Coarse Si Particles Research Articles

Investigation of Defect Dynamics in Al-Si-Mg Polycrystals by Simultaneous Measurements of Internal Friction and Acoustoplastic Effect

Dynamics of structural defects was investigatedin situduring quasistatic deformation of polycrystalline Al-1wt.%Si-0.3wt.%Mg and Al-12wt.%Si-0.3wt.%Mg alloys by means of simultaneous measurements of internal friction and acoustoplastic effect. The alloys were subjected to different heat treatments after quenching: natural ageing at room temperature (T4 treatment) and peak ageing at 433 K for 8 hrs (T6 treatment). This enabled us to study the effect of different microstructure components – solute clusters (for the T4 treatment), GP zones and β (( precipitates (for the T6 treatment), coarse Si particles (for Al-12wt.%Si-0.3wt.%Mg) – on the irreversible and reversible components of dislocation motion under the combined action of oscillatory and quasistatic stress. Based on the data obtained, conclusions have been drawn about microstructural mechanisms of the acoustoplastic effect.

Microstructure Evolution of a Fine Grain Al-50wt%Si Alloy Fabricated by High Energy Milling

Microstructure evolution of high energy milled Al-50wt%Si alloy during heat treatment at different temperature was studied. Scanning electron microscope (SEM) and X-ray diffraction (XRD) results show that the size of the alloy powders decreased with increasing milling time. The observable coarsening of Si particles was not seen below 730°C in the high energy milled alloy, whereas, for the alloy prepared by mixed Al and Si powders, the grain growth occurred at 660°C. The activation energy for the grain growth of Si particles in the high energy milled alloy was determined as about 244 kJ/mol by the differential scanning calorimetry (DSC) data analysis. The size of Si particles in the hot pressed Al-50wt%Si alloy prepared by high energy milled powders was 5-30 m at 700°C, which was significantly reduced compared to that of the original Si powders. Thermal diffusivity of the hot pressed Al-50wt%Si alloy was 55 mm2/s at room temperature which was obtained by laser method.

Sb effect on micro structural and mechanical properties of rapidly solidified Al–12Si alloy

The effects of antimony (Sb) addition on the microstructure and mechanical properties of rapidly solidified eutectic Al–Si alloy were investigated. Al–12 wt.%Si– X wt.%Sb ( X = 0, 0.5 and 1.0) alloys were fabricated using a melt-spinner at rotating speeds of 20 m/s. The as-melt-spun ribbons were characterized by a combination of scanning electron microscopy (SEM), X-ray diffraction (XRD) and depth-sensing indentation (DSI) methods. SEM observations showed that 0.5%Sb addition refined the microstructure whereas 1.0%Sb addition led to the formation of coarse Si particles. The wheel side of the ribbons exhibited a finer microstructure than those on the free or air sides exposed to the atmosphere. According to the XRD results, the diffraction patterns of melt-spun ribbons consisted solely of α-Al and Si phases, and a possible volume fraction of AlSb or an intermetallic phase in the alloy was not observed. DSI tests were performed on the ribbons under different loads ranging from 200 to 1200 mN at room temperature. The resulting indentation load-displacement ( P– h) curves were analyzed using the work of indentation approach (WIA) to evaluate the ribbons’ microhardness ( H) and reduced modulus ( Er). We found that the H and Er values of the ribbon decrease with increasing Sb content. This can be attributed to the over modification and changes in the microstructure of Al–12%Si alloy when a minimum of 1.0%Sb is present as a modifier.

