Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Effect of vinyl acetate content on the sintering behavior of hydroxyapatite–ethylene vinyl acetate copolymer composites

Ethylene vinyl acetate copolymer (EVA) alone could be used as a binder material for the fabrication of hydroxyapatite (HAP) into intricate shapes for various bone substitute applications. It was observed that as the vinyl acetate content in the polymer was increased from 12 to 28 wt % an increase in the sintered density of the HAP was observed. Retention of the shapes of HAP in the molded form was also observed.     

A novel spark plasma sintering route to process high-strength Ti–4Al–2Fe/TiB nano-composite

Ti–4Al–2Fe alloy and Ti–4Al–2Fe/TiB nano-composite were processed by a novel spark plasma sintering route. KBF4 was used as an alternative and inexpensive boron precursor to form TiB reinforcement in situ during sintering. Fe was used as an alternative to vanadium to make the (α?+?β) Ti matrix. The processed Ti–4Al–2Fe alloy exhibited excellent mechanical properties (CS?=?1798 MPa). The TiB whiskers were distributed homogeneously and were fine (widths 130?nm and lengths from 100?nm to 3?µm). No residual TiB2 was found in the composite, in contrast with other methods. The TiB homogenised and refined the microstructure, while the hardness (710?HV), compressive strength (2414?MPa) and elastic modulus (140?GPa) all increased significantly when compared to the unreinforced alloy.     

Fabrication of nano-grained Ti–Nb–Zr biomaterials using spark plasma sintering

Nanostructured near-β Ti–20Nb–13Zr at % alloy with non-toxic elements and enhanced mechanical properties has been synthesized by spark plasma sintering (SPS) of nanocrystalline powders obtained by mechanical alloying. The consolidated bulk product was characterized by density measurements and Vickers hardness (HV), and X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) combined with energy-dispersive spectroscopy (EDX), and transmission electron microscopy (TEM) for structural details. The temperature during spark plasma sintering was varied between 800 and 1200 °C, while the heating rate and holding time of 100°K/min and 10 min were maintained constant in all the experiments. The effect of SPS temperature on the densification, microstructure, and HV was discussed. The results show that a nearly full density structure was obtained after SPS at 1200 °C. The microstructure of the obtained alloy is a duplex structure with the α-Ti (hcp) region having an average size of 70–140 nm, surrounding the β-Ti (bcc) matrix. The obtained alloy was chemically homogenized with a micro hardness value, HV of 660. The developed nanostructured Ti–20Nb–13Zr alloy is suggested for biomedical use as in implant material in dental and orthopedic applications.     

Studies on the mechanical properties of steel–TiB2 composites obtained by high pressure sintering

The currently used composites produced by classical sintering methods are characterised by numerous limitations due to the difficulties in combining different materials with extreme properties. One of the ways to overcome these limitations is in the use of modern sintering methods, including the high pressure-high temperature process. This study describes the composite materials based on 316L austenitic steel reinforced with titanium diboride and examines the effect of sintering conditions on the mechanical properties and microstructure of sintered composites. It has been found that the key parameter in the manufacture of composites with optimal properties is the sintering time and temperature, while martensitic transformation taking place in the composite matrix can be controlled by the properly selected pressure applied during the sintering process.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

Effect of Fe on the sintering and thermal properties of Mo–Cu composites

Effects of Fe on the sintering and thermal properties of Mo–Cu composites have been investigated. Mo–Cu–xFe composites are fabricated by powder metallurgy techniques with addition of various Fe contents ranging from 0.4 wt% to 2.2 wt%. The thermal properties and action mechanism of Fe to Mo–Cu composites are discussed. Results have indicated that the coefficient of thermal expansion (CTE) and thermal conductivity (TC) of Mo–Cu composites are greatly affected by the addition of Fe contents. It has also been observed that the fabricated composite powders with Fe additions exhibit high sinterability. Also, the inclusion of Fe can active the sintering course in shorter times and decline the sintering temperature thus also improving the physical properties of composites. Furthermore, it is also concluded that the utilization of steel kettle and steel balls for milling the Mo–Cu powders is also beneficial to improve the physical and thermal properties of Mo–Cu alloy.     

