Glass formation in La-based La₅₅Al₂₅(NiCu)₂₀ alloys by Bridgman solidification

Five La-based La₅₅Al₂₅Ni_xCu_20-x (x = 0, 5, 10, 15, 20) alloys were subjected to Bridgman unidirectional solidification at growth velocities between 0.1 and 4.82 mm/s through a temperature gradient of 15 K/mm. The formation of the amorphous state of the resulting samples was confirmed by X-ray diffraction and differential scanning calorimetry. The critical cooling rates for glass formation in La₅₅Al₂₅Ni₂₀, La₅₅Al₂₅Ni₁₅Cu₅, La₅₅Al₂₅Ni₁₀Cu₁₀, La₅₅Al₂₅Ni₅Cu₁₅ and La₅₅Al₂₅Cu₂₀ were directly determined to be 67.5±6.8 K/s, 34.5±3.5 K/s, 22.5±2.2 K/s, 35.9±4.0 K/s and more than 72.3±8.0 K/s, respectively. With increasing Cu content the critical growth velocity shows a minimum phenomenon around 10 at.% Cu, indicating that the glass forming ability is the largest for the 10 at.% Cu containing alloy. The glass transition temperature (T_g) and the crystallization temperature (T_x) are independent of growth velocity for fully amorphous samples. The heat of crystallization (ΔH_x) increased with increasing growth velocity.