This study included a comparison of the baseline Sn-3.5Ag eutectic to one neareutectic ternary alloy, Sn-3.6Ag-1.0Cu and two quaternary alloys, Sn-3.6Ag-1.0Cu-0.15Co and Sn-3.6Ag-1.0Cu-0.45Co, to increase understanding of the beneficial effects of Co on Sn-Ag-Cu solder joints cooled at 1-3∞C/sec, typical of reflow practice. The results indicated that joint microstructure refinement is due to Co-enhanced nucleation of the Cu 6 Sn 5 phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 hr at 150∞C. The baseline Sn-3.5Ag joints exhibited significantly reduced strength and coarser microstructures.
The phase evolution during annealing of AlJNi multilayer samples prepared by ion-beam sputtering with composition modulation wavelengths A between 10 and 400 nm was determined using x-ray diffraction and differential scanning calorimeter measurements. Samples with average compositions of &n,N10,6,, and Al,,,N&,~ were investigated. For the Al,,40Ni0.60 samples the following results were obtained. A measure of the degree of periodic@ and the sharpness of the interfaces in a sample with A=80 mn was the large number (over 20) of peaks observed in small-angle x-ray scattering measurements. A sample with A=10 nm was transformed by heat treatment directly to the AlNi phase. In the A=80 nm sample, the first phase formed after annealing was the metastable Y,I phase. The r] phase was identified as Al,Ni,. In the 400 nm wavelength sample, both the metastable 17 phase and the stable Al,Ni formed after the first exothermic reaction. For the Alr,75Ni0,25 samples two results were obtained. A A=11.4 nm sample transformed directly on annealing into Al,Ni. The 77 phase was the first phase formed on annealing a A=100 nm sample. The difference in the component diffusivities and the concentration gradient play an important role in controlling phase formation and evolution.
The current options for solid-state consolidation processing of powder-based advanced aluminum alloys have been very limited and complicated, resulting in a segregation of the applications primarily to the aerospace segment. Throughout any consolidation sequence for aluminum powders, the oxide and/or hydroxide films on the typical powder surfaces can interfere with densification and interparticle bonding. In fact, the consolidation sequence for many low-strength aluminum alloy powder metallurgy parts involves transient liquid-phase sintering to massively disrupt the powder surfaces, producing improved bonding but introducing a coarsened resolidification microstructure. Although preventing aluminum oxide formation is nearly impossible during aluminum powder production, the gas atomization reaction synthesis (GARS) method -an advanced powder production technique -can modify the oxide coating and enable improved consolidation processing. This report compares the effects of the GARS process and other representative atomization processes on the surface structure and properties of aluminum powders and on their ability to sinter. The powders were characterized with transmission electron microscopy, scanning electron microscopy, Auger electron spectroscopy, quadrapole mass spectroscopy and with a new ultrasonic method for in situ sensing of the evolution of sintering. In general, a marked reduction in the surface film thickness and in the level of chemisorbed moisture and moisture-borne impurities was observed in the GARS powders. This change in powder surface characteristics also was effective in promoting sintering processes in the GARS powders, as monitored by our new technique. An initial direct comparison of explosivity for the different types of aluminum powder revealed that the GARS powder also had a reduced hazard level. All of these findings indicate that the GARS approach to aluminum powder production may enable mass-produced lightweight powder metallurgy parts from advanced aluminum alloys with simple consolidation processing techniques.
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