Proceedings of the 9th International Conference on Aluminium Alloys (2004)
Edited by J.F. Nie, A.J. Morton and B.C. Muddle© Institute of Materials Engineering Australasia Ltd582

Effects of Si and Ge Addition on the Al3Zr Precipitates in an
Al-0.6mass%Zr Alloy
F. Nakamura1, S. Hirosawa2, T. Sato2
12

Graduate student, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Dept. of Metallurgy and Ceramics Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo152-8552, Japan

Keywords: Al-Zr alloy, Precipitate, Phase stability, KKR-Green function method, Coherent potential
approximation

Abstract
Effects of Si and Ge addition on the formation of Al3Zr precipitates were investigated in achill-cast Al-0.6mass%Zr alloy. Si and Ge addition to Al-Zr binary alloys reduced theamount of cellular reaction and resulted in an increased age-hardening. The crystal
structure of the stable Al3Zr phase was also changed by the addition of Si or Ge from D023
to D022. The KKR-Green function method with coherent potential approximation (CPA),
one of the first principle calculations, revealed that the stability of D023 and D022 structureschange when the Al sites of the two structures are substituted by Si or Ge.
1. Introduction
In aluminum alloys containing Zr, the fine spherical metastable Al3Zr with the L12 orderedstructure which effectively retards recrystallization is formed. However, the amount of
unfavourable cellular reaction of the L12 ordered Al3Zr is also increased by the highconcentration of Zr. Si addition is well-known to suppress the cellular reaction and alsochange the crystal structure of the stable Al3Zr phase from D023 to D022 [1]. However, themechanism of Si addition has not been clarified yet, although this might be explained by
the interaction energies between atoms in alloys. Recently, the first principle calculations
have been developed to evaluate interaction energies in solid. In the first principle
calculations, total energy which is the sum total of occupied electronic orbital energies canbe directly obtained and easily transferred to enthalpy. Therefore, the interaction energies
are systematically derived from the total energies of ordered and disordered phases. In
this work, we firstly investigated the effects of Ge addition, which is expected to similarly
work to Si addition, on the formation of Al3Zr precipitates in an Al-0.6mass%Zr alloy. Then,the phase stability of the Al3Zr phases with Si and Ge were evaluated by using KKR-Greenfunction method based on local density approximation. The similarity of Ge to Si well
explained the reason why the stable Al3Zr phase is changed from D023 to D022.
2. Experimental Procedure and Calculation Method
Alloy specimens of Al-0.6mass%Zr binary, Al-0.6mass%Zr-(0.2 and 0.4) mass%Si ternaryand Al-0.6mass%Zr-(0.5, 1.0 and 2.0) mass%Ge ternary alloys were prepared by a chillcast method. Small plate-shaped specimens were cut from these chill-cast materials

583

and subjected directly to various heat treatments. Micro Vickers hardness was measured
to evaluate the effects of Si and Ge addition on the age-hardening behavior of the highly
concentrated Al-Zr alloys. Precipitate microstructures were observed using an optical
microscope, FE-SEM and TEM. The chemical compositions of precipitates weredetermined using an EDX analysis system attached to the TEM.
In this work, the phase stability of the Al3Zr phases with L12, D022 and D023 structures
estimated by using KKR-Green function with CPA (MACHIKANEYAMA2000) [2]. On
evaluating the phase stability, the minimum of total energies was only considered and theeffects of temperature and entropy were ignored. In addition, maximum angular momentlmax, for the Green’s function was chosen to be 2 because lmax =2 well evaluated thedifference of total energies between each phase.
3. Results and Discussion
3.1 Age Hardening Behavior
Micro Vickers hardness of the investigated alloys was measured during aging attemperatures between 673 and 773K. Figure 1 shows an example of the hardness curves
at 673K. It is obvious that Ge addition accelerates the age-hardening and increases thepeak hardness of Al-Zr alloys although such effects are less than those in the Si-added
alloy.

Figure 1: Isothermal aging curves of hardness for Al-0.6mass%Zr-Si and -Ge alloys with aged at 673K.

3.2 Microstructure
The corresponding microstructures well explain the reason why the age-hardening isincreased by Si and Ge addition. The suppression of cellular reaction is clearly observed in
Figure 2 as shown by the decrease in areas of dark contrast, resulting in the formation ofhomogenous precipitates. Note that quite fine contrast of the homogenous precipitates isvisible even in the optical micrographs of Si- and Ge-added alloys (Figure 2(b) and Figure2(c)). Therefore, suppression of the cellular reaction seems to be caused by promotion of
the homogenous precipitation which contributes to the acceleration of the age-hardening in
Figure 1584

3.3 Crystal Structure and Chemical Composition
In order to identify the crystal structure and chemical composition of the precipitates, TEM
observation was performed for the Ge-added alloys. From the bright field images of Figure
3, L12- and D022-type Al3Zr were found to co-exist; i.e. spherical and coherent L12-type
Al3Zr and semicoherent D022-type Al3Zr. The crystallographic orientation between theD022-type Al3Zr and the Al matrix was identified to be [100]D022//[100]matrix,(001)D022//(001)matrix from the corresponding diffraction pattern(Figure 3(c)). Furthermore,
the EDX analysis revealed that the L12- and D022-type Al3Zr contain Ge much higher thanthat in the Al matrix (Figure 4). By assuming that Ge substitutes the Al sites, the chemicalcompositions of precipitated particles were determined as listed in Table 1. Note that Ge
addition to Al-Zr alloys was found to show the similar effect as Si addition on theprecipitation behavior and the stability of Al3Zr phases [1].

