Critical Poisson ratio for room-temperature embrittlement of amorphous Mg85Cu5Y10

When prepared as a thin foil by rapid solidification, the Mg85Cu5Y10 amorphous alloy undergoes a 'ductile'/brittle transition upon bending after ageing at room temperature. This macroscopic behaviour arises from structural relaxation, which lowers the enthalpic content of the system and increases the elastic constants (i.e. the Young modulus and the shear modulus) as a function of ageing time. Since the shear modulus is found to increase faster than the Young modulus, the Poisson ratio decreases as the alloy ages and, at a critical value of nu similar to 0.32, the 'ductile'/brittle transition occurs. The results are discussed in terms of the annihilation of different types of defect, which characterize the disordered structure of a metallic glass.