Microstructure and properties of Sip/Al–20 wt% Si composite prepared by hot-pressed sintering technology

In the present work, 50 vol% Sip/Al–20Si composite was prepared by hot-pressed sintering technology. Si particles were uniformly distributed in the Sip/Al–20Si composite, and only the presence of Si and Al phases were detected by XRD analysis. Dislocations, twins, and stacking faults were found in the Si particles. Several Si phases were found to be precipitated between Al matrix and Si particles. Si/Al interface was clean, smooth, and free from interfacial product. HRTEM indicated that the Si/Al interface was well bonded. The average CTE and thermal conductivity (TC) of Sip/Al–20Si composite were 11.7 × 10^?6/°C and 118 W/(m K), respectively. Sip/Al–20Si composite also demonstrated high mechanical properties (bending strength of 386 MPa). Thus, the comprehensive performance (low density and CTE, high TC, and mechanical properties) makes the Sip/Al–20Si composite very attractive for application in electron packaging.     

Microstructures and properties of high-fraction Sip–6061Al composites fabricated by pressureless sintering

Pre-treated Si powder (Sip) and 6061Al powder were used to fabricate high-fraction Sip/6061Al composites via pressureless sintering, and the effects of the Sip content and the sintering temperature on the microstructures and properties of the composites were studied. The results show that in the composites, there exist MgAl₂O₄ nanocrystalline particles, and the Si phase varied from a discontinuous particulate state to a semi-continuous skeleton state as the Si content increased from 30 to 50?wt-%. Densities, bending strengths, hardness, and thermal conductivities of the composites all increased initially and then decreased with the sintering temperature. The 680°C sintered 30?wt-% Sip/6061Al composites and the 700°C sintered 50?wt-% Sip/6061Al composites have the optimal mechanical and thermophysical properties.     

Microstructure and mechanical properties of Lead-free Sn–Cu solder composites prepared by rapid directional solidification

In wave soldering Sn-Cu alloy was recommended as a promising substitute of traditional Sn–Pb alloy. Here a rapid directional solidification process was firstly adopted to prepare directionally-solidified hypereutectic Sn–Cu composites. The corresponding microstructure exhibits regular lamellar structures with alterative Sn-rich phase and intermetallic compounds. The large kinetic undercooling arising from the rapid solidification condition is the inherent mechanism to obtain directionally-solidified lamellar structures for the hypereutectic Sn–Cu solder. Additionally, the measured compressive mechanical properties of the directionally solidified solder exhibit anisotropic characteristics, that is, the compressive strength along the longitudinal direction is 1.63 times higher than that along the transverse direction.     

Microstructure and mechanical properties of cast (Al–Si)/SiCp composites produced by liquid and semisolid double stirring process

Abstract

A stirring process containing two steps, i.e. liquid and then semisolid stirring, was used to produce SiC particle reinforced aluminium matrix composites. The major advantages of this process are that full wetting of SiC particles by molten aluminium can be readily achieved at relatively low stirring rates, and undesirable Al₄ C₃ is not formed at the Al/SiC interface due to lower processing temperatures. Cast Al–Si matrix composites reinforced with 15 and 20 vol.-%SiC particles were produced in the present work. The mechanical properties of the composites were evaluated under the conditions of investment mould casting and heat treatment. For the composites obtained without employing semisolid stirring, the aggregation of SiC particles observed in the microstructure of composites resulted in quite poor mechanical properties. Observations and analyses indicated that some Al/SiC interfaces were very clean, and a reaction product of spinel MgAl₂O₄ was also found at some Al/SiC interfaces. Silicon dioxide (SiO₂ ) was found to exist on the surface of as purchased and 250°C dried SiC powders. This SiO₂ is involved in the spinel reaction at the interface between the SiC particles and the matrix in the present Al/SiC composites.     

