E L E C T R O P H Y S I C A L P R O P E R T I E S O F

T I C - - M o , A N D T i C - W C E R M E T S

G. V. S a m s o n o v , I . V . B o g o m o l , S. N. L ' v o v , a n d M. I . L e s n a y a

T i C - N b , T i C - T a ,

UDC 669.018.4:537.311.621.762.4

The e lec t rophys ica l p r o p e r t i e s of complex ca rb ides fo rmed by individual fcc NaCI type lat t ice c a r - bides exhibiting continuous mixing have a l ready been invest igated quite adequately [1], but lit t le is known of the e l ec t rophys ica l p r o p e r t i e s of c e r m e t s composed of ca rb ides of r e f r a c t o r y meta l s cemented with such me ta l s . C e r m e t s of this type constitute a c lass of m a t e r i a l s which may well find p rac t i ca l applicat ion in h igh - t empe ra t u r e s e r v i c e , in the p re sen t work, a study was made of the effect of t e m p e r a t u r e , in the range 20-1100~ on the e l ec t r i ca l r e s i s t i v i t y p and t h e r m o - e m f o~ of T i C - N b , T iC- -Ta , T i C - M o , and T I C - W c e r m e t s with var ious cement ing meta l contents, and the i r Hall coeff icients R were m e a s u r e d at r o o m t e m p e r a t u r e .

Ce rme t spec imens w e r e p r e p a r e d by the method of hot p r e s s i n g a t t e m p e r a t u r e s of 2000-2500~ and a p r e s s u r e of 300 k g / c m 2, the holding per iod being 10-15 rain. The i r r es idua l poros i ty was 2-7%. The resu l t an t a l loys w e r e subjected to ' chemica l and x - r a y diffract ion ana lyses (it is impor tan t that chemica l ana lys i s r evea l ed no f r ee carbon) . The r e su l t s of x - r a y diffract ion analys is (see Table 1) show that a l loys of the s y s t e m s inves t igated have a two-phase s t ruc tu re . One of the phases is an (Me I, MeII)c solid solution having an NaC1 type s t ruc tu re and the o ther is Me II (where Me I is Ti and Me II is Nb, Ta, Mo, o r W). F o r b rev i ty , the two phases will h e r e a f t e r be cal led phases 1 and 2, r e spec t ive ly . The method of p repa ra t ion of spec imens for m e a s u r e m e n t s was s i m i l a r to that desc r ibed in [2-4] . F o r each spec imen, p and o~ were de t e rmined in a single exper iment , a s desc r ibed in [5]. Hall coeff icient m e a s u r e m e n t s w e r e pe r fo rmed , using a d i r ec t cur ren t , in a constant magnet ic field of 15-kOe intensi ty. Cor rec t ions for spec imen poros i ty we re ca lcula ted with Ode levsk i i ' s wel l-known fo rmula for p and with the fo rmula R = Rpo r (1 - II), where II is the poros i ty e x p r e s s e d as a f rac t ion of unity, for R.

F igure 1 i l lu s t r a t e s g raph ica l ly the effect of composi t ion on p, R, and a at r o o m t e m p e r a t u r e a n d a l s o on the t e m p e r a t u r e coeff icient of r e s i s t i v i t y (TCR): ~ ' = 1 / p d p / d t . The curves in Figs . 2 and 3 show the e f fec t of t e m p e r a t u r e on p and a , f r o m which it follows that, at 20-1100~ the values of these p a r a m e t e r s for the c e r m e t s inves t iga ted change approx imate ly l inear ly with r i s e in t e m p e r a t u r e . This, as well as the i r r e l a t ive ly smal l values of R and a , is an indication that the i r conductivity has a meta l l i c c h a r a c t e r . The p vs t function for the c e r m e t s under considerat ion, like that for many meta l s and s imple ca rb ides , can be approx imate ly desc r ibed by the usual fo rmula p = P0 (1 + o~'t). S imi lar ly , the fo rmula ol = o~0(1 + fit) d e s c r i b e s the ~ vs t re la t ionsh ip .

