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 Pd₇₆Cu₆Si₁₈ 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 Pd₇₆Cu₆Si₁₅B₃ alloy crystallizes through a single-stage exothermic reaction which leads to simultaneous precipitation of Pd₄Si, Pd₃Si and Pd₂Si 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 Pd₇₆Cu₆Si₁₃B₅ alloy consists of five phases of Pd, Pd₄Si, Pd₃Si, Pd₂Si and Pd₂B. 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 Pd₇₆Cu₆Si₁₅B₃ amorphous alloy is important because of the increase in the ease of the formation of bulk amorphous alloy.