Optimum glass formation at off-eutectic composition and its relation to skewed eutectic coupled zone in the La based La-Al-(Cu,Ni) pseudo ternary system

Glass formation and its relation to the dendritic and eutectic growth has been investigated for the La_100-x[Al_0.412(Cu,Ni)_0.588]_x (x=30-56.3) and La_86-yAl₁₄(Cu,Ni)_y (y=16-29) alloy systems. The experimental results show that in the La-rich pseudo ternary La-Al-(Cu,Ni) system, optimum glass formation actually occurs at an off-eutectic composition. A nearly fully amorphous rod with 12 mm in diameter can be obtained at an off-eutectic composition near La₆₂Al_15.7(Cu,Ni)_22.3, while only a 1.5 mm diameter rod can be obtained fully amorphous for the eutectic alloy La₆₆Al₁₄(Cu,Ni)₂₀. A strong dependence of GFA on the composition is observed for these alloys. In addition, formation of a composite (i.e. αLa dendrite reinforced glass matrix) in 12 mm diameter cast rods is observed over a wide range of composition, including the eutectic composition. It has been found that the GFA does not correlate well with the extent of the undercooled liquid region (ΔT_x) and is not even closely related to the reduced glass transition temperature (T_rg). These unusual observations in glass formation and its relation to the skewed eutectic coupled zone have been explained in terms of the competition between the growth of crystalline phases (i.e. eutectic and dendritic phases) and the formation of the amorphous phase.