Neutron powder diffraction studies of transition metal hemicarbides M2C1−x—I. Motivation for a study on W2C and Mo2C and experimental background for an in situ investigation at elevated temperature

Acta metall. Vol. 36, No. 8, pp. 1891-1901, 1988 Printed in Great Britain. All rights reserved

0001-6160/88 $3.00+0.00 Copyright @. 1988 Pergamon Press plc

OVERVIEW NO. 69 NEUTRON POWDER DIFFRACTION STUDIES OF TRANSITION METAL HEMICARBIDES M 2CJ_x-I. MOTIVATION FOR A STUDY ON W2C AND Mo2C AND EXPERIMENTAL BACKGROUND FOR AN IN SITU INVESTIGATION AT ELEVATED TEMPERATURE J. D U B O I S , T. E P I C I E R , C. E S N O U F and G. F A N T O Z Z I GEMPPM, u.a. CNRS 341, INSA de Lyon, B~t. 502, 69621 Villeurbanne Cedex. France and

P. C O N V E R T Institut Laue-Langevin (ILL), BP 156 X, 38042 Grenoble Cedex, France (Received 29 December 1986; in revised form 19 June 1987)

Abstract--The crystallography of hexagonal hemicarbides of the sixth group transition metals (Mo2C, W2C) has been investigated by many workers, and an up to date review is presented herein (Section 1); it appears that controversies concerning the various ordered structures (i.e. C6, ~-Fe2N and (-Fe, N-type phases) exist in the case of W2C, while the crystallography of Mo2C seems to be more consistently established, although one can be surprised that it is significantly different from that of the similar W2C carbide (the E-Fe2N-type phase was not observed in the system Mo-C). These remarks show that a high temperature (up to 2200°C) in situ powder neutron diffraction study on these compounds is needed in order to confirm their high temperature forms. In view of the peculiarities of such an experiment, we have concentrated our attention to detail in this first part the different aspects of our experimental approach (Section 2). The results will appear as part 2 of the present article. R6smn6--La cristallographie des h6micarbures hexagonaux des m&aux de transition de la VI/m~colonne (i.e. Mo2C et W:C) a 6t6 &udi6e par de nombreux auteurs: nous mettons ici d jour une revue sur ces compos6s (Section l). Il apparait que des diff6rences significatives peuvent 6tre not6es dans le cas de W 2C, puisque plusieurs structures ordonn6es sont propos6es pour ce carbure (types C6, ~-Fe2N e t (-Fe 2N); par contre, la cristallographie de Mo2C semble mieux 6tablie, bien qu'on puisse s'&onner des diff6rences entre ces 2 compos6s similaires (la structure de type ~-Fe2N ne serait pas observ6e dans le carbure de molybd6ne). Ces remarques laissent ~ penser qu'une 6tude in situ d haute temp6rature, notamment par diffraction de neutrons, est n6cessaire afin de pr6ciser ces surstructures. Dans la mesure o£t ce type d'6tudes n'est pas tr6s courant, il nous a paru int6ressant de d6tailler, dans cette premiere partie, les diff6rents aspects exp6rimentaux d'un tel travail (Section 2); les r6sultats seront pr6sent6s dans la seconde partie de cet article. Ztr~ammenfassung--Die Kristallstruktur yon hexagonalen Subcarbiden der VI A-Metalle (M02C, W2C) wurde yon zahlreichen Autoren untersucht; in dieser Arbeit wird ein umfassender l~berblick gegeben (Section l). Daraus ergibt sich, dass verschiedene Ergebnisse f/Jr die geordnete Phase im Falle des Subearbides W2C gefunden wurden (C6, E-Fe2N und (-Fe2N-Ty p Strukturen wurden vorgeschlagen); andererseits, wurde eine gute Oberstimmung beim Vergleich der experimentellen Daten yon M02C gefunden. Es ist iiberraschend, dass/ihnliche Carbide so verschiede Verhalten zeigen (ira Mo-C System wurde keine ~-Fe2N-Typ Struktur beobachtet). Im Hinblick auf diese friiheren Arbeiten scheint es, dass eine in situ Neutronenbeugungsuntersuchung bei hohen Temperaturen n6tig ist, um die Strukturen diesen Subearbiden aufzukl/iren. Da solche Experimente nicht einfach sind, werden im ersten Teil unsere experimentellen Methoden beschrieben (Section 2). Die Ergebnisse selbst werden im zweiten Teil gegeben.

