Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC_p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC_p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC_p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC_p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC_p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.     

Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of ?1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 μm particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R=?1, the fatigue strength of the SiC_p/Al composite was the same as that of the unreinforced alloy, but at R= 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiC_p/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10⁷ cycles in the unreinforced alloy, but significantly unconservative estimation in the SiC_p/Al composite. At R= 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R=?1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiC_p/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.     

Effect of particle size on fatigue behaviour in SiC particulate-reinforced aluminium alloy composites

The effect of particle size on rotary bending fatigue behaviour was studied for powder metallurgy 2024 aluminium alloy composites reinforced with 10 wt% silicon carbide particles (SiC_p ). Average particle sizes of 5, 20 and 60 μm were evaluated. Particle size had a significant influence on fatigue strength, indicating an increased fatigue strength with decreasing particle size. The composite with 5 μm SiC particles showed higher fatigue strength than the unreinforced alloy. The incorporation of 20 μm SiC particles led to an increase in fatigue strength at a high stress level, but the improvement diminished with decreasing stress level, and a slightly decreased fatigue strength was observed at low stress level, as compared with the unreinforced alloy. The composite with 60 μm SiC particles exhibited a considerable decrease in fatigue strength. Fatigue cracks initiated at several different microstructural features, e.g. surface defects, inclusions and particle–matrix interfaces, and crack initiation was considerably affected by particle size. Fatigue strength was found to depend strongly on the resistance to crack initiation, because there was no discernible difference in small crack growth between the unreinforced alloy and the composites, particularly at a low maximum stress intensity factor.     

Incorporation of new technique for processing of Al/SiCp composites based on evaporative pattern casting (EPC) method

Abstract

A336 Al matrix composites containing different volume fraction and mean mass particle size of SiC particles as the reinforcing phase were synthesised by evaporative pattern casting (EPC) route. The process consisted of fabricating of EPS/SiC_p composite pattern followed by EPC of A336 Al alloy. The EPS/SiC_p pattern was made by blending SiC particles with expandable polystyrene (EPS) beads and placing them in expanding mould heating with steam until EPS beads expand completely. Uniform distributed SiC particles around the EPS beads and locally movement of them during pouring and degradation leads to homogenous distribution of particles in final Al/SiC_p composite. Higher modulus, strength and hardness were observed in the composites than the unreinforced Al alloy part. The fracture surfaces of the composite samples exhibited dimple surfaces and fracture in SiC particles.     

The microstructures of aluminium alloy metal-matrix composites manufactured by squeeze casting

Squeeze-cast metal-matrix composite ingots have been manufactured by recasting A356/10% SiC_p and by melt infiltrating A357/Al₂O_3f. The resulting microstructures have been examined by optical and transmission electron microscopy. The primary aluminium in squeeze-cast A356/10% SiC_p exhibits a dendritic structure with eutectic silicon particles and SiC particulate distributed throughout the interdendritic regions. Unlike squeeze-cast monolithic A357, the primary aluminium dendrite arm spacing and eutectic-silicon particle size in squeeze-cast A356/10% SiC_p are not strongly affected by an increased cooling rate at high applied pressure. With low die and preform temperature, a high applied pressure is essential to obtain complete melt infiltration in A357/Al₂O_3f. During melt infiltration, the ceramic preform acts as a filter, preferentially allowing silicon rich liquid to pass through.     

