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hase Diagram Evaluations: Section II

Ag-Al-Tb (Silver-Aluminum-Terbium) V. Raghavan

Recently, [2001Gum] determined an isothermal section for this system at 597 °C (870 K), which depicts eight ternary compounds.

Tb3Al2 (Zr3Al2-type tetragonal), TbAl (ErAl-type orthorhombic), TbAl2 (C15, MgCu2-type cubic), and TbAl3 (BaPb3-type or HoAl3-type rhombohedral). See [Massalski2] for the above diagrams.

Binary Systems The Ag-Al phase diagram depicts three intermediate phases: Ag3Al (20.5-29.8 at.% Al; denoted b, body-centered cubic; stable between 778 and 600 °C), Ag2Al (22.9-41.9 at.% Al; denoted d, close-packed hexagonal), and l (21-24 at.% Al, stable below 450 °C; A13, bMn-type cubic). The Ag-Tb diagram depicts the following intermediate phases: Ag51Tb14 (Ag51Gd14-type hexagonal), Ag2Tb (C11b, MoSi2-type tetragonal), and AgTb (B2, CsCl-type cubic). The Al-Tb phase diagram depicts the following intermediate phases: Tb2Al (C23, Co2Si-type orthorhombic),

Table 1

The known ternary compounds of this system are listed in Table 1 from [2001Gum]. The compounds labeled s1 to s8 (denoted 1 to 8 by [2001Gum]) are present at 597 °C (870 K). The last-listed compound TbAgAl3 was not found at 597 °C by [2001Gum]. It may be noted that TbAgAl3 and Tb3Ag2.5Al8.5 (s6) are both derivatives of Al4Ba-type of structure. Among the binary phases, AgTb and TbAl2 dissolve 25 at.% Al and 8 at.% Ag respectively.

Ag-Al-Tb crystal structure and lattice parameter data [2001Gum]

Phase TbAg5.2-5.8Al6.8-6.2 (s1)

Tb1.8Ag8.0-8.6Al9.0-8.4 (s2)

Tb8Ag21.1Al44.9 (s3)

TbAg2.4-3.0Al2.8-2.2 (s4)

 Tb18Ag57Al25 (s5)

Tb3Ag2.5Al8.5 (s6)

TbAg0.9Al2.1 (s7)

TbAg1.1Al0.9 (s8)

TbAgAl3

Ternary Phases

Composition, at.%

Pearson symbol

Space group

Prototype

40-44.6 Ag 52.3-47.7 Al 7.7 Tb 42.6-45.7 Ag 47.9-44.7 Al 9.6 Tb 28.5 Ag 60.7 Al 10.8 Tb 38.7-48.4 Ag 45.2-35.5 Al 16.1 Tb 57 Ag 25 Al 18 Tb 17.9 Ag 60.7 Al 21.4 Tb 22.5 Ag 52.5 Al 25 Tb 36.7 Ag 30 Al 33.3 Tb 20 Ag 60 Al 20 Tb

tI26

I4/mmm

ThMn12

a = 0.9142 c = 0.5444

hP38

P63/mmc

Th2Ni17

a = 0.92815* c = 0.90715

tI?

I4/mmm

Yb8Cu17Al49

a = 0.88099 c = 1.6771

hP?

P63/mmc

DyAg2.4Al2.6

a = 0.91739* c = 0.94151

...

...

...

...

oI28

Immm

aLa3Al11

hR12

R3m

PuNi3

oI12

Imma

CeCu2

oI?

Immm

o-CeNi2Sb2

Lattice parameter, nm

a = 0.43026 b = 1.25972 c = 1.00160 a = 0.55461 c = 2.6286 a = 0.45866 b = 0.71666 c = 0.78652 a = 0.4304 b = 0.4189 c = 1.0003

* Lattice parameters are for Tb1.8Ag8.6Al8.4 and TbAg2.7Al2.5, respectively

Journal of Phase Equilibria and Diffusion Vol. 29 No. 4 2008

361

Section II: Phase Diagram Evaluations

Fig. 1 Ag-Al-Tb isothermal section at 597 °C (870 K) [2001Gum]

Isothermal Section

ternary phases shown in Fig. 1 are from the listed values of [2001Gum].

With starting metals of 99.95% Ag, 99.95% Al, and 99.5% Tb, [2001Gum] arc-melted under Ar atm 49 alloys with Tb content up to 50 at.%. The alloys were annealed at 597 °C (870 K) for 1000 h and quenched in water. The phase equilibria were studied with x-ray powder diffraction. The isothermal section