Surrounding Al Matrix Research Articles

Microstructural characterisation of fatigue crack initiation in Al-based plain bearing alloys

The fatigue initiation behaviour of two Al-based bearing lining alloys has been assessed by image analysis tessellation techniques and subsequent adaptive numerical classification approaches. Fatigue initiation occurs by decohesion at Si particles, and in the absence of Si particles decohesion occurs at Sn particles. In both cases, relatively unclustered, large particles initiate fatigue, and the relative position of the nearest neighbour particle seems to have significance. This can be explained qualitatively in terms of strain mismatch arising from differing elastic moduli between the particles and surrounding Al matrix and local stress concentration and superposition effects. The lining alloy containing no Si and high Sn content exhibits more directionality, and a heavier dependence on angular information is found by the classifier.

Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl]tetrasulfide in neutral sodium chloride solution. Part 1: corrosion of AA 2024-T3

This study consists of two parts. In the first part, the corrosion of 2024-T3 aluminum alloy (AA 2024-T3) was studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the anodic S phase (Al 2CuMg) particles dealloyed Al and Mg during the 3.5 h of immersion in a neutral 0.6 M sodium chloride (NaCl) solution; with the dealloying of Mg being the most severe. Simultaneously, a heavy dissolution was also observed for the surrounding Al matrix of the S phase particles. This Al dissolution is likely to be caused by a local alkalization resulting from the coupled cathodic reaction (water and/or oxygen reduction). Such corrosion in AA 2024-T3, however, can be inhibited efficiently after the treatment of bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane). The associated studies on bis-sulfur silane treated AA 2024-T3 will be presented in the second part.

Localized stress near and the thermal expansion of Al2Cu precipitates in an Al thin film matrix

The thermal expansion coefficient of Al2Cu precipitates embedded in an Al–Cu thin film matrix is determined by in situ x-ray diffraction measurements on free films. The phases present and the thermal expansion or contraction are simultaneously observed during a temperature ramp. The Al2Cu precipitate has a tetragonal structure. The thermal expansion coefficient of the Al2Cu phase in the a direction is significantly smaller than that of the surrounding Al matrix while the thermal expansion in the c direction is comparable to that of the Al matrix. Upon cooling from 500 °C, the mismatch of the thermal expansion between the precipitate and the Al matrix results in compressive and tensile stresses in the aluminum matrix near the precipitates and in compressive stresses in the precipitates. Those stresses are estimated to be on the order of 100 MPa.

Microstructural aspects of the dissolution and melting of Al2Cu phase in Al-Si alloys during solution heat treatment

The dissolution and melting of Al2Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two forms of Al2Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al2Cu phase. Dissolution of the (Al + Al2Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al2Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise to massive eutectic regions of (Al + Al2Cu). Unlike the eutectic, fragments of the blocky Al2Cu phase are still observed in the matrix, even after 24 hours at 540 °C.

Heterogeneous nucleation of solidification of Si in Al-Si and Al-Si-P alloys

Heterogeneous nucleation of solidification in melt spun Al-Si and Al-Si-P has been studied using differential scanning calorimetry, and transmission, scanning transmission and high resolution electron microscopies. The microstructures of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix. The Si microstructure depends on the level of P: coarse faceted Si particles are nucleated by AlP particles in Al-Si containing 2 ppm P and Al-Si-P containing 35 ppm P whereas eutectic droplets of fine Si particles are nucleated by the surrounding Al matrix at a high undercooling in Al-Si containing 0.25 ppm P. The Si nucleation onset temperature remains approximately constant while the peak and end temperatures both decrease with increasing cooling rate, in agreement with classical nucleation theory. Kinetic analysis, using the spherical cap model gives contact angles of 10°, 43° and 10° for Si nucleation in low and high purity Al-Si and Al-Si-P respectively.

Al2 O 3 Films Formed by Anodic Oxidation of Al‐1 Weight Percent Si‐0.5 Weight Percent Cu Films

The structure of the barrier‐type anodic oxide formed on preannealed Al‐1 weight percent Cu thin film in a tartaric acid electrolyte was investigated. The oxide film is basically an amorphous layer with a thin dispersed crystalline layer interposed in the center. Pores are observed to be associated with the layer. Silicon nodules and particles originally present in the Cu film behave differently during anodization. Silicon nodules are oxidized to various degree during anodizing. The silica formed in the Si nodules is amorphous and somewhat porous possibly due to oxygen evolution associated with the Si anodization. A dark rim was found to surround each nodule in the anodic oxide film. This rim is shown to be thicker amorphous material, and its origin is attributed to the faster oxidation rate in the vicinity of Si nodules. precipitates are oxidized to form at about the same rate as the surrounding Al matrix. Copper is rejected by and accumulates at the interface.

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al.

Quantitative energy-dispersive X-ray analyses of γ′ precipitates in an Al — 4.2 at% Ag alloy

The investigation was performed in order to determine whether the metastable ..gamma..' precipitates have the composition given by the equilibrium phase diagram and if their composition varies with time. Energy dispersive x-ray spectroscopy (EDS) was used for the chemical analysis, in order to obtain accurate precipitate compositions without problems arising from fluorescence from surrounding Al matrix, the ..gamma..' precipitates were extracted from an Al - 4.2 at% Ag alloy after aging for various times at 350/sup 0/C. The 350/sup 0/C aging temperatures was chosen because it is above the G.P. zone solvus for this alloy (Fig. 1), and the ..gamma..' precipitates could be produced directly from the supersaturated solid solution without the influence of a prior G.P. zone distribution.