Processing and superplastic properties of fine grained Si3N4/Al–Mg–Si composites

Abstract

Composites with an Al–Mg–Si alloy matrix containing 20 vol.-% of either Si₃N₄ whiskers or Si₃N₄ particulates were extruded at 773 K with a reduction ratio of 100: 1, and tensile experiments were performed under conditions of constant true strain rate. Recrystallisation and dynamic precipitation occurred during hot extrusion so that very small grain sizes of less than ~ 3 ;amp;#x03BC;m were produced. The extruded composites showed superplastic behaviour at high strain rates (above 10^?1 S^?1). The high strain rate superplasticity of the composites is attributed to the very small grain sizes. Internal cavities developed during straining and density studies revealed that the rate of increase of the extent of cavitation was lower at a temperature slightly above the partial melting temperature than at a temperature lower than the partial melting temperature. It is concluded that the presence of a liquid phase restricts the development of cavities because the liquid phase serves to relax the stress concentrations.

MST/3139     

Microstructure and properties of the Si3N4/silica aerogel composites fabricated by the sol–gel method via ambient pressure drying

Si₃N₄ particle reinforced silica aerogel composites have been fabricated by the sol–gel method via ambient pressure drying. The microstructure and mechanical, thermal insulation and dielectric properties of the composites were investigated. The effect of the Si₃N₄ content on the microstructure and properties were also clarified. The results indicate that the obtained mesoporous composites exhibit low thermal conductivity (0.024–0.072 Wm^− 1 K^− 1), low dielectric constant (1.55–1.85) and low loss tangent (0.005–0.007). As the Si₃N₄ content increased from 5 to 20 vol.%, the compressive strength and the flexural strength of the composites increased from 3.21 to 12.05 MPa and from 0.36 to 2.45 MPa, respectively. The obtained composites exhibit considerable promise in wave transparency and thermal insulation functional integration applications.     

Microstructure and mechanical properties of TiC/Ti–6Al–4V composites processed by in situ casting route

Abstract

TiC/Ti–6Al–4V composites containing various volume fractions of TiC were produced by induction skull melting and common casting utilising in situ reaction between titanium and carbon powder. The microstructure and room tensile properties of as cast and heat treated TiC/Ti–6Al–4V composites were investigated. Bar-like or small globular eutectic TiC were found in 5 vol.-%TiC/Ti–6Al–4V composite, whereas the equiaxed or dendritic primary TiC particles were found to be the main reinforcements in 10 and 15 vol.-%TiC/Ti–6Al–4V composites. The as cast TiC/Ti–6Al–4V composites have shown higher strength but lower ductility than those of monolithic Ti–6Al–4V alloy. The shape and fracture of TiC particles can strongly influence the fracture and failure of the composites, and so the ultimate tensile strengths and elongations of as cast composites reduce with the increase in volume fraction of TiC. TiC particles appear to be spheroidised, and titanium precipitation can be found within large TiC particles after heat treatment at 1050°C for 8 h, which can promote the resistance to fracture of composites. Therefore, the elongations of the composites increase significantly, and the ultimate tensile strengths also have marginal increase especially for the 10 and 15 vol.-%TiC/Ti–6Al–4V composites after heat treatment.     

Processing and properties of Al–Li–SiCp composites

Al–Li–SiC_p composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 mm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al–Li–8%SiC and Al–Li–12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al–Li–8%SiC and Al–Li–12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al–Li–18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macroand microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ^' (Al₃Li) and S^' (Al₂CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al–Li–12%SiC, which shows marginally lower UTS and compressive strength than the Al–Li–8%SiC composite in extruded condition, exhibits higher strength than Al–Li–8%SiC in peak-aged condition.     

