THE ROLE OF STRUCTURAL RELAXATION IN THE PLASTIC-FLOW OF METALLIC GLASSES

The role of structural relaxation in the plastic how behavior of metal lic glasses is analyzed both theoretically and experimentally. The cha racteristic time of structural relaxation is calculated as a function of glass thermal prehistory. It is revealed that heating above the roo m temperature by several tens of Kelvins results in a sharp, by severa l orders of magnitude, decrease of this time. It is argued that locali zed ''inhomogeneous'' dislocation-like flow occurs on loading if the c haracteristic time of structural relaxation is much greater than the c haracteristic loading time, while ''homogeneous'' viscous deformation is observed in the opposite case. Precise measurements of acoustic emi ssion in a Co-based metallic glass being loaded at different temperatu res and strain rates are employed for verification of this statement. It is shown that the inhomogeneous --> homogeneous flow transition occ urs at temperatures somewhat higher than T=400 K, and the transition t emperature increases by approximate to 40 K as the strain rate increas es by two orders of magnitude. Theoretical estimations show that for t he inhomogeneous flow the characteristic time of structural relaxation in the loaded state is indeed much greater than the characteristic lo ading time. It is concluded that the kinetics of structural relaxation determines the flow mode of metallic glasses in a unique manner. The kinetically ''frozen'' structural relaxation gives rise to a crystalli ne-like localized flow under load while intensive structural relaxatio n facilitates a viscous glass-like behavior. (C) 1998 American Institu te of Physics. [S0021-8979(98)00211-4].