Wetting kinetics of a hypo-eutectic Al-Si system

This study is devoted to investigation of spreading of a hypo-eutectic Al 8Si system over a flat Al surface. Wetting kinetics was studied using a real time in situ monitoring of the triple line movement facilitated by a hot-stage microscopy system under controlled atmosphere. Analysis of the validity of the classic Tanner's law correlation developed for inert wetting systems is performed. A good agreement of the power relationship between the triple line location and time was established. The dispersion of the n-power exponent magnitude is between 9 and 13, with an average value of 11.     

Dry sliding wear in binary Al-Si alloys at low bearing pressures

On leave from: Scientific Research Council, Baghdad, Iraq.     

The properties of directionally-solidified eutectic and hypo-eutectic copper-zirconium alloys

Alloys of the copper-Cu₅Zr eutectic have been directionally solidified over a range of growth rates. The tensile strength was found to be high, 0.6 to 1.0 GN m^–2, in contrast to the poor electrical conductivity, typically 22 m ^–1 mm^–2. In order to improve the latter at the expense of the former, hypo-eutectic (copper-rich) alloys were also studied. Alloys of composition in the region of copper-4 wt % zirconium possessed a useful combination of properties but these were not superior to the properties of conventionally cast samples. These alloys were shown to be stable against exposure to elevated temperatures.     

Microstructure and dry sliding wear behavior of hot-extruded AlSiCuPb bearing alloys

The microstructures, mechanical properties and dry wear behavior of hot-extruded AlSiCuPb bearing alloys have been studied. It showed that the hot-extruded AlSiCuPb alloys possessed microstructures with uniformly distributed lead particles and fine broken grains of silicon, and exhibited further improved mechanical properties in comparison to the stir cast ones. With increasing the lead content, the wear rate decreased greatly and wear rate-load curves took different form for extruded bearing alloys containing 20% and 25% lead. Optical observation revealed the reason was formation of a black compact film of lubricant covering almost the entire worn surface of specimens at highly applied load level. This film is a mixture of different constituents containing the elements Al, Si, O, Fe and Pb.     

Nano-scale sliding contact deformation behaviour of enamel under wet and dry conditions

The abrasion response of cross sectional areas of enamel was studied by sliding a rounded diamond conical nano-indenter tip across the surface. The nano-indenter tip (radius ~1,200 nm) was scanned over a specific squared area with a load of 400 μN. Two different environments were chosen: Hank’s balanced salt solution (HBSS) and atmospheric laboratory condition. SEM (Scanning Electron Microscopy) and AFM (Atomic Force Microscopy) were used to characterize the final abraded areas. In addition, single scratches with linear incremented load were performed. The normal load and displacement data were utilized in a complementary manner to support the proposed deformation mechanisms. Greater orientation dependence for the case of the single scratches in relation to the abrasion tests was found. The latter results are discussed in terms of plastic deformation effects. The abrasion mechanisms were found to be the same for both wet and dry measurements and similar to that described in a previous study (Guidoni et al., Wear 266:60–68, 2009; Guidoni, Nano-scale mechanical and tribological properties of mineralized tissues. PhD. Montan University Leoben, Leoben, Austria, 2008). However, scratch deformation under fluid measurements shows greater recovery effects and abrasion resistance.     

Modification of hypoeutectic Al-Si alloys

Modification of the Al-Si eutectic microstructure has been studied by using the entrained droplet technique in a range of hypoeutectic Al-Si alloys doped with different levels of phosphorus and sodium. Differential scanning calorimetry has been used to investigate the kinetics of silicon nucleation during eutectic solidification, and transmission and scanning transmission electron microscopy has been used to investigate the eutectic microstructure and microchemistry. The Al-Si eutectic microstructure can be modified from coarse-faceted silicon particles nucleating at low undercooling to clusters of fine-scale silicon particles nucleating at high undercooling by either increasing alloy purity, adding sodium, or increasing cooling rate during solidification. The unmodified coarse silicon particles are heterogeneously nucleated on pre-existing AIP particles. Increasing alloy purity removes phosphorus and prevents the formation of AIP; adding sodium leads to the formation of Na₃P in preference to AIP; and increasing cooling rate allows insufficient time for the precipitation of AIP from low levels of dissolved phosphorus. At a cooling rate of 10 K min^–1, the transition from an unmodified to a modified Al-Si eutectic microstructure takes place when phosphorus levels fall below 0.25–2 p.p.m., and when sodium levels increase to 80–850 p.p.m.     

The hardness of Al-Si eutectic alloys

The Vickers hardness values of Al-Si eutectic alloys, solidified unidirectionally at rates ranging from 2.8×10^–5 to 1 cm sec^–1, have been determined. These are compared with associated tensile and compressive properties. It is shown that there is no close correlation of hardness and strength over the entire range of growth rates although similar trends are seen between hardness and compressive yield strength. It is concluded that caution should be exercised when inferring strength from hardness data.     