Optical and morphological characterization of Si nanocrystals/silica composites prepared by sol–gel processing

Linear (visible photoluminescence) and non-linear (third-order susceptibility) optical properties have been measured for laser-synthesized crystalline Si nanoparticles. The observed luminescence is compared to similar measurements on nanocomposite aerogels prepared integrating the Si-nanoparticles into a continuous silica phase by sol–gel processing. The results show that incorporation of the Si-nanoparticles into a silica matrix by sol–gel technology is not detrimental to the PL (photoluminescence) emission intensity. The effect on the nanocomposites of annealing in air (at temperatures in the range of 300–500°C) has been investigated by means of PL and Raman spectroscopy. The obtained results indicate that the oxidation treatment induces the bleaching of the smaller luminescent particles and is not effective in reducing the average size. Micro-Raman mapping and TEM analysis of the nanocomposites have shown the presence of agglomerated nanoparticles (in the sub-micrometer range), as confirmed by dynamic light scattering (DLS) measurements on colloidal suspensions of the as-synthesized powders.     

Synthesis and properties analysis of char-reinforced Al–13.5Si–2.5Mg alloy composites

The re-evaluation of previous and existing methods in materials processing is becoming ever more critical because of processing and starting materials cost factors. A study on the synthesis and properties investigation of hypereutectic Al–13.5Si–2.5Mg alloy reinforced with carbon chars using coconut shell as the organic precursor has been carried out. The low-cost, double compaction solid-state technique was used. Reinforcing the hypereutectic alloy with coconut shell char particles (size:<140 m) at 2 vol % and consolidating by reaction sintering at 600 °C in vacuum for 15 min, followed by near net-shape compaction at 250 MPa, increased the hardness of the alloy 6% while reducing its strength (UTS) by only 3%. The use of palm kernel shell char as the dispersed phase was found to yield identical results. At 2 vol % char, the mechanical properties, sintered density and dimensional changes were optimally found to be suitable for lightweight anti-friction electromechanical applications. Attempts to reinforce the alloy with 2 vol % coconut shell chars activated in CO2 reduced its strength in the range of 19 to 26% at different burn-off percentages. This is attributed to the higher amount of oxide products formed during the activation process. At 600 °C, formation of the brittle Al4C3 phase in the different sintered composites containing activated and unactivated chars was identified by X-ray studies. © 1998 Chapman & Hall     

Microstructure and Mechanical Properties of Rapidly Solidified Hypereutectic Al–Si and Al–Si–Fe Alloys

The microstructure and mechanical properties of rapidly solidified Al–18 wt% Si and Al–18 wt% Si–5 wt% Fe alloys were investigated by a combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile testing, and wear testing. The centrifugally atomized binary alloy powder consisted of the -Al (slightly supersaturated with Si) and Si phases and the ternary alloy powder consisted of the -Al (slightly supersaturated with Si), silicon, and needle-like metastable Al–Fe–Si intermetallic phases. During extrusion the metastable -Al₄FeSi₂ phase in the as-solidified ternary alloy transformed to the equilibrium -Al₅FeSi phase. The tensile strength of both the binary and the ternary alloys decreased with a high-temperature exposure, but a significant fraction of the strength was retained up to 573 K. The specific wear gradually increased with increasing sliding speed but decreased with the addition of 5 wt% Fe to the Al–18 wt% Si alloy. The wear resistance improved with annealing due to coarsening of the silicon particles.     

Microstructure and mechanical properties of Al–Si cast alloy with additions of Zr–V–Ti

The microstructure and tensile properties at temperatures up to 300 °C of an experimental Al–7Si–1Cu–0.5Mg (wt.%) cast alloy with additions of Ti, V and Zr were assessed and compared with those of the commercial A380 grade. The microstructure of both alloys consisted of Al dendrites surrounded by Al–Si eutectic containing, within its structure, the ternary Al–Al₂Cu–Si phase. Whereas the Al₁₅(FeCrMn)₃Si₂ phases were present in the A380 alloy, Ti/Zr/V together with Al and Si phases, Al(ZrTiV)Si, were identified in the experimental alloy. As a result of chemistry modification the experimental alloy achieved from 20% to 40% higher strength and from 1.5 to 5 times higher ductility than the A380 reference grade. The role of chemistry in improving the alloy thermal stability is discussed.     

