Synchrotron Strain Measurements for in situ Formed Metallic Glass Matrix Composites
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Synchrotron Strain Measurements for in situ Formed Metallic Glass Matrix Composites R.T. Ott1, F. Sansoz2, J.F. Molinari2, J. Almer3, C. Fan1, and T.C. Hufnagel1 1
Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21209 2 Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21209 3 Argonne National Laboratory, Advanced Photon Source, Argonne, IL 60439
ABSTRACT We have examined the micromechanical behavior of in situ formed metallic glass composites by performing in situ high-energy synchrotron X-ray scattering during uniaxial compression. The load partitioning between the amorphous matrix and the reinforcing particles was examined by measuring the lattice strains in the crystalline particles during compressive loading. The crystalline particles yield in compression during loading followed by tensile yielding during unloading. The large elastic mismatch between the two phases leads to large residual strains after each loading cycle. The load partitioning was also examined with finite element modeling (FEM). The predicted von Mises effective stress in the crystalline particles from the FEM calculations compares well with the experimentally determined von Mises effective stress so long as the deformation is elastic in both particles and matrix. After the particles yield, the model predicts strain hardening of the particles that is not observed experimentally.
INTRODUCTION In situ formed metallic glass composites are amorphous metallic alloys in which a crystalline second phase is precipitated during processing [1-4]. Unlike typical metal matrix composites (MMCs), metallic glass matrix composites consist of a macroscopically brittle matrix reinforced with a ductile crystalline phase that is stiffer than the surrounding glass matrix. Therefore, during loading the crystalline reinforcing phase yields first via dislocation motion, followed by yielding in the stronger glass matrix by shear band formation. The ductile crystalline second phase particles act as barriers to shear band propagation in addition to being preferential sites for shear band formation. The combination of these two processes leads to an increase in plastic strain prior to failure exhibited by the composite alloys compared to monolithic metallic glasses. The heterogeneous initiation of shear bands at the particle/matrix interface is the result of stress concentrations that develop between the crystalline phase and the amorphous matrix due to the elastic mismatch. The elastic mismatch also affects the load partitioning between the two phases. The applied stress at which the crystalline particles yield at is a function of the load transfer between the matrix and the particles. Previous studies have used high-energy X-ray scattering to measure the load partitioning among phases in MMCs[5-7]. More recently, high-energy X-ray scattering has been used for in situ strain measurements in metallic glass matrix composites produced ex situ (that is, by adding crystalline particles to the m
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