Distortion in a 7xxx aluminum alloy during liquid phase sintering

The distortion in a sintered 7xxx aluminum alloy, Al-7Zn-2.5Mg-1Cu (wt. pct), has been investigated by sintering three rectangular bars in each batch at 893 K (620 °C) for 0 to 40 minutes in nitrogen, followed by air or furnace cooling. They were placed parallel to each other, equally spaced apart at 2 mm, with their long axes being perpendicular to the incoming nitrogen flow. Pore evolution in each sample during isothermal sintering was examined metallographically. The compositional changes across sample mid-cross section and surface layers were analyzed using energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy depth profiling, respectively. The two outer samples bent toward the middle one, while the middle sample was essentially distortion free after sintering. The distortion in the outer samples was a result of differential shrinkage between their outer and inner surfaces during isothermal sintering. The porous outer surface showed an enrichment of oxygen around the large pores as well as lower magnesium and zinc contents than the interior and inner surface of the same sample, while the inner surface was distinguished by the presence of AlN. The differential shrinkage was caused by different oxygen contents in local sintering atmosphere and unbalanced loss of magnesium and zinc between the outer and inner surfaces

Pore shrinkage and Ostwald ripening in metallic systems. Progress report, July 1, 1978-June 30, 1979

The new theory of Ostwald ripening phenomena is gradually unfolding. Based on the principle of minimum rate of entropy production, the general method including the mathematical formalisms of variational calculus, is now understood and has been applied to the case of annealing of void structures, a situation somewhat simpler to model than is particle coarsening. The resultant void distribution function is compared with that expected from the LSW theory and with experiments. Developments in three areas are described: generation and annealing of voids in HVEM, distribution of Ni/sub 3/Al in Ni-Al alloys, and production of reproducible pore structures in nickel powder compacts

Pore shrinkage and Ostwald ripening in metallic systems. Progress report, July 1, 1977--June 30, 1978

A new theory of Ostwald ripening (the coarsening of particles in a matrix or on a substrate) is being developed, which is based on the principle of minimum entropy production. This theory yields particle size distribution functions whose general shape is in much closer agreement with existing observations than is that of well established theories. Progress is reported both in the theoretical development and in several areas of experiment. The goal of the experimental program is the determination of actual distributions from very large numbers of size measurements, in order to provide statistically more significant basis for comparison with the various theories than has previously characterized such measurements