| The ductility of bulk metallic glasses at room temperature is usually disappointingly low and it has limited the applications. But bulk metallic glass composites containing in-situ formed dendrites exhibits large compressive and tensile strains to failure. It's significant for theory and practice to study the stabilities of microstructure and properties of bulk metallic glass composites. In this paper, the influences of isothermal annealing and Ta micro-alloying on microstructure and mechanical properties of Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 (LM2) alloy were investigated by XRD, SEM, TEM, DSC, instrumented nanoindentation, room-temperature quasi-static compression, etc.After isothermal annealing at 503K, LM2 alloy still consisted mainly of two phases: amorphous matrix and dendrites. However, a few dendrites transformed from bcc phase to hcp phase and small amounts of intermetallic compounds were precipitated from amorphous matrix. The crystallization of LM2 alloy was not affected by heat treatments, but the glass transition temperature increased gradually as the isothermal annealing time prolonging. The supercooled liquid regions were diminished and the thermal stabilities decreased gradually.The average hardness and average elastic modulus of amorphous matrix and dendrites of as-cast LM2 alloy were measured by instrumented nanoindentation experiments. The former values were 5.38±0.4GPa and 3.47±0.3GPa respectively. The latter values were 87.4±6GPa and 61.4±6GPa respectively. The values were all increased in various degrees after isothermal annealing:average hardness for two phases was 5.53±0.5GPa and 4.09±0.2GPa respectively, and average elastic modulus were 92.8±8GPa,75.9±8GPa respectively. By room-temperature quasi-static compression, the plastic strain for as-cast LM2 alloy was measured to be 13.1%, and the yield strength and fracture strength were 1403MPa and 1604MPa respectively. The plastic strain decreased gradually while the strength was enhanced in various degrees after heat treatments. The plastic strain, yield strength and fracture strength were 2.2%,1706MPa and 1774MPa respectively after 32h annealing.The reduction of free volume was calculated by integrating the heat flow of the structural relaxation exothermic peak. The results indicated that free volume was reduced exponentially with annealing time prolonging, and it's found to obey KWW relaxation. The free volume of samples after 32h annealing was just 10% of that in as-cast samples. The thermal residual stresses in as-cast LM2 alloy was analyzed by finite element simulation. The simulation results showed that tensil residual stresses in amorphous matrix and compressive residual stresses in dendrites could reach 237MPa and-74MPa respectively. Besides, there were stress gradients and stress concentrations near the interfaces between two phases. Tensil residual stresses increased along with the volume fraction of dendrites, while compressive residual stresses decreased. It's discussed that the reduction of free volume in amorphous matrix and the reduction of dendrites ductility were primarily liable for the decrease of compressive plastic strain. In addition, the thermal residual stresses releasing also had some effects on it.When lat.% Ta added into LM2 alloy, the formation of dendrites was promoted. Their volume fraction increased from 30% to 40% and their span lengths increased from 15-30μm to 100-150μm. Besides, thermal stability of bulk metallic glass composites was improved and crystallization was influenced. The compressive plastic strain for as-cast LM2-Ta alloy was 8.2%. The yield strength and fracture strength were 1604MPa and 1740MPa respectively. |
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