Misfit accommodation in a quenched
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I.
INTRODUCTION
R A P I D solidification of aluminum alloys with a high percentage of silicon (about 20 at. pct) yields finely grained microstructures with a fine dispersion of silicon particles.tq By consolidation of the rapidly solidified ribbons or powders, aluminum alloys with improved mechanical properties as compared to conventional aluminum alloys can be produced. These improved properties, e.g., increased wear resistance and reduced thermal expansion, t2] make these alloys attractive for applications like parts of combustion engines. In many aspects, e.g., volume fraction, size, and thermal expansion of dispersed particles, the rapidly solidified aluminum alloys with a large silicon content resemble particulate-reinforced metal matrix composites (MMCs). Hence, these alloys are considered as in situ composites. The presence of reinforcing particles can influence the kinetics of precipitation in aluminum alloys, t3-6~ This influence is usually explained as follows. The difference in shrinkage on cooling, due to the difference in the respective coefficients of thermal expansion (CTE) of the dispersed particles and the surrounding matrix, introduces a thermal misfit between particles and matrix. This thermal misfit is accommodated by elastic and plastic deformation of the matrix, r7] the latter implying creation of dislocations. These act as extra nucleation sites (enhancing precipitation) or annihilate excess vacancies (retarding precipitation). Elastic stresses resulting from misfit accommodation can also influence precipitation phenomena, f81 The Al-l.3 at. pct Cu-19.1 at. pct Si model alloy produced by melt spinning and subsequent extrusion combines the attractive mechanical properties of aluminum M.J. STARINK, Scientist, and P. VAN MOURIK, Senior Scientist, are with the Laboratory of Metallurgy, Delft University of Technology, 2628 AL Delft, The Netherlands. B.M. KOREVAAR, Retired Professor, Laboratory of Metallurgy, Delft University of Technology. Manuscript submitted July 6, 1992. METALLURGICAL TRANSACTIONS A
alloys with a large silicon content with the possibility of age hardening of the matrix. The combination of a heattreatable A1 matrix with the presence of silicon particles allows the study of important effects in MMCs, such as thermal stresses and precipitation. The nonisothermal precipitation in the solid-quenched Al-l.3 at. pct Cu19.1 at. pct Si alloy was studied before, t9~ In the present work, the isothermal aging in the solid-quenched A1-1.3 at. pct Cu-19.1 at. pct Si alloy is studied by measurement of lattice parameter variations of the Al-rich and Si-rich phases. II.
EXPERIMENTAL PROCEDURES
A. Preparation of Specimens A high-purity Al-l.3 at. pct Cu-19.1 at. pct Si alloy was produced by melt spinning and subsequent extrusion. The production route of the alloy was described earlier, t91 In the extruded alloy, the Si particle size is about 0.5 to 1 /~m. E9] From the center of the extruded bars, cylindrical specimens were machined with a diameter of 0.5 mm and length of about 5 mm. The axi
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