Tensile and fracture toughness properties of SiC<Subscript>p</Subscript> reinforced Al alloys: Effects of particle size,particle volume fraction,and matrix strength

The goal of this work was to evaluate the effects of particle size, particle volume fraction, and matrix strength on the monotonic fracture properties of two different Al alloys, namely T1-Al2124 and T1-Al6061, reinforced with silicon carbide particles (SiC_p). From the tensile tests, an increase in particle volume fraction and/or matrix strength increased strength and decreased ductility. On the other hand, an increase in particle size reduced strength and increased the composite ductility. In fracture toughness tests, an increase in particle volume fraction reduced the toughness of the composites. An increase in matrix strength reduced both K _crit and δ_crit values. However, in terms of K _Q (5%) values, the Al6061 composite showed a value similar to the corresponding Al2124 composite. This was mainly attributed to premature yielding caused by the high ductility/low strength of the Al6061 matrix and the testpiece dimensions. The effect of particle size on the fracture toughness depends on the type of matrix and toughness parameter used. In general, an increase in particle size decreased the K _Q (5%) value, but simultaneously increased the amount of plastic strain that the matrix is capable of accommodating, increasing both δ_crit and K _crit values.     

Failure modes and materials performance of railway wheels

In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, K _ID , is obviously lower than static fracture toughness, K _IC , and has the same trend as K _IC . The ratio of K _ID /σ _YD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles on the microstructure and sliding wear of 7075 Al alloy manufactured by squeeze casting method

Aluminum (Al) alloy 7075 reinforced with Al₂O₃ particles was prepared using the stir casting method. The microstructure of the cast composites showed some degree of porosity and sites of Al₂O₃ particle clustering, especially at high-volume fractions of Al₂O₃ particles. Different squeeze pressures (25 and 50 MPa) were applied to the cast composite during solidification to reduce porosity and particle clusters. Microstructure examinations of the squeeze cast composites showed remarkable grain refining compared with that of the matrix alloy. As the volume fraction of particles and applied squeeze pressure increased, the hardness linearly increased. This increase was related to the modified structure and the decrease in the porosity. The effect of particle volume fraction and squeeze pressure on the dry-sliding wear of the composites was studied. Experiments were performed at 10, 30, and 50 N with a sliding speed of 1 m/s using a pin-on-ring apparatus. Increasing the particle volume fraction and squeeze pressure improved the wear resistance of the composite compared with that of the monolithic alloy, because the Al₂O₃ particles acted as load-bearing constituents. Also, these results can be attributed to the fact that the application of squeeze pressure during solidification led to a reduction in the porosity, and an increase in the solidification rate, leading to a finer structure. Moreover, the application of squeeze pressure improved the interface strength between the matrix and Al₂O₃ particles by elimination of the porosity at the interface, thereby providing better mechanical locking.     

Fatigue crack growth mechanism in cast hybrid metal matrix composite reinforced with SiC particles and Al2O3 whiskers

The fatigue crack growth (FCG) mechanism of a cast hybrid metal matrix composite (MMC) reinforced with SiC particles and Al₂O₃ whiskers was investigated. For comparison, the FCG mechanisms of a cast MMC with Al₂O₃ whiskers and a cast Al alloy were also investigated. The results show that the FCG mechanism is observed in the near-threshold and stable-crack-growth regions. The hybrid MMC shows a higher threshold stress intensity factor range, ΔK_th, than the MMC with Al₂O₃ and Al alloy, indicating better resistance to crack growth in a lower stress intensity factor range, ΔK. In the near-threshold region with decreasing ΔK, the two composite materials exhibit similar FCG mechanism that is dominated by debonding of the reinforcement–matrix interface, and followed by void nucleation and coalescence in the Al matrix. At higher ΔK in the stable- or mid-crack-growth region, in addition to the debonding of the particle–matrix and whisker–matrix interface caused by cycle-by-cycle crack growth at the interface, the FCG is affected predominantly by striation formation in the Al matrix. Moreover, void nucleation and coalescence in the Al matrix and transgranular fracture of SiC particles and Al₂O₃ whiskers at high ΔK are also observed as the local unstable fracture mechanisms. However, the FCG of the monolithic Al alloy is dominated by void nucleation and coalescence at lower ΔK, whereas the FCG at higher ΔK is controlled mainly by striation formation in the Al grains, and followed by void nucleation and coalescence in the Si clusters.     

