328 Prakt. Met. Sonderband 30 (1999) 
Thus, the main problems presented in this work are changes of microstructure and transformation of 
substrates of the heterogeneous nucleation due to grain-refinement of the high-aluminium zinc alloys, 
investigated by means of differential thermal analysis, light microscopy, XRD and Rietveld analysts. 
2. Experimental procedure 
In the presented research the binary Zn-25wt%Al alloy was grain refined by the addition of 0.05 
wt% Ti, in relation to the mass of the melt, introduced in AlTi5 and ZnTi4 grain refiner master alloy. 
The whole amount of the Zn-25wt%Al alloy examined was prepared from electrolytically refined Zn 
(99.99%) and Al (99.99%) in an induction furnace with a chamote-graphite crucible and poured into 
chill with cylindrical cavities. Next, the obtained castings were remelted in the subsequently perfor- 
med examinations. The mass of the remelted Zn-25wt%Al alloy was about 300 g. The AlTi5 and the 
ZnTi4 grain refiner alloys were prepared from Al and Zn (99.99% purity) and m3N Ti sponge ( Al- 
pha Products - England) in a vacuum induction furnace with an alumina crucible. These master alloys 
were introduced into the melt overheated to 620 °C. After addition of the grain refiner master alloy, 
the melt was held at 620 °C for 3 min, then the melt was stirred for 2 min with an alumina rod and 
poured into a dry sand mould with a cavity 36 mm in diameter and 100 mm deep. In the centre of the 
cavity there was installed NiCr-NiAl0.5 & 0.25 mm thermocouple fixing course of the poured sam- 
ple cooling curve. From the middle section (in height) of the castings, the samples of 25 mm length 
were cut for examinations of microstructure in light microscopy (LM) and scanning electron micro- 
scopy (SEM). These samples were polished with sub-microscopic ALO; abrasive slurry in etha- 
nol+water 1:1. From the top section of the castings samples were cut for macroscopic examinations. 
These samples were neither toolled nor etched. The swarf obtained during the samples cutting was 
subjected to chemical dissolution in a reactive containing 5% ethanol solution of HCL. After dissolu- 
tion of the Zn-Al-alloy matrix of the swarf, powder samples containing a high amount of the Ti-base 
compounds were obtained. This powder samples were subjected to X-ray diffractometry and X-ray 
microprobe analysis. Next, the obtained X-ray diffraction patterns were subjected to Rietveld analy- 
sis. The X-ray diffractometry was performed using a Philips PW1710 diffractometer operating at 30 
kV and 40 mA with Co-Ka radiation, with scattering angles 20 from 20° to 130°, scan-step-size 
0.04° and scan-step-time 1.25 s. The obtained X-ray diffraction patterns were subjected to a fitting 
procedure performed by using Philips PC-APD computer program. The identification of the diffrac- 
tion patterns was performed by a computer program, equipped with data from the most recent edi- 
tion of the Joint Committee on Powder Diffraction Standards. Quantitative analysis of the extracted 
Ti-base phases were carried out with EDS method (Link Isis). The Rietveld analysis was performed 
using the Fullprof computer program [5]. 
3 Results and discussion 
3.1. Structure and thermal analysis examination 
Microstructure and macrostructure of the examined Zn-25wt%Al alloy before and after grain refi- 
nement process are shown in Figs 1 and 2. The electronic picture of the examined Zn-25wt%Al alloy 
microstructure before and after grain refinement by the ZnTi4 master alloy is shown in. Fig 1. As it 
appears from Fig 1 (a) - the sample without Ti addition shows coarse dendritic structure of the o' 
ZnAl solid solution, while the sample with 0.05wt%Ti of the same alloy shows globular-like shape of 
the a' - Fig. 1 (b). The macrostructure of the alloy undergoes a significant refinement - as it is seen in 
Fig. 2. Thus, the addition of the grain refiner master alloys leads to a significant refinement both of 
macro and microstructure of the examined alloy. Cooling curves and their first derivatives registered 
during solidification of the examined alloy point out that undercooling of the alloy with Ti addition