Acoustic emission from particulate-reinforced metal matrix composites

A systematic study of the effect of microstructural parameters on the fracture behaviour of silicon carbide particle reinforced aluminium matrix composites has been carried out. Acoustic emissions have been monitored during tensile testing, giving the size and number of emmissions as a function of strain. This has been shown to be simply related to the rate of void nucleation at the reinforcing phase. Both particle fracture and particle/matrix decohesion mechanisms can be detected. Void nucleation was observed from the onset of plastic deformation and a linear relationship between damage initiation rate and strain was found. The rate of emission increased with reiforcing particle size and volume fraction but was independent of matrix alloy composition and heat treatment. These results show that the failure strain of particulate metal matrix composites is not controlled solely by the onset of void nucleation at the reinforcing phase. Local failure processes in the matrix are shown to promote void coalescence and dominate the ductility. However, suppression of void nucleation at the particles increases the ductility. It is suggested that a critical number of fractured particles is required before failure.     

In situ formation of three-dimensional TiC reinforcements in Ti-TiC composites

Three-dimensional, single-crystal reinforcements of TiC were producedin situ during manufacture of Ti-TiC composites. The composites, containing 40 to 50 vol pct TiC, were produced using standard casting procedures. The presence of aluminum in Ti-TiC composites showed enhanced strength without loss of ductility at room and elevated temperatures. Aluminum additions were found to solid solution strengthen the Ti matrix and increase the strength of the TiC phase. The morphology of the TiC, which was controlled by processing parameters, influenced the properties of the Ti-TiC composites investigated. Refinement of the secondary dendrite arm spacing of the three-dimensional (3-D) TiC particles was found to dramatically improve the ultimate tensile strength (UTS) and ductility of the Ti-TiC composites.     

Synergistic effects of wear and corrosion for Al2O3 particulate-reinforced 6061 aluminum matrix composites

Wear corrosion of alumina particulate-reinforced 6061 aluminum matrix composites in a 3.5 wt pct NaCl solution with a revised block-on-ring wear tester has been investigated. The studies involved the effects of applied load, rotational speed, and environments (dry air and 3.5 pct NaCl solution) on the wear rates of materials. Also various specimens with Al₂O₃ volume fractions of 0, 10, 15, and 20 pct were employed in this work. Electrochemical measurements and electron micrographic observations were conducted to clarify the micromechanisms of wear corrosion in such metal matrix composites. Experimental results indicated that the wear rate of monolithic 6061 Al in either dry wear or wear corrosion was reduced by adding alumina reinforcements. However, the effect of volume fraction on wear rate is only minor in dry wear, while it is significant in the case of wear corrosion. Wear-corrosion tests also showed that the corrosion potential shifted to the active side and the current density for an applied potential increased with the decrease of Al₂O₃ volume fraction in the materials and the increase in applied load and rotational speed. Although the incorporation of reinforcement in these aluminum matrix composites was deterimental to their corrosion resistance, the influence on wear corrosion was favorable.     

Optimization of the strength-fracture toughness relation in particulate-reinforced aluminum composites via control of the matrix microstructure

The evolution of the microstructure and mechanical properties of a 17.5 vol. pct SiC particulate-reinforced aluminum alloy 6092-matrix composite has been studied as a function of postfabrication processing and heat treatment. It is demonstrated that, by the control of particulate distribution, matrix grain, and substructure and of the matrix precipitate state, the strength-toughness combination in the composite can be optimized over a wide range of properties, without resorting to unstable, underaged (UA) matrix microstructures, which are usually deemed necessary to produce a higher fracture toughness than that displayed in the peak-aged condition. Further, it is demonstrated that, following an appropriate combination of thermomechanical processing and unconventional heat treatment, the composite may possess better stiffness, strength, and fracture toughness than a similar unreinforced alloy. In the high- and low-strength matrix microstructural conditions, the matrix grain and substructure were found to play a substantial role in determining fracture properties. However, in the intermediate-strength regime, properties appeared to be optimizable by the utilization of heat treatments only. These observations are rationalized on the basis of current understanding of the grain size dependence of fracture toughness and the detailed microstructural features resulting from thermomechanical treatments.     

