvious effect of Cr3C2 addition on the densification of the cermets, and all cermets are almost fully densified during sintering. The thickness of rim phase is reduced and the core size is increased remarkably in the cermets with 1 wt.% and 3 wt.% Cr3C2 addition; the grains are refined significantly in the cermets with the increase of Cr3C2 addition to 5 wt.%. The hardness and transverse rapture strength of the cermets are improved with Cr3C2 added properly. In HNO3 solution, the corrosion resistance of cermets is improved remarkably by Cr3C2 addition. The corrosion of binder phase is predominant in the cermets in which the Ni binder phase without Cr has lower corrosion resistance than the rim phase; whereas the corrosion resistance of binder phase with high Cr content is better compared to the rim phase, so that the degradation of rim phase is predominant and a reticulate binder phase forms. With the increase of Cr3C2 addition, the Mo content in rim increases, and it is bad for the corrosion resistance of rim phase. Additionally, the inner rim phase has lower corrosion resistance than the outer rim phase owing to the higher Mo content.

Effect of tungsten content on microstructure and quasi-static tensile fracture characteristics of rapidly hot-extruded W-Ni-Fe alloys

Abstract: Tungsten heavy alloys (WHAs) with three different compositions (90W–7Ni–3Fe, 93W–4.9Ni–2.1Fe and 95W–3.5Ni–1.5Fe, wt.%) were heavily deformed by one-pass rapid hot extrusion at 1100 °C with an extrusion speed of ~ 100 mm/s and an extrusion ratio of ~ 3.33:1. The influence of tungsten content on the microstructure and tensile fracture characteristics of the as-extruded alloys was investigated in detail. The results show that the tungsten particles in the as-extruded 95W have the largest shape factor compared to the as-extruded 90W and 93W alloys and this implies that the tungsten particles in the as-extruded 95W alloy were subjected to the heaviest plastic deformation. In addition, ultimate tensile strength (UTS) and hardness (HRC) are significantly improved after rapid hot extrusion. The as-extruded 95W alloy processes the highest strength (1455 MPa) and hardness (HRC40) but the lowest elongation (5%), followed by the as-extruded 93W (UTS1390MPa; HRC39; 7%) and 90W alloys (UTS1260MPa; HRC36; 10%). The fracture morphology shows the distinct fracture features between the as-sintered alloys and the as-extruded alloys. For the as-sintered alloys, the fracture modes are various while transgranular cleavage of tungsten particles is the main characteristic in the as-extruded alloy. Meanwhile, the fracture modes of the three as-extruded alloys vary slightly with the tungsten content. TEM bright field images indicate that many lath-like subgrains with the width of 150–500 nm are present in the three as-extruded alloys, particularly in the as-extruded 93W and 95W alloys. Furthermore, the dislocations are absent in the γ-(Ni, Fe) phase. This means that dynamic recovery–recrystallization process took place during rapid hot extrusion.

Optimization of the Ti3SiC2 MAX phase synthesis

Abstract: The synthesis of Ti3SiC2 MAX phase by self-propagating high-temperature synthesis (SHS) and pressureless argon shielding synthesis has been investigated following different pathways pertaining to the reactant systems Ti/Si/C, Ti/SiC/C and Ti/TiC/Si. Silicon in excess ranging from 10 to 50 mol% was employed to obtain powders mainly constituted by Ti3SiC2. Optimizing the excess of silicon and the pressing technique, the resultant powders with Ti3SiC2 content near to 100% were obtained. Result was consequent to the use of pressureless argon shielding synthesis obtained with 30 mol% of silicon excess in the examined different systems. The Ti3SiC2 was also obtained by SHS, but with lower proportion (88% and 86% from 3Ti + 1.2SiC + 0.8C and 3Ti + 1.3Si + 2C respectively). These results driving from XRD patterns were confirmed by FESEM observations and the EDAX analyses.

Synthesis of monodispersed La2Ce2O7 nanocrystals via hydrothermal method: A study of crystal growth and sintering behavior

Abstract: Monodispersed and uniformly distributed La2Ce2O7 nanocrystals were synthesized via the hydrothermal method using polyethyleneglycol (PEG) as surfactant. X-ray diffraction (XRD), Thermogravimetric analysis/Differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and High resolution transmission electron microscopy (HRTEM) were utilized to characterize the thermal decomposition, phase structure and morphology of the products. Qualitative analysis indicates that the products are comprised of well-dispersed square particles with cubic fluorite structure. The specific surface area and the average crystallite size of the as-prepared products are 195.59 m2 g− 1 and 10–15 nm, respectively. The low effective activation energy (15.27 ± 0.03 kJ mol− 1) for crystal growth was obtained in the calcination temperature range of 700–1300 °C. The sintering behavior of the compacted body was also investigated, revealing that La2Ce2O7 has a low relative density and open channel morphology.

