Aging Behavior of High Volume Fraction SiC Particles Reinforced 2024Al Composite

2024 Al matrix composite reinforced by SiC particles with 45% volume fraction and 1 μm diameter was successfully fabricated by squeeze-exhaust casting method. The aging behavior of SiCp/2024Al composite at four temperatures was investigated and compared to 2024 alloy. It was found that the addition of high volume fraction SiC particles does not alter the aging sequence, but it significantly accelerates the kinetics of precipitation in the composite matrices.Therefore, the aging peak of the composite appears earlier than that of 2024Al alloy. This is attributed to the decrease in the activation energy for the precipitate formation and the increase in the precipitate growth rate due to the high density dislocations in the composite with high volume fraction particles. The high density dislocations, as preferential nucleation sites for precipitates, bring about the tiny and dense precipitates in the composite.     

位错对高体积分数SiCP/2024Al时效行为的影响

采用粒径为1μm的SiC颗粒,用挤压铸造法制备出体积分数为45 %的SiC_P/2024Al复合材料,研究了位错对高体积分数SiC_P/2024Al时效行为的影响。结果表明,复合材料中的高密度位错可以湮灭大量的淬火空位,这在一定程度上抑制了GP区的析出。但是,高密度位错的存在降低了其它析出相的热扩散激活能,促进了析出相形核;还能为原子的管道扩散提供通道,促进了溶质原子的扩散,加速析出相的长大,在宏观上表现为对时效行为的促进,使峰时效提前。高密度的位错为强烈依赖于位错等缺陷形核的θ'和S'相提供许多优先形核的场所,使复合材料中的形核密度增加,同时使析出相的尺寸减小,所以复合材料中的析出相呈现细小弥散的分布特点。      

高体积分数挤压铸造铝基复合材料时效特征

采用挤压铸造法制备40 ％SiC_P/LD2复合材料,并对其在不同温度下的时效特征进行了研究。结果表明,高体积分数SiC颗粒的加入带来的弥散强化作用,可以大幅度提高基体合金的硬度和强度,但区别于基体合金和低体积分数复合材料,复合材料本身时效强化效果不明显。高体积分数复合材料峰时效时间较低体积分数缩短,且随时效温度的提高峰时效时间缩短。低温时效时峰值硬度最高且时效动力学较基体合金提前幅度较大。在时效析出过程中,高体积分数复合材料G.P.区的形成受到完全抑制,而β'相热扩散激活能降低,易于析出。      

An experimental study on deformation behavior below 0.2% offset yield stress in some SiCp/Al composites and their unreinforced matrix alloys

The deformation behaviors below 0.2% offset yield stress in some silicon carbide particulate reinforced aluminum composites (SiCp/Al) and their unreinforced matrix alloys were investigated experimentally in this work. The results of the study showed that incorporation of SiC particulate into aluminum matrix can enhance the plastic flow stress (PFS) in macroplastic stage but slightly lower PFS in microplastic stage. With increase in the volume fraction of SiC particulate (V_p), the 0.2% offset yield stress (σ_0.2) increases while the resistance to microplastic deformation (σ_10⁻⁵) first decreases and then increases. The composite with smaller particle size presents higher PFS both in micro- and macro-plastic stages. It was also found that heat treatment remarkably influence both micro- and macro-plastic behaviors of the composites. Quenching followed by artificial aging can significantly enhance PFS both in micro- and macro-plastic stages for the age hardened alloy based composites (SiCp/2024Al) but has no obvious effect for the non-age hardened alloy based composites (SiCp/Al). For both the SiCp/2024Al composite and unreinforced 2024Al alloy, PFS exist a ‘peak value’ with variation of aging time, implying that like the conventional yield strength, PFS in microplastic stage of the composite is also strongly controlled by the precipitates formed in matrix during aging treatment. The effects of thermal cycling on PFS are dependent to the V_p. In large V_p case (35%), with increase in cyclic number PFS slightly decreases but in small V_p case (15%) PFS slightly increases as the cyclic number increases. The PFS in microplastic stage is very sensitive to the microstructure features. The lower residual thermal stresses, small density of moveable dislocations and harder matrix would be beneficial to the increase of PFS in microplastic stage in the composites.     