The d i a g r a m s in Fig. 1, i l lus t ra t ing the effect of composi t ion on the e lec t rophys ica l p r o p e r t i e s of the c e r m e t s , exhibit m a x i m a of p cor responding to the composi t ions 50 TIC-50 Nb, 50 TIC-50 Ta, 75 TIC-25 Mo, and 75 TIC-25 W (at. and mole%). It is c h a r a c t e r i s t i c of al l the c e r m e t s invest igated that the i r R changes sign, f r o m negat ive to posi t ive, with r i s e in Me II content. Apar t f r o m this , the m a x i m u m values of p fo r these a l loys cor repond to the m a x i m u m posi t ive values of R, ex t r eme values of ~, and the min imum TCR ~ ' . The change in the sign of the Hall coefficient in the alloy s y s t e m s inves t iga ted is p r e s u m a b l y an indication tha t the i r conductivity is of a dual, e l ec t ron-ho le type, the predominance of one type of conduc- t ivi ty being rep laced by that of the o ther with change in the propor t ions of the al loy components . F o r this r ea son , it is imposs ib le to uti l ize the values of R obtained for de te rmin ing c a r r i e r concent ra t ions . The

Insti tute of Mate r ia l s Science, Academy of Sciences of the Ukrainian SSR. Kherson Pedagogica l Inst i tute . T rans l a t ed f r o m Poroshkovaya Metal lurgiya, No. 10 (118), pp. 62-67, 1972. Original a r t i c le submi t ted November 16, 1971.

�9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. /t copy of this article is available from the publisher for $15.00.

824

TABLE I. Ta, T i C - M o , and T i C - W Cermets

Ailoy'"comp:, ' ....... mole and at.%

Pha~e compodtion 1 TiC I MeII *

10O 75 25

5O 50 25 75

100

75 25 50 50 25 75

100

75 25 5O 50 25 75

I00

75 25 50 5O 25 75

100

Phase Compositions of TiC-Nb, TiC-

System TiC--Nb TiC (Ti, Nb)C s. soln.'~ +

Nb (tr.) (Ti, Nb)C s. sob1. + Nb The same Nb

System T I C - - T a

(Ti, Ta)C s. soln.+Ta(tr.) (Ti,Ta)C s. soln. + Ta The same

n H

System TiC -- Mo

(Ti, Mo)C s. soln. + Mo The same

w

Mo

System T i C - W (J:i, W)C s. soin, + W The same

it

w

Solid solu- tion lat- flee COoU- stant, A

4,380 4,371

4,398 4,395 3,290

4,350 4~366 4,365 3,313

4,286 4.285 4,288 3,150

4,300 4,293 4,298 3,144

*Me II is Nb, Ta, Mo, and W, respect ively . tAl l the solid solutions have NaC1 type s t ruc- tures .

same problem was encountered in studies of defective binary monocarbides of Groups IV-V metals [6, 7] and oxycarbide and oxynitride alloys of titanium [8], in which the difficulty was noted of employing simple zonal considerat ions for the interpreta t ion of the kinetic proper t ies of these mate r ia l s . In the ease of the ce rme t s under investigation, the difficulty is fur ther aggravated by the fact that the i r s t ructure is not a s ingle-phase one.

On passing f rom one composit ion to another, the p roper t i es of the ce rmets change, which is evidently due on the one hand to the change in the relat ive proport ions of the phases present ha them and on the othhr hand to a change in the p roper t i e s of the phases themselves , o r at least one of them. Varying the amount of phase 1, i .e. , the metal, ha the alloys under investigation c lear ly has no effect on its proper t ies , but the proper t ies of phase 2, i.e., an (Me I, MeII)c solid solution, can change appreciably depending on the relat ive amounts of the metals and carbon, which may vary f r o m alloy to alloy. It is quite natural that ra is ing the amount of Me II in an alloy increases its concentrat ion in the solid solution; as a result , the defectiveness of the i a t t e r ' s carbon sublattice grows, because the solution is a substitutional solution with r e spec t to the metal .

F r o m the theory of overal l conductivity [9], it follows that with r i se in the relat ive amount of a tow- res i s tance phase (ha our case, phase 2, i.e., MeII) in a s ta t is t ical mixture of any invariable phases, the e lec t r ica l res i s t iv i ty of the mixture may be expected to increase . This, however, is not observed with the ce rmets under examination. On the cont rary , the res i s t iv i ty at f i r s t substantially grows, which can, of course , be at tr ibuted only to an increase in the e lec t r ica l res is t iv i ty of phase 1 (solid solution) itself, the increase being so pronounced that it more than makes up for the fall in res i s t iv i ty of the alloys due to the r i se ha the amount of phase 2 ( low-res is tance metal) . The increase in the e lec t r ica l res is t iv i ty of phase 1 - a highly complex solid solution - may be linked in the f i r s t place with a growth in its res idual part , because with r i se in the MeII content of an alloy the solution becomes more and more saturated with ex- traneous M II atoms, and under these conditions its lattice becomes p rogress ive ly more deficient in carbon.

