S T R U C T U R A L S E G R E G A T I O N P H E N O M E N A

IN D E N S E Z I R C O N I U M C A R B I D E

V. N. T u r c h i n , A. B~ E m e l V y a n o v , G~ A. R y m a s h e v s k i i , and A. G. L a n i n

UDC 621.762

The process of sin~ering leads, as was f i rs t pointed out by M. Yu. Balishin, not only to a consolidation of particles but also to their spontaneous segregation into individual zones (or, using the terminology applied by E. F. Vegraan to ore sintering, %locks ~) and then to the segregation of these zones [1, 2]. The reason why such segregated zones form during sintering is that in each of them the particles shrink in an inward direction, as if toward its geometric center. The boundaries of the segregated zones are characterized by lower density because of the movement of part icles in the boundary regions of neighboring zones in opposite directions. The extent to which these defects manifest themselves will vary depending on many factors, such as uneven compact density and nonuniform sintering shrinkage, which in turn are determined by variations in particle size and shape, impurity content, etc. Zonal segregation phenomena have been observed not only in single-phase (copper [1]) but also in multiphase (copper- lead [1], ore sinter [2]) systems.

In the present work metallographic examinations revealed that zonal segregation is characterist ic of zirconium carbide produced by sintering from pressed compacts. It was found that the degree of zonal seg- regation in ZrC may vary within wide l imits, but the extent to which changes in pressing and sintering condi- tions affect this phenomenon was not investigated. However, i t was considered of interest to study the effect of the structural imperfections discovered on mechanical characteris t ics , since structural micro- and macro - defects are known to exer t a strong influence on the strength and ductility of zirconium carbide [3, 4].

Zonal segregation regions in ZrC resemble in shape the grains of a cast material . Their size in var - ious batches and batches produced by different processes is ~ 30 ~m and shows very little variation, which points to a definite regularity of segregation during the sinf~ring of part icles of approximately the same size. The structure of segregated zones does not change significantly as one moves away from the surface toward the center of a blank. The structure of a segregated zone in zirconium carbide may be formed by r e - gions differing sharply in density and hence porosity, althollgh groups of regions of the same density can also be encountered.

The boundaries of the regions can vary in width and character . Usually they a re loosely structured, fair ly porous interlayers of material between denser shrinkage regions. Sometimes single cracks or crack networks are found at the boundaries of the regions. Individual zones or zone boundaries differ not only in size and porosi ty but also in grain size. To determine the effects of zonal segregation on the mechanical propert ies of carbide mater ials , a study was made of the temperature dependence of strength and of the char- acter of plastic deformation in tension for four batches of zirconium carbide produced by sintering rolled {P1 and P2) and pressed (P3 and P4) blanks for 1 h in an argon atmosphere at temperatures of 2400 and 2500~ respectively (Table 1). P2 and P4 materials had a well-developed zonal segregation structure, while in P1 and P3 materials no zonal segreg3tion structure could be detected metallographically (Fig. 1).

The effect of temperas on strength* and elongation was very s~milar for specimens of all the batches, although absolute values of these propert ies showed appreciable differences (Fig. 2). Thus, the materials with the well-developed zonal segregation structure exhibited lower strength over a wide temperature range (1600-2500~ and their maximum strength was observed at 2600~ (as against 2200~ for the materials in which no zonal segregation structure was detected)~ At temperatures above 2600~ the strength propert ies of all the materials were virtually ident2cal. Characteristically, the materials with well-developed zonal segregation structure showed a considerable scat ter of strength values right up to 2600-2700~

* Testing was performed, by the method described in [5], in a URE-402 machine with electron-beam heating.

Podolsko Translated from Poroshkovaya Metallurgiya, No. 1(205), pp. 31-34, January, 1980. Original article submitted August 1, 1978.

24 0038-5735/80/1901-0024507.50 �9 1980 Plenum Publishing Corporation

% 2f!"- ,k,7 e4

, Is N IO | ~ I ~ f \ \ i ~e g

1500 2000 2500 5g00 Temp., ~

Fig. I Fig. 2

Fig. i. lYHcrostructures of PI, P2, P3, and P4 materials: a) unetehed, • b) etched, x 1000. '

Fig. 2. Effect of temperature on strength and ductility exhibited by materials during ten- sile testing at deformation rate of 3 �9 10 -3 1/see.

