Prakt. Met. Sonderband 30 (1999) 327 
Research of phase transformation in high-aluminium zinc foundry alloys modified by Ti 
addition 
W.K. Krajewski 
University of Mining and Metallurgy, Faculty of Foundry Engineering. PL 30-059 Cracow 
Abstract 
In order to increase the elongation of high-aluminium zinc-base foundry alloys the grain refinement 
of the of primary dendrites of ZnAl solid solution is now used in foundry practice. Commonly 
employed grain refiner Al-Ti master alloy introduces binary Al;Ti phase, while Zn-Ti master alloy 
introduces TiZn; phase - and both act as substrate of the heterogeneous nucleation of o'. In the melt 
AlL;Ti and TiZn; undergo phase transformation into ternary Ti(Al,Zn); phase, whose fcc crystal 
structure and small lattice mismatch with o causes high efficiency of the o nucleation. 
Kurzfassung 
Zur Verbesserung der Duktilitit von hochaluminiumhaltigen ZinkguBlegierungen wird im 
GieBereiwesen eine Kornfeinung der o’-Dendriten des Aluminiummischkristalles durchgefiihrt. Im 
Allgemeinen werden Al-Ti und Zn-Ti-Vorlegierungen angewendet, die zur Entstehung von Al;Ti 
und TiZnz führen - beide wirken als heterogene Keime für die Bildung der @’-Phase. Al3;Ti und 
TiZnz wandeln zur ternéren Phase Ti(Al,Zn); um, deren kfz Struktur und geringe Fehlpassung zum 
Gitter der o’-Phase eine hohe Keimbildungseffizienz bewirken. 
1. Introduction 
High-aluminium zinc foundry alloys show very good strength and bearing properties. However, 
after casting in sand-mould they also show a coarse dendritic structure of the o' solid solution of Zn 
in Al. This type of microstructure leads to a decrease of the alloy plastic properties. For this reason 
these alloys are grain-refined before pouring into foundry mold. Main agents which are 
commercially used for the grain refinement are AlTi-base binary or ternary master alloys. As the 
result of the grain refinement one can observe a significant increase of grains number per 1 sqr cm 
of the alloy surface as well as significant decrease of the alloy supercooling during crystallization. 
These two phenomena prove that the grain-refinement process proceeds due to heterogeneous 
nucleation. The effective substrate of the nucleation are binary titanium phases introduced with the 
master alloys. However, in the centres of the grain-refined dendrites one can observe ternary Ti-Al- 
Zn phases, which was first stated as simple cubic phase of AlsTiZn [1] or Ti(AlZn); [2] 
composition and in [3, 4] as Ti(Al,Zn); cubic L1, super-structure. Thus, during the grain refinement 
process the binary D0, Al;Ti phase introduced with the ATi master alloy undergoes structural and 
chemical transformation changing its crystal structure from tetragonal to cubic one. It is well known 
that the Zinc-base alloys can not be overheated during the melting process. Thus, the described 
above grain refinement must be carried on in temperature level about 580-600°C, which is 
unfavourable for quick dissolution of the AITi master alloy. On the other hand, the significant 
difference between the melt (about 5 g/cm’) and the master alloy (about 3 g/cm’) causes some 
technical difficulties during the modification process. For this reason this author uses ZnTi master 
alloy, which shows a good solubility in the range of temperature proper for Zn-Al alloys and also 
shows high grain-refining activity. The Zn-Ti master alloy introduces TiZn; phase (having L1, 
crystal structure) into modified melt. The TiZn; phase also undergoes transformation into 
Ti(Al,Zn), but without change of its crystal structure [4].