Tensile and wear properties of rolled Al5Mg-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> or C particulate composites

Al5Mg alloy matrix composites reinforced with different percentages of Al₂O₃ (60 μm) or C (90 μm) particulates were prepared by the vortex method. The composites were then subjected to hot or cold rolling with different reduction ratios. The microstructures of the rolled composites revealed that the matrix grains moved around the particulate causing deformation. By continuing deformation, the particulates rearranged themselves in the matrix, leading to lensoid distortion. It was found that the addition of Al₂O₃ or C particulates increased the 0.2% proof stress and reduced both the tensile strength and ductility, compared with the monolithic alloy. Scanning electron microscopy (SEM) fractographic examinations showed that the composites reinforced with Al₂O₃ particulates failed through particulate fracture and matrix ligament rupture. However, the failure of the composites reinforced with C particulates was through particulate decohesion, followed by ductile failure of the matrix. Abrasive wear results showed that the wear rate of the Al5Mg alloy decreased with the addition of C particulates. However, increasing the volume fraction of C particulates did not have a prominent effect on the wear rate. The composites reinforced with Al₂O₃ particulates exhibited a higher wear rate than that of the unreinforced alloy. Furthermore, addition of both C and Al₂O₃ particulates into the Al5Mg matrix alloy did not significantly improve the wear resistance. For all composites studied in this work, hot or cold rolling had a marginal effect on the wear results.     

Microstructure-based modeling of the influence of particle spatial distribution and fracture on crack growth in particle-reinforced composites

The crack growth behavior of particle-reinforced composites is determined by several factors, such as volume fraction, particle size, particle morphology, spatial distribution and particle strength. Thus, an accurate and robust numerical model must incorporate the true microstructure of the particles. It will be shown that the strength of the reinforcement particles is also an important factor. Hence, the model must be able to simulate particle fracture. In this paper, the crack growth behavior of SiC particle-reinforced Al matrix composites was modeled using actual microstructures. Linear elastic fracture mechanics principles were used to propagate the crack and obtain the local stress intensity values. The effect of particle fracture on crack growth was studied. It will be shown that spatial distribution and shape of the particles, as well as particle fracture ahead of the crack tip, significantly affect the crack trajectory and the stress distribution at the crack tip.     

Discrete dislocation plasticity modeling of short cracks in single crystals

The mode-I crack growth behavior of geometrically similar edge-cracked single crystal specimens of varying size subject to both monotonic and cyclic axial loading is analyzed using discrete dislocation dynamics. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation incorporated through a set of constitutive rules. The fracture properties are specified through an irreversible cohesive relation. Under monotonic loading conditions, with the applied stress below the yield strength of the uncracked specimen, the initiation of crack growth is found to be governed by the mode-I stress intensity factor, calculated from the applied stress, with the value of K_init decreasing slightly with crack size due to the reduction in shielding associated with dislocations near a free surface. Under cyclic loading, the fatigue threshold is ΔK-governed for sufficiently long cracks. Below a critical crack size the value of ΔK_I at the fatigue threshold is found to decrease substantially with crack size and progressive cyclic crack growth occurs even when K_max is less than that required for the initiation of crack crack growth in an elastic solid. The reduction in the fatigue threshold with crack size is associated with a progressive increase in internal stress under cyclic loading. However, for sufficiently small cracks, the dislocation structure generated is sparse and the internal stresses and plastic dissipation associated with this structure alone are not sufficient to drive fatigue crack growth.     

