Studies on machinability of Al/Si<Subscript>p</Subscript> + SiC<Subscript>p</Subscript> composite materials

This paper presents the experimental results on the machinability of silicon and silicon carbide particles (SiC_p) reinforced aluminium matrix composites (Al/Si_p + SiC_p) during milling process using a carbide tool. The total volume fraction of the reinforcements is 65 vol%. The milling forces, flank wear of the tool and the machined surface quality of composites with different volume fraction of SiC_p were measured during experiments. The machined surfaces of composites were examined through SEM. The results showed that the flexural strength and Vickers hardness are improved when certain volume fraction of silicon particles are replaced by silicon carbide particles with the same volume fraction and particle size and the effect of SiC_p on machinability is optimal when 9 vol% silicon particles in Al/Si_p was replaced by silicon carbide particles with the same volume fraction and the same particle size. Cracks and pits were found on the machined surfaces of composites due to the intrinsic brittleness of silicon particles.     

Effects of Sr on the microstructure and mechanical properties of in situ TiB2 reinforced A356 composite

In situ A356–3 wt.% TiB₂ composites were fabricated via a remelting and diluting (RD) approach, to investigate the effect of Sr on the modification of in situ A356–TiB₂ composites with respect to the composite prepared by the conventional flux assisted synthesis (FAS) approach. The tensile properties of the composites were tested to evaluate the modification efficiency of Sr in different approaches. The results demonstrated that the RD composite can achieve fully modified eutectic structures than the FAS one owing to avoidance of the Sr–B interaction, which is commonly encountered in the FAS composites. The addition of Sr greatly improves the mechanical properties (especially the elongation) of thus prepared composites, only when the composites are in a fully modified state. Optimum modification of in situ A356–3 wt.% TiB₂ composite was obtained with Sr addition in the range around 0.03 wt.%. The elongation of the 0.03 wt.% Sr modified RD A356–3 wt.% TiB₂ composite are 6.6% and 5.6%, in as-cast and T6 states, respectively. The improvements in strength and ductility are attributed to the morphology change of Si as well as the improved melt cleanliness.     

Y<Subscript>2</Subscript>O<Subscript>3</Subscript> and Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> co-stabilized ZrO<Subscript>2</Subscript>-WC composites

Y₂O₃ + Nd₂O₃ co-stabilized ZrO₂-based composites with 40 vol% WC were fully densified by pulsed electric current sintering (PECS) at 1350 °C and 1450 °C. The influence of the PECS temperature and Nd₂O₃ co-stabilizer content on the densification, hardness, fracture toughness and bending strength of the composites was investigated. The best combination of properties was obtained for a 1 mol% Y₂O₃ and 0.75 mol% Nd₂O₃ co-stabilized composite densified for 2 min at 1450 °C under a pressure of 62 MPa, resulting in a hardness of 15.5 ± 0.2 GPa, an excellent toughness of 9.6 ± 0.4 MPa.m^0.5 and an impressive 3-point bending strength of 2.04 ± 0.08 GPa. The hydrothermal stability of the 1 mol% Y₂O₃ + 1 mol% Nd₂O₃ co-stabilized ZrO₂-WC (60/40) composites was compared with that of the equivalent 2 mol% Y₂O₃ stabilized ceramic. The double stabilized composite did not degrade in 1.5 MPa steam at 200 °C after 4000 min, whereas the yttria stabilized composite degraded after less than 2000 min. Moreover, the (1Y,1Nd) ZrO₂-WC composites have a substantially higher toughness (~9 MPa.m^0.5) than their 2Y stabilized equivalents (~7 MPa.m^0.5).     