825

,ooy \ ,oo ' \ ,oo �9 OJ . . . . v Ol ~ '~" O} , . , ~ 0

O ~ .-v,,~ . O " - c = ~ - O L'~r . . 0 R " I0"4. 16 ]R k

om'/C . -8 / \ - 8 8 O ~ ~ 7 ~ 'Y 0 ; ' ' ~ 0

-8' c -8 -8' -8

b

/b c

0 6 0 0 ~ d -8 -~ . , . - 8 . . . . - 8

. . . . ! l

1"/[ 25 5075/Vb i;C 25 50 75 Tt/ T/C 2 5 50 75 No ~'r 25 5015 W at.%Nb at.%Ta at.%Mo at.%W

Fig. 1. Effect of composi t ion on e l ec t r i ca l r e - s i s t iv i ty (a), t e m p e r a t u r e coefficient of r e s i s - t ivi ty (b), Hall coefficient (c), and t h e r m o - e m f (d) of T i C - N b , T iC- -Ta , T i C - M o , and T i C - W e e r m e t s .

j~, /~ fl-cm yOT/C-~50,Vb - 50TIC-SuTa

200

iO0 ~ T i C - O U R

25~C 7 5 T a 2 5 ~

5o

@ f

Fig. 2.

pV/deg C

o

-4

.8

-12

"20 . !

0 ~00 ~00 & ebO 7000 t 'C ]

Fig. 2 Fig . 3

Effect of t e m p e r a t u r e on e l ec t r i c a l r e s i s t i v i t y of T i C - N b , T i C - T a , T i C - M o , and T i C - W c e r m e t s .

F ig . 3. Effect of t e m p e r a t u r e on t h e r m o - e m f of c e r m e t s : 1) 75 T I C - 2 5 W; 2) 75 TIC--25 Mo; 3) 75 T I C - 2 5 Ta; 4) 25 T I C - 7 5 Mo; 5) 50 T I C - 5 0 Mo; 6) 25 T I C - 7 5 Ta; 7) 75 TIC--25 Nb; 8) 25 T I C - 7 5 W; 9) 50 T I C - 5 0 W; 10) 50 T i C - 50 Ta; 11) 50 T I C - 5 0 Nb; 12) 25 TIC--75 Nb.

Both these f ac to r s produce , in quali tat ive a cco rd with Nordheim' s theory [10], addit ional sca t t e r ing of c u r - ren t c a r r i e r s and an i n c r e a s e in r e s i s t i v i t y . It is a l so poss ib le that the concentra t ion of Me II in the so lu- tion and the carbon def ic iency in the l a t t e r ' s la t t ice affect the concentra t ion of cu r r en t c a r r i e r s , the reby influencing the r e s i s t i v i t y of phase 1. A growth in the res idua l r e s i s t i v i t y of al loys is c l ea r ly shown in Fig . 2, where the p vs t cu rves for 50 T I C - 5 0 Nb (Ta) and 75 T I C - 2 5 Mo (W) c e r m e t s lie well above the o ther curves and, in the i r l inear pa r t s , have the leas t s lope. Some red i s t r ibu t ion of cu r r en t c a r r i e r s in the p r o c e s s of e l ec t r i ca l r e s i s t i v i t y inc rease is indicated by the change in the magnitude and even sign of the Hall coef f ic ien t . The change unquestionably tes t i f ies to a re la t ive d e c r e a s e in n - type conductivity and an i nc r ea se in p - type conductivity. In view of the complexi ty of zonal s t r u c t u r e which apparen t ly c h a r a c - t e r i z e s an (MeI, MeII )c sol id solution, i t is difficult to draw" any conclusions concerning the red i s t r ibu t ion of e l ec t rons among the zones with r i s e in Me II content. However, in the light of the configurat ional model

826

of matter [ii], the change in the sign of R may be attributed to a fall in conduction electron concentration brought about, during the formation of an (Me I, MeII)c solid solution, by a redistribution of unlocalized electrons most likely to give rise to the formation of the energetically stable d 5 configurations. On the same basis [ii], it may be asserted that the rising branches of the electrical resistivity curves for these alloys do in fact correspond to an increase in the degree of defectiveness of the carbon sublattices of (Me I, MeII)c solid solutions. The reason for this increase is that, owing to the formation of TiC-Nb (Ta, Me, W) bonds, which are stronger than the Ti--Ti bonds (because the degree of localization of valence electrons increases in the order Ti, Nb, Ta, and W [ii]), the proportion of unlocalized electrons falls. Under these conditions, stabilization of the sp 3 configurations of carbon atoms becomes less marked than that in TiC and there is a corresponding weakening of the predominantly covalent Ti--C bond [II], which is respon- sible for local displacement of carbon atoms, with the formation of defects in the carbon sublattice.