TABLE 1o

I Mate-i

rial i

Chemical Compositions of Specimens Investigated

zr T~ Free] r % t Method of C C w o N Porosity, manufacture

wt. %

I

Grain size, ~m

P1 Rolling + sintering 89,0 11,0 0,1 0,2 0,08 0,024 14 12 P2* The same 87,2 l l ,0 0,1 2,0 0,2 - - 6 10 P3 Pressing + stntering 88,0 11,1 0,1 0,4 0,12 0,024 7 8 P4* The same 87,7 11,9 0,6 0,3 0,08 0,008 6 12

* Materials having a zonal segregation structure.

The batches of specimens investigated differed very markedly in their duct21ity properties - absolute magnitude of permanent strain and temperature of its appearance. The materials with the well-developed zonal segregat2on structure had a smaller elongation, and their initial plastic deformation temperature was 400-500~ higher than that of the other mater ia ls .

An ex.-unination of the influence exerted by additional factors associated with differences in grain size, porosity, and chemical compositiononthe behavior under load of specimens of the batches investigated revealed that the difference in their mechanical properties was essentially linked with the extent of zonal segregation in their s tructures. Indeed, the materials investigated were characterized by almost identical C:Me ratios, s imilar conoentrations of oxygen, free carbon, kmgsten, and other impurities, and almost the same grain sizes and hardness levels, and differed only slightly in porosity (Table 1). These minor fluctuations in chem- ical composition and structural parameters did not affect the behavior of the specimens investigated during testing. Thus, P2 and P4 mater ia ls , which differed appreciably in their tungsten, oxygen, and free carbon contents, were characterized by a well-developed zonal segregation strucktre and resembled each other in their temperat~tre dependence of strength (Fig. 2). By contrast, P1 and P4 materials , which contained approx- imately the same amounts of. tungsten and oxygen but whose zonal segregation structures were developed to markedly different degrees, differed sharply intheir behavior under load. An analysis of the influence of structural parameters (porosity and grain size) leads to the same conclusion.

Let us consider the probable mechanism by which the zonal segregation structure affects the strength and ductility of zirconium carbide. The network of defects located at the boundaries of zonal segregation regions constitutes an assembIy of stress ra i sers , which lower the level of s t reng~ of these materials in the brittle sta~e. For the same reasons, the ductility and maximum strength linked with stress relaxation and redistribution manifest themselves in the materials investigated at higher temperatures. Raising the tem- perature above 2600~ changes the character of their fracture; loss of strength is observed, caused by the processes of intragramtlar slip, sliding on grain boundaries, and formation and coalescence of intergranular

25

Fig. 3. Character of deformation and high-temperature fracture of zircon- ium carbide with well-developed zonal segregation s t r u ~ r e during tensile testing at 2600~ Magnification: a) • 5; b) • 1~0; c) • d) x 300.

Fig. 4. Fracture surfaces of P1 material without zonal segregation (a) and P2 material with well-developed zonal segregation structure (b). Magnification: a) • 5000; b) x 10,000. The structure observed is charac- ter is t ic of a discontinuity surface in zirconium carbide.

cracks. Because of the presence of the crack network and pore concentration at the segreg~ion zone bound- aries, sliding and plastic flow along these boundaries proceed more vigorously, as a result of which local- ized shrinkage zones in the specimen are shi_~d and turned (Fig. 3), intragranular slip is inhibited, and zones of high-temperature fracture appear and grow at these boundaries. Final fracture is brought about by crack coalescence. The fracture has a cellular structttre, the cell size being commensurable with the segregation zone size.

An electron-fractegraphic analysis of specimens revealed the presence of structural formations charac- ter is t ic of the free surface of internal voids, which occupied a large part of the fracture surface (Fig. 4). Evidently, the crack propagated partly over existing crevicelike discontinuities in the material - boundaries of zonal segregation regions. The observed character of deformation and fracture of the materials with the well-developed zonal segregation structure was linked with their lower strength and duc~lity and increased tendency toward brittle rupture.

C O N C L U S I O N S

Experiments have revealed the existence of a zonal segregation structure in pressed and siutered zir- conium carbide specimens. Zonal segregation has a pronounced effect on absolute values of strength and duc- t i l i ty and their scatter as well as on the character of fracture over a wide range of temperature.

The authors wish to thank R. A. Andrievskii and E. F. Vegman for their helpful discussion of the exper- imental results .

o

2,

LITERATURE CITED

M. Yu. Bal'shin, in: Scientific Principles of Powder and Fiber Metallurgy [in Russian], Metallurgiya, Moscow (1972), p. 304, 330, E. F. Yegman, Theory and Practice of the Ore Sintering Process [in Russian], Metallurgiya, Moscow (1974), p. 121,

26

o

4.

