Easy glass formation in La55Ni20Al25 by Bridgman solidification

Easy glass-forming alloy La55Ni20Al25 was subjected to Bridgman unidirectional solidification at growth velocities between 0.34 mm/s and 4.5 mm/s through a temperature gradient of 15 K/mm. Results of X-ray diffraction confirm that the sample grown at 4.5 mm/s was fully amorphous whereas sample grown at 2.3 mm/s was partially amorphous with some crystalline structure and sample grown at 0.34 mm/s was basically crystalline. DSC results showed typical glass transition and crystallization for samples grown at 4.5 mm/s and 2.3 mm/s, in contrast to no transformation characterized by a flat line for sample grown at 0.34 mm/s. Values of T-g and T-x obtained from the fully amorphous sample grown at 4.5 mm/s are 493 K and 557 K respectively, resulting in a temperature span of 64 K between these two temperatures. These results confirm a critical cooling rate of 62.5 K/s for glass formation in La55Al25Ni20 alloy and this rate is compared with critical cooling rates obtained by other methods for the same alloy as well as for other easy glass forming materials. (C) 1998 Elsevier Science B.V.