X-ray investigation of phase relations and crystal structures in the binary system ReAl

Journal of the Less-Common

Metals, 98 (1984)

215 - 220

X-RAY INVESTIGATION OF PHASE RELATIONS STRUCTURES IN THE BINARY SYSTEM Re-Al

215

AND CRYSTAL

J. C. SCHUSTER Institut fiir Physikalische Chemie der Universitiit Wien, Wtihringerstrasse 42, A-l 090 Vienna (Austria) and Institute of Materials Science, University of Connecticut, U-136, Storrs, CT 06268 (U.S.A.) (Received August 1,1.983)

summary The binary system Re-Al was investigated using X-ray diffraction examination at temperatures above 1273 K in the composition range 20 - 80 at.% Re. Four intermediate phases were observed: Re,Al, ReAl, ReA13 and ReAL,. Re2A1 is tetragonal with lattice constants a = 0.29802 nm and c = 0.975 96 nm (isotypic with MoSi,); ReAl is tetragonal with lattice constants a = 0.307 85 nm and c = 0.595 15 nm. ReAl, is isotypic with TcAl,. A complete binary phase diagram is proposed.

1. Introduction To synthesize the binary Re-Al phases which occur as constituents in ternary Re-Al alloys at temperatures of 1273 K and above and to obtain reference X-ray powder patterns, it was initially intended to prepare binary alloys of the nominal composition ReO.&lO.sO, Reo.26sA10.735and ReO.~oAlO,~o only, On identification of the binary Re-Al phases by matching the experimental X-ray powder pattern with that c~cula~d from crystal data given in the literature only the ReA14 phase was found. This partial failure to reproduce published data led to a more detailed X-ray investigation of the binary Re-A1 system in the composition range between 20 and 80 at.% Re. The Re-Al phase diagram was first investigated by Savitskii et al. [l]. The existence of four intermediate phases was reported: a x phase at 60 at.% Re with the o-Mn structure (a = 0.958 nm) suggesting a formula ResAl,; a phase Re,Al, with a composition close to the equiatomic ratio (45 - 50 at.% Re) which did not have the CsCl-type structure reported earlier by Obrowski [Z]; a phase ReA12 at 33 at.% Re; a phase ReAlI at 5.6 at.% Re (b.c.c. structure with a = 0.7528 nm which is isotypic with WA112). All phases were found to melt incon~uently and their decomposition temperatures were given. K~pyakevich and Kuz’ma 133 confirmed the ReAlI st~ct~e and derived a formula Rez4A1, for the x phase based on structural arguments, 0022-5088/‘84/$3.00

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216

although they observed this phase at 63.5 at.% Re. D’Alte da Veiga [4, 51 reported the additional intermetallic phases ReAl, (space group, Ccmm; a = 0.66117 nm; b = 0.760 91 nm; c = 0.9023 nm) isotypic with MnA16, which was later confirmed by Wilkinson [ 61, and ReA14 (triclinic structure; a = 0.913 nm; b = 1.38 nm; c = 0.516 nm; CY= 99.5”; fl= 94”; y = 103.5”), and proposed a modification of the aluminum-rich part of the phase diagram which is incorporated in the complete phase diagram compiled by Elliott [7]. Edshammar [8] reported another phase with a triclinic unit cell (space group, Pi; a = 0.497 nm; b = 0.880 nm; c = 0.490 nm; (x = 85”; fl= 100”; y = 106”) isotypic with Mn4A1 r1 [9, lo] and suggested that the correct formula of the ReAlz phase reported by Savitskii et al. [l] was RegAlI,. However, Edshammar obtained only a poor reliability factor (R = 0.2) for his structural analysis. Several investigations in ternary systems with the binary Re-Al as the boundary system have shown five intermediate phases ReAlI*, ReAl,, ReAla, Re,,Al,, and ResA12 which are stable at 773 K [ll] and 873 K [12 - 141. ReAla, RedAll and ResAl* were observed in an isothermal section at 1273 K [141. 2. Experimental details Aluminum (Koch-Light Ltd.; purity, 99.999%) and rhenium (Chase Brass and Copper; purity, 99.99%) were used as starting materials. Appropriate mixtures of the metal powders were cold pressed and arc melted under argon (minimum purity, 99.999%). The ingots were then cut and checked for homogeneity which was satisfactory in all cases. The alloys were heat treated at 1273 K (300 h) or 1473 K (200 h) in molybdenum crucibles which were sealed in evacuated quartz tubes. Heat treatment at 1773 K (50 h) was performed in molybdenum crucibles under pure argon at a pressure of lo4 Pa using a high frequency furnace. After quenching, all alloys were analyzed by X-ray powder diffraction using Cu Kcr or Cr KCYradiation. 3. Results and discussion Four intermediate phases were identified and characterized by X-ray diffraction in the composition range between 20 and 80 at.% Re (Table 1). Rhenium dissolves only a small amount of aluminum and virtually no change in the lattice parameters with respect to those of the pure element was observed . The inter-metallic phase with the highest rhenium content (which is also in equilibrium with the terminal solid solution Re(ss)) is present as a single phase in the alloy Reo.ssAlo.s5. This agrees with the results reported by Savitskii et al. [l]. The X-ray powder pattern is indexed on the basis of a body-centered tetragonal unit cell with a = 0.298 02 nm and c = 0.975 96 nm. This phase is isotypic with MoSiz (space group, I4/mmm) with two aluminum atoms in the 2a positions and four rhenium atoms in the 4e