I. GENERAL INTRODUCTION The present article has been divided into two separate parts; the first one (I) deals with the need and the general background for undertaking the study: on the one hand, it justifies this investigation (Section l) and on the other hand it reports the experimental procedure followed in order to prepare the carbides

powders of this study (Section 2.1) and gives the necessary explanations concerning the high temperature and neutronic environments (respectively Sections 2.2 and 2.3). In the second part (II), the results (Section l) concerning the high temperature neutron diffraction investigation are presented for both W2C ( l . l ) and Mo2C (1.2) powders; these features are discussed in

1891

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OVERVIEW NO. 69--I

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0

0 •

0

0

o o .

o • e|~ll~o

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•

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0 0

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0 g} ~

0

0 •

Fig. 1. Distribution of the carbon atoms in the different M2C structures. (a) Host metallic h.c.p, sublattice (solid circles; the open ones indicate the octahedral interstitial sites). (b) Disordered L'3-type structure (hexagonal symmetry, space group P63/mmc) projected onto the (0001) plane. (c) C6-type or Cdl2-antitype (hexagonal symmetry, P~ml). (d) Co2C-type or CaCl2-antitype (orthorhombic symmetry, Pnnm). (e) E-Fe2N-type (hexagonal symmetry, P~Im). (f) (-Fe2N-type (orthorhombic symmetry, Pbcn). (g) ~-Nb2C-type (orthorhombic symmetry, Prima). In drawings (b) to (g), the solid and open circles correspond to carbon atoms respectively in the ,,-plane at z = 0 and in the//-plane at z = I/2; a distortion of the metallic sublattice is expected within the structures of orthorhombic symmetry.

section 2, while some additive informations are detailed in Appendices 1 to 4. The pursuit of the work which is presently in progress will be published later.

(c) a precise determination of the structures of W:C~_x and M o 2 C 1 - x at room temperature.

1.I. Aim of this work

The crystal structure of transition metal hemicarbides M2C may be described on the basis of a hexagonal close-packed (h.c.p.) metal sublattice in which the carbon atoms occupy half of the octahedral interstitial sites: the stacking sequence of (0001)-type planes is then

In the last years, a few studies have been made on the mechanical behaviour of transition metal carbides with f.c.c, structures (NaCI type) [1]. On the contrary, the plasticity of hexagonal carbides has been hardly explored (except in the cases of Mo2C and WC). In our laboratory, mechanical tests on the tungsten hemicarbide have been carried out recently [2]. For explaining the mechanical behaviour, a knowledge of the atomic structure of these hemicarbides is essential; it is shown in the next paragraph (1.2) that only few references concerning the determination of the crystal structure of the close packed transition metal hemicarbides at elevated temperature are available, and consequently the position of the carbon atoms in these structures is not well established. Thus, the problem of the atomic structures of hemicarbides (particularly of W2C~ _~ and of Mo2C1_~) is not at all resolved; hence we have carried out a complete study of the polymorphism of W~C and Mo2C, by high temperature powder neutron diffraction. This paper is then based on 2 sets of experiments which were perfomed at the High Flux Reactor of the Institute Laue-Langevin (ILL), Grenoble, F/f:~ These studies consist of: (a) a determination of the various atomic carbon distributions (ordered or random) as function of temperature, including the determination of the transition temperatures and the kinetics of transformation; (b) the effect of the carbon concentration on the transitions; "fILL experiment No. 5-21-175, ILL Report (1984). ~:ILL experiment No. 5-22-260, ILL Report (1985).

