Fig. 14. c lattice spacings in the lanthanum-holmium system. The straight lines represent the Vegard's law relation- ships for the spacings in the Sm-phase and dhcp regions based on the lattice parameters given in table 2.
presence of the samarium-type structure in this system. The X-ray pattern from a 34at% La-66 at% Lu alloy could be indexed on the basis of the coexistence of lanthanum (dhcp) and lutetium (hcp) solid solutions. These data suggest that a two-phase immiscibility gap exists in the low temperature region of this system.
Lundin proposed that the most probable phase equilibria to accommodate this immiscibility gap would be a eutectoid reaction of the high temperature body- centered cubic allotrope to the two terminal (lanthanum plus lutetium) solid solu- tions. On the basis of the two-phase character of the microstructure, Lundin deduced that the assumed eutectoid would be near 34 at% La-66 at% Lu.
The reviewers, however, suggest an alternate model for the phase equilibria in this region of the lanthanum-lutetium system. At high temperature we propose that the dhcp La and hcp Lu phases form a continuous series of solid solutions, which upon cooling form a solid miscibility gap of the two terminal solid solutions. This construction would be essentially identical to the phase relationships observed in the N d - S c system by Beaudry et al. (1965) (see section 2.37 and fig. 65) and similar to that observed in the L a - G d system in that the dhcp La phase forms a continuous series of solid solutions with the hcp Gd phase at high temperatures (see section 2.4 and fig. 5). Clearly a careful experimental study needs to be made of the 20 to 50 at%
La region of the lanthanum-lutetium system, especially at high temperatures.
Anderson et al. (1958) measured the effect of lutetium additions on the supercon- ducting transition temperature of lanthanum and reported that alloys containing 55 and 80 at% La each had the lanthanum dhcp structure. These results are consistent with those reported above by Lundin.
2.8.2. Lattice spacings
Lundin (1966) reported lattice spacings for the two phases found in a 34 at%
La-66 at% Lu alloy. For the lanthanum solid solution he found a = 3.727 Ä and c = 12.028 Ä. For the lutetium solid solution he reported a and c lattice spacings of 3.547 and 5.642 Ä, respectively.
References
Anderson, G.S., S. Legvold and F.H. Spedding, 1958, Phys. Rev. 109, 243.
Beaudry, B.L, M. Michel, A.H. Daane and F.H. Spedding, 1965, in: Eyring, L., ed., Rare Earth Research III (Gordon and Breach, New York) p. 247.
Lundin, C.E., 1966, Denver Research Institute Rept. AD-633558, University of Denver, Denver, CO (also given as DRI-2326).
2.9. La-Sc: Lanthanurn-scandium 2.9.1. Phase diagram
Lundin (1970), in an investigation of the formation of samarium-type structure in intra rare earth binary alloys included six compositions in the lanthanum-scandium system ranging from 10 to 85 at% La. Lundin prepared his alloys using 99.8 (wt?)%
pure lanthanum metal (major impurities, 330ppm other rare earths, 510ppm O, 50 ppm each Si, Mg and Zn) and 99 + (wt?)% pure scandium for which there were no details given on the impurities. Lundin found two-phase immiscibility at low temperatures in the lanthanum-scandium system and, since no samarium-type structure was found, he concluded that scandium behaves more like the neighboring transition elements than it does as a rare earth metal.
The phase diagram for this system, as reported by Naumkin et al. (1970) and Savitskii et al. (1970) is shown in fig. 15. Savitskii et al. did not reveal the purity of their alloying materials but Naumkin et al. reported a 99.7wt% purity for their lanthanum (impurities, reported in wt%, were 0.04 Ce, 0.07 each Pr and Nd and 0.012 Fe). Their scandium was reported to be 99.7wt% pure (impurities included 0.11 wt% O). Their alloys were formed by arc-melting the metals under an atmo- sphere of purified helium. Below the solidus, the body-centered cubic "/La and/3Sc form a solid solution. Two eutectoid transformations at about 750°C and 45 at% Sc and 233°C and 14 at% Sc result in regions of two-phase immiscibility.
2.9.2. Thermodynamic properties
DeBoer et al. (1980) presented calculated values for the enthalpy of formation, the limiting partial heats of solution and the heat of mixing of several binary alloys based on scandium. The authors pointed out that the scarcity of experimental information on the heat of alloying of scandium alloys makes the comparison of the
2 6
1 6 0 0
1 4 0 0
1200
I 0 0 0
u 80O
0
h l r «
~_ 6 0 o
bi a.
~ 40o
I-- w
200
K.A. GSCHNEIDNER and F.W. CALDERWOOD L A N T H A N U M - SCANDIUM
I ' ' ' ' 1541 ° 1
L I Q U I D
( y L a , ~ Sc ) / / /
1537"
- 14 % 233" ~ 8 6 %
i i i I I I t I
0 2 0 4 0 6 0 8 0 I 0 0 Fig. 15. Phase diagram of the La ATOMIC PERCENT SCANDIUM Sc lanthanum-scandium system.
calculations with experimental results difficult. They assumed that enthalpies of formation and of mixing are independent of temperature. For compounds, the reference state used was the pure solid metals and for liquid alloys, the reference states used were the pure liquid metals.