Effect of rare earth elements on plasma electrolytic carbonitriding of Ti–6Al–4V alloy

The paper aims to research the effect of rare earth elements on the carbonitriding layer of titanium alloy by establishing a theoretical model between rare earth concentration and atomic diffusion efficiency. It shows that adding rare earth can not only refine the discharge holes, but also reduce surface cracks, which significantly improve the surface quality of the strengthening layer. It is also found that an appropriate amount of rare earth can greatly increase the growth rate of the carbonitriding layer and carbon content near the surface. Furthermore, compared with the conventional plasma electrolytic carbonitriding treatment, adding 2?g?L^?1 cerium oxide can effectively reduce the activation energy of carbon, so as to improve its diffusion ability.     

Effect of Al content on hot tearing behaviour of Mg–xAl–2Ca–2Sm alloys

Mg–xAl–2Ca–2Sm (x?=?3, 5, 9 and 15) alloys were tested using an ‘L’-shaped sand mould serving as a hot tearing testing system. The experimental results showed that the solidification range of the Mg–xAl–2Ca–2Sm alloys first decreased and then increased as the Al content was increased. Furthermore, by increasing the Al content, the dendritic arms of the α-Mg phase become more developed, and the hot tearing tendency of the Mg–xAl–2Ca–2Sm alloys increased. In addition, the variety of precipitated phases was seen to be affected by the Al content and the tendency for hot tearing depended on the precipitated phase. The tendency of the Mg–xAl–2Ca–2Sm alloys for hot tearing first decreased and then increased with increasing Al content.     

Effect of post-curing on properties of carbon fibre–epoxy composites

Abstract

The effect of post-curing on the moisture absorption characteristics of Fibredux 914/T300 carbon fibre–epoxy composites, and hence on their thermomechanical behaviour, has been examined. Laminates 1 mm thick were post-cured at 190 or 210°C for 4 or 10 h. The various cross-link densities thus established had almost no effect on the moisture absorption behaviour. Interlaminar shear strength and torsion pendulum tests gave similar results, in that the cross-link density had almost no influence on the dynamic shear modulus or the mechanical dissipation factor. From these findings, environmental degradation of the composite is shown to depend on the content of absorbed water. The behaviour of the composite in hot, humid conditions therefore cannot be improved by post-curing treatment.

MST/400     

Effects of heat treatments on microstructure and tensile properties of as-extruded TiBw/near-α Ti composites

The effects of different heat treatments on the microstructure and tensile properties of the as-extruded TiBw/Ti60 composites have been investigated. Without solution treatment, the α₂ phase precipitated from α phase and mounts of elliptical S₂ silicide are formed at the interface of α/β phases after aging at 700 °C for 5 h, which results in clear enhancement of strength at 700–800 °C. After the solution and aging treatment, more α₂ phase precipitates from α_p phase and less spherical S₂ silicide precipitated from the body of both α_p and transformed β phase. Moreover, some small silicide precipitates are formed from α matrix near the TiBw due to the stacking faults and the interface effect. The TiBw are stable without any interfacial reaction during the heat treatments. The results show that the composites performed by solution and aging treatment exhibit high strength (1377 MPa) and low elongation (2%). The annealing treatment can soften the composites and increase the elongation to 7%.     

Effect of silicon content on the microstructure and properties of Fe–Cr–C hardfacing alloys

In this study, the surface of St52 steel was alloyed with preplaced powders 55Fe39Cr6C, 49Fe39Cr6C6Si, and 45Fe39Cr6C10Si using a tungsten-inert gas as the heat source. Following surface alloying, conventional characterization techniques, such as optical microscopy, scanning electron microscopy, and X-ray diffraction were employed to study the microstructure of the alloyed surface. Microhardness measurements were performed across the alloyed zone. Room-temperature dry sliding wear tests were used to compare the coatings in terms of their tribological behavior. It was found that the as-deposited coatings contained higher volume fractions of carbides (Cr₇C₃). The presence of 6%Si in the preplaced powders caused an increase in microhardness and wear resistance.     