Enhanced Young’s Modulus of Al-Si Alloys and Reinforced Matrices by Co-continuous Structures

In the present work, the elastic behavior of different hypoeutectic and hypereutectic Al-Si alloys and different Al2O3 short fiber and SiC particle reinforced materials (SFRM and PRM, respectively) is studied. The effective Young’s modulus (E) of materials was experimentally measured and compared with the different theoretical predictions of Hashin-Strikman, Tuchiniskii, shear lag, and the rule of mixtures (ROM). The unreinforced alloys present an interconnected lamellar Si structure after fast solidification, which increases the Young’s modulus up to that of the Tuchiniskii prediction for interpenetrating skeletal structures. On the other hand, alloys presenting isolated and coarse Si particles (after spheroidization treatment at 540°C) are well described by the lower bound of the ROMs. Similarly, the interconnected Si-SiC structure observed in 10 and 70 vol% SiC reinforced AlSi7Mg and AlSi7 matrices in the as cast condition is responsible for the higher stiffness of the composite, if compared with that of Al99.5 or spheroidized AlSi7 matrices. An analogous behavior is observed in the SFRMs in the as cast condition, where the Si lamellae bridge the Al2O3 fibers, increasing the Young’s modulus of the composites, if compared with the conditions of spheroidized Si. Furthermore, the primary Si particles produce an improvement in the Young’s modulus by connecting several fibers in the case of a short fiber-reinforced hypereutectic AlSi18 matrix.

Effect of rapid solidification on the microstructure and mechanical properties of hot-pressed Al–20Si–5Fe alloys

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al–20Si–5Fe–2X (X = Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 × 10 5 K/s and 5 × 10 7 K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 °C/250 MPa/1 h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated. Results showed a profound effect of the cooling rate, consolidation stage, and transition metals on the microstructure and mechanical strength of Al–20Si–5Fe alloys. While microstructural refining was obtained at both cooling rates, the microstructure of the atomized powder exhibited the formation of fine primary silicon (~ 1 μm), eutectic Al–Si phase with eutectic spacing of ~ 300 nm, and δ-iron intermetallic. Supersaturated Al matrix containing 5–7 at.% silicon and nanometric Si precipitates (20–40 nm) were determined in the microstructure of the melt-spun ribbons. The hot consolidation resulted in coarsening of Si particles in the atomized particles, and precipitation of Si and Fe-containing intermetallics from the supersaturated Al matrix in the ribbons. The consolidated ribbons exhibited higher mechanical strength compared to the atomized powders, particularly at elevated temperatures. The positive influence of the transition metals on the thermal stability of the Al–20Si–5Fe alloy was noticed, particularly in the Ni-containing alloy.

Effect of grain size and indium addition on tensile characteristics of Al–1Si alloy

The effect of grain size and indium addition on the workhardening characteristics of Al–1Si (wt-%) alloy has been investigated at room temperature (RT). The samples were preaged at different temperatures in the range 523–623 K. The yield stress, the fracture stress, the fracture time and the linear workhardening coefficient generally decreased with increasing temperature and/or grain size, while the fracture strain and dislocation slip distance increased. The yield and fracture stresses for different grain sizes at different temperatures were found to be linearly related to grain diameters. Indium addition caused general increase for all the measured strength parameters. As concluded from transmission electron microscope (TEM) investigations, In addition to Al–Si alloy may retard the coarsening of Si particles. The energies activating the operating fracture mechanisms were found to be 79·6±0·4 and 32·4±0·4 kJ mol−1 for alloys Al–1Si and Al–1Si–0·2In respectively. This suggests a value of 47·2 kJ mol−1 as a binding energy between Si and In atoms in Al matrix.

Sliding wear behaviour of thixoformed AlSiCuFe alloys

The effect of Si content on the wear properties of thixoformed AlSiCuFe alloys was investigated in the present work. Wear rates, which decrease with increasing Si content until 12 wt%, start increasing at hypereutectic Si levels. The inferior wear resistance of thixoformed hypereutectic alloys is linked with the response of the coarse Si particles to severe wear conditions. The primary Si particles fracture and eventually drop out during the wear test leaving large cavities on the surface. The T6 heat treatment provided considerable hardening yet failed to improve wear properties. It is inferred from the wear rates of thixoformed and heat-treated samples that the wear properties of the thixoformed AlSiCuFe alloys is governed largely by the size and distribution of coarse Si particles and that the impact of hardness is only secondary.