Statistical analysis of the fracture strengths of aluminum alloy–alumina (Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) particulate composites

The mechanics of composite materials and their “fracture behaviors” are relatively complex phenomena to analyze and establish due to their inconsistent process stability and reliability, combined with production and related processing problems. In this work, an attempt has been made to statistically analyze the tensile behavior of metal matrix composites. Composites of aluminum alloy containing 5–20% volume fraction of Al₂O₃ particles of 15 μm size were prepared by adding alumina particles to a vigorously agitated semi-solid aluminum alloy. Prior to this, alumina particles were subjected to preheating at 800 °C for 5 h. Particles were then added to the aluminum alloy and further heated to 850 °C by using a mixer in a nitrogen medium. A total of 20 tension tests were performed for each volume fraction according to ASTM Standards B557 and using these test data, the initial estimators for an empirical model were obtained. Using this empirical model, the reliability of the composite characteristics in terms of its tensile strength was assessed. Another significant implication of the present study is proving the ability and utility of the Weibull statistical distribution for describing the experimentally measured data on the tensile strength of metal matrix composites, in a more appropriate manner.     

Microstructure and abrasive wear behaviour of B<Subscript>4</Subscript>C particle reinforced 2014 Al matrix composites

In this study, the microstructure and abrasive wear properties of varying volume fraction of particles up to 12% B₄C particle reinforced 2014 aluminium alloy metal matrix composites produced by stircasting method was investigated. The density, porosity and hardness of composites were also examined. Wear behaviour of B₄C particle reinforced aluminium alloy composites was investigated by a block-on-disc abrasion test apparatus where the samples slid against the abrasive suspension mixture (contained 10 vol.% SiC particles and 90 vol.% oil) at room conditions. Wear tests performed under 92 N against the abrasive suspension mixture with a novel three body abrasive. For wear behaviour, the volume loss and specific rate of the samples have been measured and the effects of sliding time and the content of B₄C particles on the abrasive wear properties of the composites have been evaluated. The dominant wear mechanisms were identified using SEM. Microscopic observation of the microstructures revealed that dispersion of B₄C particles was generally uniform while increasing volume fraction led to agglomeration of the particles and porosity. The density of the composite decreased with increasing reinforcement volume fraction but the porosity and hardness increased with increasing particle content. Moreover, the specific wear rate of composite decreased with increasing particle volume fraction. The wear resistance of the composite was found to be considerably higher than that of the matrix alloy and increased with increasing particle content.     

Synthesis of nanocrystalline TiB<Subscript>2</Subscript> powder from TiO<Subscript>2</Subscript>, B<Subscript>2</Subscript>O<Subscript>3</Subscript> and Mg reactants through microwave-assisted self-propagating high-temperature synthesis method

In this research work, microwave-assisted self-propagating high-temperature synthesis (SHS) process was employed for the fabrication of titanium diboride (TiB₂) compound from TiO₂–B₂O₃–Mg mixtures. Thermodynamic evaluations of this system and its relevant subsystems revealed that TiB₂–MgO composite powder can be easily produced by a SHS reaction. However, experimental results of a TiO₂ : B₂O₃ : 5Mg mixture heated in a domestic oven showed the formation of some intermediate compounds such as Mg₃B₂O₆, presumably due to some degree of Mg loss. The optimum amount of Mg in TiO₂ : B₂O₃ : xMg mixtures, yielding the highest amount of TiB₂ phase, was found to be around 7 mol, i.e., 40 mol% more than the stoichiometric amount. Experimental results revealed that a pure TiB₂ compound could be obtained by leaching the unwanted by-products in an HCl acid solution. Scanning electron microscopic observations and Scherrer calculations showed that the produced TiB₂ contains sub-micron (150–200 nm) particles, where each particle consists of a number of nanosized (32 nm) crystallites.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> particles on microstructure formation and shear properties of Sn-58Bi solder

In the present study, varying weight percentages of Y₂O₃ particles (3–5 μm) from 0 to 3% were incorporated into eutectic Sn-Bi solder matrix to form composite solders. It is found that the reinforcement particles were well dispersed in the solder matrix. They depressed the growth of intermetallic compound (IMC) layers and reduced the size of IMC grains. Since the Y₂O₃ particles serve as additional nucleation sites for the formation of primary Bi-rich phase, the size of both Bi-rich phase and the IMCs were decreased gradually with the Y₂O₃ content increasing. Shear tests were also conducted on as—soldered joints. The growth of solid-state intermetallic compounds layer was examined by thermal aging of the solder/Cu couple for a temperature range from 60 to 120°C and time periods from 50 to 500 h. Compared with Sn-58Bi solder, finer eutectic microstructures were obtained with Y₂O₃ addition after long time aging. The apparent activation energies calculated for the growth of the intermetallic compound layers were 72 ± 5 kJ/mol of Sn-58Bi, 74 ± 4 kJ/mol of Sn-58Bi-0.5wt%Y₂O₃, 81 ± 5 kJ/mol of Sn-58Bi-1wt% Y₂O₃ and 81 ± 7 kJ/mol of Sn-58Bi-3wt.% Y₂O₃, respectively.     

