Crystallization pathway in the bulk metallic glass Zr_41.2Ti_13.8Cu_12.5Ni_10Be_22.5

摘要

A new family of multicomponent metallic alloys exhibits an excellent glass forming ability at moderate cooling rates of about 10K/s and a wide supercooled liquid region. These glasses are eutectic or nearly eutectic, thus far away from the compositions of competing crystalline phases. The nucleation of crystals from the homogeneous amorphous phase requires large thermally activated composition fluctuations for which the time scale is relatively long, even in the supercooled liquid. In the Zr_41.2Ti_13.8Cu_12.5Ni_10Be_22.5 alloy therefore a different pathway to crystallization is observed. The initially homogeneous alloy separates into two amorphous phases. In the decomposed regions, crystallization probability increases and finally polymorphic crystallization occurs. The evolution of decomposition and succeeding primary crystallization in the bulk amorphous Zr_41.2Ti_13.8Cu_12.5Ni_10Be_22.5 alloy have been studied by small angle neutron. Samples an-nealed isothermally in the supercooled liquid and in the solid state exhibit interference related wavelengths increase with temperature acocrding length density. The related wavelengths increase with temperature according to the linear Cahn0Hilliard therory for spinodal decomposition. Also the time evolution of the interference peaks in the early stages is consistent with this theory. At later stages. X-ray diffraction and transmission electron microscopy investigations confirm the formation of nanocrystals in the decomposed regions.