Strength and plastic flow in “in situ” TiC reinforced aluminum composites

The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiC _p ) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried out at 293 and 623 K and at two strain rates (3.7×10⁻⁴ and 3.7×10⁻³ s⁻¹). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at 623 K, increasing the rate of straining from 3.7×10⁻⁴ s⁻¹ to 3.7×10⁻³ s⁻¹ lowered the yield strength, particularly in 10 vol pct TiC _p -Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiC _p composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛ ⁿ ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic strain was increased while the opposite was true for the 10 vol pct TiC _p composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiC _p -Al composites, with the effect being minor at room temperature.     

Corrosive wear of SiC Whisker-and particulate-reinforced 6061 aluminum alloy composites

Wear tests on SiC whisker- and SiC particulate-reinforced 6061-T6 aluminum matrix composites (SiCw/Al and SiCp/Al), fabricated using a high pressure infiltration method, were performed in laboratory air, ion-exchanged water and a 3 pct NaCl aqueous solution using a block-on-ring type apparatus. The effects of environment, applied load, and rotational (sliding) speed on the wear prop-erties against a sintered alumina block were evaluated. Electrochemical measurements in ion-ex-changed water and a 3 pct NaCl aqueous solution were also made under the same conditions as the wear tests. A comparison was made with the properties of the matrix aluminum alloy 6061-T6. The SiC-reinforced composites exhibited better wear resistance compared with the monolithic 6061 Al alloy even in a 3 pct NaCl aqueous solution. Increase in the wear resistance depended on the shape, size, and volume fraction of the SiC reinforcement. Good correlation was obtained between corrosion resistance and corrosion wear. The ratios of wear volume due to the corrosive effect to noncorrosive wear were 23 to 83 pct, depending on the wear conditions.     

TiC颗粒增强铜基复合材料的研究

以电解铜粉和TiC粉为原料, 采用粉末冶金法制备了增强体质量分数为5%、10%、15%、20%的TiC颗粒增强铜基复合材料。通过对显微组织的观察和对相对密度、硬度、电导率、磨损率、摩擦系数的测试, 研究了增强相质量分数、烧结温度对复合材料组织性能的影响。研究结果表明, TiC颗粒除少量团聚外均匀分布在基体上, 并与基体结合良好; 随烧结温度升高, 铜基复合材料的密度和硬度均有所增加; 随增强相质量分数的增加, 硬度增加, 相对密度和电导率均有所下降; 磨损率则表现为先降低后有所增加的趋势, 磨损率在TiC质量分数为15%时最低; 铜基复合材料的摩擦系数明显低于纯铜, 其磨损机制主要以磨粒磨损为主。     

Recycling of aluminum matrix composites

Separation of matrix metals in composites was tried on alumina short fiber-reinforced aluminum and 6061 alloy composites and SiC whisker-reinforced 6061 alloy composite for recycling. It is possible to separate molten matrix metals from fibers in the composites using fluxes that are used for melt treatment to remove inclusions. About 50 vol pct of the matrix metals was separated from the alumina short fiber-reinforced composites. The separation ratio of the matrix from the SiC whisker-reinforced 6061 alloy composite was low and about 20 vol pct. The separation mechanism was discussed thermodynamically using interface free energies. Since the flux/fiber interface energy is smaller than the aluminum/fiber interface energy, the replacement of aluminum with fluxes in composites takes place easily. Gases released by the decomposition of fluxes act an important role in pushing out the molten matrix metal from the composite. The role was confirmed by the great amount cavity formed in the composite after the matrix metal flowed out.     

Failure of SiC particulate-reinforced metal matrix composites induced by laser thermal shock

Thermal failure of SiC particulate-reinforced 6061 aluminum alloy composites induced by both laser thermal shock and mechanical load has been investigated. The specimens with a single-edge notch were mechanically polished to 0.25 mm in thickness. The notched-tip region of the specimen is subjected to laser beam rapid heating. In the test, a pulsed Nd:glass laser beam is used with duration 1.0 ms or 250 μs, intensity 15 or 70 kW/cm², and spot size 5.0 mm in diameter. Threshold intensity was tested and fracture behavior was studied. The crack-tip process zone development and the microcrack formation were macroscopically and microscopically observed. It was found that in these materials, the initial crack occurred in the notched-tip region, wherein the initial crack was induced by either void nucleation, growth, and subsequent coalescence of the matrix materials or separation of the SiC particulate-matrix interface. It was further found that the process of the crack propagation occurred by the fracture of the SiC particulates.     

