Application of isochronal dilatation measurements for determination of viscosity of amorphous alloys

Abstract

The viscosity of metallic glasses and its temperature dependence is well descriptive of the amorphous structure and reflects the structural changes or the influence of preparation conditions on the thermodynamic state of these materials. Determination of temperature dependence of viscosity from isothermal measurements is rather tedious. The present work describes a method of obtaining the values of viscosity from measurement of dilatation and isoconfigurational flow in isochronal regime. The viscosity η( T) is calculated from the sample length changes, which are given as a sum of contributions due to thermal dilatation, flow and elasticity. Thermal dilatation is determined from temperature dependence of dilatation of crystallized sample. Elastic length changes are expected to be small enough to be negligible. The viscosity η( T) is then η( T)= Kl( T) σ/[d l( T)/d t], where K is a numerical constant, l( T) the sample length and σ is the applied specific load. This method of dilatation determination has been tested on Ni 25Zr 55Al 20 metallic glass. The measured viscosity curves were found to be in excellent agreement with isothermal viscosity values and furthermore exhibited smooth temperature dependence allowing to determine processes taking place in the material, e.g. glass transition.