217

TABLE 1 Results

of the X-ray analysis

of binary

Re-A1 alloy

Alloy composition (at.% Re)

Phases identified after annealing under the following condition9

80

Re(ss) Re(ss) + RezAl Refss) + RezAl Re, Al + trace Re(ss) Rez Al Re*Al + trace ReAl RezAl + ReAl ReAl + trace Rez Al ReAl ReAl + trace ReA13 ReAl + ReA13 ReAl + ReA13 ReA13 + ReAl ReAl3 ReAl3

75 70 67 65 60 55 50 45 40 35 33 30 26.5 25 20

1273 K, 300 h

1373K,30h

1773K,30h

As arc melted

Re(ss)

Re(ss) + Re2Al Re(ss) + Re2Al Rez Al Re,Al+

RezAl + trace ReA13 Re2Al + trace ReA13

Re2Al + trace ReA13

RezAl + ReA13 Regal + ReA13

Re2Al + ReA13 ReA13

ReA13 ReAl4

ReA13

RezAl + ReA13 Re7Al + ReA13 RezAl + ReA13 Re2Al + ReA13 ReA13 ReAlj

as~, solid solution.

positions (z = l/3) and thus has the formula Re2A1. The experimental and calculated X-ray diffraction patterns are given in Table 2. Darby et al. [15] reported an analogous phase in the Tc-Al system. The RezAl phase is found in the arc-melted alloys and in all the annealed samples (Table 1). After annealing at 1273 K a low temperature phase is observed at an alloy composition slightly richer in ~uminum than the equiatomic ratio (Table 3). This phase appears to correspond to the Re,Al, phase reported by Savitskii et al. [ 11. The X-ray powder pattern can be indexed on the basis of a tetragonal unit cell with a = 0.307 85 nm and c = 0.595 15 nm. The X-ray intensities calculated for an assumed formula ReAl match the experimental intensities when the (0,0,O) positions are occupied by rhenium atoms, the (O,O, 1/2)positionsareoccupiedbyaluminumatomsandthe(1/2, l/2,2) -2) positions are occupied by a statistical mixture of both and (l/2,1/2, atoms (z = 0.25). This model probably describes only the subcell of the structure since it is expected that the low temperature ReAl phase has an ordered arrangement of rhenium and aluminum atoms rather than a statistical occupation. However, neither the gumption of ordered or semi-ordered occupancy with a doubled or tripled c axis nor variation of the z parameter between 0.2 and 0.3 gives a pattern which matches the experimental data.

218 TABLE 2 X-ray diffraction pattern (Cu Ka radiation) of ReossA1e_35 (arc melted and annealed at 1273 K for 300 h) hkl

sin20 X 1 O4 (o bserued)

00 2 10 1 00 4 10 3 11 0 11 2 10 5 00 6 11 4 20 0 20 2 21 1 11 6 20 4 10 7 21 3 00 8 21 5 20 6 22 0 11 8 22 2 10 9 30 1 22 4 0010 21 7 30 3 31 0 20 8 31 2 30 5 22 6 31 4 1110 1011 21 9 32 1 31 6 2010 30 7 32 3 0012 22 8

261 737 1039 1255 1337 1596 2288 2326 2373 2676 2934 3405 3668 3710 3837 3922 4956 5000 5349 5462 5596 5907 6065 6378 6508 6591 6677 6808 6938 7624 7679 a 8578 8751 9004 9272 b

9479

I (observed)

I (calculated)

ms ms

30 34 5 100 45 11 6 10 6 15 4 7 18 3 3 33 0 4 12 6 2 2 11 2 2 0 3 10 10 2 3 2 11 4 2 2 28 5 34 3 3 40 5 4

sin28 X 1 O4 (calculated) 259

733 1034 1250 1336 1595 2284 2327 2370 2672 2931 3405 3663 3714 3836 3922 4138 4956 4999 5344 5474 5603 5905 6077 6378 6465 6508 6594 6680 6810 6939 7628 7671 7714 7801 8491 8577 8750 9007 9137 9180 9266 9310 9482

VW VS s

mw W

mw W

m W W ms VW VW S

W m W VW VW m VW VW

VW m m VW VW VW m s VW s S VW

vw, very weak ; mw, medium weak; w, weak; m, medium; s, strong; ms, medium strong, vs, very strong. aCoincidence with the (226) Ka2 line. bCoincidence with the (323) K@z line.