Application of the generalized method of cells principle to particulate-reinforced metal matrix composites

A three-dimensional model based on the generalized method of cells (GMC) principle has been used to predict the effective properties of particulate-reinforced metal matrix composites (PMMCs). The effects of constituent phases on the elastic properties of PMMCs are predicted using GMC. The predictions are compared with an assortment of finite-element predictions and experimental results available in the literature. The accuracy and the computational efficiency of the GMC model are also discussed. Moreover, the effect of particle shape and orientation on the elastic properties of PMMCs has been predicted and analyzed. Cubical and parallelepiped shaped particles having different orientations are considered for this study. Significant variations are noted on the elastic properties of the PMMC systems by altering the shape and orientation of the particles.     

高温钛合金和颗粒增强钛基复合材料的研究和发展

简要回顾了高温钛合金的研究和发展历程,指出现代高温钛合金进一步发展需要解决的主要难题.综述了颗粒增强钛基复合材料的研究现状,从基体的选择、增强相的选择和制备工艺等3个方面,较详细地阐述了颗粒增强钛基复合材料设计中的基本任务.最后对今后的发展趋势进行了展望.     

Microstructure and mechanical properties of in situ (TiC + SiC)/FeCrCoNi high entropy alloy matrix composites

In situ (TiC + SiC) particles (5 vol.％ and 10 vol.％, respectively)-reinforced FeCrCoNi high entropy alloy matrix composites were fabricated via vacuum inductive...     

Effect of microstructure (particulate size and volume fraction) and counterface material on the sliding wear resistance of particulate-reinforced aluminum matrix composites

The effects of microstructure (namely, particulate volume fraction and particulate size) and the counterface materials on the dry-sliding wear resistance of the aluminum matrix composites 2014A1-SiC and 6061Al-Al₂O₃ were studied. Experiments were performed within a load range of 0.9 to 350 N at a constant sliding velocity of 0.2 ms^-1. Two types of counterface materials, SAE 52100 bearing steel and mullite, were used. At low loads, where particles act as loadbearing constituents, the wear resistance of the 2014A1 reinforced with 15.8 μm diameter SiC was superior to that of the alloy with the same volume fraction of SiC but with 2.4 μm diameter. The wear rates of the composites worn against a steel slider were lower compared with those worn against a mullite slider because of the formation of iron-rich layers that act asin situ solid lubricants in the former case. With increasing the applied load, SiC and A1₂O₃ particles fractured and the wear rates of the composites increased to levels comparable to those of unreinforced matrix alloys. The transition to this regime was delayed to higher loads in the composites with a higher volume percentage of particles. Concurrent with particle fracture, large strains and strain gradients were generated within the aluminum layers adjacent to contact surfaces. This led to the subsurface crack growth and delamination. Because the particles and interfaces provided preferential sites for subsurface crack initiation and growth and because of the propensity of the broken particles to act as third-body abrasive elements at the contact surfaces, no improvement of the wear resistance was observed in the composites in this regime relative to unreinforced aluminum alloys. A second transition, to severe wear, occurred at higher loads when the contact surface temperature exceeded a critical value. The transition loads (and temperatures) were higher in the composites. The alloys with higher volume fraction of reinforcement provided better resistance to severe wear. Wearing the materials against a mullite counterface, which has a smaller thermal conductivity than a counterface made of steel, led to the occurrence of severe wear at lower loads.     

