Morphology Evolution Of Ductile Crystalline Precipitates And Its Effects On Mechanical Behavior Of BMG Composites

Bulk metallic glass(BMG) possesses a series of excellent properties,such as high strength and hardness,corrosion resistance and excellent magnetic properties.However,the application of BMG is restricted severely by room temperature brittleness.The innovation of BMG composites with ductile crystalline particles can improve the room temperature ductility.The ductility of this kind of composite is influenced dramatically by the ductile crystalline precipitates,especially the morphology,size,distribution,volume fraction and etc. In the present work,Zr₆₀Ti_14.67Nb_5.33Cu_5.56Ni_4.44Be₁₀ was used to investigate the effects of processing parameters on the microstructure evolution of in situ ductile phase and consequently on the mechanical behavior of BMG composites with in situβ-Zr(Ti,Nb) crystalline phase.And then the corresponding mechanisms were discussed.The effects of holding time under a fixed temperature on the evolution of in situβphase in BMG composites,especially,the evolution from dendritic to spheroidal,and their effects on mechanical behavior were investigated systematically.The results show that both the microstructures and mechanical behavior are influenced significantly by the holding time.The size ofβphase increases and the morphology ofβphase changes from dendritic to spheroidal gradually with prolonging the holding time,while the volume fraction ofβphase do not change obviously,maintaining approximately 50%.However,when the holding time is prolonged to 20min,theβphase reaches a stable state,and the size and morphology ofβphase have no longer obvious change with prolonging the holding time.This phenomena was explained based on the diffusion theory of curved interface in a multi-phase system.There is a similar change of the mechanical property with the evolution of in situ ductile phase.The plastic strain increases with prolonging the holding time and also reaches a stable value.The effect of cooling rate on the microstructure evolution of in situ ductile crystalline inclusions and its effect on the mechanical properties were also investigated systematically. The results show that only dendritic-like in situβphase can be obtained in our studies. However,with the decreasing of cooling rate,the size and volume fraction of in situβphase increase and the morphology transforms from fine to coarse dendritic.When the withdrawing rate decreases to 1mm/s,a new intermetallic phase occurs.Meantime,with the decrease of cooling rate,the plastic strain increases.At 1mm/s,the elongation of the BMG composites decreases dramatically due to the occurrence of the new intermetallic phase.