1.2. Crystal structure of tungsten and molybdenum hemicarbides--a review

Act Bfl A~ B//. Where a and // represent the two types of basal layers in the interstitial simple hexagonal sublattice (A and B represent the metallic layers). Several distributions of the carbon atoms in the a and //-planes may exist: five ordered structures are proposed for M:C compounds, which could be described as regular arrangements of rows of interstitial carbon atoms parallel to the c-axis of the h.c.p. metallic cell (see Fig. 1). A nearly complete survey of structural studies devoted to close packed transition metal carbides has been published [3]; an up to date review will be reported hereafter for the two particular compounds of interest, i.e. W2C (Section 1.2.1) and Mo:C (1.2.2). 1.2.1. W2C compound. Several crystallographic studies have been performed on the tungsten hemicarbide, as reported in Table 1. It comes out from the results of these studies that 3 ordered structures seemingly exist for W~C, i.e. the C6, E-Fe2N and ~-Fe2N types, but the stability of respective types with temperature cannot be consistently discussed in view of the discrepancy in conclusions reported by different authors (see Section 1.2.3). Two constitutional diagrams have been proposed for the W - C system (see Fig. 2); these diagrams differ essentially by the existence of one or several W : C structures.

DUBOIS et al.: OVERVIEW NO. 69---I

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Table 1. Structural studies devoted to W2C. Not~ that the 0rll~¢~ i t~aperature in sire,analysis was performed by neutron diffraction [91 Form"

-W: C

Temperature b 20 20 (< 2250)

Structure

Parameters

Observation technique

Hexagonal C6

a = 2.99 A c = 4.72 a = 2.98 c = 4.71

Electron Diffraction

[4]

X-ray diffraction

[5]

X-ray diffraction

[6]

X-ray diffraction

[7]

X-ray diffraction

[5]

Neutron diffraction Neutron diffraction

[8] [9]

20 ( < 2400)

20 (800-1000)

Orthorhombic ~-Fe2N

20 (2300-2450) /~-W2C

20 (< 2400) 1750c

a ~ 5.18 c ffi4.72 Hexagonal -Fe2N

20 20 (1600)a 20c

)'-W2C

20 (> 2400) > 1750~

a = 4.728 b ~ 6.009 c = 5.193 a = 4.72 b = 6.04 c =5.19 a = 4.72 b = 5.98 c=5.17

a = 5.19 c ~ 4.72 a ~ 5.1852 c = 4.7232 a = 5.1809 c = 4.7216 a = 2.99 c ~ 4.72

Hexagonal L'3

20 (> 2550)

a ffi 3.00 c ffi4.72

Reference

Neutron diffraction & electron diffraction

[10]

Neutron diffraction

[I 1]

Neutron diffraction

[12]

X-ray diffraction Neutron diffraction

[6] [9]

X-ray diffraction

[5]

"The designation ,,, #, ~,-W2C (although not really adequate) is employed here to allow an easy comparison with the author's own notations.

bThe true temperature (°C) of the determination is indicated first; the second (in parenthesis) concerns the annealing temperature. Cln presence of WC.

~W2C0s9. 'W 2C084.

1.2.2. Mo2C compound. T h e structure a n d the phase t r a n s i t i o n s o f m o l y b d e n u m hemicarbide, Mo2C, have been studied by different a u t h o r s , as indicated in T a b l e 2. All these workers claim a n o r t h o r h o m b i c ( - F e 2 N - t y p e structure at low temperatures, which t r a n s f o r m s into a disordered structure a b o v e 1400°C nominally; this o p e r a t i o n involves a displacive transf o r m a t i o n ( o r t h o r h o m b i c distortion of the h.c.p. metallic sublattice), as well as a n o r d e r - d i s o r d e r transition within the interstitial c a r b o n sublattice. F o r the c o m p o u n d which t r a n s f o r m s at the highest t e m p e r a t u r e (Mo2C~0.975 at 1430°C a c c o r d i n g to R u d y et al. [16], a n d M o 2 C ~ l at 1430°C according to B o w m a n a n d A r n o l d [9], the order--disorder transition is a first o r d e r transition, as s h o w n by differential t h e r m a l analysis a n d direct s u p e r s t r u c t u r e reflexion intensity m e a s u r e m e n t s respectively from the two a b o v e m e n t i o n e d studies. The p h a s e d i a g r a m s p r o p o s e d for the M o - C system are s h o w n in Fig. 3; the d i a g r a m o b t a i n e d from n e u t r o n experiments (b) differs from the o n e arrived at by c o n v e n t i o n a l techniques (a): for hypo-