Interpenetrating Hybrid Reinforcement in Al2O3 Short Fiber Preforms Infiltrated by Al-Si Alloys

In this work, the effect of the Si content and its morphological features on the mechanical properties of different short fiber-reinforced Al-Si matrices is studied. Al-Si alloys with Si contents of 7 wt pct and more contain a percolating Si network in the as-cast condition. The addition of alumina fibers into these Al-Si matrices increases the mechanical properties by forming an interconnected Al2O3-Si network. Both elastic modulus and maximum strength (from tensile and compression tests) increase with increasing Si content, which reinforces the interpenetration. However, Si diffuses toward the fibers’ surface during solution treatment forming coarse Si particles, reducing considerably the connectivity of the Al2O3-Si-hybrid network and thus the mechanical properties of the composites. The mechanical properties of the Al2O3-Si network itself are studied by means of compression tests on extracted hybrid reinforcement samples and are related to the global behavior of the composites. The strength vs true strain curves depict a strain-controlled softening of the composite material, which proceeds the faster, the higher the interconnectivity of the hybrid reinforcement, due to the propagation of damage of the Al2O3-Si network.

Microstructural modification of as-cast Al-Si-Mg alloy by friction stir processing

Friction stir processing (FSP) has been applied to cast aluminum alloy A356 plates to enhance the mechanical properties through microstructural refinement and homogenization. The effect of tool geometry and FSP parameters on resultant microstructure and mechanical properties was investigated. The FSP broke up and dispersed the coarse acicular Si particles creating a uniform distribution of Si particles in the aluminum matrix with significant microstructural refinement. Further, FSP healed the casting porosity. These microstructural changes led to a significant improvement in both strength and ductility. Higher tool rotation rate was the most effective parameter to refine coarse Si particles, heal the casting porosity, and consequently increase strength. The effect of tool geometry was complicated and no systematic trend was observed. For a standard pin design, maximum strength was achieved at a tool rotation rate of 900 rpm and traverse speed of 203 mm/min. Post-FSP aging increased strength for materials processed at higher tool rotation rates of 700 to 1100 rpm, but exerted only a marginal effect on samples prepared at the lower rotation rate of 300 rpm. Two-pass FSP with 100 pct overlapping passes resulted in higher strength for both as-FSP and post-FSP aged conditions.

A cyclic process for the nitridation of Si powder

The presence of coarse Si particles contributes a hindrance to achieving fully reacted Si 3N 4 by a reaction-bonded silicon nitride (RBSN) technique for use in real net-shape parts, such as turbo-charger rotors. A new process is described, in which the nitridation rate is enhanced by cyclic heating and the cyclic and isothermal heating processes were compared. When the nitridation was conducted at various temperatures and times (1370–1420 °C for times up to 60 h), the reaction rate of the cyclic process was three times faster than that of the isothermal reaction. This can be attributed to the cracking of the reacted Si 3N 4 shell during nitridation, which exposes unreacted liquid Si in the core. Cracking was found to occur by the internal stress resulting from the volume mismatch between Si 3N 4 and liquid Si.

Liquid formation and microstructural evolution during re-heating and partial melting of an extruded A356 aluminium alloy

Thixoforming processes require feedstock having a non-dendritic, equiaxed microstructure, which in some cases can be obtained by a deformation–recrystallisation technique followed by partial melting. From the study of an extruded, re-heated and partially re-melted A356 alloy, a number of phenomena were observed and analysed: grain and particle growth, texture and low angle grain boundary formation, primary phase coarsening during isothermal holding in the semi-solid state and uneven distribution of Si particles and liquid phase. Both grain growth (solid state) and particle growth (semi-solid state) obey a R 3 against time relationship. It was also found that microstructural coarsening follows an Ostwald ripening type of growth and that coalescence was also present, although exerting a minor role. Finally, phenomena such as the evolution with time of the proportion of low angle grain boundaries, and the relationship between recrystallisation, the onset of liquid formation and the coarse Si particle precipitation and dissolution are presented and discussed.

