Ordered AB and AB3 phases in T6-T9 alloy systems and a modified Mo-lr phase diagram

SHORTCOM~~UNICATXO~S

147

Thanks are due to Messrs H. A. C. M. BRUNING and A. I. LUTEIJN for their assistance. J. H. N. VANVUCHT

Phili@ Research Laboratories, N.V. Philips’ Gloeilampenfabrieken, Eindhoven (The Netherlands)

I J.H.N.vAN VUCHTAND K.H.J.BuscHow,J. Less-CommonMetaZs,~o (Ig66)g8-107. 2 V. I?. NOVY, R.C. VXKERVAND E.V. KLEBEK, Trans.AIME. zzr(Ig6I) 580-584. 3 V.F. Novu,R. C. VICKERYANDE.V.KLEBER,T~U+ZS. AIME, 221(Ig6I) 585-587. 4 V. F. NOVY, R.C. VICKERYANDE.~. KLEBER,~'YWLS. AIME, zzr(Ig6r) 588-590. 5 D. T. CROMER AND C. E. OLSEN, Acta C~yst., rz (1959) 689694. Received June r7th, 1965 J. i%SS-COmW&OlZkf&Uk, IO (1965) 146-147

Ordered AB and ABa phases in Te-Tg alloy systems and a modified Mo-lr phase diagram The Te-Tg phase diagrams Mo-Rhl, Mo-Ir2, W-Rh3, and W-Ir3 have as a common feature an intermediate, disordered, close-packed hexagonal E phase with a broad stoichiometry range of 20-40 at. %. These phases are commonly regarded as equivalent in a crystal-chemical sense to Ru, OS, and the limited solid solutions of MO, Rh, and Ir in Ru and OS, because they occur at the same average group number (agn.) as Ru or OS, have similar subnormal axial ratios, e.g., cja=1.6o2 for Moo_so Iro.so (F phase) as compared to cla=x.584 for Ru, and have similar solidus points. No ordered, c.p. phases had been reported in the Te-Te systems with MO, WZ Rh, and Ir. However, in the Mo-Pt system a disordered E phase and ordered phases &‘-MoPt (Do19)435, e”-MoPt (&‘19)~,~ exist. We have re-examined the Te-TV systems to establish whether ordering of the E phase exists in the concentration range AB-AB3. Parts of our crystallographic data have been listed in our abstracts. Experimental procedure and results Experimental procedures of arc-melting, heat-treatments, metallographic and X-ray powder-diffraction techniques (with 114.6 mm camera and GE-XRD5 diffractometer) have been described by RXTTER et aE.7. Alloys with Mo+~o at.% Rh; Mof47,jo,62.5,7o,and75at.%Ir;W~5oand75at.%Rh;andW+$oand75at.o/, Ir were made from metals of greater than 99.9% purity. Macroscopic~ly homogeneous specimens were homogenized at 16oo’C for 24 h to eliminate coring and were then annealed to permit ordering. Except for MoRh, all AB and AB3 phases were fully ordered after 24 h at 1200°C; MoRh was then annealed for 36 h at 95o”C, after which it, too, was ordered. Additional heat treatments were carried out with MoIr to determine approximately the order-disorder transition temperature for this alloy. The specimens were then crushed to powder and stress relieved at the original heattreating temperatures (IO min at 16oo’C, I h at 1200°C, or 3 h at 950°C) before X-ray examination. The error limits are + I at.% for compositions and < 5 x 10-4 for lattice parameters. The occurrence of ordered structures was established for MoRh, MoIr, WIr, Morn, and WRh3, as summarized in Table I. J. Less-Common

Metals,

10 (x905) x47-150

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148 TABLE NEW

I AB

ORDERED

AND

AB3

PHASES

IN

Alloy

Structure type

aa

MoRh MoIr WIr

B 1g-M&d B1g-M&d Brg-M&d

2.752 2.760

MoRha** MoIra WRh3

DowMgCda DowMgCdz DowMgCd3

::%*** 5.453

WIr3**

DowMgCd3

Mo+62.5 at.% lr Mo+7o at. o/0 Ir

Do18+trace Do18

Ts-Ts

2.745

B19

ALLOY

SYSTEMS

Ordering temp. TO(T) -._

bia 1.607 1.609 1.613

g50-1200 1570-1650 >I200

4.350*** 4.385 4.350

1.595*** 1.598 1.596

> 1600 > 1200

5.496

4.390

1.597

-

5.5oSt

4.401 t

1.598

> 1200

5.499

4.389

1.596

> 1200

4.785 4.804 4.811

4.413 4.429 4,452

* 2 c/a for DOIS phases. ** 0018 type assumed by analogy, lattice parameter a~ = 2 *** Calculated from data of ANDERSON AND HUME-ROTHERY~ t Lattice constants correspond approximately to MO + 66

1.743 1.746 1.743

. a,. for MO + at. y0 Ir.