stoichiometric compositions, either a second order [16] or a first o r d e r [9] transition is proposed.

1.2.3. Preliminary discussion o f the above results In conclusion o f this presentation, one can notice t h a t the preceding features are n o t consistent, especially c o n c e r n i n g W2C, for which different ordered structures have been proposed. A l t h o u g h the results o b t a i n e d for M o 2 C a p p e a r to be m o r e coherent, one can be surprised t h a t the *-Fe2N-type structure, which has been clearly observed in W2C, a n d also in o t h e r transition metal hemicarbides (V2C [19], N b 2 C [20,2i]) is n o t present in the M o - C system. Moreover, the study o f eventual t r a n s i t i o n s would be unsuccessful at r o o m t e m p e r a t u r e if these t r a n s f o r m a t i o n s are fully reversible a n d the corres p o n d i n g superstructures are n o t retainable by quenching. Consequently, it is o u r feeling t h a t a s y s t e m a t i c / n situ high t e m p e r a t u r e structural study has to be u n d e r t a k e n o n b o t h W2C a n d M o 2 C c o m p o u n d s .

1894

et al.:

DUBOIS

OVERVIEW

3000

NO. 69--1

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Neutron diffraction is required for this investigation since it is well known that the scattering amplitude of carbon with respect to the one of the metal is very weak in the case of X-rays or even electrons, but both values are approximatively equal in the case of neutrons (see Table 3). Therefore, it appears that X-ray or electron diffraction techniques are not adequate to study the long-range ordered structures encountered in M2C carbides, although the orthorhombic phase(s) can clearly be identified through the distortion of the metallic sublattice, which induces a splitting of some

peaks which may be detectable by careful X-rays measurements. In addition to the above, one can add that the good penetration of neutrons compared to that of other particles is more favourable and allows to work on a greater quantity of material, and reduces the perturbations due to bulk sample inhomogeneities or to superficial contamination of the specimens at high temperature; this particularity also offers obvious technical advantages in a high temperature environment. Considering all the previous remarks, we have

Table 2. Structural studies devoted to M o : C (notations as in Table 1) Temperature

Structure

Parameters

Observation technique

a = 4.724 A

Neutron diffraction

b =6.o04

20 C

Reference

psi

c = 5.199 20 (1000) ~-Mo2C Orthorhombic 20 ( ~ 1000-1430)

a =4.735 b = 6.038 c = 5.208

X-ray diffraction

a = 4.733 b = 6.024 c = 5.202

X-ray diffraction and differential thermal analysis

[16]

a = 4.732 b = 6.037 c = 5.204

X-ray and neutron diffraction

ll 7]

[7]

~-Fe2N 20 T < 1200-1400' Neutron diffraction

[9]

X-ray diffraction and differential thermal analysis

p61

T > 1200-1400~ /LMo2C 20 ( > 1400)

L'3

a = 2.990 - 3.010' c = 4.730 4.778a --

a = 3.003 c = 4.729 'Value depending upon the exact composition Mo2C,

_~.

