Microstructure Evolution in Al-Ag Alloys Under Severe Plastic Deformation
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ABSTRACT We present results on the microstructure evolution in an A1Ag alloy after plastic deformation under fatigue at different temperatures, strain amplitudes and strain rates. Both the dislocation structure and the precipitate morphology and size distribution were analyzed. At low temperature, particle elongation and fragmentation was observed while at higher temperature an accelerated growth due to vacancy supersaturation was observed. INTRODUCTION Alloys under extreme external forcing are of both technical and fundamental interest technical, because processing involving severe plastic deformation or irradiation can modify the microstructure considerably, leading to a change in materials properties - fundamental, since such materials are examples for driven systems far from equilibrium, where phase stability, kinetics of phase formation and microstructure evolution are not governed by thermodynamics and linear response, but has to be described by kinetic equations. An example, which will be described here are the changes in a material due to plastic shear. Despite the fact that plastic deformation by dislocations is usually considered a conservative process, it can create a multitude of lattice defects as soon as nonconservative
processes are taken into account:
"* climb of dislocations by itself emits or absorbs vacancies, "* double cross slip creates jogs or superjogs, which under an applied stress emit vacancies,
"* cutting of dislocations
also can create jogs,
"* in multiphase alloys
(e.g. precipitate hardened alloys containing particles of a second phase) cutting may alter the particle morphology and even destroy the particles.
These defects are similar to those created by high energy irradiation. Nevertheless a detailed description of all the phenomena is more complicated, since the dislocation structure and kinetics are not independent of the defects. Quite often, deformation tends to concentrate in spatial regions due to local softening of the lattice, thus creating a complicated structure of high and low defect densities. Experimental work on the changes in phase stability caused by heavy plastic deformation as is reached in ball milling experiments often concentrated on average properties measured e.g. by X-ray-diffraction or calorimetry [1]. The modeling was mostly based on thermodynamic considerations, neglecting the highly non-equilibrium, dissipative nature of the underlying process. For a review see e.g. [2]. 643 Mat. Res. Soc. Symp. Proc. Vol. 481 ©1998 Materials Research Society
The aim of our current work was to better understand the elementary processes of phase transformations by plastic deformation in a simple case and to mimic these transformations in a crude Monte-Carlo simulation. We chose the alloy Al-Ag, since its thermal decomposition behaviour is well studied [3, 4, 5]. Unmixing in a supersaturated Al-Ag solid solution occurs via a sequence of metastable phases. It starts with spherical Guinier-Preston zones, the so-called in-phase with a Agconcentration between 30% and 60%
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