5o

A. G. Lanin, M. A. Fed, toy, and V. V. Glagolev, in: Hochtemperaturwerkstoffe, Plansee Seminar, VoL 6, Spri~ger-Verlag, Vienna (1969), p. 105. A. G. Lanin, M. A. Fed, toy, and Vo V. Glagolev, ~Effect of surface condition on the strength of cermet specimens, n Poroshk. Metall., No. 5, 97-101 (1968). A. B. Emel'yanov, A. G. Lanin, and A. N. Kolesnichenko, in: Methods and Apparatus for the Investigation of Materials of Structures Operating under Conditions of Irradiation [in Russian], u 1, Atomizdat, Moscow (1973), pp. 47-51.

E F F E C T OF P A L L A D I U M ON THE P R O C E S S OF

D I F F U S I O N A L A L L O Y F O R M A T I O N AND

M I C R O S T R U C T U R E D URING THE S I N T E R I N G OF

T U N G S T E N - R H E N I U M P O W D E R S W I T H

W E L L - D E V E L O P E D S U R F A C E S

L . I . S h n a i d e r m a n a n d V. Vo S k o r o k h o d UDC 621.762:669.018.4:669.2-8

To lower the temperature at which alloys of refractory metals can be produced by sintering, it is nec- essary to en,mre that the metallic reactants are in a state in which their atoms exhibit high diffusional activ- ity. Additions of some Group VIII metals activate the processes of diffusional alloy formation during the sin- tering of tu~.~sten and molybdenum with other refractory metals [1, 2]. In an investigation of the tungsten- rhenium system the addition of 0.5-1% palladium to mechanical mixtures of tungsten and rhenium powders was found to lower by 600~ the initial alloy formation temperature during the sintering of compacts from these mixtures. A further intensification of the alloy formation process could probably be achieved by adding activators to powders exhibiting increased sintering activity.

In the !present work an investigation was carried out into the influence exerted by palladium on the pro- cess of diffusional alloy formation and microstructure during the sintering of tungsten-rhenium powders with well-developed surfaces. As starting mater ia ls , tungsten trioxide prepared by calcining ammonium para- tungstate at 500~ and ammonium perrhenate were used. Alloy formation was activated with palladium [1, 2], added in the form of a solution of palladium chloride, which was subsequently reduced in hydrogen. Mixtures were prepared so that the resultant alloys contained 5, 10, and 20% rhenium and 1% palladium. In the preparation o f tungsten powders by the reduction of tungsten trioxide in hydrogen, heating was performed for 4 h at a temperature of 500~ and 1.5 h at a temperature of 800~ [3]. Rhenium-containig tungsten powders were prepared by coreduction with hydrogen in the following stages: 2 h at 250~ 2 h at 500~ and 1~5 h at 800~ The specific surfaces of the powders were measured by the thermal gas desorption method in a gas-chromuLographic apparatus.

X-ray diffraction analysis was carr ied out, using Cu K a radiation , in a DRON-0.5 apparatus. Alloy formation was followed by observing changes in the profile of the (321) line. Metallographic examinations were made with a Neophot microscope. Electrolytic etching of specimens in 2% NaOH solution was employed.

The rhenium-containing powders had a larger surface and exhibited better sinterability than the rhenium- free powders (Table I). X-ray diffraction studies of the process of diffusionai alloy formation in the reduced powders were hindered by the fact that the powders had distorted crystal lattices. The density of dislocations in the tungsten powders was, according to the results of a substruc~_val study, 1.6.10 il 1/cm 2, and this was reflected in the profile of the x- ray line [the (321) line in Fig. 1 is diffuse] owing to the presence of micro- distortions. The microdistortions were annealed out only at a temperature of 1200~ In order to determine the effect of palladium on the process of alloy formation at 800~ it was necessary to study the influence ex- erted by it on the recovery process. It was found that tungsten powders prepared by low-temperature reduc- tion in the presence of palladium were practically free from microst resses , i .e., their recovery process had

Institlrte of Materials Science, Academy of Sciences of the Ukrainian SSR. Translated from Porosh- kovaya Met:~Ilurgiya~ No. 1(205), pp. 36-40, January, 1980. Original article submitted April 19, 1979.

0038-5735/80/1901-0027507.50 �9 1980 Plenum Publishing Corporation 27