Complex intermetallic phases in the Al-Pd-Ru and Al-Pd-Ir alloy systems
B. Grushko
1,D. Pavlyuchkov
1,2,T. Ya. Velikanova
21 IFF-8: Microstructure Research
2 I.N. Frantsevich Institute for Problems of Materials Science, Kiev, Ukraine
Basing on the updates of the Al-Ru and Al-Ir con- stitutional diagrams the Al-rich parts of the Al-Pd- Ru and Al-Pd-Ir constitutional diagrams were de- termined in the temperature range up to 1100◦C.
The study was carried out using powder XRD, DTA, SEM/EDX and TEM. Both alloy systems ex- hibit formation of complex intermetallic phases.
Known to date binary and ternary alloy systems of aluminum with platinum metals (Ru, Rh, Pd, Os, Ir and Pt) usually contain structurally complex inter- metallics, including stable ternary quasicrystals (see [1] for references). The title ternary alloy systems, studied for the first time, are linked to either Al-Pd-Fe (Ru and Fe belong to the same column in the peri- odic table) or Al-Pd-Co and Al-Pd-Rh (Co, Rh and Ir belong to the same column in the periodic table) pre- viously also studied in FZJ [1].
Basing on the updated Al-Ru constitutional diagram (see in [1]), the partial isothermal sections of Al-Pd- Ru were determined at 1000, 1050 and 1100◦C in [2]
and completed with the partial isothermal sections at 790 and 900◦C. The latter is presented in Fig. 1.
FIG. 1: Partial isothermal section of Al-Pd-Ru at 900◦. The isostructural binary AlPd and AlRu phases form a continuous β-range of the CsCl-type solid solu- tions. A number of ternary phases were revealed.
Between 66 and 75 at.% Al, three structurally re- lated cubic phases: C (primitive,a=0.7757 nm), C1
(bcc,a=1.5532 nm) and C2 (fcc,a=1.5566 nm) are formed. The same structures are also typical of the Al-Pd-Fe alloy system [1]. Although their composi- tional regions were somewhat different from those in Al-Pd-Fe, the “chain” arrangement of these regions
and their sequence were the same in both these sys- tems.
FIG. 2: Electron diffraction patterns of the: (a-c) P20- phase, (d-f) P40-phase and (g-i) F40-phase [1] along the [1 0 0], [1 1 0], and [1 1 1] zone axes.
A stable icosahedral quasicrystalline I-phase is formed below 1080◦C around the Al71.5Pd17Ru12.5
composition. Similarly to that concluded for other Al- TM alloy systems [1], the stable ternary Al-Pd-Ru I- phase is actually a ternary extension of a metastable Al-Ru icosahedral phase stabilized by Pd. At nearby compositions complex cubic phases were observed (see Fig. 2): primitive P20with the lattice parameter a≈2.0 nm, P40witha≈4.0 nm and fcc F40also with a≈4.0 nm. Despite their definite periodicity, these phases exhibit powder X-ray diffraction patterns very similar to that of the quasiperiodic I-phase, and the phase boundaries between these periodic phases and the I-phase are not clearly detectible.
The complex ε-phases, also structurally related to quasicrystals, widely extend from “Al3Pd” to ternary compositions. Similarly to that in Al-Pd-Fe or Al-Pd- Mn (see [1] for references), the orthorhombicε6,ε16, ε22andε28phases were observed. Their lattice pa- rametersa≈2.34 andb≈1.62 nm are essentially the same, while thecparameters are∼1.23, 3.24, 4.49 and 5.70 nm, respectively. Apart from these regu-
FIG. 3: Overall compositions of the Al-Pd-Co (a), Al-Pd-Rh (b) and Al-Pd-Ir (c) phases.
lar structures, also structures aperiodic along the c- direction were revealed at intermediate compositions.
Thus, inside the wideε-phase range only slight con- tinuous variation of the orthorhombicaandbcell pa- rameters are accompanied by complicated modula- tions of theccell parameter.
Theε-range in Al-Pd-Ru broadens up to 15 at.% Ru.
In Fig. 1 only its high-temperature part is shown:
at lower temperatures it links to the Al-Pd terminal.
At compositions close to the high-Ru limit of the ε- range the electron diffraction patterns of complex orthorhombic structures and one-dimensional qua-
sicrystalline structure were revealed. These struc- tures are formed in a small compositional region des- ignated E in Fig. 1.
The Al-Ir phase diagram was specified in the range from 65 to 90 at.% Al [3]. At∼1600◦C the congru- ent Al2.7Ir phase forms a eutectic with the congruent AlIr phase. At higher Al concentrations four interme- diate phases were found to be formed by a cascade of peritectic reactions: Al3Ir at 1466◦C, Al28Ir9(χ)at 1446◦, Al45Ir13(φ)at 993◦C and Al9Ir2at 877◦C.
Basing on the updated Al-Ir constitutional diagram, the partial isothermal sections of Al-Pd-Ir were deter- mined at 1100, 1000, 900 and 790◦C [4]. As in Al-Pd- Ru, Al-Pd-Co and Al-Pd-Rh, the isostructural binary AlPd and AlIr phases (probably) form a continuous β-range of the CsCl-type solid solutions (see Fig. 3).
The above-mentioned complexε-phases extend from
“Al3Pd” up to 22 at.% Ir, i.e. almost up to the Al-Ir terminal. Also the Al4Pd phase (λ-phase) dissolves up to 15.5 at.% Ir, which significantly increases its higher existence temperature limit. As a result, this phase only forming in Al-Pd in the solid state, can be in equilibrium with the liquid at its high-Ir concentra- tions. The C-phase, similar to that observed in Al-Pd- Ru at ternary compositions, is already forms in the bi- nary Al-Ir alloy system (above-mentioned Al2,7Ir) and it can dissolve up to 15 at.% Pd. The C2-phase is also formed in Al-Pd-Ir at ternary compositions, while the C1-phase was not observed in this alloy system. In- stead, a hexagonal C3-phase (a=1.09135,c=1.3418 nm), structurally related to the cubic C, C1 and C2
phases, was revealed. The ternary C2phase is also formed in Al-Pd-Co, while both C2and C3phases are formed in Al-Pd-Rh.
The overall compositions of the phases in the Al-rich parts of the Al-Pd-Co, Al-Pd-Rh and Al-Pd-Ir alloy systems are compared in Fig. 3. In contrast to Al- Pd-Ru, neither of these alloy systems contain stable quasicrystals. In Al-Pd-Rh the isostructuralε-phases form a continuous range of solid solutions between the binary terminals. Since in Al-Pd-Ir the Al-Pdε- phases extend almost up to the Al-Ir terminal, this is plausible to suggest that theε-phases are also typ- ical of this binary alloy system. In contrast to Al-Pd and Al-Rh, in Al-Ir theε-phases are metastable but are stabilized by only a few at.% Pd. In Al-Pd-Co the ε-phases “only” extend up to∼16 at.% Co.
[1] B. Grushko and T. Velikanova, CALPHAD,31, 217- 232 (2007).
[2] D. Pavlyuchkov, B. Grushko and T. Ya. Velikanova, J. Alloys Comp.464, 101-106 (2008).
[3] D. Pavlyuchkov, B. Grushko and T. Ya. Velikanova, In- termetallics.16, 801-806 (2008).
[4] D. Pavlyuchkov, B. Grushko and T. Ya. Velikanova, J. Alloys Comp.453, 191-196 (2008).