219 TABLE

3

X-ray diffraction pattern 1273 Kfor 300 h)

(Cu KCKradiation}

of Re 0,45A10.55 (arc melted

and annealed

at

__.

hkl

sin’0 (o

001 100 002 101 110 102 111

-

X IO4

sin28

bserued)

380

1769 2776 2876 3135 3348 -

003 112 103 200 2011 004) 113 210 202 211 104 212 114 203 005

6108 -

Abbreviations

as in Table

4726

I

I

(calculated)

(observed)

(calculated)

371 1382 1484 1753 2764 2864 3135 3339 4248

ms

4721 5528 58991 59361 6103 6910 7012 7281 7318 8394 8700 8867 9275

5530 5911

7282 8391 8693 8865 -

X IO4

m ms VS

mw W W

m m W -

W S S

mw -

53 0 0 30 53 100

12 3 0 7 ,T7” 16 6 0 0 7 0 57 64 10

3

2.

This indicates that the ordering of the rhenium and aluminum atoms is a more complex structural modulation along the c axis and focusing camera techniques are required to detect very weak superstructure lines, The lattice parameters of ReAl do not vary within the error limits, indicating a very small homogeneity range for this phase. This phase was not observed at 1373 K. Thus the peritectoid reaction must take place between 1273 and 1373 K. A third intermediate phase occurs between the 25 and 30 at.% Re compositions (Table 1). This includes the composition ReAls (or Re,Al,,) but not ReAl*. It is not possible to identify the observed powder diffraction pattern with the pattern calculated using the crystal structure data given by Edshammar [8]. However, the pattern is analogous to that observed by Darby et al. [15] for the TcAl, phase. Thus the formula ReAls is assumed. At 1373 K ReAls coexists with RezAl while at 1273 K ReAl, and ReAl are in equ~ib~um . The phase with the highest aluminum content found at temperatures above 1273 K is ReAl, at 20 at.% Re. The very complex powder pattern can be indexed using the unit cell data given by D’Alte da Veiga [ 51, but further X-ray work is necessary to elucidate the crystal structure.

Al

ReA16 ReA13 ReAlI ReAt&

Lo

ReAl at.

Fig. 1. Proposed

60 Re2AI

% Rhenium

80

Re

-

Re-AI phase diagram.

The number and temperature range of the isothermal reactions of the phase diagram of Savitskii et al. [ l] are compatible with all the results of this investigation. A new Re-Al phase diagram based on the present structural information for the phase composition and the isothermal reaction temperatures reported by Savitskii et al. [l] is proposed (Fig. 1). References 1 E. M. Savitskii,

2 3 4 5 6 7 8

9 10 11 12 13 14 15

M. A. Tylkina and K. B. Povarova, Zh. Neorg. Khim., 6 (1961) 1962. W. Obrowski, Naturwissenschaften, 47 (1960) 14. P. I. Kripyakevich and Yu. B. Kuz’ma, Sou. Phys. - Crystallogr., 7 (2) (1962) 240. L. M. D’Alte da Veiga, Philos. Msg., 7 (1962) 1247. L. M. D’AIte da Veiga, Philos. Mug., 8 (1963) 1241. C. Wilkinson, Acta Crystallogr., 22 (1967) 924. R. P. Elliott, Handbook of Alloy Phase Diagrams, McGraw-Hill, New York, 1st Suppl., 1965, p. 53. L.-E. Edshammar, in A, Magngli, L.-E. Edshammar and T. Dagerhamn (eds.), Phase Analysis and Crystal Structure Studies on Binary Alloys of Aluminum with Transition Metals, Final Tech. Rep. 1, 1963, p. 43 (Institute of Inorganic Chemistry, University of Stockholm, Stockholm) (Contract DA-91-591-EUC-2734 (AD 426927)). J. A. Bland, Acta Crystaflogr., I1 (1958) 236. A. Kontio, E. D. Stevens, P. Coppens, R. D. Brown, A. E. Dwight and J. M. Williams, Acta Crystallogr., Sect. B, 36 (1980) 435. N. F. Chaban and Yu. B. Kuz’ma, Izv. Aknd. Nauk S.S.S.R., Neorg. Mater., 8 (6) (1972) 1065. Yu. B. K&ma, V. N. Gurin, I. F. Shilov and V. V. Milyan, izu. Akad. Nczuk S.S.S.R., Met., (8) (1977) 232. A. P. Prevarskii, Yu. B. Kuz’ma and M. S. Zrada, Izv. Akad. Nauk S.S.S.R., Met., (4) (1979) 199. V. V.‘Burnashova, P. K. Starodub, G. R. Struganov and V. N. Doronin, Izu. Akad. Nauk S.S.S.R., Met., (4) (1977) 211. J. B. Darby, Jr., J. W. Downey and L. J. Norton, J. Less-Common Met., 8 (1965) 15.