Processing and microstructural characterisation of RBSiC-TaSi2 composites

Reaction bonded silicon carbide (RBSiC) ceramics typically contain 10 vol% silicon inherent to the reaction bonding process. However, the relatively low melting point (1410°C) of the silicon phase is a limiting factor in the high temperature use of RBSiC materials. The application temperature can potentially be enhanced by replacing the silicon with more refractory metal disilicide phases. In this paper we report the infiltration of SiC-graphite compacts with alloyed Si-Ta melts in an attempt to precipitate TaSi2 (Tm=2040°C) in place of the residual silicon. High density RBSiC-TaSi2 ceramics with virtually no porosity were readily produced, but subsequent XRD and SEM examination revealed that the silicon phase was not completely removed. In addition, the materials possessed complex, inhomogeneous microstructures and were susceptible to various types of crack formation phenomena.     

Processing of Al/B4C composites by cross-roll accumulative roll bonding

In the present study, Al/B₄C composites were successfully produced in the form of sheets, through accumulative roll bonding (ARB) and cross-roll accumulative roll bonding (CRARB) processes. The CRARB process was performed in two steps. In the first step, the strips were roll-bonded with a draft percentage of 66% reduction, while in the second step the strips were roll-bonded with a draft percentage of 50%. The results indicated that the dispersion of the B₄C particles in the CRARB process is more homogeneous than the ARB process. In addition, the tensile strength of the CRARBed composite is higher than that of the ARBed composite.     

Microstructural evolution and mechanical properties of ultrafine grained Al3Ti/Al–5.5Cu composites produced via hot pressing and subsequent friction stir processing

In situ Al₃Ti/Al–5.5Cu composites fabricated by powder metallurgy and subsequent forging were subjected to multiple pass friction stir processing (FSP) with and without active cooling. The forged sample exhibited lower strength and ductility due to the presence of coarse Al₃Ti clusters with a size range of 50–100 μm and coarse matrix grains. Four-pass FSP in air resulted in the refinement and redistribution of the Al₃Ti clusters, and the generation of micron matrix grains, thereby increasing the strength and ductility of the composites. Furthermore, coarse Al₂Cu particles dissolved and re-precipitated due to a relatively long duration of thermal exposure. Additional two pass FSP with rapid water cooling (FSP-water) dissolved most of the Al₂Cu into the matrix and retained the solutes in solution due to the short duration of thermal exposure. Meanwhile, ultrafine matrix grains with a high density of dislocations were obtained. These microstructural changes led to significant increase in strength and a decrease in ductility in the FSP-water sample. After aging, the FSP-water sample exhibited further increased yield strength and ultimate tensile strength due to the precipitation of metastable Al₂Cu phases. However, the ductility did not decrease due to the decrease of dislocation density after aging.     

Fabrication of toughened Cf/SiC whisker composites and their mechanical properties

In this paper, dense short carbon fiber reinforced silicon carbide matrix composites had been fabricated by hot-pressed (HP) sintering using Al₂O₃ and La₂O₃ as sintering additives. The results showed that the combination of Al₂O₃ and La₂O₃ system was effective to promote densification of short cut carbon fiber reinforced silicon carbide composites (Cf/SiC). The whisker structure of silicon carbide was formed during the annealed treatment at 2023 K for 1 h. However, it was noted that this structure was not observed in the as-received HP material. The mechanism of forming whisker structure was not clear, but this kind of whisker structure was helpful to improve mechanical properties. The combination of grain bridging, crack deflection and whisker debonding would improve the fracture toughness of the Cf/SiC composites.     

Microstructure and room temperature fracture toughness of cast Nbss/silicides composites alloyed with Hf

The effect of Hf addition on microstructure and room temperature fracture toughness of cast Nb-16Si alloy was investigated. The Hf addition changes significantly the microstructural morphology of Nb-16Si alloys, which includes microstructure refinement and disappearance of eutectic colonies. Fracture toughness of the alloys improves with increasing Hf content. The improvement in fracture toughness is mainly attributed to the microstructural change by Hf addition. The Hf addition leads to a transition of Nb solid solution fracture manner from brittle cleavage to plastic stretching.     