Aging behavior of a 2024 Al alloy-SiCp composite

In the present research work the 2024 aluminum alloy was reinforced with SiC particles via powder metallurgy method. The effect of heat treatment conditions on artificial aging kinetics was investigated. The solution treatment of the composite sample and the unreinforced alloy was carried out at 495 °C for 1, 2 and 3 h followed by aging at 191 °C for various aging times between 1 and 10 h. The existence of SiC particles led to increasing the peak hardness of the alloy. The peak hardness of the composite sample took place at shorter times than that of the unreinforced alloy for the samples solution treated for 2 and 3 h, but took place at longer times for the samples solution treated for 1 h. The suitable solution treating time was about 2 h for both the composite and the unreinforced alloy that led to the fastest aging kinetics and the maximum hardness. At the solution treating time shorter than 2 h due to incomplete dissolution of precipitates, the aging kinetics decelerated and the hardness values decreased. X-ray diffraction studies indicated the presence of precipitation phases such as CuAl₂ and CuMgAl₂ in the composite in both as-extruded and solutionized conditions. For the samples solution treated more than 2 h, hardness values decreased due to the grain growth of matrix but no change occurred in the aging kinetics.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Micro-yield property of sub-micron Al2O3 particle reinforced 2024 aluminum matrix composite

The microstructure and micro-yield strength of sub-micron Al₂O₃ particle reinforced 2024Al composites and the effect of the thermal-cold cycling treatment on the microstructure and properties were studied. The results show that the dislocations are rare in the microstructure of the sub-micron Al₂O_3p/2024Al composite in the squeeze casting condition. Aging and thermal-cold cycling treatment does not change this phenomenon. The Al₂O₃ particles are fine, so the thermal misfit between particles and the matrix is very small during the temperature change, resulting in decreased dislocations. The tiny and uniformly dispersed S′ precipitates and sub-micron particles can effectively pin dislocations, therefore, the micro-yield strength of the composite increases. Depending on the condition of the thermal-cold cycling treatment after aging, both the size and distribution of the S′ precipitates in the composite change, and they have great effect on the micro-yield strength of the composite.     

Precipitation sequence in a SiC/Al-Mg2Si alloy composite material

The precipitation sequence of an Al-1.0mass%Mg₂Si composite material having 8 vol.% SiC particles was investigated by Vickers micro hardness and specific electrical resistivity measurements, and by TEM observation. The formation of GP zones was suppressed in the composite material and its age-hardenability was reduced. Distribution of precipitates in the composite material was coarser and their size was larger than that in the Al-1.0mass%Mg₂Si alloy (base alloy). Some types of precipitates in the composite material were not similar to those found in the base alloy but were similar to those in an Al-1.0mass%Mg₂Si alloys with excess silicon (the excess Si alloy). Especially, the metastable phases in the composite material aged at 473 K belonged to the type-A, type-B and type-C precipitates, which are typical metastable phases in the excess Si alloy, instead of the phase that is a typical metastable phase in quasi-binary Al-Mg₂Si alloys. The dislocations had little effect on the aging process of this composite material, because of the small number of dislocations introduced by the quenching after solution treatment.     

Evolution of Thermoplastic Shear Localization and Related Microstructures in Al／SiCp Composites Under Dynamic Compression

The localized shear deformation in the 2024 and 2124 Al matrix composites reinforced with SiC particles was investigated with a split Hopkinson pressure bar (SHPB) at a strain rate of about 2.0×103 s-1. The results showed that the occurrence of localized shear deformation is sensitive to the size of SiC particles. It was found that the critical strain, at which the shear localization occurs, strongly depends on the size and volume fraction of SiC particles. The smaller the particle size, the lower the critical strain required for the shear localization. TEM examinations revealed that Al/SiCp interfaces are the main sources of dislocations. The dislocation density near the interface was found to be high and it decreases with the distance from the particles. The Al matrix in shear bands was highly deformed and severely elongated at low angle boundaries. The AI/SiCp interfaces, particularly the sharp corners of SiC particles, provide the sites for microcrack initiation. Eventual fracture is caused by the g     

Production and properties of SiCp-reinforced aluminium alloy composites

A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content.     