Microstructure and mechanical properties of SiC whisker reinforced ZrO2–6 mol.-%Y2O3 composites

Abstract

Microstructure, mechanical properties, fracture behaviour, and toughening mechanisms of hot pressed SiC whisker (SiC_w)

reinforced ZrO₂–6 mol.-%Y₂O₃ composites were investigated via transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and mechanical testing. The experimental results show that there is a continuous increase in the Vickers hardness, elastic modulus, and fracture toughness of the composites with increasing SiC_w content, and an addition of 30 vol.-%SiC_w increases the fracture toughness from 3·42 MN m^?3/2 for the unreinforced matrix to 5·83 MN m^?3/2. The flexural strength is increased from 293 MN m^?2 for the unreinforced matrix to a maximum of 372 MN m^?2 by an addition of 10 vol.-%SiC_w, then it is significantly decreased by further increasing the SiC_w content. Observations via transmission electron microscopy show that no distinct second phase or intermediate layers form at the SiC_w/ZrO₂ interface. Diffusional separation of tetragonal phase from the cubic matrix occurred during cooling after hot pressing. Whisker bridging and crack deflection are the main toughening mechanisms, but whisker pull-out, crack branching, and refinement of the matrix particles also contribute to the improvement in the fracture toughness.

MST/1747     

Mechanical properties of monolithic AIN and SiCw/AIN composites

Flexural, tensile, and high cycle fatigue test data are presented for pressureless sintered aluminium nitride (AIN) and hot-pressed aluminium nitride reinforced with silicon carbide whiskers (SiC_w/AIN). Tests were conducted at ambient temperature. The SiC_w/AIN composites consisting of 30wt% SiC_w produced significant increases in flexural strength, tensile strength, and tensile fatigue strength compared to monolithic AIN. Increases were nearly double in all cases. Corresponding strain-to-failures measured in tensile tests increased from 0.04% in monolithic AIN to 0.10% in the SiC_w reinforced composite. Fracture surfaces showed evidence of whisker-toughening mechanisms due to additions of SiC_w whiskers. High-cycle fatigue results indicated that both materials have the ability to sustain higher stress levels in the cyclic tests compared to the tensile experiments. The improved performance under cyclic testing is explained in terms of strain-rate effects. The times at or near peak stress are considerably less under high-cycle fatigue testing (20 Hz) compared to tensile tests (strain rate = 0.5%min^–1).     

The relationship between TiB<Subscript>2</Subscript> volume fraction and fatigue crack growth behavior in the in situ TiB<Subscript>2</Subscript>/A356 composites

The relationship between TiB₂ volume fraction and fatigue crack growth behavior in the A356 alloy matrix composites reinforced with 3, 5.6, and 7.8 vol% in situ TiB₂ particles has been investigated. The mechanisms of crack propagation in the TiB₂/A356 composites were also discussed. The results show that the 3 vol% TiB₂/A356 composite has nearly the same crack growth behavior as the matrix alloy, while the 5.6 vol% TiB₂/A356 composite exhibits a little bit faster crack growth rate. The 7.8 vol% TiB₂/A356 composite presents the lowest resistance to crack growth, indicating that the crack growth is accelerated by increasing TiB₂ volume fraction. Fractographies reveal that an increase in TiB₂ volume fraction results in a change from the formation of striation and slip to the failure of voids nucleation, growth, and coalescence. Cracks tend to propagate within the matrix and avoid eutectic silicon and TiB₂ particles in the intermediate ΔK region, while prefer to propagate along interfaces of eutectic silicon and TiB₂ particles and link the fractured eutectic silicon particles in the near fractured ΔK region. Furthermore, the propensity for the separation of TiB₂ increases with the increase in TiB₂ volume fraction. The massive voids caused by fractured eutectic silicon and separated TiB₂ particles propagate and coalesce, and then accelerates the crack growth in TiB₂/A356 composites.     

