Investigation of Strain Accommodation upon Phase Transformation of Small Inclusions in Aluminum
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Investigation of Strain Accommodation upon Phase Transformation of Small Inclusions in Aluminum L. H. Zhang1, E. Johnson2, U. Dahmen1 1 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA94720, USA 2 Nano Science Center, Niels Bohr Institute, University of Copenhagen, and Department of Materials Research, RISØ National Laboratory, Denmark. ABSTRACT The evolution of elastic strain caused by melting and solidification of small inclusions in aluminum was investigated by in-situ transmission electron microscopy. The appearance and subsequent decay of elastic strain during phase transformation of inclusions around 100nm in size were observed directly, and the decay rate was determined as a function of temperature. The mechanism of strain accommodation was studied by determining the activation energy of the process using alloy composition and inclusion size to control the transformation temperature. INTRODUCTION Small inclusions embedded in a solid matrix are subject to crystallographic and elastic constraints from the matrix [1,2,3,4,5]. These constraints can have a significant effect on thermodynamic and kinetic behavior of materials. Several such effects have been demonstrated for lead inclusions in aluminum. For example, it was found that residual elastic strain energy was responsible for magic-size behavior of solid Pb inclusions in Al [6], or that the anisotropy of interfacial energy can cause particles to be frozen in non-equilibrium [7,8], or that premelting – the initiation of melting below the bulk melting point - is controlled by the crystallographic orientation of an inclusion [9]. When inclusions melt or solidify, the shape or volume change leads to an elastic strain, which can be observed in a transmission electron microscope under two-beam dynamical diffraction conditions [10,11]. Our previous work showed the appearance and later decay of strain around a small Pb72Cd28 inclusion in an Al matrix during solidification of the particle [1]. It was hypothesized that the strain decay was due to vacancy transport, similar to that which operates during annealing of vacancy defects such as dislocation loops or voids. The study of defect shrinkage kinetics has proven to yield accurate measurements of self-diffusion coefficients, stacking fault and surface energies, and binding energies of impurity atoms [12]. However, to date the accommodation of elastic strain by vacancy transport has not been investigated in detail. In the present work we have examined the evolution of this strain systematically for a range of temperatures and sizes during in-situ heating and cooling experiments. By varying alloy composition and inclusion size we were able to control the transformation temperature and thus study the mechanism of strain accommodation by determining the activation energy of the process.
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EXPERIMENTAL PROCEDURE Alloys with nominal composition Al99.5Pb0.5, Al99.5(Pb72Cd28)0.5, Al99.5(Pb50Cd50)0.5, and Al99.3(Pb50In50)0.7 were prepared from high purity elements (
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