73.9 at. y0 Rh.

The ordered AB phases MoRh, MoIr, and WIr have an orthorhombic structure c of the Brg-M&d type, space group Dih-Pmcm, 4 atoms per unit cell with A in z(e): + (0, ye, a) ; B in z(f) : f (i, yf, t), with yew Q and ypw4. Good agreement between calculated and observed line positions and intensities was found. The two variable parameters were not refined further; however, the true values do not seem to be appreciably different from the ideal positions assumed above. The powder pattern is not reproduced here; for an indexed pattern see the literature7. For MoRh and WIr only the orthorhombic line splitting could be observed, owing to the similar scattering powers of the constituents, and the B19 type was assumed by analogy. The alloy at W+~O at.% Rh was two-phase W solid solution + h.c.p. E, in agreement with the phase diagrams; the coexisting .Sphase is too far from the ideal composition WRh to allow ordering, at least at IZOO’C. The ordered AB3 phases MoIr3 and WRh3 are of the DolQ-MgCd3 type, space group D,4,-P6+nmc, 8 atoms per unit cell, with A in z(c) : f (4, $, 4) ; and B in 6(h) : +(zi!, 2, $; x, 2, 4; x, zx, +), with xm -4. Calculated line positions and intensities assuming the ideal parameter x~ - 4 a g reed with those observed. For MoRh3 and WIr3 the similar scattering factors precluded the observation of ordering; however, the occurrence of both phases appears likely by analogy, and the lattice parameters of WIr3 as determined here, and of MoRh3, as calculated from ANDERSON AND HUMEROTHERY~, are included in Table I. To decide whether the new ordered phases have appreciable homogeneity ranges the system Mo-Ir was checked more closely. The results are incorporated in Fig. I as tentative modifications of the published phase diagram2. The sample MO + 62.5 at.0/b Ir consisted of a-MoIr3 and some MoIr, placing the maximum solubility of MoIrs for MO at about 66+ z at.% Ir at ~zoo”C. The invariance of the lattice parameters of MoIr in the samples Mo+47 at.% Ir and Mo+62.5 at.% Ir proves a small homogeneity range of MoIr of 50 Ifi 2 at.% Ir at most, in contrast to MoIr3. Assuming J. Less-Common

Metals, IO (1965) 147-150

SHORT COMMUNICSTIONS

0 MO

IO

20 ATOMIC

30

I49

40 PERCENT

50

60

70

IRlDtLlM

Fig. I, Tentative Mo--1r constitution diagram. Solid and dotted lines, diagram proposed by MIcHhtaK AND BROPHY~; dashed lines, proposed modifications.

two eutectoid reactions, E= MoaIr + MoIr and F= MoIr +n-MoIra, the proposed diagram (Fig. I) was constructed; however, other possibilities exist, e.g., a peritectoid reaction E+a-MoIr3 = MoIr, followed by E= MoaIr + MoIr. This possibility is indicated by the existence in the Mo-Pt system of the semi-ordered phase e’(Doi9 type with random substitution of MO for Pt in 5 4 of the Pt sites) in a temperature range intermediate between e”(Bx9) and 443). Mo I ra was assumed to be stable up to the solidus temperature. Discussion

