Dissolution of particles in binary alloys: part II. experimental investigation on an Al-Si alloy
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I.
INTRODUCTION
IN the preceding article, r~] two models for the dissolution of particles in binary alloys were presented. The models were based on a model for the dissolution of one single particle in a finite matrix, and the extension of this model covers the dissolution kinetics of a size distribution of particles. In the present article, an experimental investigation of the dissolution kinetics of Si particles in an AI-Si alloy has been carried out in order to test the validity of the models introduced in the preceding article. II.
EXPERIMENTAL PROCEDURE
The A1-Si alloy used in the investigation was produced from high-purity aluminum and a master alloy containing 12.5 wt pct silicon. The alloy was produced in laboratory scale by directional solidification. The chemical composition of the alloy, obtained by spectrographic analysis, was 0.77 at. pct (0.80 wt pct) Si and 0.012 at. pct (0.024 wt pct) Fe. The amount of each of the other elements was in the parts per million range. After solidification, the casting was homogenized for 48 hours at 580 ~ The annealing procedure for obtaining spherical particles is shown schematically in Figure 1. If the alloy is quenched from the one-phase region to the two-phase region, the particles would mainly be plate-like or equiaxed. [2) The plastic deformation introduced by cold rolling after the initial anneal at 490 ~ was carried out in order to break down the orientation relationship between the particles and the matrix and thus to obtain spherical particles. The long annealing time at 450 ~ was carried out to ensure a uniform concentration of silicon in the matrix. After the annealing process, the
ULF H. TUNDAL, Research Scientist, is with the Division of Metallurgy, SINTEF, N-7034 Trondheim, Norway. NILS RYUM, Professor, is with the Department of Metallurgy, The Norwegian [nstitute of Technology, N-7034 Trondheim, Norway. Manuscript submitted April 10, 1991. METALLURGICAL TRANSACTIONS A
average grain size w a s / ) ~ 500 p m , which is an order of magnitude larger than the interparticle spacing. Specimens of size 15 • 15 • 2 n t l n 3 w e r e cut from this material for annealing experiments. The dissolution reaction was studied by up-quenching the specimens in a salt bath to various temperatures above the solvus temperature. The heating-up rate of the specimens was very high when the specimens in the salt bath were stirred. The time for the specimen to reach a temperature of 2 deg within the temperature of the salt bath was about 4 seconds. The starting point for isothermal annealing was thus fixed to 4 seconds after the start of the up-quenching. After annealing, the specimens were quenched in cold water. Due to a relatively large difference in hardness between the matrix phase and the silicon particles, some problems arise regarding metallographic preparation of specimens for particle size measurements. Care had to be taken in order to prevent any excavation of the matrix around the particles. If excavation occurs, the area fraction of particles will be overestimat
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