Psi

DUBOIS et al.: OVERVIEW NO. 69--I

1895

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Fig. 3. Proposed phase diagrams for Mo-C. (a) From Rudy etal. [16]. (b) Detail of the Mo2C region as deduced from neutron experiments [9]. Table 3. Qualitative comparison between carbon and metal (Mo, W) concerning their ability to scatter particles of different nature rc.Mo rc.w Electrons 0.28 0.2 X-rays 0.14 0.08 Neutrons 1 1.38 Notations are: rc, M= fc(sin 0 = 0)/fM(sin0 = 0) for electrons and X-rays (f is the atomic scattering amplitude). rc.M=bc/bM for neutrons (b is the coherent scattering length, deduced from Ref. [22] for C, Mo and W). carried out neutron diffraction experiments as a function of temperature, in order to specify the carbon atoms distribution (ordered, partially-ordered or random) in W2C]_ x and MO2Cl_ x compounds between room temperature and 2200°C.

2.

EXPERIMENTAL

The various experimental aspects of this work are tln

fact some qualitative microprobe analysis was performed on a Castaing microsonde--laboratoire de M6tallurgie, Ecole Centrale de Lyon--to check that the re-homogenization heat treatment at 2100°C had averaged the carbon distribution through the thickness of the samples. This technique has however already been used in the course of a crystallographic characterization of veining and band-type substructures in Mo2C single crystals [25]; in our case, attempts were made to determine precisely the carbon content of W2C powders of various compositions: if the sensitivity of the electron microprobe measurements is enough to compare the W2C~_ ~ compounds ( ~ 9 7 w t % W) to pure tungsten, the technique remains largely unsuccessful in detecting x variations of less than 0.1 nominally.

detailed in the following sub-sections; as a complement t o the specific presentation o f the high temperature and neutron environments (Section 2.2 and 3), one can find interesting features in a recent review concerning the high temperature devices developed for structural studies involving X-ray or neutron scattering [23].

2.1. Samples preparation and characterization 2.1.1. W2C powders. W2C was studied in our laboratory with a view to determine its mechanical behaviour at high temperatures (see [2] and references therein); hence, the powders used in the present work were prepared from the polycrystalline sheets used for the 4 point-bending deformation tests. The detailed experimental procedure for the preparation of massive W2C sheets has already been published [24]; nevertheless, it can be summarized herein for completeness: tungsten plates were embedded into carbon powder of spectrographic quality; then these compacts were heated to 2200°C during 3 0 m i n nominally, and annealed at 2100°C for a few hours under argon. The W2C samples contained a rather low impurity content, as shown in Table 4. X-ray analysis confirm that W2 C was the major constituant, the only other crystallographic phases being W C and W in very minor quantities. The determination of the composition of the W2C~_ ~ samples has required a technique which allows one to measure the average carbon content: no "'microscopic scale" methods, as the electron microprobe analysis was undertaken.t Three different techniques were used: (i) weight gain measurements of the quantity of

1896

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OVERVIEW

N O . 69---I

Table 4. Chemical composition of the W2C powders (particle size ~<500 #m; important impurities [24]: M o < 50 ppm, Fe < 20 ppm, Ta < 2 0 p p m , Ti < 2 0 p p m , Cr < 1 0 p p m ) Before the neutron study Powder

Element

wt%

C

3.16 -+0.05

O N

< 0.003 <0.001

B

C O N

C

C O N

AI

Composition

After the neutron study Technique"

wt%

Composition

Technique*

W2 C0.92± 0003

CA

2.89 ±°°~

W2 Ci ± o.01s

WGM

< 0.025 <0.001

2.89 ± 0.05 < 0.003 <0.001

W2 Co.91s ± o.o,6

WGM

2.96 ~ o.01 < 0.005 <0.005

W2 Co.937± o.oo3

CA

2.55 ± 0.os < 0.003 <0.00l

W2 Co.sJ ~:o.ot6

WGM

2.635 ± o.01 < 0.006 <0.005

W2 Cos3 ± 0.002

CA

"WGM ffi weight gain measurement; CA ffi chemical analysis.

