Calorimetric investigation of structural relaxation in supercooledNi75Al22Zr2B amorphous alloy

Abstract

A differential scanning calorimetric (DSC) study is performed on a supercooled ${\mathrm{Ni}}_{75}$${\mathrm{Al}}_{22}$${\mathrm{Zr}}_{2}$B alloy to present an example for identification between amorphous and microcrystalline structures of a structurally disordered solid. The supercooled ${\mathrm{Ni}}_{75}$${\mathrm{Al}}_{22}$${\mathrm{Zr}}_{2}$B alloy exhibits a weak and broad exo- therm (which is followed by the pronounced crystallization exotherm) due to enthalpy or structural relaxation, which masks the glass-liquid transition (${\mathit{T}}_{\mathit{g}}$) characteristic of an amorphous solid. Thermal treatment of the alloy (performed at T=${\mathit{T}}_{\mathit{g}}$) removes the exothermic step, and the DSC output recorded after the thermal treatment showed a characteristic ${\mathit{T}}_{\mathit{g}}$ at around 228 \ifmmode^\circ\else\textdegree\fi{}C. The signal presents a monotonically decreasing enthalpy release (\ensuremath{\partial}H/\ensuremath{\partial}t${)}_{\mathit{T}}$ in an isothermal calorimetric measurement, which is modeled successfully by using the existing mechanism of structural relaxation. The microstructure of the specimens studied before and after the thermal treatments confirms the origin of the DSC exotherm and isotherm signal to be structural relaxation.