Figure 2: Optical micrographs of (a)Al-0.6mass%Zr, (b)Al-0.6mass%Zr-0.2mass%Si and (c)Al-0.6mass%Zr2.0mass%Ge alloys aged at 773K for 86.4ks. The dark contrast shows the cellular reaction areas. (d)highmagnification of the dark contrast areas of Al-0.6mass%Zr alloy aged at 773K for 86.4ks obtained by FESEM.

3.4 Calculation of the Phase Stability of Al3Zr Phases
The calculation results of equilibrium lattice constant of a and c/a ratio for Al3Zr phases arelisted in Table 2. The corresponding structural energy - lattice parameter curves provedthat the minimum phase stability of each Al3Zr phase is obtained at the equilibrium lattice
constant. By comparing the phase stability, furthermore, the minimum total energy wasobtained for the D023 structure (Figure 5). This indicates that the calculation well
reproduced the experiment data within a small amount of errors as shown in Table 2. As
for the effects of Si and Ge addition on the phase stability of Al3Zr phases, all possibilitiesof the substitutions of Si and Ge to Al or Zr sites were considered. Figure 6 shows anexample of structural energy - lattice parameter curves for (Al0.72,Si0.28)3Zr and585(Al0.84,Ge0.16)3Zr. The D022-type Al3Zr was found to be most stable if Si and Ge substitutethe Al sites. On the other hand, the curves for Al3(Zr0.56,Si0.44) and Al3(Zr0.68,Ge0.32) inFigure 4 indicate that the D022 structure is no longer estimated as the most stable phasealthough Si and Ge slightly change the stability of Al3Zr phases. Note that chemicalcompositions in Figure 6 and Figure 7 were calculated from the EDX analysis results in
Figure 3 and reported results [1]. From the calculated results, therefore, it was concluded
that Si and Ge addition is attributed to the formation of the most stable D022- type of Al3Zr
phase, if Si and Ge substitute the Al-sites of Al3Zr phases.

Figure 3: Transmission electron micrographs of Al-0.6mass%Zr-2.0mass%Ge alloys aged at 773K for345.6ks showing (a) spherical L12-type Al3Zr and (b) D022-type Al3Zr with (c) the diffraction pattern from D022
-type Al3Zr.

Figure 4: EDX analysis data from L12- and D022-type Al3Zr precipitates in Al-0.6mass%Zr-2.0mass%Gealloys aged at 773K for 345.6ks.
Table 1: Chemical compositions of Al3Zr phases and matrix for the Al-0.6mass%Zr-2.0mass%Ge alloys aged
at 773K for 345.6ks (at%).

Al3ZrAlGeZr

L12-type Al3Zr

72.7

3.325.0D022-type Al3Zr
Matrix

62.6
99.2

12.4
0.7

25.0
0.1

Table 2: The calculation and experimental results of equilibrium lattice constant of a and c/a ratio for L12and D023-type Al3Zr in Al-Zr binary system [3]. The experimental values of D022-type Al3Zr also listed from AlZr-V system [4].

Al3Zrcalculate a/nm
calculate c/a
experiment a/nm
experiment c/a

L120.406
1.00.405
1.000

D022
0.390
2.30.391
2.250

D023
0.398
4.30.400
4.330

586

Figure 5: Structural energy-lattice parameter curves for L12-, D022- and D023-type Al3Zr phases obtained by
the KKR-Green function method. 1 a.u. converts into 0.0529nm.
(Al0.72 ,Si0.28 )3 Zr(Al0.84 ,Ge 0.16 )3 Zr

Figure 6: Structural energy-lattice parameter curves for L12-, D022- and D023-type Al3Zr phases with Si andGe. Note that composition of (Al0.84,Ge0.16)3Zr was determined from EDX analysis results in Figure 3 byassuming that Ge substituted Al sites. (Al0.72,Si0.28)3Zr is a reported composition in ref.[1].

4. Conclusions
(1)
(2)
(3)

Ge addition to an Al-0.6mass%Zr alloy increased the hardness during aging andsignificantly inhibited cellular reaction similar to Si addition.
EDX analysis revealed that both of the metastable L12-type Al3Zr and subsequentD022-type Al3Zr contain Si and Ge.
KKR-Green function method well reproduced not only the phase stability of theAl3Zr phases but also the effects of Si and Ge on the stability of the phases. From587

the calculated results, it was found that Si and Ge substitute the Al-sites in the Al3Zrphases.
Al3 (Zr 0.56 ,Si0.44 )Al3 (Zr0.68,Ge 0.32)

Figure 7: Structural energy-lattice parameter curves for L12-, D022- and D023-type Al3Zr phases with Si andGe. Note that composition of Al3(Zr0.68,Ge0.32) was determined from EDX analysis results in Figure 3 byassuming that Ge substituted Zr sites. Similarly the composition of Al3(Zr0.56,Si0.44) was determined from thecomposition of (Al0.72,Si0.28)3Zr in ref.[1] by assuming that Si substituted Zr sites.

References
[1]
[2]
[3]
[4]

T.Sato and A.Kamio, Proc. of Int. Conf. of Science and Engineering of Light Metals, Japan Inst. ofLight Metals, 971-976(1991)
H.Ebert, B.Drittler, H.Akai, J. Magnetism and Magnetic Mat, v104-07 733-734(1992)
N.Ryum, Acta Meta. 17,269-278(1969)
S.Z.Han, S.I.Park, J.-S.Huh, Z.-H.Lee and H.M. Lee, Material Science and Engineering, A230 100106 (1997)