Effects of sintering temperature on fine-grained tungsten heavy alloy produced by high-energy ball milling assisted spark plasma sintering

Abstract: Fine-grained tungsten heavy alloys (WHAs) were successfully produced using the high-energy ball milling assisted spark plasma sintering (SPS) method. The effects of increasing sintering temperatures on the microstructure and mechanical properties of the alloy were studied in detail. The hardness of the alloy was found to continuously decrease from 79.3 to 63.8 HRA. In contrast, the bending strength continuously increased from 353.6 to 954.5 MPa. W grain size increased with increased sintering temperature. The temperature ranges from 1000 to 1100 °C and 1150 to 1200 °C were a period of rapid growth of W grain. According to the color change in the scanning electron microscope (SEM) image, the W alloy microstructure were classified into white W grains, off-white W-rich particles, dark grey matrix γ-(Ni, Fe, W), as well as pitch-black W- and O-rich particles. The bending fracture of the alloy mainly displays the features of intergranular fracture. The microporosity of different sizes was distributed on the bending fracture, and grew with increased sintering temperature.

A multivariate robust parameter design approach for optimization of AISI 52100 hardened steel turning with wiper mixed ceramic tool

Abstract: This paper presents an experimental study of AISI 52100 hardened steel turned with wiper mixed ceramic (Al2O3 + TiC) inserts coated with TiN, using Multivariate Robust Parameter Design (MRPD). The main characteristic of this new optimization approach consists of considering both controllable (xi) and noise (zi) variables of the hard turning process to find out the parameter levels which minimize the distance of each response (yi) from its respective targets (Ti) while keeps each variance caused by the noise variables as low as possible. Using a crossed array, a response surface design formed by cutting speed (Vc), feed rate (f) and depth of cut (d) is submitted to the influence of four scenarios built with an 22 full factorial design of two noise factors — workpiece hardness decreasing (Z1) and tool flank wear (Z2). This experimental arrangement allows the generating of mean, variance and mean square error (MSE) of five surface roughness parameters (Ra, Rz, Ry, Rt and Rq). As these responses are highly correlated, to extract and employ this information, Principal Component Analysis (PCA) was used. Adopting the Multivariate Mean Square Error (MMSE) as optimization criteria, a robust solution could be found. Theoretical and experimental results were convergent and confirmed. With Vc = 199.9 m/min, f = 0.191 mm/rev and d = 0.190 mm, the five surface roughness parameters and respective variances were minimal, with better results than those obtained with individual optimization.

Effects of processing parameters of tungsten–copper composites

Abstract: Fabrication of net-shape tungsten–copper (W–Cu) composites has attracted more attention in recent years due to its good performance for wide applications. In this research, W–Cu composite including 20 wt.%, 25 wt.%, or 30 wt.% Cu. was produced by powder metallurgy technique using wet mixtures of elemental powders. Cold compaction was carried out under pressures from 300 to 1200 MPa, while sintering was achieved in vacuum at 1400 °C for 1 h, and 2 h. The particle size and shape of powders, as well as the microstructure after wet mixing, compaction, and sintering were investigated by using SEM. Wet mixed powders were proved to be homogeneous, and the copper flake particles were found semi-coated by fine spherical tungsten particles. The relative green and the as-sintered densities were found to increase with increasing compaction pressure and copper content. After sintering, the composite revealed homogeneous structure. The relative as-sintered density was found to increase with sintering time. Tungsten–copper composite compacted under 1200 MPa and sintered at 1400 oC for 2 h exhibited the highest relative sintered density, hardness, and compression strength. Also, it exhibited the lowest electrical resistivity and coefficient of thermal expansion.

Inverse core-rim microstructure in (Ti,Ta)(C,N)-based cermets developed by a mechanically induced self-sustaining reaction

Abstract: Cermets with a nominal composition (Ti0.8Ta0.2C0.5N0.5— 20 wt.% Co) were synthesised by a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental powder blends. The MSR allowed the production of a complex (Ti,Ta)(C,N) solid solution, which was the raw material used for the sintering process. The pressureless sintering process was performed at temperatures between 1400 °C and 1600 °C in an inert atmosphere. The microstructural characterisation showed a complex microstructure composed of a ceramic phase with an unusual inverse core–rim structure and a Ti–Ta–Co intermetallic phase that acted as the binder.