Microstructural studies and wear assessments of Ti/TiC surface composite coatings on commercial pure Ti produced by titanium cored wires and TIG process

Tungsten Inert Gas (TIG) process and titanium cored wires filled with micro size TiC particles were employed to produce surface composite coatings on commercial pure Ti substrate for wear resistance improvement. Wire drawing process was utilized to produce several cored wires from titanium strips and titanium carbide powders. Subsequently, these cored wires were melted and coated on commercial pure Ti using TIG process. This procedure was repeated at different current intensities and welding travel speeds. Composite coating tracks were found to be affected by TIG heat input. The microstructural studies using optical and scanning electron microscopy supported by X-ray diffraction showed that the surface composite coatings consisted of α′-Ti, spherical and dendritic TiC particles. Also, greater volume fractions of TiC particles in the coatings were found at lower heat input. A maximum microhardness value of about 1100 HV was measured which is more than 7 times higher than the substrate material. Pin-on-disk wear tests exhibited a better performance of the surface composite coatings than the untreated material which was attributed to the presence of TiC particles in the microstructure.     

Formation mechanism of in situ Al3Ti in Al matrix during hot pressing and subsequent friction stir processing

In situ Al₃Ti/Al composites were fabricated by a combination of vacuum hot pressing (VHP) and friction stir processing (FSP). The formation mechanism of the Al₃Ti and the effect of VHP and FSP parameters on the resultant microstructure and mechanical properties were investigated. The Al₃Ti formed due to the reactive diffusion between Al and Ti during VHP, and the number of Al₃Ti particles increased with increasing the temperature and holding time of the VHP. FSP not only induced the Al–Ti reaction, but also resulted in significant refining of the Al₃Ti, thereby creating a homogeneous distribution of Al₃Ti particles in the Al matrix. These microstructural changes led to significant improvement in the tensile properties of the in situ Al₃Ti/Al composite. However, the change trends of the tensile properties of the FSP samples were dependent on the extent of the Al–Ti reaction during VHP.     

Synthesis of a nanostructured MgH2-Ti alloy composite for hydrogen storage via combined vacuum arc remelting and mechanical alloying

To improve the hydrogen kinetics of magnesium hydride, TiCr_1.2Fe_0.6 alloy was prepared by vacuum arc remelting (VAR) and the alloy was co-milled with MgH₂ to process nanostructured MgH₂-5 at.% TiCr_1.2Fe_0.6 powder. The hydrogen desorption properties of the composite powder were studied and compared with pure magnesium hydride. X-ray diffraction (XRD) analysis showed that the composite powder prepared by VAR/mechanical alloying (MA) procedure consisted of β-MgH₂, γ-MgH₂, bcc Ti-Cr-Fe alloy, and small amount of MgO. The average size of particles and their grain structure after 4 h MA were determined by a laser particle size analyzer and XRD method and found to be 194 nm and 11 nm, respectively. The hydrogen desorption temperature of magnesium hydride decreased from 426 °C to 235 °C by the bcc Ti alloy and the utilized processing method, i.e. combined VAR/MA.     

Improved mechanical property of foam glass composites toughened by glass fiber

The mechanically improved foam glass composite toughened by glass fiber was prepared by sintering technique, using waste sodium-calcium silicon flat glass powder as main raw materials. In this study, the preparation and properties of the samples were characterized by differential thermal analysis (DTA), field-emission scanning electron microscopy (FESEM) and mechanical property test. The specific strength of the composite was defined for the first time, and applied into the investigation of mechanical property. The results show that the specific improved bending strength of 10.45-22.26 MPa/(g cm^− 3), and the specific compressive strength of 30.45-34.34 MPa/(g cm^− 3) can be displayed when sintered at 790-815 °C with the addition of 5-25 wt.% glass fiber. Good correlations between the microstructure (in particular the fiber distribution), the high specific strength and the high modulus of elasticity of glass fibers.     