Effects of Si p size and volume fraction on properties of Al/Si p composites

Al–12 wt.% Si alloy matrix composites reinforced with high volume fraction of Si p were fabricated by squeeze infiltration. The effects of the compacting pressure on the volume fraction of Si p in preforms, and the influences of Si p size and volume fraction on the properties of Al/Si p composites were examined through this study. Si particles were compacted at different pressure of 40–130 MPa followed by sintered at 1000 °C for 7 h to obtain preforms containing 60–70 volume fraction (vol.%) of Si p. The sintered preforms were then infiltrated with Al–12 wt.% Si alloy at 750 °C under a 75 MPa squeeze infiltration pressure. It was found that lower coefficient of thermal expansion (CTE) and smaller density may be obtained with higher Si p volume fraction, yet increasing Si p volume fraction leads to higher amount of porosities in the composites and thus lowers the thermal conductivity (TC) and flexural strength. Besides, with the same Si p volume fraction, coarse Si particles result in higher CTE and TC, while finer Si particles may lower CTE and enhance the flexural strength of the composites effectively. From the results obtained in this study, it is expected that the high volume fraction Si p reinforced Al/Si p composites posses good potential in electronic packaging applications.

High temperature mechanical and microstructural behavior of A356\\15 vol% SiCp and A356 alloy

A metal-matrix composite (MMC, 15 vol% SiCp\\A356 Al) and its matrix alloy were subjected to hot torsion over the range 300–540°C and 0.1–5.0 s −1. Flow stresses of the A356 MMC were found to be much higher than A356 alloy at low temperatures but the difference was quite small at higher temperatures. Flow stresses were found to depend on the strain rate through a sinh function and on temperature through an Arrhenius term with activation energies of 263 kJ\\mol for the composite and 161 kJ\\mol for the matrix ; the increased value for the composite suggests that the SiC particles cause the matrix to undergo additional strain hardening. The substructures in both materials increase in cell size and decrease in internal and wall density, as temperature T rises and strain rate ε falls ; the composite shows much greater and less uniform dislocation density to which the strengths of the two materials are related. Dynamic recovery seems to be predominant in A356 ; however, dynamic recrystallization likely nucleates in the vicinity of silicon carbide particles in 15 vol% SiCp\\A356 Al. Ductility of the composite, about 25% below that of the alloy, rose by a factor of 4 between 400 and 500°C to become higher than many wrought alloy composites. The low ductility of A356 was shown to result from linking up of the cracks nucleated at coarse Si particles, whereas linkage of the decohesion voids at the SiC was associated with more plastic flow in the matrix which had much finer Si particles than the bulk alloy.

FATIGUE CRACK INITIATION AND GROWTH MECHANISMS IN PARTICLE-REINFORCED ALUMINUM MATRIX COMPOSITES

Reversed plane bending fatigue tests were carried out on three kinds of particle-reinforced aluminum matrix composites, Si/A2024, SiC/A2024 and SiC/AC4CH. SiC/A2024 and two kinds of Si/A2024 which have different size and distribution of Si particles were consolidated by Powder Metallurgy(PM) process, and SiC/AC4CH was made by casting process. The initiation and growth behavior of small surface cracks was continuously monitored by the replication technique and investigated in detail. It was found that both fatigue crack initiation strength and crack growth resistance became higher in the order, SiC/AC4CH, , Si/A2024 with coarse Si particle, Si/A2024 with fine Si particle, SiC/A2024. When stress amplitude was normalized by tensile strength, the difference in the fatigue strength of P/M composites was scarcely observed, but the casting composite exhibited lower strength than P/M one.