Effect of microstructure and load ratio on fatigue crack growth behavior of advanced Al-Cu-Li-Mg-Ag Alloys

Fatigue crack growth (FCG) behavior of recently developed three Al-Cu-Li-Mg-Ag alloys, Weldalite 049, X 2095 and MD 345, was examined in air at load ratios of 0.1 and 0.75. It was found that all three alloys showed better resistance to fatigue crack growth than conventional high strength Al alloys. The morphologies of crack growth paths were generally linear, but some showed deflection and branching. And the alloys revealed rough and transgranular fracture surfaces. Among the factors contributing to the excellent resistance of Al-Cu-Li alloys to fatigue crack growth are enhanced slip reversibility and high surface roughness causing a high crack closure level, thus reducing ΔK_eff for crack extension. The fatigue threshold decreased and fatigue crack growth rates increased significantly with increasing the load ratio. This is caused by the decrease in crack closure level at high load ratio. But the fracture mode did not show a significant change with increasing the load ratio.     

Measurements of fibre bridging during fatigue crack growth in Ti/SiC fibre metal matrix composites

High spatial resolution synchrotron X-ray strain mapping has been used to map the elastic matrix and fibre strains in the vicinity of a fatigue crack in a Ti–6Al–4V/SCS6 SiC fibre composite. A 0.61 mm fatigue crack was initiated and grown in three-point-bending. By using an in-situ loading stage it was possible to map the crack opening (longitudinal) strain distribution at K_appl=K_max and K_appl=0. In the far field region, significant thermally induced stresses were evident, being compressive in the fibres and tensile in the matrix. Around the notch and in the wake of the crack tip essentially no residual strain and only small interfacial shear stresses were found in the unloaded case, indicative of a debonded/damaged interface. At K_max the maximum tensile stress in the matrix is in the vicinity of the crack tip, whereas for the SiC fibres the maximum stress is in the bridging zone in the wake of the crack. The perturbed zone extends about ±1.5 mm either side of the crack. It was at the boundary of this zone that the maximum interfacial shear stresses (∼80 MPa) were measured in the loaded stage. A small area of tensile strain in front of the crack tip in the unloaded condition suggests frictional resistance from the bridging fibres acts to keep the crack slightly open. A simple three-dimensional finite element model has been developed to help interpret the results. The crack is introduced statically by node release and the Coulomb friction law governs the interface strength. The results of the model are compared to the synchrotron strain measurements. This comparison confirms the degradation of the interface strength in the wake of the crack.     

Thermally Stable Aluminum-Lithium Alloy 1424 for Application in Welded Fuselage

Sheets of Al – Li alloy 1424 have high fracture toughness and diminished rate of fatigue crack growth, high corrosion resistance, and excellent weldability. The chemical composition and the regime of the hardening heat treatment of alloy 1424 are shown to provide good thermal stability of the sheets with respect to fracture toughness (K _c ^f) and growth rate of fatigue cracks (GRFC) after a hold of up to 3000 h at 85°C. These characteristics decrease only after a hold of 4000 h and only in the L – T direction. The suggested process for manufacturing sheets diminishes the anisotropy of the strength characteristics and the elongation due to the formation of a partially recrystallized structure.     

A Fatigue Model for Discontinuous Particulate-Reinforced Aluminum Alloy Composite: Influence of Microstructure

In this paper, the use of a microstructure-sensitive fatigue model is put forth for the analysis of discontinuously reinforced aluminum alloy metal matrix composite. The fatigue model was used for a ceramic particle-reinforced aluminum alloy deformed under conditions of fully reversed strain control. Experimental results revealed the aluminum alloy to be strongly influenced by volume fraction of the particulate reinforcement phase under conditions of strain-controlled fatigue. The model safely characterizes the evolution of fatigue damage in this aluminum alloy composite into the distinct stages of crack initiation and crack growth culminating in failure. The model is able to capture the specific influence of particle volume fraction, particle size, and nearest neighbor distance in quantifying fatigue life. The model yields good results for correlation of the predicted results with the experimental test results on the fatigue behavior of the chosen aluminum alloy for two different percentages of the ceramic particle reinforcement. Further, the model illustrates that both particle size and volume fraction are key factors that govern fatigue lifetime. This conclusion is well supported by fractographic observations of the cyclically deformed and failed specimens.     