高体积分数SiCp/A356复合材料的显微组织和电导率

铝基复合材料在电子封装领域存在着潜在的应用前景。为获得高体积分数的铝基复合材料,利用压力浸渗法制备了高体积分数SiC颗粒增强A356复合材料(SiC_p/A356),通过金相显微镜、XRD、SEM和EDS等分析手段对其物相、显微结构和电导率进行了表征。结果表明:用该方法制备的SiC_p/A356复合材料组织致密,颗粒分布均匀,界面结合性能较好;SiC增强颗粒与A356基体界面反应控制良好,仅有少量Al4C3脆性相生成。SiC粉体经颗粒表面氧化处理在其表面生成一层SiO_2薄膜,虽抑制了界面反应的发生,但也使复合材料的收缩减小,电阻率增大,导电性能变差。     

Thermal mismatch stress in SiC whisker reinforced aluminium composite: new measurement method by X-ray diffraction

Abstract

A new X-ray diffraction method for characterising thermal mismatch stress (TMS) in SiC_w–Al composite has been developed. The TMS and thermal mismatch strain (TMSN) in SiC whiskers are considered to be axis symmetrical, and can be calculated by measuring the lattice distortion of the whiskers. Not only the average TMS in whiskers and matrix can be obtained, but the TMS components along longitudinal and radial directions in the SiC whiskers can also be deduced. Experimental results indicate that the TMS in SiC whiskers is compressive, and tensile in the aluminium matrix. The TMS and TMSN components along the longitudinal direction in the SiC whiskers are greater than those along the radial direction for a SiC_w–Al composite quenched at 500°C.     

Microstructure and mechanical properties of SiC<Subscript>P</Subscript>/SiC and SiC<Subscript>W</Subscript>/SiC composites by CVI

37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were prepared by chemical vapor infiltration (CVI) process through depositing SiC matrix in the porous particulate and whisker preforms, respectively. The particulate (or whisker) preforms has two types of pores; one is small pores of several micrometers at inter-particulates (or whiskers) and the other one is large pores of hundreds micrometers at inter-agglomerates. The microstructure and mechanical properties of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were studied. 37.2 vol.% SiC_P/SiC (or 25.0 vol.% SiC_W/SiC) consisted of the particulate (or whisker) reinforced SiC agglomerates, SiC matrix phase located inter-agglomerates and two types of pores located inter-particulates (or whiskers) and inter-agglomerates. The density, fracture toughness evaluated by SENB method, and flexural strength of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were 2.94 and 2.88 g/cm³, 6.18 and 8.34 MPa m^1/2, and 373 and 425 MPa, respectively. The main toughening mechanism was crack deflection and bridging.     

Creep properties of (Si–Al–O–N)SiC whisker composites

(Si–Al–O–N) (sialon)–SiC whisker (SiC_w) composites containing up to 10 mass% SiC_w were prepared by hot isostatic pressing. The strengths and the fracture toughness of composites remained relatively unchanged with the amount of SiC_w. The addition of SiC_w enabled us to improve the creep properties of sialon ceramics. The total creep strain and steady-state creep rate at 1473 K under a stress of 400–500 MPa decreased with increasing the amount of SiC_w. The experimental creep exponent values of monolithic sialon and sialon–SiC_w composites were nearly 1. It is supposed that the creep of both monolithic sialon and sialon–SiC_w composites are dominated by the viscous flow of the interglanular glassy phase.     

Preparation and properties of nano-SiCp/A356 composites synthesised with a new process

Nano-SiC_p reinforced A356 aluminium alloy composites were prepared by solid–liquid mixed casting. Nano-SiC particles 40?nm in diameter were pre-oxidised at 850°C. Millimetre-sized composite granules were then fabricated by milling a mixture of nano-SiC_p and Al powders, and then remelting to form an alloy melt, which was treated by mechanical stirring and ultrasonic vibration to prepare the composites. Results showed that nano-SiC particles were dispersed pretty well within the matrix and no serious agglomeration was observed. As the weight fraction of SiC_p increased, the tensile strength, yield strength and elongation of the composites increased as well, by 22%, 62% and 24%, respectively, when compared to A356 alloy with 2wt-% nano-SiC_p.     