Effect of Powder Particle Shape on the Properties of In Situ Ti–TiB Composite Materials Produced by Selective Laser Melting

This work studied the preparation of starting powder mixture influenced by milling time and its effect on the particle morphology(especially the shape) and, consequently, density and compression properties of in situ Ti–Ti B composite materials produced by selective laser melting(SLM) technology. Starting powder composite system was prepared by mixing 95 wt% commercially pure titanium(CP-Ti) and 5wt% titanium diboride(TiB 2) powders and subsequently milled for two different times(i.e. 2 h and 4 h).The milled powder mixtures after 2 h and 4 h show nearly spherical and irregular shape, respectively.Subsequently, the resultant Ti–5 wt% TiB 2 powder mixtures were used for SLM processing. Scanning electron microscopy image of the SLM-processed Ti–Ti B composite samples show needle-shape TiB phase distributed across the Ti matrix, which is the product of an in-situ chemical reaction between Ti and TiB 2during SLM. The Ti–Ti B composite samples prepared from 2 h and 4 h milled Ti–TiB 2 powders show different relative densities of 99.5% and 95.1%, respectively. Also, the compression properties such as ultimate strength and compression strain for the 99.5% dense composite samples is 1421 MPa and 17.8%, respectively, which are superior to those(883 MPa and 5.5%, respectively) for the 95.1% dense sample. The results indicate that once Ti and TiB 2 powders are connected firmly to each other and powder mixture of nearly spherical shape is obtained, there is no additional benefit in increasing the milling time and, instead, it has a negative effect on the density(i.e. increasing porosity level) of the Ti–Ti B composite materials and their mechanical properties.     

Effect of cyclic heat treatment parameters on the grain refinement of Ti–48Al–2Cr–2Nb alloy

This work presents the influence of individual parameters of a cyclic heat treatment, i.e. upper cycle temperature, heating and cooling rates between room temperature and an upper temperature and number of cycles, on the grain refinement of a Ti–48Al–2Cr–2Nb alloy. The grain size as determined by the value of the average plane section diameter and indices describing its distribution (standard deviation) is sufficient to define grain refinement when the refinement obtained as a result of the treatment concerns the whole section of a sample. In the event when the refinement process only takes place partially or locally it becomes necessary to present distributions of the grain plane section area as a function of the frequency of occurrence and area fraction. The paper presents a possible mechanism of grain refinement of Ti–48Al–2Cr–2Nb alloy. The use of cyclic heat treatment provides an increase in the mechanical parameters of the alloy.     

Effect of filler type on the mechanical properties of self-reinforced polylactide–calcium phosphate composites

Bioabsorbable polymers are of interest as internal fracture fixation devices. Self-reinforcement has been developed to improve the mechanical properties of the material and the addition of calcium phosphate fillers improves the bioactivity. Composite plates, produced by compression molding preimpregnated sheets of polylactide fibers coated in a polylactide matrix have been degraded in simulated body fluid for up to 12 weeks. Some samples also contained hydroxyapatite or tricalcium phosphate filler particles. Degradation was measured by monitoring the water uptake and mass decrease of the samples, as well as carrying out four point bend tests to assess the mechanical properties of the material. By 12 weeks, it was found that the unfilled samples absorbed more water and showed greater mass loss than the samples containing calcium phosphate fillers. Also, the flexural modulus and yield stress decreased significantly at week 12 for the unfilled samples. Adding hydroxyapatite (HA) or tricalcium phosphate (TCP) to the composite increased the flexural modulus and yield strength to values within the range of those reported for cortical bone and these values were maintained over the 12-week period.     

Effect of matrix strength on the mechanical properties of Al–Zn–Mg/SiCP composites

The mechanical properties of Al–Zn–Mg alloy reinforced with SiC_P composites prepared by solidification route were studied by altering the matrix strength with different heat treatments. With respect to the control alloy, the composites have shown similar ageing behaviour in terms of microhardness data at 135°C. It was shown that although composites exhibited enhanced modulus values, the strengthening was found to be dependent on the damage that is occurring during straining. Thus the initial matrix strength plays an important role in determining the strengthening. Consequently, compression data had shown a different trend compared to tension.     