Vibration milling of TiB<Subscript>2</Subscript> + TiNi powder mixtures

We have studied how the duration of the vibration comilling of a 80 vol % TiB₂ + 20 vol % TiNi powder mixture influences the particle size, morphology, and fine-structure parameters of its components. At a milling time of 60 h, we obtained a mixture containing 27 vol % nanoparticles, in which the cubic TiNi phase had a crystallite size of 1.1 nm. We believe that vibration-milled TiB₂ + TiNi mixtures are potentially attractive for the fabrication of composite materials by powder metallurgy methods.     

Strengthening mechanism of nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles reinforced Sn3.5Ag0.5Cu lead-free solder

To improve the properties of the eutectic Sn3.5Ag0.5Cu lead-free solder, various amounts of mixed nano-Al₂O₃ particles were added. The microstructure, thermal analysis, density, thermal expansion coefficient (CTE), and mechanical behavior were studied. The results of differential scanning calorimetry (DSC) indicate that the melting point of the composite solder doped with nano-Al₂O₃ particles is slightly higher that of the Sn3.5Ag0.5Cu lead-free solder and has a eutectic peak. The Sn3.5Ag0.5Cu composite solders exhibited lower density values and thermal expansion coefficient (CTE) values than did the unreinforced solder matrix. Compared to solder without the addition of nano-Al₂O₃ particles, the formation of dendritic β-Sn grains, the Ag₃Sn phase average size, and the spacing of lamellae decreased significantly in the composite solder matrix. The mechanical properties also improved with increasing weight percentages of nano-Al₂O₃ particles. However, the ductility of the Sn3.5Ag0.5Cu composite solder decreased. For the addition of 1 wt% nano-Al₂O₃ particles, microporosity was observed both at and along the grain boundary regions, coupled with the presence of second-phase particles (i.e. nano-Al₂O₃ and Ag₃Sn).     

Oxidation behavior of TiB<Subscript>2</Subscript> micro- and nanoparticles

The oxidation of TiB₂ particles (75 to 1500 nm in size) has been studied at temperatures of up to 1000°C by thermogravimetry, X-ray diffraction, X-ray photoelectron spectroscopy, IR frustrated total internal reflection spectroscopy, and energy dispersive X-ray analysis. The oxidation onset was observed between 210 and 475°C, depending on the particle size. This distinction can presumably be accounted for in terms of the deformation produced by the Laplace pressure. Oxidation at temperatures under 1000°C leads to the formation of the rutile phase of TiO₂ and boron oxide (B₂O₃). Moreover, at a temperature of ? 1000°C titanium borate, TiBO₃, was observed to form. Under all of the conditions examined, the oxidation reaction does not reach completion and the oxidation products contain unreacted TiB₂.     

Effect of shot peening on surface mechanical properties of TiB<Subscript>2</Subscript>/Al composite

The influence of shot peening on the surface mechanical properties of the TiB₂/6351Al composite has been investigated. The microstructures were determined by X-ray diffraction line profile analysis. The results showed that the increment of hardness was about 50% in the top surface layer. The matrix proof stress σ _0.2 of the shot peened surface had been increased by 27% and the whole strength increment was about 21% by considering the contribution of the reinforcements. The domain size and the dislocation density in the strengthened surface were 55 nm and 3.67 × 10¹⁵ m⁻², respectively. The mechanical properties improvement of the modified surface was partially due to the reinforcements but mainly due to the fine domains, high value of dislocation density induced by shot peening.     

