Elasto-plastic load transfer in bulk metallic glass composites containing ductile particles
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Elasto-Plastic Load Transfer in Bulk Metallic Glass Composites Containing Ductile Particles DORIAN K. BALCH, ERSAN ÜSTÜNDAG, and DAVID C. DUNAND In-situ diffraction experiments were performed with high-energy synchrotron X-rays to measure strains in crystalline reinforcing particles (5 and 10 vol. pct W or 5 vol. pct Ta) of bulk metallic glass composites. As the composites were subjected to multiple uniaxial tensile load/unload cycles up to applied stresses of 1650 MPa, load transfer from the matrix to the stiffer particles was observed. At low applied loads, where the particles are elastic, agreement with Eshelby elastic predictions for stress partitioning between matrix and particles is found, indicating good bonding between the phases. At high applied loads, departure from the elastic stress partitioning is observed when the particles reach the von Mises yield criterion, as expected when plasticity occurs in the particles. Multiple mechanical excursions in the particle plastic region lead to strain hardening in the particles, as well as evolution in the residual strain state of the unloaded composite.
I. INTRODUCTION
BULK metallic glasses (BMGs) are multicomponent alloys capable of retaining an amorphous structure upon quenching at low to moderate cooling rates (1 to 100 K/s), resulting in maximum specimen thickness up to several centimeters. Optimized BMGs have an attractive combination of properties, e.g., very high strength (around 2 GPa), high fracture toughness (20 to 55 MPa-m1/2), excellent wear and corrosion resistance, and very high elastic limits (2 pct).[1] However, BMGs also have the tendency to fail catastrophically, when unconstrained, by shear banding after very limited plastic deformation at room temperature. Composites, consisting of a BMG matrix and a metallic second phase in the form of particles or fibers, exhibit improved strength and ductility, likely due to load transfer to the stiffer metallic reinforcement and arresting of shear bands by the more ductile metallic reinforcement.[2–5] These BMG composites show important differences in their mechanical behavior as compared to conventional metal matrix composites (MMCs) consisting of a metallic, ductile matrix containing stiffer but brittle reinforcement, e.g., Al/SiC. The main difference is that the matrix of BMG composites is brittle while that of most MMCs is ductile. This leads to different situations upon cooling from the processing temperature, where usually tensile thermal mismatch strains are developed in the matrix of both types of composites. These strains are easily relaxed by matrix plasticity in MMCs but build up elastically in the BMG matrix, thus decreasing the applied stress at which shear bands are formed upon subsequent tensile loading. Second, upon mechanical loading, the matrix of DORIAN K. BALCH, formerly with the Department of Materials Science and Engineering, Northwestern University, is Postdoctoral Appointee, Engineered Materials Department, Sandia National Laboratories, Livermore, CA 94
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