Fatigue crack retardation by silicon carbide powder paste infiltration under different cyclic stress conditions*

Fatigue crack retardation with infiltrated SiC paste into a crack is examined in low carbon structural steel. Two different sizes of SiC powders, whose average diameters are 15 and 53 μm, are used. The SiC powder mixed with oil is infiltrated into a through thickness fatigue crack from the crack mouth. Fatigue crack growth retardation is examined by the ΔK increasing test of R = 0.1, comparing with the base plate property, where ΔK is stress intensity factor range and R is stress ratio. Crack growth is retarded just after infiltrating SiC paste into the crack mouth, and the deceleration of crack growth rate to 1/50 of the base plate appears in the maximum. It is revealed that this crack retardation behaviour results from the crack closure induced by the wedge effect of the SiC particle into a crack. The crack retardation effect is investigated with several combinations of SiC particle size and cyclic stress conditions. The crack growth rate, da/dn and stress intensity factor, K_cl for the crack closure depend on both the maximum stress intensity factor, K_max, and the stress ratio, R. While the better retardation effect can appear in the higher K_max and the higher R ratio, it disappears in the R ratio over 0.7. The SiC paste with 15 μm powder brings the crack retardation effect in the wider cyclic stress condition more stable than in the SiC paste with 53 μm powder.     

SiC颗粒增强铝基复合材料的连接现状

SiC颗粒增强铝基复合材料因其具有成本低、耐磨性好、高比强度和比刚度、高谐振频率等优良的性能受到关注,但由于难于机械加工,特别是焊接性较差制约其在工程中的应用推广. 文中通过对国内外SiC颗粒增强铝基复合材料的连接现状(焊接方法主要集中于熔化焊、扩散焊、搅拌摩擦焊和钎焊等)进行综述和评价. 结果表明,SiC颗粒和Al基体的较大物理化学性能差异是影响该种复合材料焊接性的主要因素;当SiC颗粒体积分数低于35%时,目前已取得令人基本满意的焊接效果,已具有小批量生产的趋势;但当SiC颗粒体积分数大于35%时,特别是针对高体积分数(55% ~ 75%)的复合材料而言,传统的熔化焊方法很难获得高质量的接头,因此选择合适的连接方法和特殊的焊料成分则成为该种材料的重要创新方向.     

High Wear-Resistant Aluminum Matrix Composite Layer Fabricated by MIG Welding with Lateral B<Subscript>4</Subscript>C Powder Injection

In this work, a low-cost technique combining MIG welding and lateral powder injection was developed to fabricate B₄C particles-reinforced aluminum matrix composite (AMC) layer on a T6 heat-treated 7075 aluminum alloy (AA7075-T6) substrate. The AMC layer was 6-7 mm thick and well bonded to the substrate. The B₄C particles were dispersed throughout the AMC layer with an average content of approximately 7 vol.%. No significant reaction products existed either at the particle–matrix interface or in the Al-matrix. In pin-on-disk dry sliding wear tests against Al₂O₃ grinding wheels, the AMC layer exhibited excellent wear resistance with volume wear rate approximately 1/10-3/10 that of the quenched AISI 1045 steel and only approximately 2-7% that of the AA7075-T6 alloy under the same wear conditions. A small addition of ceramic particles can greatly improve wear resistance, suggesting that this technique has good prospects for a wide variety of applications.     

Microstructure and Mechanical Behavior of Microwave Sintered Cu<Subscript>50</Subscript>Ti<Subscript>50</Subscript> Amorphous Alloy Reinforced Al Metal Matrix Composites

In the present work, Al metal matrix composites reinforced with Cu-based (Cu₅₀Ti₅₀) amorphous alloy particles synthesized by ball milling followed by a microwave sintering process were studied. The amorphous powders of Cu₅₀Ti₅₀ produced by ball milling were used to reinforce the aluminum matrix. They were examined by x-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness and compression testing. The analysis of XRD patterns of the samples containing 5 vol.%, 10 vol.% and 15 vol.% Cu₅₀Ti₅₀ indicates the presence of Al and Cu₅₀Ti₅₀ peaks. SEM images of the sintered composites show the uniform distribution of reinforced particles within the matrix. Mechanical properties of the composites were found to increase with an increasing volume fraction of Cu₅₀Ti₅₀ reinforcement particles. The hardness and compressive strength were enhanced to 89 Hv and 449 MPa, respectively, for the Al-15 vol.% Cu₅₀Ti₅₀ composites.     

