Sheng-Long Lee

Effects of be and fe content on plane strain fracture toughness in A357 alloys

The effect of Be and Fe content on the plane strain fracture toughnessKIC of aluminum-based A357 alloys is investigated. The fracture behavior of A357 alloys has been evaluated as a function of both the magnitude and morphology of iron-bearing compounds and silicon particles. Addition of Be is beneficial for tensile properties and fracture toughness in the case of alloys containing intermediate (0.07 pct) and higher (0.15 pct) Fe levels. On the other hand, Be added to alloys containing the lower Fe (0.01 pct) level appears detrimental to tensile strength, but the quality index, notch-yield ratio (NYR), and plane strain fracture toughness were improved. Fractographic analysis reveals that crack extension of A357 alloys occurs mainly in an intergranular fracture mode. The fracture processes are initiated by void nucleation at iron-bearing compounds or irregularly shaped eutectic silicon particles as a result of their cracking and decohesion from the matrix. Then, void growth and coalescence result in growth of the main crack by shear-linkage-induced breakdown of submicronstrengthening particles. The effect of Be on increasingKIC is more apparent in the higher Fe alloys than in the lower Fe alloys. Superior toughness obtained by microstructural control has also been achieved in the intermediate and higher Fe levels of Be-containing alloys, with values equal to those obtained in alloys of lower Fe content.

Effects of Be and Fe additions on the microstructure and mechanical properties of A357.0 alloys

A357 hypoeutectic alloy is a heat-treatable Al-Si-Mg system with a nominal composition of Al-7 pct Si and about 0.6 pct Mg have widespreaded applications, especially in the aerospace and automotive industries. The purpose of this study was to determine the influences of Be and Fe content on the microstructure and mechanical properties of A357.0 alloys. Distinct morphologies were discerned between Be-containing and Be-free alloys. The Be-free alloys contain larger amount of iron-bearing phases with Mg than in Be-containing alloys. The addition of Be can change the plateletlike structure of iron-bearing phases to a comparatively harmless round nodular form. Also, the amounts of iron-rich phases are significantly lower and the silicon particles are smaller and more spherical in the Be-containing alloys. Small amounts of Be in A357.0 caused significant increases in the precipitation kinetics of Mg2Si. It was found that the addition of Be lowers the ternary and binary eutectic melting point. The amount of Mg available to form the major strengthening phase Mg2Si is increased promoting the tensile strength of A357.0 casting. The tensile properties were improved with decreasing Fe content and the addition of Be. The effect is more apparent in the higher Fe alloys than that in the lower Fe alloys.

Solid–liquid interdiffusion bonding between In-coated silver thick films

Abstract Solid–liquid interdiffusion (SLID) bonding between two pieces of In-coated silver thick films was investigated. The silver thick film was prepared by screen-printing of silver paste on an aluminum oxide substrate. Indium was coated in thickness ranging from 3 to 12 μm onto the silver thick films using thermal deposition (in vacuum). Two pieces of the In-coated thick films were placed in intimate contact (with a compressive stress of 0.04 MPa) and heated (at 180–250 °C) for various durations (600–3600 s) to undergo SLID bonding. Measurement of the bonding strength indicated that the SLID bond in 3-μm-In-coated thick-film couples is stronger than that in 8-μm-In-coated couples. X-Ray diffraction, scanning electron microscopy and electron probe microanalysis were used to analyze the bond. The stronger bond comprises a central zone of γ-Ag 2 In sandwiched by two Ag films; the weaker bond comprises a central zone of AgIn 2 sandwiched by two layers of γ-Ag 2 In. In the latter case, diffusion of excess indium from the coat into the interface between the silver film and alumina substrates is responsible for bond weakening. The development of microstructures in the SLID process for both strong and weak bonds is proposed.

Effects of beryllium on fatigue crack propagation of A357 alloys containing iron

An investigation of iron-bearing compounds, silicon particles and Mg2Si precipitates on fatigue crack growth behaviour in A357 alloys has been performed. The effect of Be on increasing ΔKth is more apparent in alloys with higher Fe content than in alloys with medium and lower Fe content. The fatigue life of the higher and medium Fe-content Be-containing alloys will be improved much as well as that of in the lower Fe content of alloys. Major fatigue cracks induced by cracking of iron-bearing compounds cracking were found in the medium and higher Fe-content alloys. In the lower Fe-content alloys, irregular silicon particles appear to be the main source of fatigue crack nucleation. In region I of low fatigue crack growth rates, the higher threshold stress intensity is attributed to the greater mechanical strength and to enhanced roughness-induced crack closure and deflection from more tortuous crack paths. The superior fatigue crack propagation resistance at higher growth rates of regions II and III is attributed to greater fracture toughness and to reduced crack growth rates in the Be-containing alloys regardless of Fe levels. The fracture surface morphologies of near-threshold crack growth contain stepped patterns and cleavage-like facets. With increasing crack growth rate, the fracture surface roughness increases and striations are formed in region II. At higher fatigue crack growth rates, the formation of a dimple surface and cleavage plane replaces striation formation.

Effect of trace Ce addition on the microstructures and mechanical properties of A356 (AL–7SI–0.35 Mg) aluminum alloys

A356 hypoeutectic alloy is a heat-treatable Al–Si–Mg system with a normal composition of Al–7 wt.% Si and about 0.35 wt.% Mg which has seen widespread applications in the aerospace and automotive industries. The purpose of this study is to determine the influence of different amounts of Ce on the microstructures and mechanical properties of A356 alloys. The findings indicate that modification efficiency in microstructures and mechanical properties of A356 alloy are greatly enhanced by adding 1.0 wt.% Ce. Two kinds of intermetallic compounds are found in the study, including Ce–23Al–22Si and Al–17Ce–12Ti–2Si–2 Mg (in wt.%) phases. Moreover, though the ductility of the Ce modified alloys are both greatly improved, there is no positive effect on the ultimate tensile strength that can be attributed to the formation of an Al–17Ce–12Ti–2Si–2 Mg phase. Furthermore, the results of thermal analysis reveal that there is no direct relationship between eutectic growth temperature and silicon morphology/modification rating.

