Structural Origins Of Liquid-solid Transitions And Primary Crystallization Behaviors Of Al-Ni-La-based Amorphous Alloys

During solidification of metallic melts,if the cooling rate is large,the glass transition happens,or if the cooling rate is small,crystal phases form.Given an un-crystallized amorphous alloy,the crystallization may also occur at the temperature over the primary crystallization temperature T_x1 of amorphous alloys.Using experimental/simulation methods,many researchers concluded that a large number of short-range ordered clusters exist in liquid and amorphous alloys,which will affect the processes of liquid-solid transition and crystallization of amorphous alloys,but the mechanism of influences of the topological structure of these clusters on the above two processes remains unknown.To investigate the structural origins of the liquid-solid transitions and primary crystallization behaviors of amorphous alloys,the effects of the topological structure of the short-range ordered clusters on the liquid-solid transitions and primary crystallization behaviors were reviewed from:（1）the types and sequence of the precipitates during slow cooling of Al_85.5Ni_9.5La₅alloy;（2）the evolution of atomic structure during liquid-glass transition and liquid-crystal transition;（3）the origin of the primary crystallization behaviors of Al_85.5Ni_7.5La₅M₂（M=Si,Fe,Co,Ni）amorphous alloys.Differential scanning calorimeter,synchrotron high energy x-ray diffraction,reverse Monte Carlo and Voronoi tessellation were used as main analysis methods.The conclusions can be drawn as follows:1.The types and sequence of the precipitates during the solidification of liquid Al_85.5Ni_9.5La₅ on cooling with the cooling rate of 20 ^oC?s^-1 are:Liquid->Liquid+Al₁₁La₃->Liquid+Al₁₁La₃+Al₃Ni->Al+Al₁₁La₃+Al₃Ni;2.The shoulder at the right side of the structure factor patterns is correlated with Al-M（M=Si,Fe,Co,Ni）bond of Al_85.5Ni_7.5La₅M₂ amorphous alloys while the same shoulder couldn’t be observed in liquid Al_85.5Ni_9.5La₅alloy.The density and order degree of liquid Al_85.5Ni_9.5La₅ both increase as temperature decreases.The coordination number of liquid Al_85.5Ni_9.5La₅ has a linear correlation with temperature;3.The solidification pathway of liquid Al_85.5Ni_9.5La₅ relies on the number fraction variation of crystal-like clusters rather than non-crystal-like ones,but the thermal stability of Al_85.5Ni_7.5La₅M₂（M=Si,Fe,Co,Ni）depends on mixing enthalpy instead of short-range ordered clusters.The correlation between mixing enthalpy and T_x1 is parabolic.