Effect of weight percentage and particle size of B4C reinforcement on physical and mechanical properties of powder metallurgy Al2024-B4C composites

In this study, Al2024-B₄C composites containing 0, 5, 10 and 20 wt% of B₄C particles with two different particle sizes (d₅₀=49 μm and d₅₀=5 μm) as reinforcement material were produced by a mechanical alloying method. Two new particle distribution models based on the size of reinforcement materials was developed. The microstructure of the Al2024-B₄C composites was investigated using a scanning electron microscope. The effects of reinforcement particle size and weight percentage (wt%) on the physical and mechanical properties of the Al2024-B₄C composites were determined by measuring the density, hardness and tensile strength values. The results showed that more homogenous dispersion of B₄C powders was obtained in the Al2024 matrix using the mechanical alloying technique according to the conventional powder metallurgy method. Measurement of the density and hardness properties of the composites showed that density values decreased and hardness values increased with an increase in the weight fraction of reinforcement. Moreover, it was found that the effect of reinforcement size and reinforcement content (wt%) on the homogeneous distribution of B₄C particles is as important as the effect of milling time.     

双相钢搭接点焊接头疲劳寿命分析

研究了双相钢焊点特征,对不同匹配双相钢搭接焊点进行了疲劳试验,获得了焊点的载荷寿命曲线.研究了双相钢焊点的疲劳裂纹扩展及失效形式,分析和解释了疲劳过程中的现象,并根据裂纹的实际扩展路径,提出了局部等效张开应力强度因子keq,从断裂力学的角度对双相钢焊点的疲劳失效进行了分析.结果表明,keq能够有效地关联具有不同厚度,不同熔核直径的搭接焊点试样的疲劳寿命,是反映双相钢焊点疲劳强度的有效参量,能够用来预测焊点疲劳寿命.     

Micromechanisms of fatigue crack growth in a single crystal Inconel 718 nickel-based superalloy

The fatigue crack growth behavior of an experimental, single crystal alloy, of equivalent nominal chemical composition to Inconel 718 is presented. Fracture modes under cyclic loading were determined by scanning electron microscopy. The results of the fractographic analyses are presented on a fracture mechanism map that shows the dependence of the fatigue fracture mechanisms on the maximum stress intensity factor, K_max, and the stress intensity factor range, ΔK. Crack-tip deformation mechanisms associated with fatigue crack growth were studied using transmission electron microscopy. The relative effects of ΔK and K_max on the fatigue crack growth behavior of this material are discussed within the context of a two-parameter crack growth law. The influence of grain boundaries on the fatigue crack growth resistance of materials such as Inconel 718 is also discussed in light of the results of this investigation.     

Microstructural and mechanical behavior of blended powder semisolid formed Al7075/B4C composites under different experimental conditions

This work aimed to fabricate B₄C reinforced aluminum matrix composites via blended powder semisolid forming that is an implementation of the benefits of semisolid forming to the powder metallurgy. Al7075 elements were incrementally added to ethanol solution under mechanical mixing. Al7075 constituents and B₄C particles were blended in a high energy ball mill. Cold compacted Al7075/B₄C blends were pressed at semisolid state. The effects of the size of the matrix (20, 45 and 63 μm), reinforcing volume fraction (5%, 10% and 20%) and semisolid compaction pressure (50 and 100 MPa) on the morphology, microstructure, density, hardness, compression and bending strength were thoroughly analyzed. Experimental results revealed that the highest microstructural uniformity was achieved when large B₄C particles (45 μm) were distributed within the small particles (20 μm) of the matrix phase. Composites with matrix particles larger than reinforcing phase indicated agglomerations in loadings more than 10% (volume fraction). Agglomerated regions resisted against penetration of the liquid phase to the pores and lowered the density and strength of these composites. Composites with 20 μm Al7075 and 20% (volume fraction) 45 μm B₄C powder pressed under 100 MPa exhibited the highest values of hardness (HV 190) and compressive strength (336 MPa).