The ordered phases at AB and AB3 follow the principle of forming a maximum of unlike first nearest neighbors, i.e., 8 out of 12 for the B19 type and 12 for the MgCds type. This leads to the orthorhombic distortion for AB with #/a = x.743-1.746 (>)/3) and cla=I.603-1.607(<~~). The b/ a ratios put MoRh, MoIr, and WIr into the category of Br9 phases, Ts-Tro: NbPt*, and Ts-Tro: e”-MoPt*B5, where b/a= 1.75-x.80 (>1/3). This contrasts with IF19 phases such as AuCd, M&d, and (Nb0.85Rho.15) Rh7, where b/a<1/3, a distortion leading over the Bz-CsCl structure, for which bla=1/2 (in this setting). For these latter Brg phases, typical deviations of the free y parameters from ideal values are observed7 that put the atoms into a more &z-like configuration. This is not observed for the present Br9 phases which must, therefore, represent c.p. ordered phases rather than transitions between A3 and B2. On the other hand, the c/a ratios of the new B19 phases with Ts-Tg are considerably lower than those with Ts-Tio quoted above, which are all >l/$. This low axial ratio, which is due to the similarity of the disordered E parent phase to Ku or OS, may be the reason for the relatively low disordering temperatures of these B19 phases, because it results in rather short distances (&-x0 = 2.73 A in MoIr, assuming ~1~” 4) between nearest MO neighbors belonging to different close-packed layers; this might be unfavorable microelastically. In the [IOO] direction MO or W chains with greatly reduced interatomic distances (&,-M~ = 2.75 A in MoIr) are formed, as compared to the disordered E j. Less-Cornsnon Met&,

IO (1965) 147--150

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150

phases. In this context, it is interesting to note that the superconducting transition temperature of Moa.&ro.so changes on passing from disordered E to ordered MoIrg. Acknowledgements

The authors gratefully acknowledge financial support of this research under ARPA Contract SD-90 and a donation of noble metals by the International Nickel Company. B. C. GIESSEN U. JAEHNIGEN N. J. GRANT

Metallurgy Department, Massachusetts Institute of Technology, Cambridge, Mass. (U.S.A.)

I E.

ANDERSON AND W. HUME-ROTHERY,

I. Less-Common

Metals, z (1960) 19,

J. MICHALEK AND J. H. BROPHY, Trans. AIME, 227 (1963) 104j _ 3 E. J. RAPPERPORT AND M. J. SMITH, ASD Tech. Rept. 60-132, 1962, p. 8. 4 A. RAMAN AND K. SCHUBERT, Z. Met&k., 55 (1964) 619. 5 B. C. GIESSEN, L. DARDI AND N. J. GRANT, unpublished work. 2 S.

6 B. C. GIESSEN AND N. J. GRANT, Acta Cryst., 18 (1965) 1080. 7 D. L. RITTER, B. C. GIESSEN AND N. J. GRANT, Trans. AIME, 230 (1964) 1250 and 1259. 8 B. C. GIESSEN AND N. J. GRANT, Acta Cryst., 17 (1964) 615. g V. SADAGOPAN, E. R. POLLARD, B. C. GIESSEN AND H. C. GATOS, Appl. Phys. Letters, 7 (1965) 73.

Received July z?th, 1965 J.

LeSS-COWnON

Metals, 10 (1965) 147-150

The metallographic

preparation

of some rare-earth

metals

Gadolinium, erbium, holmium and dysprosium, all rare-earth metallic elements of the lanthanide series, have in recent years attracted the attention of the research scientist and as a result of this interest a need has arisen to prepare reliable microstructures for evaluation. This has presented a challenge to the metallographer, since little information is available in the published literature on their preparation or etching, and furthermore most of the material obtained from commercial sources contains large amounts of oxide which tend to cause difficulties during preparation. The metallographic preparation of gadolinium is complicated by the fact that the substance is rather reactive in water and extremely soft. Previous work on the metallography of the rare-earth metals has been summarized by LOVE~. Preparation is begun by mounting the specimen in epoxy resin and grinding in turn on 180, 400, and 600 grit silicon carbide abrasive papers using kerosene as a lubricant. After a thorough cleansing with acetone, the specimen is coarse polished on Buehler Metcloth with 9 ,u diamond paste, long enough to completely remove the 600 grit silicon carbide grinding marks. Again the specimen is carefully washed with acetone and the procedure repeated using 3~ diamond paste on Buehler Metcloth. The specimen is now polished with 3 p diamond paste on Buehler Microcloth, using a rotary motion, until the specimen surface is free of any deep scratches caused by the previous polishing steps. If upon examination the sample shows only a small amount of oxide inclusions, it can be chemically polished in a solution containing : 20 ml lactic acid, 5 ml phosphoric acid, IO ml acetic acid, 15 ml nitric acid, I ml sulfuric acid. J. Less-Common

Metals, IO (1965) 150-152