Microstructure and ductile-brittle transition of as-cast Zr-based bulk glass alloys under compressive testing

This paper investigates mechanical properties and fracture mechanisms of Zr 52.5Cu17.9Ni14.6Al10Ti5 alloys with various volume fractions of quenched-in crystalline. The alloys with various volume fractions of quenched-in cryst alline were prepared by controlled oxygen content of alloys and overheating of the pouring. The phase structure, particle size and volume fraction of all samples were identified by X-ray diffraction, differential scanning cal orimeter DSC curves and scanning electron microscopy (SEM) photographs. The mean sizes of crystalline increased from 0.3 to 1.3 mum with increasing vo lume fraction of crystalline from 4 to 13%. The compressive mechanical test s show a ductile-brittle transition with significant decrease in the fractu re stress and ductility. Detailed observations in the flow deformation and fracture surface illustrate the relationship between the quenching-in cryst alline and the mechanical behavior. The full bulk amorphous Zr-based alloy exhibits typical ductile deformation and fracture behavior. The torn shear bands form the typical vein patterns on the fracture surface. The effects o f quenching-in crystalline on the flow deformation and fracture behavior de pend on the nature, size, volume fraction and distribution. The particle si ze of the crystalline in the sense of the width of shear bands is critical. When the size is larger than the width of the shear bands the particles in duce an obvious inhomogeneity of the flow deformation and more microcracks by the separation of the interfaces. Nano-scale particles, on the other han d, may increase the viscosity of the flow but do not form microcracks, resu lting in particle strengthening of the metallic glass. Increasing the volum e fraction of large-scale particles is favorable to leaking the microcracks and brittle fracture. With increasing particle size and volume fraction up to two times the width of the shear band and 10% vol., respectively, the d uctile fracture of bulk amorphous alloy completely transforms to brittle fr acture under compressive testing. (C) 2001 Elsevier Science B.V. Ali rights reserved.