CORROSION FATIGUE FRACTURE BEHAVIOUR OF A SiC WHISKERALUMINIUM MATRIX COMPOSITE UNDER COMBINED TENSIONTORSION LOADING

We describe an investigation into the fatigue fracture behaviour under combined tension–torsion loading of a SiC whisker-reinforced A6061 aluminium alloy fabricated by a squeeze casting process. Special attention was paid to the environmental effects on fatigue fracture behaviour. Tests were conducted on both the composite and its unreinforced matrix material, A6061-T6, under load-controlled conditions with a constant value of the combined stress ratio, α = τ_max /σ_max in laboratory air or in a 3.5% NaCl solution at the free corrosion potential. The corrosion fatigue strength of both the matrix and composite was less in the solution than in air. The dominating mechanical factor that determined the fatigue strength in air was either the maximum principal stress or the von Mises-type equivalent stress, depending on the combined stress ratio. However, in the 3.5% NaCl solution, the corrosion fatigue strength of both materials was determined by the maximum principal stress, irrespective of the combined stress ratio. In the case of the matrix material, crack initiation occurred by a brittle facet normal to the principal stress due to hydrogen embrittlement. However, in the composite material, the crack was initiated not at the brittle facet, but at a corrosion pit formed on the specimen surface. At the bottom of the pit, a crack normal to the principal stress was nucleated and propagated, resulting in final failure. Pitting corrosion was nucleated at an early stage of fatigue life, i.e. about 1% of total fatigue life. However, crack initiation at the bottom of a pit was close to the terminal stage, i.e. about 70% or more of total fatigue life. The dominating factor which determined crack initiation at a pit was the Mode I stress intensity factor obtained by assuming the pit to be a sharp crack. Initiation and propagation due to pitting corrosion and crack growth were closely examined, and the fatigue fracture mechanisms and influence of the 3.5% NaCl solution on fatigue strength of the composite and matrix under combined tension–torsion loading were examined in detail.     

Mechanical properties of Al2O3 and Al2O3 + ZrO2 ceramics reinforced by SiC whiskers

The effect of the SiC whisker content on the mechanical properties of Al₂O₃ and Al₂O₃ + 20 vol% ZrO₂ (2 mol% Y₂O₃) ceramic composites has been investigated. It is shown that the strength and fracture toughness of the composites are increased by the addition of 0–30 vol% SiC whiskers with only one exception that 30 vol% SiC whisker leads to a decrease in the flexure strength. The addition of 20 vol% ZrO₂ (2 mol% Y₂O₃) significantly improves the mechanical properties of the Al₂O₃ + SiC whisker (SiC_w) composites and the t-m phase transformation of ZrO₂ is enhanced by the residual stresses caused by the thermal incompatibility between the SiC_w and the matrix. The toughening effect of both SiC whiskers and the t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) is shown to be additive, but the addition of ZrO₂ decreases the strengthening effect of the SiC whiskers.     

Effect of size of reinforcement on thickness of anodized coatings on SiC/Al matrix composites

The effect of size of reinforcements on morphology and thickness of anodic coatings on 3.5 μm and 10 μm SiC particles reinforced 2024Al metal matrix composites (SiC_p/Al MMCs) formed in sulfuric acid was investigated with optical microscopy and scanning electron microscopy. The thickness of anodized coating on the MMCs is strongly dependent of size of SiC particles, and it is smaller for the MMC with smaller SiC particles because growth of more pores is affected when the concentration of SiC particles is fixed. The oxide/substrate interface became locally scalloped, and the anodized coatings formed on the MMCs were non-uniform in thickness, especially for the MMC reinforced by bigger particles.     

