Mechanical and wear behaviour of high-speed steels reinforced with TiCN particles

Metal matrix composites (MMCs), based on M3/2 high-speed steel (HSS) and reinforced with two different percentages of TiCN (2.5% and 5% by wt), were manufactured following a conventional powder metallurgy (P/M) route: mixing, compacting and sintering. The carbide and base material powders were dry mixed and uniaxially compacted at 700 MPa. After this, vacuum sintering was carried out at 1275 °C, determined as optimal sintering temperature in a previous sinterability study. Sintered materials were characterised by measuring hardness, transverse rupture strength (TRS) and wear behaviour. The study is completed with a microstructural analysis by scanning electron microscopy (SEM) along with energy dispersive X-ray analysis (EDXA).     

Dispersion and Damage of Carbon Nanotubes in Carbon Nanotube/7055AI Composites During High-Energy Ball Milling Process

High-energy ball milling (HEBM) combined with powder metallurgy route was used to fabricate carbon nanotube (CNT) reinforced 7055Al composites.Two powder morphology evolution processes (HEBM-1 and HEBM-2) were designed to investigate the dispersion and damage of CNTs during HEBM process.HEBM-1 evolution process involved powder flattening,cold-welding and fracture,while HEBM-2 evolution process consisted of powder flattening and fracture.For HEBM-1,the repetitive fracture and cold-welding process was effective for dispersing CNTs.However,the powder flattening process in HEBM-2 was unsuccessful in dispersing CNTs due to two reasons: (1) the thickness of flaky Al powders exceeded the critical value,and (2) the clustered CNTs embedded in flaky Al powders could not be unravelled.Because of the broadening of D band and the appearance of a new defect-related D'band,product of ID/IG and full width half maximum of D band,rather than ID/IG,was used to evaluate the actual damage of CNTs.It indicates that the damage of CNTs was severe in powder flattening and fracture stages,while the damage of CNTs was small in powder cold-welding stage.     

Microstructural Characteristics and Mechanical Behavior of Spark Plasma-Sintered Cu–Cr–rGO Copper Matrix Composites

Copper matrix composites were prepared through spark plasma sintering(SPS) process, mixing fixed amount of reduced graphene oxide(rGO) with the different amounts of Cr. In the sintered bulk composites, the layered rGO network and uniform Cr particles distributed in the Cu matrix. Both of mechanical blending and freeze-drying stages of the wet-mixing process obtained the Cu/Cr/rGO mixture powders, and then SPS solid-phase sintering realized the faster densification of these mixture powders. The hardness and compressive yield strength of the Cu–Cr–rGO composites depicted the higher values than those of pure Cu and single rGO-added composite, and they were gradually increased with increasing Cr. The rGO/Cr hybrid second-phases are believed to be beneficial to strengthening Cu matrix. The relevant formation and strengthening mechanisms involved in Cu–Cr–rGO composites were discussed.     

A Novel Cu–GNPs Nanocomposite with Improved Thermal and Mechanical Properties

Classical powder metallurgy followed by either hot isostatic pressing(HIPing) or repressing–annealing process was used to produce Cu–graphene nanoplatelets(GNPs) nanocomposites in this work. A wet mixing method was used to disperse the graphene within the matrix. The results show that a uniform dispersion of GNPs at low graphene contents could be achieved, whereas agglomeration of graphene was revealed at higher graphene contents. Density evaluations showed that the relative density of pure copper and copper composites increased by using the post-processing techniques.However, it should be noticed that the efficiency of HIPing was remarkably higher than repressing–annealing process, and through the HIPing, fully dense samples were achieved. The Vickers hardness results showed that the reconsolidation steps can improve the mechanical strength of the specimens up to 50% owing to the progressive porosity elimination after reconsolidation. The thermal conductivity results of pure copper and composites at high temperatures showed that the postprocessing techniques could enhance the conductivity of materials significantly.     