Ａｌ‐ＳｉおよびＡｌ‐Ｆｅ系粉末冶金合金の破壊特性

Static and dynamic fracture toughness tests were carried out on Al-18mass%Si and Al-8mass%Fe powder metallurgy (P/M) alloys. The fracture mechanisms of the P/M alloys were investigated which was related to the microstructure. Both static and dynamic fracture toughness are greater in the Al-Si alloy than in the Al-Fe alloy. Crack propagates in the matrix or along the interface between matrix and particles according to the size of the particles under both static and dynamic conditions. Crack path which acrosses the coarse Si particles is also observed in the Al-Si alloy. The characteristic distance, l∗0, which dominates the fracture initiation is round from two through four times of the interparticle spacing of coarse particles under static conditions while that is equivalent to the distance between the fatigue precrack and the nearest coarse particle under dynamic conditions.

Ａｌ‐２５Ｓｉ粉末鍛造アルミニウム合金における表面微小疲労き裂の発生および進展挙動

Reversed plane bending fatigue tests were carried out on two kinds of forged Al-25Si P/M (Powder Metallurgy) aluminum alloys having different sizes and distributions of Si particles. The initiation and growth behaviors of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail.It was found that the crack initiation strength of the material with fine Si particles was higher than that with coarse Si particles. Although little difference in strength was observed between the forged and transverse directions in the former material, some difference was found depending on the specimen orientations in the latter material. When the stress intensity range was calculated by considering the difference of the projected area of Si particles to the plane perpendicular to the loading axis, the difference in fatigue strength among the specimens oriented differently could be explained.The macroscopic crack growth rate, da/dn, could be expressed by the Paris equation in terms of the maximum stress intensity factor, Kmax, irrespective of stress level and specimen orientation. The fatigue crack growth rate was found faster in the coarse grained material than that in the fine grained material, and as the crack propagated along the Si particles, more deflective behavior of crack growth was observed in the material with fine Si particles, which was thought to result in the increase in crack growth resistance.

溶湯鍛造アルミニウム合金鋳物の室温および４２３Ｋにおける破壊靭性と疲労強度

Fracture toughness, S-N curve and fatigue crack propagation behavior in melt-forged aluminum casting alloys such as AC4C-T6 and AC8A-T6 were investigated at room temperature and 423K, and were compared with these in a commercial 6061-T6 wrought alloy. The static or dynamic fracture toughness, JIC or Jd for AC4C-T6 and AC8A-T6 alloys was lower than that for a 6061-T6 alloy. The S-N curves for AC4C-T6 and AC8A-T6 alloys were situated lower than that for a 6061-T6 alloy. This was because of a pre-existed surface flaw effect by coarse Si particles in the casting alloys. The fatigue crack propagation in AC4C-T6 and AC8A-T6 was unstable in a high stress intensity range, because a large number of fracture-initiation sites such as Si and compounds were distributed in these alloys comparing with a 6061-T6 alloy. On the contrary, the value of ΔKth increased in a lower stress intensity range because Si particles and compounds acted as obstacles against crack propagation.

Influence of Modification on the Solution Heat-treatment Response of Cast Al-Si-Mg Alloys

A preliminary investigation has been conducted to evaluate the influence of Sr modification on the solution heat-treatment response of hypoeutectic Al-Si-Mg (A356.0) alloys. Modified and unmodified test castings were produced in zircon-coated steel moulds and were solutionised at a constant temperature for varying times. The microstructural changes in the Si-particle morphology were analysed as a function of solution time. Machined samples were obtained from test castings to determine the tensile properties. The results indicate that the solution treatment is accompanied by spheroidisation and coarsening of Si particles. The kinetics of spheroidisation are much higher in modified than in unmodified samples. Because of the diversity in particle size distribution, however, greater coarsening rates are observed in unmodified specimens. The tensile properties of the casting increase with the duration of solution treatment and upon modification of the melt. The effect of modification is to lower the solution time necessary to obtain the desired mechanical properties in the casting.