Corrosion behavior of SiC p /6061 Al metal matrix composites in chloride solutions

The corrosion behavior of silicon carbide particulates-aluminum metal matrix composites was studied in chloride solution by means of electrochemical techniques, scanning electron microscope (SEM), transmission electron microscope (TEM) and optical microscope. The materials under investigation were compocasting processed 6061 Al reinforced with increasing amounts of SiC particulates. Electrochemical tests such as potentiostatic polarization were done in 0.1 kmol·m^–3 NaCl solutions that were aerated and deaerated to observe overall corrosion behavior. In addition, pit morphology was observed after immersion tests. It was seen that the pitting potentials did not vary greatly or show definite trends in relation to the amounts of SiC _p reinforcement. However, the degree of corrosion increased with increasing SiC _p content; probably mainly due to galvanic couple. No intermetallics layer was found at the SiC _p /Al interface. Based on pitting potentials of Al-Si alloys, a pitting process around SiC particulate was proposed.Abbreviations SiC _p (silicon carbide particulates) - SiC _f (silicon carbide fibers) - SiC _w (silicon carbide whiskers) - E_pit (pitting potential) - E_prot (protection potential) - E _corr (corrosion potential) - i _galv (galvanic current density) - E _galv (galvanic potential)     

Fabrication,characterization and mechanical properties of SiC-whisker-reinforced Y-TZP composites

The microstructure and mechanical properties of hot-pressed yttria-stablized tetragonal zirconia polycrystals (Y-TZP) reinforced with up to 30 vol % SiC whiskers were investigated. The homogeneously dispersed and fully dense SiC whisker/Y-TZP composites were fabricated by wet-mixing the constitutents and uniaxially hot-pressing the resulting powder. The grain size of the matrix depended on the whisker volume fraction and the hot-pressing temperature. The significant increase of fracture toughness of about MPa m^1/2 at 10 Vol % SiC and a small increase in strength were achieved by uniformly dispersing the whiskers in the Y-TZP matrix. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, pullout, and crack bridging by the whiskers and also a phase transformation of ZrO₂. The observed increase in the fracture toughness of Y-TZP due to the addition of SiC whiskers was correlated with existing models of toughening mechanisms. Good agreement was achieved between the theoretical predictions and the experimental toughness values, obtained from the Y-TZP/SiC_w composites.     

Influence of SiC whisker on planar slip in Al-Li based alloys

The tensile deformation microstructure characteristics of silicon carbide whisker (SiC_w) reinforced Al-Li and Al-Li-Cu-Mg-Zr composites prepared by squeeze casting technique were studied by means of transmission electron microscopy (TEM), in order to evaluate the influence of SiC whisker on planar slip in Al-Li based alloys. For the purpose of comparison, the microstructural features of the unreinforced matrix alloys with the identical fabrication, thermal processing and tensile deformation history were also investigated. Dislocation pairs and intense slip bands originated from cutting of (Al₃Li) phases by moving dislocations, could be found in the specimens of Al-Li and Al-Li-Cu-Mg-Zr alloys, whereas absent in either SiC_w/Al-Li or SiC_w/Al-Li-Cu-Mg-Zr composites. The results demonstrate that the addition of SiC whisker to Al-Li based alloys, has a considerable effect on suppressing planar slip which is a general phenomenon in Al-Li based alloys resulting from the interaction between phase and dislocations.     

Particle size dependence of fatigue crack propagation in SiC particulate-reinforced aluminium alloy composites

Fatigue crack propagation (FCP) and fracture mechanisms have been studied for two orientations in powder metallurgy 2024 aluminium alloy matrix composites reinforced with three different sizes of silicon carbide particles. Particular attention has been paid to make a better understanding for the mechanistic role of particle size. The FCP rates of the composites decreased with increasing particle size regardless of orientation and were slightly faster in the FCP direction parallel to the extrusion direction. After allowing for crack closure, the differences in FCP rate among the composites and between two orientations were significantly diminished, but the composites showed lower FCP rates than the corresponding unreinforced alloy. Fracture surface roughness was found to be more remarkable with increasing particle size and in the FCP direction perpendicular to the extrusion direction. Taking into account the difference in the modulus of elasticity in addition to crack closure, the differences in FCP rate between the unreinforced alloy and the composites were almost eliminated.