Investigation of wear behaviour and microstructure of hot-pressed TiB2 particulate-reinforced magnesium matrix composites

In this study, magnesium and magnesium metal matrix composites (Mg-MMCs) reinforced with 10, 20 and 30 weight (wt.)% TiB₂ particulates were produced by powder metallurgy using the hot pressing technique. The hardness, density, wear behaviour and microstructure of samples were investigated. Uniform distributions of TiB₂ particulates were observed except with some partial agglomeration for 30 wt-% TiB₂. When compared to pure Mg, the hardness increment of Mg-MMCs reinforced with 10, 20 and 30 wt-%TiB₂ particulates was 10.7, 31.9 and 65.3%, respectively. As compared to pure Mg, under load of 20 N, the decrease in wear rate in 10, 20 and 30 wt-% reinforcement was 28.71, 34.98 and 42.92%, respectively. It is believed that the reason of decrease in wear rate was the presence of harder TiB₂ particulates, which resisted to wear and plastic deformation. For pure Mg, oxidative wear changed to oxidative and mild abrasive wear transition from 10 to 20 N. Mg/TiB₂ composites exhibited abrasive wear mechanism under load of 10 and 20 N except 30 wt-% TiB₂ composite indicated oxidative and adhesive wear. However, a transition from mild abrasive wear to severe abrasive wear was observed with increasing load in composites.     

In situ combustion synthesis of dense ceramic and ceramic-metal interpenetrating phase composites

A model exothermic reaction is used to demonstrate the application of simultaneous combustion synthesis, conducted under a consolidating pressure, as a one-stepin situ synthesis technique for the production of dense ceramic and ceramic-metal interpenetrating phase composites (IPC). The addition of an excess amount of metal,e.g., Al, and/or a diluent,e.g., Al₂O₃, lowers the combustion temperature and aids in the refinement of the microstructure, facilitating an increase in compressive strength and elastic modulus. The effects of the important process parameters,e.g., reaction stoichiometry and diluents, green density, pressure, and temperature, on microstructure and properties of these high-performance composites are discussed.     

Melt-Processed Ni3Al matrix composites reinforced with TiC particles

TiC-reinforced Ni₃Al metal matrix composites have been investigated. The composites were prepared by in situ precipitation of either 0.01, 0.05, or 0.1 vol fraction TiC in molten Ni3Al. These precipitates are thermodynamically stable in the Ni₃Al at 1000 °C up to 50 hours. Although almost ideal precipitate distribution was achieved at low volume fraction of TiC, at higher volume fraction, there was a tendency toward agglomeration. Room and hightemperature mechanical testing showed significant improvement in modulus, yield strength, and ultimate tensile strength, while still maintaining a 2.5 pct elongation at 0.1 vol. fraction TiC and 500 °C. The density of precipitates (number/m³) is strongly dependent on the fraction of TiC, on temperature, and on the holding time at that temperature. The critical nucleation temperature for these precipitates was determined to be at 1475 °C. Experimentally evaluated growth rate constants indicate a diffusion-controlled coarsening mechanism along with coalescence and multiparticle interaction at higher volume fractions of TiC. This is in accordance with either the Lifshitz Slyozov Encounter Modified (LSEM) model or with the Voorhees and Glicksman (VG) model.     

Transformation superplasticity of iron and Fe/TiC metal matrix composites

Unreinforced iron was thermally cycled around the α/γ phase field under an externally applied uniaxial tensile stress, resulting in strain increments which could be accumulated, upon repeated cycling, to a total strain of 450 pct without failure. In agreement with existing theory attributing transformation superplasticity to the biasing of the internal allotropic strains by the external stress, the measured strain increments were proportional to the applied stress at small stresses. However, for applied stresses higher than the nominal yield stress, strain increments increased nonlinearly with stress, as a result of strain hardening due to dissolved carbon and iron oxide dispersoids. Also, the effects of transient primary creep and ratchetting on the superplastic strain increment values were examined. Finally, partial cycling within the α/γ phase field indicated an asymmetry in the superplastic strain behavior with respect to the temperature cycling range, which is attributed to the different strengths of ferrite and austenite. Transformation superplasticity was demonstrated in iron-matrix composites containing 10 and 20 vol pct TiC particles: strain increments proportional to the applied stress were measured, and a fracture strain of 230 pct was reached for the Fe/10TiC composite. However, the strain increments decreased with increasing TiC content, a result attributed to the slight dissolution of TiC particles within the matrix which raised the matrix yield stress by solid-solution strengthening and by reducing the transformation temperature range.