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 (Tg) and crystallization temperature (Tx), ΔTx(=Tx-Tg) 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 Tg and an increase in Tx, leading to the maximum value of 70 K for ΔTx. 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 ΔTx 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.