Interconnection of thermal parameters, microstructure, macrosegregation and microhardness of unidirectionally solidified Zn-rich Zn-Ag peritectic alloys

In this work, the microstructural evolution of Zn-3.2 wt%Ag (hypoperitectic) and Zn-8 wt%Ag (hyperperitectic) alloys during transient unidirectional solidification is investigated. The experimental results include solidification thermal parameters such as the growth rate (V-L), thermal gradient (G(L)) and tip cooling rate (T), which are related to the microstructural interphase spacing (lambda) by proposed experimental growth laws. It is shown that, the classical lamellar eutectic growth law lambda V-2 = constant, applies to the growth of the peritectic Zn-Ag alloys examined, despite the different values of the constant associated with each alloy composition. In contrast, it is shown that identical functions of the form lambda = constant (G(L)) (14)(V-L) (1/8), and lambda = constant (T-1/3) can be applied to both alloys examined. Positive solute macrosegregation was observed in regions close to the bottom of the castings. The dependence of microhardness (HV) on the length scale of the microstructures (including that of a single phase Zn 0.8 wt%Ag alloy: lambda(C) cellular spacing) is examined. An experimental Hall-Petch type power law is proposed relating the resulting microhardness to lambda(C) for the single phase alloy, and despite the segregation profiles and the alloying differences of the hypoperitectic and hyperperitectic alloys, the average microhardnesses of these alloys is shown to be essentially constant and similar along the castings lengths. (C) 2014 Elsevier Ltd. All rights reserved. (AU)