carbon diffusing into the tungsten sheets during the experiment; (ii) coulometric analyses (Titrimetre-Coulometre 2 TI.Eraly); (iii) infrared absorption analysis combined with a combustion method [apparatus Leco O/N--Ugicarb Morgon, Grenoble (France)~also allowing the oxygen and nitrogen analysis]. All the methods gave the same results, within a precision range of 0.05 wt% for the carbon content. Since the two last techniques are destructive, and considering the limited quantity of material available for this study, we preferred the weighting method before the neutron diffraction experiments. Three batches of polycrystalline sheets with different compositions (W2Cl ± 0.0~, W2 C0.9~5±0.0~6, W2C0.8~±0.0~6 respectively) were then ground in a corindon mortar in order to prepare ~ 2 - 5 g of powders (labelled A, B, C in Table 4). Concerning the stoichiometric A-powder, two specimens were prepared: one part (A~) was used for an extensive study from 20 to 2200°C, the other part (A2) was annealed 100 h at 1200°C under vacuum to allow to investigate an eventual structural evolution at this low temperature. The granulometry of these powders was rather rough (see Table 4), but the grindirig was limited in order to avoid an undesirable contamination of the specimens. In the end, a final annealing of the Ai, B and C powders was performed during 4 h at 1200°C under vacuum (the specimens being enclosed in a tungsten container): this will constitute our state of reference.

2.1.2. Mo2C powders. A commercial Mo:C~~ powder was subjected to a pre-homogenization at 1200°C under good vacuum; a first sample (labelled A in Table 5) was obtained by selecting 3 g after this treatment, its composition being Mo2C0993±o.00~-The powder B was obtained by a decarburizing heat treatment under a nitrogen flux at 1100°C for 1 h; the carbon content was determined by weight loss measurements and chemical analysis, both results being confistent (see Table 5). A new annealing of this powder was performed during 4 h at 1200°C under vacuum; the final composition of this sample is Mo2C09,±OOl. The X-ray diffraction diagram of the A-sample revealed Mo2C as the only detectable crystalline phase, while the B-powder exhibits a very small amount of Mo in addition to MoeC. According to the Mo-C phase diagram (see Fig. 3), the region of the Mo2C phase does not extend below ~32% at. C at room temperature (Mo2 C~o.96), and this explains the presence of the molybdenum phase in our B sample; for this compound Mo2C~0.94 (31% at. C), the composition will vary with temperature, and the whole material will be transformed into the Mo:C~-x phase at 1700°C nominally: thus, if the thermodynamical equilibrium is reached during the neutron study, we can expect to get the lowest stoichiometry of the Mo2Cj_x up to this mentioned temperature, and this represents a sufficiently large temperature range to cover the crystallographic transition "~-Mo2C--, fl-Mo2C" reported in the literature. 2.1.3. Summarization of the chemicalanalysis of the powders. The Tables 4 and 5 summarize the previous data on W:C~ _ x and Mo2Ct _ x powders respectively.

Table 5. Chemical composition of the Mo2C powders (free carbon ~0.03 wt% for both A and B powders; particle size < 10pm) Before the neutron study Powder

A

B

Element

wt%

C

5.84 ± o.os

O N

0.25 ± o.ol 0.02 ± o.oos

C

5.53 ± o.os

O N

Composition

After the neutron study Technique"

wt%

Composition

Technique'

Mo2 Co99 ± oool

CA

Mo2Co.9~ ±o.ool

CA

5.82 ± o.ol

Mo2 Co.~3 ± o.ool

CA

0.28 ± o.ol 0.03 ± 0,005

Mo2Co~±oo, b

WLM

0.16 ± ° ' ° ° 5 0.03 ± o.o~

5.61 ± o.o,

°WLM = weight loss measurement (other abbreviations as in Table 4). bComposition Mo2C0.gs determined by CA.