Delamination buckling growth in laminated composites using layerwise-interface element

In this paper, a numerical investigation on the buckling of composite laminates containing delamination, under in-plane compressive loads, is presented. For this purpose, delamination propagation is modeled using the softening behavior of interface elements. The full layerwise plate theory is applied for approximating the displacement field of laminates and the interface elements are considered as a numerical layer between any two adjacent layers where the delamination is expected to propagate. A non-linear computer code was developed to handle the numerical procedure of delamination buckling growth in composite laminates using layerwise-interface elements. The load/displacement behavior and the contours of embedded and through-the-width delamination propagation for composite laminates are presented. It is shown that delamination growth can be well predicted using this layerwise-interface elements with decohesive law.     

Rapid fabrication of in situ TiC particulates reinforced Fe-based composites by spark plasma sintering

TiC particulates reinforced Fe-based composites have been fabricated using ferrotitanium and carbon black powders with the combination of in situ and spark plasma sintering (SPS) technique. The sintering and densification behaviors were investigated. The results show that when the composite was sintered at 1150 °C for 5 min, the maximum relative density and hardness are 99.2% and 83.2 HRA, respectively. The phase evolution during sintering indicates that the in situ reaction occurs evidently between 850 °C and 1050 °C. The microstructure investigation demonstrates that with the rapid in situ SPS technique, fine TiC particulates with a size of ~ 1 μm are homogeneously distributed in the matrix.     

Synthesis and characterization of conductive copper-based metal-organic framework/graphene-like composites

Metal-organic framework (MOF) incorporating conductive graphene-like layers were synthesized and characterized. The selected MOF, HKUST-1, combines high surface area, water stability, simple preparation and low costs. Graphene-like layers incorporated into the MOF structure were obtained by a two-step oxidation/reduction wet treatment of a high surface carbon black. MOF composites were produced at different carbonaceous layers content. It was shown, through a wide characterization of the samples, that the composites preserve the main features of the parent MOF, additional exhibiting a tunable electrical conductivity.     

Oxidation of Au4Al in gold ballbonds

Two different 4N (99.99% purity) gold wires were ballbonded on 1 μm thick Al-1 wt.% Si-0.5 wt.% Cu bondpad metallisation and subjected to high temperature storage (HTS) at 175 °C in air. Each wire type showed ball lift failures, Type A after 500 h and Type B after 1500 h, which in both cases was a result of Au₄Al oxidation. With wire Type A the dominant compound underneath the ball was Au₈Al₃. A thin layer of Au₄Al (≈ 1 μm thick) was observed between the Au₈Al₃ and the gold ball. Ball lift failures occurred in the Au₄Al layer, which appeared to disintegrate due to oxidation and the resulting by products of oxidation were deposited on the underlying and unoxidised Au₈Al₃. With wire Type B, a double layer Au₄Al was dominant after long term ageing and Au₈Al₃ was confined to the ball periphery. Consequently, because of the much greater volume of Au₄Al, compound oxidation resulted in the formation of a large amount of a completely new microstructure consisting of gold precipitates embedded in a dark oxide matrix. The Au₈Al₃ compound remained unoxidised. It is speculated that internal stress and contamination may accelerate the oxidation reaction.     

Bending properties of epoxy resin matrix composites filled with Ni-Mn-Ga ferromagnetic shape memory alloy powders

Two types of epoxy resin matrix composites filled with Ni-Mn-Ga ferromagnetic shape memory alloy powders, with and without magnetic field during curing, were prepared, aiming to create anisotropic and isotropic alignments of the Ni-Mn-Ga particles. The bending properties of the composites are found to be significantly influenced by the distribution of Ni-Mn-Ga particles in the epoxy matrix. The composites without magnetic field during curing exhibit larger bending strength and fracture strain than the composites with magnetic field during curing. The fractography reveals that the Ni-Mn-Ga particle chains formed in the composites with magnetic field during curing facilitate the initiation and propagation of cracks, thus weakening the bending properties.     

Effect of C/Ti ratio on the laser ignited self-propagating high-temperature synthesis reaction of Al-Ti-C system for fabricating TiC/Al composites

TiC/Al composite was successfully synthesized utilizing laser ignited self-propagating high-temperature synthesis (SHS) of Al-C-Ti system with the different C/Ti molar ratio. When the molar ratio of C to Ti is below 1:1 in the starting materials, in addition to fine TiC particulates, a large amount of Al₃Ti phase was found in the composites; however, when the molar ratio of C to Ti is 1:1 in the starting materials, the Al₃Ti phase was almost completely eliminated and the distribution of TiC particulates generally appeared to be more homogeneous throughout the products synthesized.     