Phase quantification of impulse atomized Al<Subscript>68.5</Subscript>Ni<Subscript>31.5</Subscript> alloy

Powders of Al_68.5Ni_31.5 were produced using the impulse atomization technique. The molten droplets were cooled in-flight by the stagnant helium or nitrogen in the atomizing chamber, and the resulting powders were sieved into different size ranges. Scanning electron microscopy, X-ray diffraction, and neutron diffraction were used in order to study the microstructure and to quantify the phase fractions in the samples. The computer software GSAS was used to calculate the weight fraction of the existing phases, namely Al₃Ni₂, Al₃Ni, and Al, by the profile refinement method. X-ray micro-tomography and optical microscopy were used to study the porosity formation inside the particles. It was found that for particles having sizes decreasing from 925 to 256 μm (increasing cooling rate), the weight fraction of Al₃Ni and eutectic Al decreases while that of Al₃Ni₂ increases. Furthermore, the droplets formed at higher cooling rates yielded a lower volume fraction of porosity.     

Fabrication of TiC and TiB<Subscript>2</Subscript> locally reinforced steel matrix composites using a Fe–Ti–B<Subscript>4</Subscript>C–C system by an SHS-casting route

Steel matrix composites locally reinforced by in situ TiC and TiB₂ particulates were successfully fabricated using self-propagating high-temperature synthesis (SHS) in a Fe–Ti–B₄C–C system during casting. The locally reinforced steel matrix composites consist of three distinct regions: (i) a TiC and TiB₂ particulate-reinforced region, (ii) a transition region, and (iii) a steel matrix region. The TiC and TiB₂ particulates in the locally reinforced regions display a relatively uniform distribution, and their sizes decrease with the increase in Fe content from 10 wt.% to 40 wt.%. The wear resistance of the locally reinforced region of the steel matrix composites is much higher than that of the unreinforced steel matrix.     

Dielectric and ferromagnetic properties of BaTiO<Subscript>3</Subscript>/Ni<Subscript>0.93</Subscript>Co<Subscript>0.02</Subscript>Cu<Subscript>0.05</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> composites

xBaTiO₃ + (1 − x)Ni_0.93Co_0.02Cu_0.05Fe₂O₄ (x = 0.5, 0.6, 0.7, 0.8) composites with ferroelectric–ferromagnetic characteristics were synthesized by the ceramic sintering technique. The presence of constituent phases in the composites was confirmed by X-ray diffraction studies. The average grain size was calculated by using a scanning electron micrograph. The dielectric characteristics were studied in the 100 kHz to 15 MHz. The dielectric constant changed higher with ferroelectric content increasing; and it was constant in this frequency range. The relation of dielectric constant with temperature was researched at 1, 10, 100 kHz. The Curie temperature would be higher with frequency increasing. The hysteresis behavior was studied to understand the magnetic properties such as saturation magnetization (M _s). The composites were a typical soft magnetic character with low coercive force. Both the ferroelectric and ferromagnetic phases preserve their basic properties in the bulk composite, thus these composites are good candidates as magnetoelectric materials.     

Electrical and thermal properties of PTFE-Sr<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> composites

A new polymer-ceramic composite was prepared using PTFE and low loss Sr₂ZnSi₂O₇. The dielectric properties of the composite were studied in the microwave and radiofrequency ranges. The relative permittivity (ε_r) and dielectric loss (tan δ) increased with the filler loading from 0.10 to 0.50 volume fractions (v_f). The observed values of ε_r, thermal conductivity and coefficient of thermal expansion (CTE) were compared with the corresponding theoretical predictions. The ability of the composite towards moisture absorption resistance was studied as a function of filler loading. It was also found that the variation of ε_r was less than 2% in the temperature range 25–90 °C, at 1 MHz. For a filler content of 0.50 v_f, the PTFE/Sr₂ZnSi₂O₇ composite exhibited ε_r = 4.4_, tan δ = 0.003 (at 4–6 GHz), CTE = 38.3 ppm/°C, thermal conductivity = 2.1 W/mK and moisture absorption = 0.09 wt%.     