Their calculated values for the heat of formation, AHf, for the compositions ScLa», ScLa2, ScLa, Sc2La and ScsLa were, respectively, +4, +9, +11, +10 and + 6 k J per mole of atoms. These positive values for AHf are consistent with the known L a - S c phase diagram in that no intermediate phases are known. The limiting partial heats of solution, A H °, for Sc in La and for La in Sc were, respectively, + 27 and + 35 kJ/mol, and the integral heat of mixing A Hmix, for ScLa was calculated to be + 8 kJ/mol.
References
DeBoer, F.R., R. tloom and A.R. Miedema, 1980, Physica 101B, 294.
Lundin, C.E., 1970, in: Les Elements des Terres Rates, Vol. 1 (Centre National de la Recherche Scientifique, Paris) p. 151.
Naumkin, O.P., V.F. Terekhova and E.M. Savitskii, 1970, Izv. Akad. Nauk SSSR, Met. [4], 137 [English transl.: Russ. Metall. [4], 99].
Savitskii, E.M, V.F. Terekhova, R.S. Torchinova, I.A. Markhova, O.P. Naumkin, V.E. Kolesnichenko and V.F. Stroganova, 1970, in: Les Elements des Terres Rares, Vol. I (Centre National de la Recherche Scientifique, Paris) p. 47.
2.10. La-Y: Lanthanum-yttrium 2.10.1. Phase diagram
T h e phase diagram for the l a n t h a n u m - y t t r i u m alloy system, fig. 16, is based on the d a t a of Spedding et al. (1962), but has been revised somewhat in light of the investigation b y Lundin (1966) on the formation of the samarium-type structure in intra rare earth binary alloys. The metals used by Spedding et al. were more than 99.9 (wt?)% p u r e with respect to other rare earths a n d contained small amounts of t a n t a l u m and oxygen as impurities. In the case of yttrium, the oxygen and tantalum contents were 1800 and 2000 ppm, respectively, and in the l a n t h a n u m each was less t h a n 5 0 0 p p m . Their alloys were prepared b y co-melting the components in a t a n t a l u m crucible using an induction furnace. Lundin used 99.9 wt% pure lanthanum
L A N T H A N U M - Y T T R I U M
1 6 0 0 , l l i
1522 = . 1400 -
1200 -
I 0 0 0 - S . ~ / ~ / / 7 ~ ° ' L a ' ~ Y) BCC - 86,5 =)
8 0 0
I II _73»° -
( ù,~'
~n~ I ß ( / ~ La) FCC , 1 [ ~ \ (aY)HCP
~_600t--II t I [~ -
I I I I f i t
I// i ~ l '
L / / ( = La) DHCP I I [ 4 0 0 I-// . . . . I I I I.- J/l~310 o I l B I
200 F - , i I I
1 1 ' I
0 I t l I
0 2 0 4 0 6 0 8 0 I 0 0
La A T O M I C P E R C E N T Y T T R I U M Y 1478 °
Fig. 16. Phase diagram of lanthanum- yttrium system.
28 K.A. GSCHNEIDNER and F.W. CALDERWOOD
that contained 520ppm O, 330ppm other rare earths and 50ppm each Si, Mg and Zn. His yttrium which was 99.4wt% pure, contained 1850ppm O, 3000ppm Zr, 3 3 0 p p m other rare earths, 80ppm C and 6 0 p p m each Fe, Ta and Ni. Melting of c o m p o n e n t metals was done under a positive pressure of purified argon in a n o n c o n s u m a b l e electrode arc furnace. As proposed by Spedding et al., the phase diagram showed a peritectoid reaction which formed the intermediate samarium-type phase from f l L a and «Y. An extensive investigation, that included thermal analyses, dilatometry, resistivity measurements and isothermal annealing and quenching treat- ments, failed to confirm the peritectoidal reaction. Lundin concluded that the samarium-type structure results from a recrystallization phenomenon formed by strains set up within the solid solution in the composition range where this structure exists, also see sections 2.1.3 and 2.1.4. In fig. 16 the peritectoid reaction has been replaced b y a congruent transformation. Lundin reported the compositional range for the Sm-type structure in the L a - Y system to run from - 40 at% La to - 65 at%
La, a range that is somewhat wider than that given by Spedding et al. Jayaraman et al. (1966) reported that a 60 at% Y alloy has the hcp phase, which is consistent with the results shown in fig. 16. Some of the melting and transformation tempera- tures of the pure metals have been adjusted to conform with table 1.
z
( . . )
13_ O3 I.lJ I-- I-- .._1
o
3.76
3.72
3.68
3.64 0 La
LANTHANUM- YTTRIUM
I I i !
3. 7740
o ~ o 5 9 V a l
0 %
A 66 Lun
• 6 6 Jay
o ~ o ; 6 4 Har
t A . ~ . t ,
Sm - typ e re qion
3,648,"
I I I I
20 4 0 60 8 0
ATOMIC PERCENT YTTRIUM
Fig. 17. a lattice spacings in the lanthanum-yttrium system. The o straight line represents the Vegard's ] law relationship for the ä latfice IO0 spacings based on the latfice parame-
Y ters given in table 2.
LANTHANUM-YTT R I U M
12.2,1, u ,
o ~ DHCP
1 2 . 0 - ~ o
26.8
o< 2 6 . 6 -
d z õ
cn 5.81 -o 59 V«l 66 Lun
• 66 Joy I- •
5 . 8 4 - 64 Hor J
5.8C -
5.-?6
5.72 0 Lo