Microstructure evolution and mechanical properties of Nb-alloyed Cu–Zr–Al–Ni large-sized metallic glass composites

The effects of Nb on the microstructures and mechanical properties of large-sized (Cu_0.47Zr_0.47Al_0.06)_99???xNi₁Nb_x (x?=?0, 0.5, 1, 2?at.-%) bulk metallic glass composites were investigated. It is verified that the liquidus temperature (T_l) of the Nb-added alloys decreases to cause the increase of glass-forming ability (GFA). The addition of Nb adjusts the distribution and the volume fraction of B2-CuZr phase in the Cu–Zr–Al–Ni large-sized composites by changing the GFA of the alloys. The mechanical properties of the composites strongly depend on the volume fraction and distribution of B2-CuZr phase in the glassy matrix. The alloy with 0.5?at.-% Nb addition exhibits the high mechanical properties, which should be attributed to the uniform distribution and the proper volume fraction of B2-CuZr phase in the glassy matrix.     

Microstructure and magnetic properties of nanostructured Fe–Co powders prepared by series of milling and annealing treatments

Fe_1−xCo_x(x = 0.1, 0.15, 0.2, 0.25, 0.3 and 0.5) powders were prepared by different milling-annealing treatments, and magnetic properties were investigated based on microstructure. Elevated heating times led to an increase in crystallite size, and decrease in lattice parameter. Up to 20 min annealing, series 3 powders showed a decrease in microstrain 2.5 times more than series 2. The coercivity (H_C) of 1-step milled and 60 min annealed Fe₅₀Co₅₀ alloys decreased rigorously from 60 Oe to 19 Oe due to strain relief (from 0.3% to 0.08%) and grain growth (from 30 nm to 40 nm). For series 2 alloys, the H_C (up to 60 min heating) increased from 72 Oe to 90 Oe, and decreased (up to 100 min heating) to 70 Oe. Compared to series 1, extra milling treatment of series 2 causes an increase in magnetization saturation (M_S) due to completion of alloying and grain refinement. Also, compared to series 2, extra annealing treatment for series 3 resulted in larger values of M_S caused by stain relief.     

Microstructure evolution and elemental diffusion of SiCp/Al–Cu–Mg composites prepared from elemental powder during hot pressing

15vol%SiCp/Al–Cu–Mg composites were fabricated by hot pressing method using pure elemental powders. Microstructure evolution and elemental diffusion of Cu and Mg were studied. The microstructure of as-hot pressed composites and the elemental distribution of the composites before and after solution treatment were also investigated. The results showed that there were two types of eutectic liquid phases with different compositions after the compact was heated to 580 °C. After the compact was held at 580 °C for 60 min, the eutectic liquid was absorbed into the Al matrix and some equilibrium liquid phases formed in the boundaries of the initial Al particles. Meanwhile, Cu was homogeneously distributed in the Al particles while Mg tended to be distributed near the boundaries of the initial Al particles and in the SiC clusters. The presence of Al₂Cu, Mg₂Si, and some oxides of Mg was identified in the as-hot pressed composite. After solution treatment, Al₂Cu dissolved into the Al matrix, however, some Mg-rich compounds (silicide and oxide of Mg) did not dissolve into the matrix completely.     

Investigation of thermal properties of Al–Si matrix reinforced fine SiCp composites

Abstract

The characterisation of thermal expansion coefficient and thermal conductivity of Al–Si matrix alloy and Al–Si alloy reinforced with fine SiC_p (5 and 20 wt-%) composites fabricated by stir casting process are investigated. The results show that with increasing temperature up to 350°C, thermal expansion of composites increases and slowly reduces when the temperature reaches to 500°C. The values of both thermal expansion and conductivity of composites are less than those for Al–Si matrix. Microstructure and particles/matrix interface properties play an important role in the thermal properties of composites. Thermal properties of composites are strongly dependent on the weight percentage of SiC_p.     