Using microwaves to synthesize pure aluminum and metastable Al/Cu nanocomposites with superior properties

The processing of aluminum using powder metallurgy techniques is a challenging task due to the presence of thin oxide films on the surface of the metal particles which prevent strong bonding between particles during sintering. In this study, an innovative hybrid microwave sintering technique is utilized to process pure aluminum and Al/Cu nanocomposites without the need of any protective atmosphere. Hybrid microwave sintering involves the heating of aluminum compacts using both microwave energy and radiant heating. Significant savings in time and energy can be achieved using the fabrication methodology. Results revealed that processing methodology used in this study provides a simple and inexpensive option to existing processing methodologies to synthesize pure Al and Al/Cu nanocomposites with superior combination of properties.     

Effects of C particle size on the ignition and combustion characteristics of the SHS reaction in the 20 wt.% Ni–Ti–C system

C particle size plays an important role in the ignition and combustion characteristics of the SHS reaction in the 20 wt.% Ni–Ti–C system. When coarse C particles (38 and 75 μm) are used, the SHS reactions consist of two different combustion stages with different brightness intensity of the combustion wave; XRD results indicate that the first and second combustion stages mainly correspond to the formation of Ni–Ti compounds and TiC ceramics, respectively. However, the final reaction is incomplete with a few Ni–Ti compounds and unreacted C. In contrast, when the fine C particle (1 μm) is used, the SHS reaction consists of only one combustion stage with high brightness intensity of the combustion wave; XRD result indicates that final products consist of TiC and Ni, without any intermediate phase. With the decrease of C particle size, the wave velocities increase, and the ignition time becomes shorter. In addition, the morphology of TiC particulate changes to near-spherical, as C particle size decreases.     

Microstructure and kinetics of a functionally graded NiTi–TiCx composite produced by combustion synthesis

Production of a NiTi–TiC_x functionally graded material (FGM) composite is possible through use of a combustion synthesis (CS) reaction employing the propagating mode (SHS). The NiTi–TiC_x FGM combines the well-known and understood superelastic and shape memory capabilities of NiTi with the high hardness, wear and corrosion resistance of TiC_x. The material layers were observed as functionally graded both in composition and porosity with distinct interfaces, while still maintaining good material interaction and bonding. XRD of the FGM composite revealed the presence of TiC_x with equi-atomic NiTi and minor NiTi₂ and NiTi₃ phases. The TiC_x particle size decreased with increasing NiTi content. Microindentation performed across the length of the FGM revealed a decrease in hardness as the NiTi content increased.     

Microstructure and thermal properties of copper matrix composites reinforced with titanium-coated graphite fibers

Milled form of mesophase pitch-based graphite fibers were coated with a titanium layer using chemical vapor deposition technique and Ti-coated graphite fiber/Cu composites were fabricated by hot-pressing sintering. The composites were characterized with X-ray diffraction, scanning/transmission electron microscopies, and by mea-suring thermal properties, including thermal conductivity and coefficient of thermal expansion (CTE). The results show that the milled fibers are preferentially oriented in a plane perpendicular to the pressing direction, leading to anisotropic thermal properties of the composites. The Ti coating reacted with graphite fiber and formed a continuous and uniform TiC layer. This carbide layer establishes a good metallurgical interfacial bonding in the composites, which can improve the thermal properties effectively. When the fiber content ranges from 35 vol% to 50 vol%, the in-plane thermal conductivities of the composites increase from 383 to 407 Wá(máK) -1 , and the in-plane CTEs decrease from 9.5 9 10-6 to 6.3 9 10 -6 K-1 .     

Synthesis of ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders by low-energy milling and subsequent carbothermal reduction–nitridation reaction

Ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders with globular-like particles of 50–300 nm were synthesized at static nitrogen pressure from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation (CRN) techniques. Reaction path of the (Ti, W, Mo, V)(C, N)–Ni system was discussed by X-ray diffraction (XRD) and thermogravimetry–differential scanning calorimetry (TG–DSC), and microstructure of the milled powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that CRN reaction has been enhanced by nano-TiO₂ and nano-carbon powders. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N)–20Ni is at only 1300 °C for 1 h. During synthesizing reaction, Ni solid solution phase forms at about 700 °C and reduction–carbonization of WO₂ and MoO₂ occurs below 900 °C. The reactions of TiO₂ → Ti₃O₅, Ti₃O₅ → Ti(C, O) and Ti(C, O) → Ti(C, N) take place at about 930 °C, 1203 °C and 1244 °C, respectively.