The crystal growth of silicon in Al-Si alloys

Some crystallographic observations of primary silicon and unmodified eutectic silicon in aluminium-silicon alloys are summarized and used to support the theory that sodium modification is due to the poisoning of active growth centres. The implications of endogenous growth in unmodified aluminium-silicon sand castings are explored and mechanisms are proposed for the flake-fibre transition in chill-modified and sodium-modified alloys.     

Antiphase domain morphologies in Ti3Al-Si alloys

Ti₃Al based alloys with additions of Si, V, and V + Si were prepared in ingot form by argon arc co-melting and in the form of rapidly solidified ribbons by chill block melt spinning. Binary Ti₃Al and ternary Ti₃Al-Si arc-melted ingots showed equiaxed antiphase boundaries (APBs) in the ₂ (DO₁₉) grain interiors with columnar APBs adjacent to the grain boundaries. In the melt spun ribbons, which undergo high cooling rates, three types of antiphase domain (APD) morphologies (fine, columnar, and coarse equiaxed APDs) were observed in any single ₂ grain. Addition of 15 at.%V to these alloys led to retention of the ordered (B2) phase with a relatively large antiphase domain size.     

Wettability of SiC by aluminium and Al-Si alloys

The variations with time of the contact angle formed by molten pure aluminium or Al-Si alloys with single crystalline SiC were measured by the sessile drop method in a vacuum of 10^–4 to 10^–5 Pa at temperatures ranging from 933 to 1200 K. In the Al/SiC system, a non-wetting-wetting transition was observed at a temperature that decreases as time increases. After holding times of about 2 h, contact angles were stabilized to acute angles even at the aluminium melting point. Although additions of silicon to aluminium were in such amounts as to prevent Al₄C₃ formation at the interface, wettability in both pure Al/SiC and Al-Si alloys/SiC systems was not observed to differ appreciably.     

制取铸造Al-Si合金EBSD样品的方法

铸造Al-Si合金的组织特殊,制取EBSD样品时难度较大.本文综述了目前常用的制取铸造Al-Si合金EBSD样品的方法,包括电解抛光、离子铣削和震动抛光,对不同方法在实验周期、可操作性、成本及样品表面质量等几方面的表现进行了对比分析.并以ZL114A合金作为实际操作合金,利用上述方法制备样品,进行EBSD分析,实验结果...     

Cyclic plasticity at pores and inclusions in cast Al-Si alloys

Finite element analyses of micronotches including pores and silicon particles of an A356 aluminum alloy were performed to elucidate microstructure-property relations for fatigue crack incubation. Several important findings resulted. By varying the particle and pore size, spacing, aspect ratio, and clustering, the relative microstructural differences were quantified related to micronotch root cyclic plasticity. Results from realistic two-dimensional microstructures showed that minimal microstructure-scale cyclic plasticity corresponds well to the measured fatigue strength at 10⁷ cycles for low porosity A356 aluminum alloy specimens. “Realistic” and idealized particles/pores simulations were used to formulate a local Coffin-Manson type law for crack incubation.     

Influence of oxides on porosity formation in Sr-treated Al-Si casting alloys

The strength and quality of an Al-Si alloy casting are determined by its microstructure and the amount of porosity present in the casting. Modification is one of the processes used to improve the microstructural quality, where the addition of a modifying agent alters the shape of the eutectic Si from an acicular to a fibrous form that is extremely beneficial to the mechanical properties. Among various modifiers, strontium has been used extensively, as it is easier to handle and more resistant to fading. However, its addition is also associated with porosity formation in these alloys. The porosity formed in Sr-modified castings has been variously related to an increase in the hydrogen level of the melt, feedability problems in the mushy zone during solidification, and changes in the mode of eutectic nucleation—from near the -Al dendrites in the Sr-free alloy, to within the eutectic liquid itself in the Sr-containing alloy. The present study was carried out to determine the influence of oxides on the porosity characteristics observed in Al-Si alloys containing strontium. A series of experimental and industrial alloys viz., Al-7%Si, Al-12%Si, 319 and 356 were selected, to cover a variety of alloy freezing ranges. The techniques of thermal analysis, optical microscopy, and SEM/EDX and EPMA analyses were employed to obtain the results presented here. It is seen how the presence of oxides (Sr and Al) is responsible for the porosity formation observed in Al-Si alloys, and that the difference in porosity characteristics with the addition of Sr depends on the amount of Sr oxides present in the solidified structure. The presence of aluminum oxide films leads to the formation of large pores that are often linked together. Both aluminum and strontium oxides are favorable sites for the nucleation of other microconstituents.