Influence of Ag replacement on the formation and heating-induced phase decomposition of Zr65Al7.5Co27.5-xAgx (x=5 to 20 at%) glassy alloys

摘要

A glass-type alloy with glass transition (GT) and supercooled liquid region was formed at a limited Ag/Co content ratio around 1.2 in melt-spun Zr65Al7.5Co27.5-xAgx (x = 5-20 at%) alloys. This alloy crystallized through two-stage exothermic reactions where an icosahedral (I) phase appears as a primary precipitation phase, followed by the second-stage reaction from glass + I-phase to four crystalline phases. The I-phase has a spherical morphology and its size is about 8 nm. The Zr65Al7.5Co17.5Ag10 alloy without GT keeps a glass-like structure even after heating for 3.6 ks at 750 K above the first exothermic peak, indicating the formation of a cluster-like glassy phase. The Zr(65)Al(7.5)Co(17.5)Ag(10 )alloy rod keeps a glassy phase in the diameter up to 1.7 mm, though the rod does not show the first-stage exothermic reaction. These features are the same as those for the annealed ribbons, indicating that a clustered glassy phase is also formed for the bulk alloy. This alloy satisfies with the formation criterion of the clustered glassy phase, i.e., large atomic size mismatch and positive heat of mixing for solute elements. The high stability of glassy phase leading to the formation of the clustered glassy phase is due to the resistance to atomic rearrangement of unlike Co and Ag elements. The Zr65Al7.5Co15.5Ag15 alloy exhibits the lowest decomposition temperature (T-x) and the highest hardness, being opposite to the previous relation for hardness to increase with increasing T-x. The relationship between GT and I-phase precipitation suggests that the GT is attributed to the development of I-like medium-range ordered structure. (C) 2019 Elsevier B.V. All rights reserved.