Unfortunately, full data upon the solubility of metals in carbides are not yet available. However, the results obtained for the growth of electrical resistivity in the alloys investigated provide an indirect basis for the assumption that the metals added to these alloys (Nb, Ta, Mo, and W) differ in their solubilities. Admittedly, it cannot be claimed with confidence that the Me K concentrations in the alloys at which maxima of electrical resistivity have been shown in the diagrams (Fig. i) are the optimum concentrations ensuring the highest electrical resistivity in the corresponding solid solutions. We cannot rule out the possibility that the points at which the maximum values of p are in fact attained are slightly displaced to the left or right. Their positions could, of course, be determined more accurately by investigating systems of alloys with finer gradations of composition. Nevertheless, analysis of the configurational model [ii] enables us to conclude that, owing to the large difference in the degrees of localization of valence electrons in Ti-Mo and Ti-W alloys on the one hand and Ti- Bib and Ti--Ta alloys on the other, the solubility of Mo and W in TiC will be less than that of Nb and Ta. In view of this, the optimum concentrations beyond which the prop- erties of the eermets begin to approach those of the pure metals Me II would be expected to be less with Mo and W additions than with Nb and Ta additions. This is apparently borne out by the property vs com- position diagrams in Fig~ i.

The monotonic decrease in the electrical resistivity of the alloys from their maxima to the values characteristic of pure Me II is qualitatively in accord with the theory of overall conductivity of two-phase systems. Clearly, when some limiting concentration of Me II in these alloys is reached, the properties of their high-resistance phase 1 become stabilized, and for this reason any further increase in the amount of the low-resistal]ce phase 2 causes the resistivity of the alloys to approach monotonically the resis- tivity of this phase. The same apparently applies to the other properties investigated (R, ~, and o~').

Characteristically, alloys with the maximum values of p exhibit the minimum values of TCR. Their behavior in this respect is analogous to that of the majority of metallic alloys [12].

CONCLUSIONS

I. A study was made of the effect of temperature, in the range 20-II00~ on the electrical resis- tivity and thermo-emf of TiC-Nb, TiC--Ta, TiC-Mo, and TiC-W cermets with metal-to-carbide ratios of 25 �9 75, 50 : 50, and 75 : 25 at. and mole %, and their Hall constants at 20~ were determined.

2. It was established that property vs composition curves for these materials exhibit extreme points at 50 at.%. Nb and Ta and 25 at.% Mo and W. The plots of p vs t and o~ vs t obtained for these cermets were found to be straight lines, demonstrating that their conductivity has a metallic character. The elec- trical resistivity of the TiC-Nb and TiC-Ta cermet s i nvestigat ed is seven to fourteen times that of the starting metals (16 for Nb and 14.7 p~-cm for Ta) and two to four times that of titanium carbide (53 p~q-em). The electrical resistivity of cermets of the systems TiC-Mo and TiC-W is one order higher than that of the starting metals and up to three times higher than that of titanium carbide, the only exceptions being 25 TIC-75 Me II cermets (where Me II is Mo or W), whose electrical resistivity is slightly less than that of titanium carbide.

I.

2.

LITERATURE CITED

E. N. Denbnovetskaya and S. N. L'vov, in: Refractory Carbides [in Russian] (ed. G. V. Samsonov), Naukova Dumka, Dumka, Kiev (1970), p. 163.

S. N. L'vov and V. F. Nemchenko, Informpis'mo IMMS AN USSR, No. 272, Kick- (1960).

827

3. S.N. L'vov, V. F. Nemchenko, and V. I. Marchenko, Pribory i Tekh. ~ksperim., No. 2, 159 (1961). 4. S.N. L'vov, V. F. Nemchenko, aJld G. V. Samsonov, Poroshkovaya Met., No. 4, 3 (1962). 5. S.N. L'vov, P. I. Mal'ko, and V. F. Nemchenko, Poroshkovaya Met., No. 9, 89 (1966). 6. O.A. Golikova et al., Fiz. Tverd. Tela, No. 7, 12 (1965). 7. A.I. Avgustinik et al., Izv. Akad. Nauk SSSR, Neorgan. Mat., No. 3, 286 (1967). 8. V.S. Neshpor, G. M. Klimashin, and V. P. Ni|dtin, in: Refractory Carbides [in Russian] (ed. G. V.

Samsonov), Naukova Dumka, Kiev (1970), p. 169. 9. V.I. Odelevskii, Zh. Tekh. Fiz., 2__1, 667 (1951).

i0. F. Seitz, Modern Theory of the Solid State [Russian translation], IL, Leningrad (1949), p. 571. ii. G.V. Samsonov, I. F. Pryadko, and L. F. Pryadko, Configurational Model of Matter [in Russian],

Naukova Dumka, Kiev (1971). 12. B. Chalmers, Physical Metallurgy [Russian translation], Metallurgizdat, Moscow (1963), p. 102.

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