Effect of rare earth elements addition on microstructures and mechanical properties of A356 alloy

Abstract A356 is a heat treatable Al–Si–Mg hypoeutectic alloy with a normal composition of 7 wt‐%Si and ∼0·3 wt‐%Mg which has widespread applications in the aerospace and automotive industries. The influence of the rare earth metals La and Ce on the microstructures and mechanical properties of A356 alloy has been investigated. The findings indicate that the modification efficiency of 1 wt‐%La is similar to that achieved by commercial modification with 0·02 wt‐%Sr, but the modification efficiency of Ce is lower than those of La and Sr. Two kinds of La or Ce rich intermetallics are found in this study: AlTiLa(Ce)Mg and AlSiLa(Ce) phases. Although the ductility of the rare earth modified alloys are both greatly enhanced, no positive effect on ultimate tensile stress that can be attributed to the formation of AlTiLa(Ce)Mg phase was observed.

Effects of Be addition on microstructures and mechanical properties of B319.0 alloys

Abstract An investigation of iron intermetallics, silicon particles and strengthening precipitates on the microstructures and mechanical properties in B319.0 alloys has been performed. Distinct morphologies were discerned between Be-containing and Be-free alloys at various solution temperatures. Solution treatment was carried out in the temperature range 480°C–530°C for 8 h. Dissolution of the iron intermetallics takes place at temperatures close to 510°C through fragmentation of the phases. The dissolution of iron intermetallics is accelerated with increasing solution temperature. The addition of Be to the alloy will raise the Al-Al 2 Cu eutectic melting temperature to 522°C compared with 515°C for Be-free alloys. The Be-free alloys show larger amounts of iron intermetallics than the Be-containing alloys. The addition of Be can change the platelet-like shape of iron intermetallic phases to a comparatively harmless Chinese-script shape. Also, the amount of iron-bearing phases and silicon particles are lower and silicon particles are smaller and more spherical in the Be-containing alloys. Small amounts of Be in B319.0 alloys can also increase the precipitation kinetics of strengthening precipitates. Tensile properties could be improved with optimal solution treatment and the addition of Be.

Effect of Ethanol on the Photoelectrochemical Fabrication of Macroporous n-Si(100) in HF Solution

The effect of ethanol on the photoelectrochemical fabrication of macroporous n-type Si(100) (pore diameter > 50 nm) in 2.0 M hydrofluoric acid was investigated. A cross-sectional scanning electron microscope examination revealed the formation of rough bigger pores (diameter ≈ 7-8 μm) in the absence of ethanol but smooth smaller ones (diameter ≈ 3-4 μm) in the presence of ethanol when the silicon was etched at 0.250 V (vs saturated calomel electrode) under 50 W illumination for 3 h. Characteristic electrochemical properties, such as limiting current density (i limit ), half-wave current density (i 1/2 ), transition potential (E trans ), and half-wave potential (E p/2 ) were derived from dc polarization. Electrochemical impedance spectroscopy conducted at E trans and E p/2 was helpful to illustrate the kinetics of the photoelectrochemical reaction. An additional inductive loop in the Nyquist plot occasioned at low frequencies in the presence of ethanol was attributed to the relaxation of the adsorption of ethanol in the pores. Addition of ethanol in the etching solution led to a decrease of contact angle between the solution and the silicon. Wetting behavior of ethanol plays an important role in the formation of smooth and small macropores.

Effect of Natural Aging and Cold Working on Microstructures and Mechanical Properties of Al-4.6Cu-0.5Mg-0.5Ag alloy

This research investigates the effects of natural aging and cold working prior to artificial aging on microstructures and mechanical properties of Al-4.6Cu-0.5Mg-0.5Ag alloy. Mechanical properties relative to microstructure variations were elucidated by the observations of the optical microscope (OM), differential scanning calorimeter (DSC), electrical conductivity meter (pct IACS), and transmission electron microscopy (TEM). The results showed that natural aging treatment has little noticeable benefit on the quantity of precipitation strengthening phases and mechanical properties, but it increases the precipitation strengthening rate at the initial stage of artificial aging. Cold working brings more lattice defects which suppress Al-Cu (GP zone) and Mg-Ag clustering, and therefore the precipitation of Ω phase decreases. Furthermore, more dislocations are formed, leading to precipitate the more heterogeneous nucleation of θ′ phase. The above-mentioned precipitation phenomena and strain hardening effect are more obvious with higher degrees of cold working.

Elastic interaction between screw dislocations and a blunting interfacial crack

Abstract Using the conformal mapping technique, the elastic interaction between a screw dislocation and an interfacial crack emanating from a semielliptic hole is investigated. We have derived the complex potential, the stress field, the strain energy, the force on the dislocation, the stress intensity factor at the crack tip, the critical stress intensity factor KD for dislocation emission and the critical stress intensity factor Kc for brittle fracture. From the image force, we find that there are zero-force points near the surface of the semielliptic hole if the applied stress is sufficiently large. The ductile vs. brittle behaviour of two-phase material can be determined by the relative magnitudes of D and Kc. A two-phase material with a larger shear modulus ratio and sharp crack will be more brittle. Finally, our results can be reduced to several special cases.