搅拌铸造SiCp/2024复合材料热挤压-轧制变形组织及性能

利用搅拌铸造-热挤压-轧制工艺制备SiCp/2024复合材料薄板。通过金相观察(OM)、扫描电镜(SEM)及力学测试等手段研究了该复合材料在铸态、热挤压态及轧制态下的显微组织及力学性能,分析了材料在塑性变形过程中显微组织及力学性能的演变。结果表明,该复合材料铸坯主要由80～100μm的等轴晶组成,粗大的晶界第二相呈非连续状分布,SiC颗粒较均匀地分布于合金基体中;热挤压变形后,晶粒沿挤压方向被拉长,SiC颗粒及破碎的第二相呈流线分布特征;轧制变形后,基体合金组织进一步细化,晶粒尺寸为30～40μm,SiC颗粒破碎明显,颗粒分布趋于均匀,轧制变形对挤压过程中形成的SiC颗粒层带状不均匀组织有显著的改善作用。数学概率统计指出,塑性变形有利于提高颗粒分布的均匀性。力学测试表明,塑性变形后,复合材料的抗拉强度、屈服强度和延伸率显著提高。SiCp/2024铝基复合材料主要的断裂方式为:合金基体的延性断裂、SiC颗粒断裂及SiC/Al界面脱粘。     

Composite precipitates in a commercial Al–Li–Cu–Mg–Zr alloy

Composite precipitates with duplex and triplex association of single precipitates commonly found in commercial Al–Li alloys were observed by transmission electron microscopy (TEM) in a re-solution and quenched AA2090 alloy after aging. These novel precipitates were the duplex association of δ′(Al₃Li) and T₁ (Al₂CuLi) as well as the triplex δ′/β′(Al₃Zr)/T₁ and δ′/β′/θ′(Al₂Cu). The reduction of elastic energy associated with composite precipitate formation is suggested as a basic mechanism with plate-like T₁ and θ′ particles serving as preferential sites for nucleation of either δ′ or δ′/β′ spherical particles, which although in much smaller numbers, represent a volume fraction of 10 to 20% of all precipitates.     

Aging behaviour of Al–Cu–Mg alloy–SiC composites

The age-hardening kinetics of powder metallurgy processed Al–Cu–Mg alloy and composites with 5, 15 or 25 vol.% SiC reinforcements, subjected to solution treatment at 495 °C for 0.5 h or at 504 °C for 4 h followed by aging at 191 °C, have been studied. The Al–SiC interfaces in composites show undissolved, coarse intermetallic precipitates rich in Cu, Fe, and Mg, with its extent varying with processing conditions. Examination of aging kinetics indicates that the peak-age hardness values are higher, and the time taken for peak aging is an hour longer on solutionizing at 504 °C for 4 h, due to greater solute dissolution. Contrary to the accepted view, the composites have taken longer time to peak-age than the alloy, probably due to lower vacancy concentration, large-scale interfacial segregation of alloying elements, and inadequate density of dislocations in matrix. The composite with 5 vol.% SiC with the lowest inter-particle spacing has shown the highest hardness.     

Effect of Thermal Cycling Treatment on Microstructure and Mechanical Properties of AlNp/2024Al Composite

The AlNp/2024 composite with a high volume fraction (50%) was fabricated by squeeze casting. Its mechanical properties were investigated by tensile tests at ambient temperature and the microstructure was observed by a transmission electron microscope (TEM). It was indicated that after six thermal cooling cycles treatment the AIN/2024Al composite exhibited higher elastic limit (σ0.01), tensile strength (UTS) and elastic modulus (E). TEM observation showed that there was higher dislocation density both in the matrix and the particle. δ′ phase is the mainly precipitate.     

Effect of Thermal Cycling Treatment on Microstructure and Mechanical Properties of AlNp/2024A1 Composite

The A1NP/2024 composite with a high volume fraction (50%) was fabricated by squeeze casting. Its mechanical properties were investigated by tensile tests at ambient temperature and the microstructure was observed by a transmission electron microscope (TEM). It was indicated that after six thermal cooling cycles treatment the AIN/2024Al composite exhibited higher elastic limit (ac.ol), tensile strength (UTS) and elastic modulus (E). TEM observation showed that there was higher dislocation density both in the matrix and the particle. Δ’ phase is the mainly precipitate     

Effects of pre-treatments on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy

The effects of pre-treatments (solution and retrogression) on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy are investigated by means of transmission electron microscope (TEM), scanning electron microscope (SEM) and cyclic potentiodynamic polarization experiments. It is found that the aging precipitates and corrosion resistance are greatly affected by the pre-treatments. For the creep-aged alloy after solution pre-treatment, fine aging precipitates with high density are formed within grains. Meanwhile, large and continuously-distributed aging precipitates appear along grain boundaries. Also, this creep-aged alloy is strongly sensitive to the electrochemical corrosion, and the corrosion pits are easily induced in the 3.5 wt.% NaCl solution. For the creep-aged alloy after retrogression pre-treatment, when the retrogression pre-treatment time is increased, the density of intragranular aging precipitates first increases and then decreases, while the size of grain boundary precipitate and the width of precipitate free zone continuously increase. Compared with the creep-aged alloy after solution pre-treatment, the corrosion resistance of the creep-aged alloy after retrogression pre-treatment is greatly improved.     

