In-Situ Fracture Observation and Fracture Toughness Analysis of Zr-Based Amorphous Alloys Containing Ductile Dendrites

Abstract

Effects of dendrite size on fracture properties of Zr-based amorphous alloys containing ductile beta dendrites were explained by directly observing microfracture processes using an in-situ loading stage installed inside a scanning electron microscope (SEM) chamber. Three amorphous alloy plates having different thicknesses were fabricated by varying cooling rates after vacuum arc melting. The effective size of beta dendrites was varied from 14.7 to 30.1 mu m in the alloy plates, while their volume fraction was almost constant. According to microfracture observation of the alloy containing fine beta dendrites, shear bands initiated at the amorphous matrix were connected with the notch tip as they were deepened through dendrites, which led to abrupt crack propagation. In the alloy containing coarser beta dendrites, shear bands were initiated at the amorphous matrix to form a crack near the notch tip region and were expanded over large matrix areas. The crack propagation was frequently blocked by beta dendrites, and many shear bands are formed near or in front of the propagating crack, thereby resulting in stable crack growth, which could be confirmed by the fracture resistance curve (R-curve) behavior. This increase in fracture resistance with increasing crack length could be explained by mechanisms of blocking of crack growth, multiple shear band formation, and crack blunting.