Prakt. Met. Sonderband 30 (1999) 203 
04S on the oY . : 
aly by he Diborides present in Al-Ti-B alloys annealed at 1600 °C 
ough, these 
0 the free 
Ry Franc Zupanié&', Savo Spaic’, Alojz Krizman' oo 
of te me 1) Faculty of Mechanical Engineering, University in Maribor, Slovenia 
sal i 2) Department of Materials and Metallurgy, University in Ljubljana, Slovenia 
th the wide 
It grains 
Sit intrasion Abstract 
1 Abstract 
en This paper deals with the composition and morphology of the diboride phase present in some 
+ column Al-Ti-B alloys annealed at 1600 °C for 10 hours. The results of EDS and XRD indicate that almost 
ermined pure TiB, exists in the equilibrium with the melt at 1600 °C. The morphology of the diboride 
the ate particles depends on Ti/B ratio. An excess of Ti (B) over that needed to form TiB, tends to stabilise 
crystal planes with the planar arrangement of Ti (B) atoms. 
Introduction 
3 Yh ly It is believed that particles of the diboride phase (Al, Ti)B; present in Al-Ti-B alloys play the most 
send on te important role in the grain-refining process in aluminium alloys (1). Therefore, they were the 
subject of several investigations (1-4). The results show that during manufacturing of Al-Ti-B 
alloys by an aluminothermic synthesis the diboride phase is formed with compositions ranging from 
: te id the composition of stoichiometric AlB; to the composition of stoichiometric TiB, (1, 2). All 
hil or diborides, pure AIB, and TiB, as well as the mixed diboride (ALTi;x)B, (© <x <1) possess the 
0 on same crystal structure (space group P6/mmm, Ref 5) and very similar lattice parameters (see Fig. 
1). On this ground it was suggested that the mixed diboride phase (Al,Ti;)B, represents a 
: thermodynamically stable phase in the aluminium rich corner of the Al-Ti-B ternary system (6). 
VTS However, some experimental results do not support this assumption (1, 7). Johnsson and Jansson 
(1) found out that the composition of the mixed diboride moves toward the compositions of pure 
TiB; and AIB; during holding in the liquid aluminium. The investigation of Zupani¢ et al. (7) has 
ony. for revealed that during synthesis of Al-Ti-B alloys by arc melting almost pure AIB; and TiB, are 
esol formed. Furthermore, the formation of the mixed diboride (AlcTi;.x)B, from pure diborides AIB, 
and TiB; has not been observed even after 1000 h exposure at 800 °C. This temperature is relatively 
low (~0,3 Tr, of TiB;; Tm — melting temperature of pure TiB,: 3225 °C), thus - even in the case if 
the mixed diboride is a stable phase - the transformation of pure AIB; and TiB; to the mixed 
diboride might be suppressed by kinetic reasons. Annealing of Al-Ti-B alloys containing both pure 
41) 227-289. AlB, and TiB; is not possible at much higher temperatures that should allow completion of the 
transformation in shorter times, because AlB, transforms peritectically to the liquid phase and 
o-AlB1; above 900 °C (8). Thus, we are restricted to study the diboride phase in the equilibrium 
11990 985 with other phases. 
In this work we investigate the diboride phase after annealing at 1600 °C (> 0,5 Ty, of TiB;). The 
pr main purpose is to determine the composition of the diboride phase coexisting with the liquid phase 
to contribute to the constitution of the ternary system Al-Ti-B. Additionally, we also investigate the 
influence of Ti/B ratio on the morphology of diboride particles.