Effect of B addition on the extension of supercooled liquid region in Zr-Cu-Al base amorphous alloys

Abstract

The replacement of Al by B for Zr65Cu27.5Al7.5-xBx amorphous alloys was found to cause an extension of the supercooled liquid region before crystallization. The largest value of the supercooled liquid region (ΔTx,) denned by the difference between crystallization temperature (Tx) and glass transition temperature (Tg) is 100 K for Zr65Cu27.5B4Al3.5. The AT], value is much larger than that (72 K) for the Zr65Cu27.5Al7.5 alloy. The increase is due to the significant increase in Tx by the addition of B exceeding the degree of the increase in Tg. The crystallization from the supercooled liquid takes place through a single exothermic reaction in the B concentration range less than 4 at%. The crystallized structure consists of Zr2Cu, Zr2Al and Zr5Al3 for the Zr-Cu-Al alloy and changes into more multiple phases of Zr2Cu, Zr2Al, Zr3Al, Zr5Al3 and ZrB2 for the Zr65Cu27.5B4Al3.5 alloy. Besides, the temperature dependence of the storage and loss moduli (E′ and E″) in the supercooled liquid also changes from the single stage for the Zr-Cu-Al alloy to the two stages for the Zr-Cu-Al-B alloy. The first-stage changes in the E′(T) and E″(T) fit between both alloys and the second-stage change for the B-containing alloy takes place in the temperature range above Tx of the Zr-Cu-Al ternary alloy. It is therefore concluded that the generation of Zr-B atomic pair with stronger bonding nature and longer relaxation time by the addition of B causes the increase in the thermal stability of supercooled liquid region through the retardation of the crystallization reaction. Furthermore, this is the first evidence on the synthesis of the amorphous alloy with the large ΔTx, above 100 K for the Zr-Cu-B-Al alloy containing higher B content as compared with Al content.