SiC颗粒增强6061Al基复合材料的动态拉伸性能Ⅱ应变速率效应

用拉伸split Hopkinson bar实验装置进行了SiCp/6061Al得合材料及基体合金的动态拉伸实验，研究了材料的应变速率敏感性。结果表明，SiCp/6061Al复合材料及其基体合金均具有明显的应变速率效应，且复合材料的应变速率敏感性高于基体合金的应变速率敏感性，根据位错机制对此进行了解释。     

Characterizing and modeling the precipitation of Mg-rich phases in Al 5xxx alloys aged at low temperatures

Al 5083 alloys(5.25 at.% Mg) of different tempers(H131 and H116) were aged at low temperatures(50and 70?C) for 41 months. Scanning transmission electron microscopy(STEM), energy-dispersive X-ray spectroscopy(EDS), and atom probe tomography(APT) were applied to characterize precipitates formed in the sensitized samples. Experimental results revealed that the size of Mg-rich precipitates increased with aging time at 70?C for both alloys. APT results showed that Mg-rich precipitates of different Mg concentrations and morphologies formed in Al matrix and on the interface of Al matrix/pre-existing particles. In addition, a model based on local equilibrium of chemical potential and multi-class precipitates number evolution was adopted to predict the multiphase precipitation process in the Al-Mg binary system. The overall trend of precipitate radius and number density predicted by the model matched well with the experimental results. Moreover, modeling results revealed that nucleation and coarsening occurred faster in Al 5083 H131 than in Al 5083 H116 when aged at same temperature. The high density of dislocations and the pipe diffusion mechanism of dislocations can be used to explain such behavior.     

Effect of SiC particles on fatigue crack propagation in SiC/Al composites

Fatigue crack propagation has been studied in two SiC particulate-reinforced aluminium-matrix composites with differing matrix alloys and composite heat treatments. Results indicate that the fatigue crack propagation rate (FCPR) of aged SiC/LY12 Al composites decreases with increasing volume fraction (V_f) of SiC particles; for composites containing 15 volume % SiC particles in an LY12 Al matrix the FCPR is independent of heat treatment (ageing or annealing). Annealed SiC/5083 Al composite has a higher FCPR than annealed SiC/LY12 AI. The influence of SiC particles on crack path is briefly discussed.     

High strain rate superplastic flow stress and post-deformation mechanical properties of mechanically alloyed 2024 aluminium alloy reinforced with SiC particles

Abstract

Composites consisting of 2024 aluminium alloys reinforced with volume fractions of 0, 5, 10, and 15 vol.-% of SiC particles were fabricatedfrom the mechanically alloyed powders by an optimised hot compaction and prestraining process. Fine and equiaxed grain structures with grain sizes of <1 μm were observed within the matrix of each alloy. The composite specimens were compressed at temperatures between 733 and 813 K with a wide strain rate range from 10^?3 to 10 s^?1. Two strain rate regions with different slopes from ～ 5 × 10^?1 s^?1 were found in log (true stress–log (strain rate) curves. In the lower strain rate region of each alloy, the strain rate sensitivity values m were 0.03–0.16. The threshold stress σ_th for each alloy was estimated using an extrapolation procedure. A linear relationship was found between <disp-formula><graphic href="splitsection10-m1.tif"/></disp-formula> and σ_th where V_f is the volume fraction of SiC particles. In the higher strain rate region of each alloy, m values greater than 0.3 were obtained at 773 K, which is very close to the solidus temperature of 775 K for 2024 aluminium alloy. Moreover, the maximum yield strength and elongation for each alloy at room temperature were also obtained in the specimens compressed at 773 K. Thus, it was found that the optimum temperature for the high strain rate superplastic processing of the composites was just below the solidus temperature of the 2024 aluminium alloy. The grain coarsening resulted in the decrease of post-deformation strength and ductility as well as the m value in hot compression above the solidus temperature.