Electrochemical corrosion behavior of Al<Subscript>18</Subscript>B<Subscript>4</Subscript>O<Subscript>33</Subscript>w/Al composite

The effects of the volume fraction of alumina borate (Al₁₈B₄O₃₃) whisker and the scan rate of potentiodynamic technique on the localized corrosion behaviors of Al₁₈B₄O₃₃w/Al composite were investigated. Potentiodynamic polarizations and cyclic polarizations were performed to examine the electrochemical corrosion behavior of the composites. The surface morphologies of the as-cast composite were observed by optics microscopy and the surface morphologies of the composite after corrosion tests were observed by scanning electronic microscopy (SEM). The results of electrochemical measurement indicated that the anodic passive region reduced with the increasing in the volume fraction of Al₁₈B₄O₃₃ whisker in the composites (at the same scan rate), and the anodic passive region reduced with the decreasing in the scan rate of potentiodynamic technique (in the same volume fraction of whisker). The results of cyclic polarization indicated that increase in the volume fraction of Al₁₈B₄O₃₃ whisker in the composites results in a significant decrease of protection potential and an increase of the area of cyclic hysteresis loop.     

Fabrication and mechanical properties of WC particulate reinforced Cu<Subscript>47</Subscript>Ti<Subscript>33</Subscript>Zr<Subscript>11</Subscript>Ni<Subscript>6</Subscript>Sn<Subscript>2</Subscript>Si<Subscript>1</Subscript> bulk metallic glass matrix composites

The mechanical behavior of the WC particulate (WC_p) reinforced Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ bulk metallic glass (BMG) matrix composites has been examined. The mechanical properties are improved with increasing WC_p content up to 20 wt%. The ultimate compression strength and plastic strain of the composite containing 20 wt% WC_p are 2.4 GPa and 2.4%, while those of the monolithic BMG are 1.6 GPa and ∼0%, respectively. The multiple shear band formation and crack deflections through WC particles have been identified as the main mechanism for the improved toughness.     

Accelerated hydrogen desorption from MgH<Subscript>2</Subscript> by high-energy ball-milling with Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The effect of the addition of Al₂O₃ (50 wt%) on the dehydrogenation of MgH₂ was investigated. Composites of the oxide and the hydride were prepared in two ways: by milling the components separately or by co-milling them together in a gear-driven planetary ball mill for 10 min. The co-milled composite (MgH₂–Al₂O₃) released approximately 90% of the maximum hydrogen storage capacity within 30 min under a pressure of 0.003 MPa at 250 °C. In contrast, the composite of the separately milled components did not release hydrogen even after 2 h under the same conditions. BET measurement with nitrogen gas showed a negligible difference in the specific surface areas between the co-milled and separately milled composites. However, the saturation amount of hydrogen gas for the co-milled composite was 30% larger than that of the mixture of separately milled hydride and oxide. The activation energy for hydrogen desorption from the co-milled composite, calculated on the basis of the surface-controlled model was 80 kJ mol⁻¹, a value that is 50 kJ mol⁻¹ lower than that of mixture of the separately milled MgH₂ and Al₂O₃.     

In situ TiB2 particulate reinforced near eutectic Al–Si alloy composites

In situ TiB₂ particulate reinforced near eutectic Al–Si alloy composites fabricated by the melt reaction composing (MRC) methods have been investigated. It has been shown that minute TiB₂ particles (less than 1 μm) uniformly distribute in the eutectic structure and they are interlaced with the coralline-like eutectic Si, while there are very few TiB₂ particles in α-Al. It has been also shown that in situ TiB₂ particles can enhance the tensile strength of the Al–Si alloy matrix. The strengthening effect increases with increasing TiB₂ content. The ultimate tensile strength (UTS) at room temperature of as-cast 6%TiB₂/Al–Si–Mg composite is 296 MPa, that is a 14.7% increase over the matrix, and its elongation at fracture is 5.5%. After heat-treatment (T6), the UTS of the composites reaches 384 MPa. The strengthening mechanism has been discussed.