Characterization of hydroxyapatite– and bioglass–316L fibre composites prepared by spark plasma sintering

To improve the mechanical properties of pure hydroxyapatite (HA) ceramics and pure 45S5 bioglasses, HA–316L fibre composites and bioglass 45S5–316L fibre composites were produced by spark plasma sintering (SPS) at 950 and 850 °C, respectively. While the HA phase in the HA–316L fibre composites did not decompose after the SPS process, microcracks were found around the 316L fibres in the composites. Consequently, the HA–316L fibre composites could not effectively improve the mechanical properties of the pure HA ceramics. In contrast, the bioglass 45S5–316L fibre composites showed no microcracks around the 316L fibres and thus exhibited bending strengths of up to 115 MPa.     

Effect of addition of graphite particulates on the wear behaviour in aluminium–silicon carbide–graphite composites

Aluminium matrix composites with multiple reinforcements (hybrid AMCs) are finding increased applications because of improved mechanical and tribological properties and hence are better substitutes for single reinforced composites. Few investigations have been reported on the tribological behaviour of these composites with % reinforcement above 10%. The present study focuses on the influence of addition of graphite (Gr) particulates as a second reinforcement on the tribological behaviour of aluminium matrix composites reinforced with silicon carbide (SiC) particulates. Dry sliding wear tests have been performed to study the influence of Gr particulates, load, sliding speed and sliding distance on the wear of hybrid composite specimens with combined % reinforcement of 2.5%, 5%, 7.5% and 10% with equal weight % of SiC and Gr particulates. Experiments are also conducted on composites with % reinforcement of SiC similar to hybrid composites for the sake of comparison. Parametric studies based on design of experiments (DOE) techniques indicate that the wear of hybrid composites decreases from 0.0234 g to 0.0221 g as the % reinforcement increases from 3% to 7.5%. But the wear has a tendency to increase beyond % reinforcement of 7.5% as its value is 0.0225 g at.% reinforcement of 10%. This trend is absent in case of composites reinforced with SiC alone. The values of wear of these composites are 0.0323 g, 0.0252 g and 0.0223 g, respectively, at.% reinforcement of 3%, 7.5% and 10% clearly indicating that hybrid composites exhibit better wear characteristics compared to composites reinforced with SiC alone. Load and sliding distance show a positive influence on wear implying increase of wear with increase of either load or sliding distance or both. Whereas speed shows a negative influence on wear indicating decrease of wear with increase of speed. Interactions among load, sliding speed and sliding distance are noticed in hybrid composites and this may be attributed to the addition of Gr particulates. Such interactions are not present in composite reinforced with SiC alone. Mathematical models are formulated to predict the wear of the composites.     

Effect of fabrication parameters on the microstructural quality of fibre–foil titanium metal matrix composites

The microstructure of fibre–foil Ti–6Al–4V (composition in weight per cent) and IMI 834 matrix metal matrix composites (MMCs), and corresponding foil-bonded alloys, are investigated in relation to fabrication parameters. Higher fabrication temperatures are required in IMI 834 MMCs, which results in a thicker interfacial reaction layer than in Ti–6Al–4V MMCs. The matrix microstructure in all materials is predominantly with intergranular , as a result of the slow cooling rate. MMCs reinforced with SM1240 fibres exhibit boron precipitates along foil bond lines, owing to diffusion during consolidation. Fabrication using fibre mats with 7.1 fibres per millimeter (FPM) results in an excellent microstructure in (Ti–6Al–4V)–SM1240. The larger diameter of the SM1140+fibre compared with SM1240 means that (Ti–6Al–4V)–SM1140+requires FPM significantly below 7.1 in order to produce acceptable microstructural quality. The higher residual stresses in IMI 834 MMCs result in cracking of the matrix and fibre–matrix interfacial region when a FPM of 7.1 is used. Acceptable microstructural quality is observed in IMI 834 MMCs when the FPM of fibre mats is reduced to 6.3. Interfibre cracking in IMI 834–SM1140+is enhanced by a higher matrix microhardness than the other materials. This high hardness may be caused by a high matrix carbon content.