An analysis of intermetallics formation of gold and copper ball bonding on thermal aging

In IC packages, thermosonic wire bonding is the preferred method for making electrical connections between the die pad and lead frame. These interconnect are made using fine metal wires. On thermal aging (under 175 °C for 5 h) gold aluminide easily forms in gold ball bonds while formation of intermetallic compound is absent in case of copper ball bonds. An analysis of the atomic property of the elements bonded explains that atomic radii and electronegativity factors favor gold aluminide formation.     

Design and optimization of symmetric laminated composites using a variable neighbourhood search-based model

The development of society is still marked by the need for lighter and stronger structures. The materials that respond best to these needs are composite materials. Designing composite materials is difficult as it involves designing the geometry and their composition. Traditionally, the design tasks have been based on approximate methods; the possibility for creating composite materials is almost unlimited, characterization by testing is very expensive and it is difficult to apply the results to other contexts. This article proposes a variable neighbourhood search-based model for the design of symmetric laminated composites, a general encoding for the design of composites, an evaluation function that has taken into consideration cost and safety criteria in design, the neighbourhood structures and a set of local search operators. The proposed model has been applied to different real-world problems and the results have been compared with other well-known design methods.     

Investigation of thermal stability of Mo thin-films as the buffer layer and various Cu metallization as interconnection materials for thin film transistor-liquid crystal display applications

Cu/Mo/Si multi-layer structures were fabricated to investigate diffusion behaviors and thermal stability between Cu and Mo. Physical vapor deposition (PVD), chemical vapor deposition, electroplating and electrolessplating were used to grow 100 nm thick Cu films as interconnection materials, and radio-frequency sputtering system was introduced to grow 37.5 nm thick Mo films as a buffer layer. All Cu/Mo/Si multi-layer specimens were annealed at 350 to 700 °C for 30 min. When the annealing temperature was over 600 °C, the Cu diffused through Mo into Si, and the Cu₃Si phase and Mo-Si intermetallic compounds formed at the Mo/Si interface. The diffusion mechanism is the grain boundary diffusion. The results indicate that Cu film deposited by PVD had best crystallinity, lower roughness, large adhesive energy and resistivity. The values of the resistivity, diffusion activity energy and large adhesive energy are 5.47 μΩ-cm, 0.948 eV and 2.46 N/m, respectively.     

WC–Co particulate reinforcing Cu matrix composites produced by direct laser sintering

Experimental investigations on the development of the WC–10%Co particulate reinforcing Cu matrix composite material have been conducted using direct laser sintering. The chemical compositions and microstructures of the laser processed material have been characterized using X-ray diffraction (XRD), energy disperse X-ray (EDX) spectroscopy, scanning electron microscopy (SEM), and atomic force microscope (AFM). An excellent interfacial bonding between the reinforcement and the matrix was obtained. The WC reinforcing particulates typically had two distinct morphologies, i.e., partially dissolved and smoothed or completely dissolved and refined. The effects of the WC–Co content on the microstructural characteristics and resultant properties of the laser sintered parts have been studied. It was found that lowering the amount of WC–Co resulted in the insufficient reinforcement, while at a higher amount of WC–Co the significant agglomeration of the WC reinforcing particulates occurred. A homogeneous sintered structure with a high average hardness of HV_0.1384.6 was obtainable using 30 wt.% WC–Co.     

Mechanical properties of ABOw+MWNTs/Al hybrid composites made by squeeze cast technique

Multi-wall carbon nanotubes (MWNTs) have been considered a realistic kind of reinforcement for composite materials. In this paper, microstructure and mechanical properties of the aluminum borate whisker (ABOw) and MWNTs hybrid composites were investigated. The results show that MWNTs decrease the compressive deformation of the hybrid preforms and are kept intact in the matrix during squeeze cast processing. A small amount of MWNTs may effectively improve the modulus, strength and elongation of the hybrid composite. Decreasing micropores and strengthening the matrix, high strength MWNTs make the mechanical properties of the hybrid composite superior to the singularly reinforced ones. This makes MWNTs a promising material for novel micro/nanohybrid composite.