Microstructure and mechanical properties of directionally solidified Al–Si eutectic alloys with and without antimony

Abstract

Hardness H, interjlake spacing λ, and tensile properties are reported for Al–12·7Si and Al–12·7Si–0·2Sb (all wt-%) eutectic alloys directionally solidified at growth velocities of up to 250 μm s^?1 and under temperature gradients in the liquid of up to 12·9 K mm^?1. The hardness is related to interflake spacing by the equation H=H^o+Kλ^?0·2, where H^o is the initial hardness of the alloy. This behaviour contradicts previous results, which suggest that a Hall–Petch relationship is followed. The tensile properties are shown to follow similar behaviour, confirming that hardness shows the same dependence as proof stress on interflake spacing. However, the nature of the relationship depends on the Si morphology and caution should be exercised in using hardness or interflake spacing to indicate proof stress.

MST/1585     

Microstructure and mechanical properties of Al-base composites by addition of Al–Ni–Co decagonal quasicrystalline particles through a mechanical stirring route

In the present study, the A356Al-base composite materials were fabricated by introducing 2.5, 5, 7.5, 10 mass% of Al–Ni–Co decagonal quasicrystalline particles using the mechanical stirring method. The microstructures, mechanical properties, and Brinell hardness of these composites were investigated in detail by means of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that serious compositional diffusion occurs between the Al₇₂Ni₁₂Co₁₆ quasicrystalline particles and the Al melt. Microstructural analysis of all as-cast composites shows that the structure of the quasicrystal disappears and is replaced by the formation of two crystalline phases, Co-rich θ-phase and Ni-rich γ-phase which all contain Al, Si, Ni, and Co. The particle sizes of the two crystalline phases are much smaller than that of the original decagonal quasicrystalline phase. The composites exhibit improvement of 10.5–24% and 20–25% in yield strength and Brinell hardness, respectively, while the percent elongation decreases obviously. Examination of the fracture surface of the as-cast A356Al-base composites shows that they exhibit typical brittle fracture mode.     

Microstructure and mechanical properties of zirconia-toughened lithium disilicate glass–ceramic composites

The lithium disilicate glass–ceramics composites reinforced and toughened by tetragonal zirconia (3Y-TZP) were prepared by hot-pressing at 800 °C with varying zirconia content from 0 to 30 wt.%. In the case of the composites of small zirconia content (below 10 wt.%), zirconia acted as nucleation agent primarily, and the microstructure was refined continuously. The morphology of Li₂Si₂O₅ crystals transformed from rod-shaped to spherical structure, and the mechanical properties decreased inevitably. For the composites of large zirconia content (from 15 wt.% to 30 wt.%), however, zirconia restrained the phase separation of glass. The morphology of Li₂Si₂O₅ crystals transformed to rod-shaped structure again. The mechanical properties of the composite at zirconia content of 15 wt.% increased up to 340 MPa and 3.5 MPa m^1/2 which were much higher than those of zirconia-free glass–ceramics. The improved properties were attributed mainly to compressive stress reinforcement, phase transformation and bridging toughening mechanisms.     

Microstructure and mechanical properties of Al–Si–Mg–Cu–Ti alloy with trace amounts of scandium

The effect of Sc on the microstructure and mechanical properties of Al–Si–Mg–Cu–Ti alloy was investigated. Results obtained in this research indicate that, with increasing Sc content, the microstructure of the investigated alloys exhibits finer equiaxed dendrites with rounded edges and the morphology of the eutectic Si shows a complete transition from a coarse needle-like structure to a fine fibrous structure upon modification of eutectic Si. Subsequent T6 heat treatment had further induced the precipitation of nano-scaled secondary Al₃(Sc, Ti) phase, as well as spheroidisation of eutectic Si. Combined with T6 heat treatment, the ultimate tensile strength, yield strength, percentage elongation and hardness were achieved in 0.20?wt-% Sc-modified alloy.