Effect of B Addition on Extension of Supercooled Liquid Region before Crystallization in Pd–Cu–Si Amorphous Alloys

Abstract

The supercooled liquid region defined by the difference between glass transition temperature (T g ) and crystallization temperature (T x ), ΔT x (= T x - T g ) for a Pd 76 Cu 6 Si 18 amorphous alloy was found to be significantly extended by the replacement of 3 at%B for Si. The replacement causes a decrease in T g and an increase in T x , leading to the maximum value of 70 K for ΔT x . The Pd 76 Cu 6 S 15 B 3 alloy crystallizes through a single-stage exothermic reaction which leads to simultaneous precipitation of Pd 4 Si, Pd 3 Si and Pd 2 S i phases. With further increasing replacement amount of B, the ΔT x decreases significantly, accompanying the change in the crystallization mode to two stages including the primary precipitation of metastable supersaturated Pd solid solution. The crystallized structure after the two-stage reaction for the Pd 76 Cu 6 Si 13 B 5 alloy consists of five phases of Pd, Pd 4 Si, Pd 3 Si, Pd 2 Si and Pd 2 B. By the dissolution of an appropriate amount (about 3 at%) of B element with smaller atomic size and negative heats of mixing against the other constituent elements, the amorphous structure is presumed to change into a higher degree of dense random packed state. As a result, the long-range atomic rearrangement for crystallization required for the precipitation of the three compound phases in the single-stage exothermic reaction becomes difficult, leading to the extension of the supercooled liquid region. The increase in the thermal stability of the supercooled liquid in the Pd 76 Cu 6 Si 15 amorphous alloy is important because of the increase in the ease of the formation of bulk amorphous alloy.