DUBOIS et al.: OVERVIEW NO. 69--I

2

~@~

1

Fig. 4. Simplified scheme of the high temperature neutron furnace ILL equipment: (1) cooling water connections; (2) power supply cables; (3) vacuum exhaust; (4) tube for the thermocouple and for pyrometric temperature measurements; (5) system to rotate the sample (not used here); (6) tungsten container; (7) tungsten shields [(7') resistor)]; (8) window for the neutrons ("a" indicates the neutron axis). In order to verify that the composition did not fluctuate during the structural study, ultimate chemical analysis were performed after the high temperature neutron diffraction experiments: the results reported in the second column of the tables show that the carbon concentration remains nearly the same for each sample, except for the W2C A~-powder, which was heated more than 50 h (at least at 1200°C) for the purpose of in situ annealing (see Part II for details); the observed decarburization is easily explained by a correlated carburization of the tungsten container used during the study (see Section 2.2). 2.2. Available high temperature equipment A high temperature furnace (see Fig. 4) has been provided by the ILL to perform the neutron diffraction experiments. This device is a slightly modified version of the initial furnace developed for structural studies of oxides at high temperature (see [26] and references within). The low resistance heating element has a cylindrical shape (16 mm in inner diameter, 185 mm in height and 0.5 mm in thickness) and is made of tungsten. In the final version of the furnace, 6 thin radiation shields (tungsten-made) are set around the heating element in order to reduce the heating loss. Three circular plate shields are fixed at the top and at the bottom of the heated zone. To minimize the absorption of the neutron beam going through the furnace, an aluminium window, 1 mm thick, is put in a median position of the body furnace and allows about 300 ° access for the scattered neutron beam in the horizontal plane. This area is cooled by water (nevertheless the power consumption

1897

it l~ted tO 3.7 k W at,2200°C). All connections into the furnace such as power supply cables, vacuum exhaust, cooling water connections are done through the top flange. The sample holder has been manufactured in a rod of tungsten: it has 6 ram in diameter and 30 m m in height. It is mounted at the end of a tungsten rod of 2 m m diameter, in order to limit the conduction heat loss as well as the thermal gradient in the sample holder. The sample rod is fixed at the bottom of the furnace on a movable element. The furnace is combined with an appropriate temperature controller (Drusch All). The thermal detector is a chromel-alumel thermocouple closely wound around a molybdenum rod. This detector is set near the heating element; when the temperature controller is correctly adjusted, a temperature stability of better than 3°C is obtained. In order to measure the sample temperature we have chosen two types of thermal detectors. To work below 1900°C we use a tungsten rhenium thermocouple (W-Re 5%, W - R e 26°,/0)with a hot junction set at about 5 m m from the top end sample holder. To work above 1900°C we use a bichromatic infrared pyrometer ( I R C O N Modline R) with a silicon detector~ the pyrometer is adjusted to a sighting post set at the top of the sample holder. The temperature is subordinated to the rate spectral energies at 0.95 and 1.05/~m. This is interesting for two reasons: firstlyit is not necessary to known the value of the factor of emissivity (in the case of grey body), and secondly the target can be small compared with the detector area. During the working of the furnace it is possible to measure with both thermal detectors, thus enabling comparisons to be made: the temperature measured with the thermocouple was systematically higher than the one measured with the infra-red pyrometer; indeed the latter one is probably closer to the temperature of the sample. Moreover the thermal gradient in the empty sample holder has been determined with the thermocouple: its value is about 50°C at 1900°C. In fact the thermal gradient in the sample is weaker if we account for the mismatch in height between the sample and the sample holder (about 1/3). If all these things are taken into consideration, the sample temperature may be estimated at +20°C in the working temperature range. Figure 5 gives a general view of the furnace in its working configuration. 2.3. Neutron environment

It seems to be useful to introduce this section with a brief overview of the different experimental requirements which have guided us to the choice of the most convenient instrument available at the ILL for conducting this study. As already mentioned in Section 1, the temperatures which have to be explored for a crystallographic study of M2C compounds are very high, up

1898

DUBOIS et al.: OVERVIEW NO. 69--1

Fig. 5. Assemblage of the high temperature device on the experimental stage; (a) furnace; (b) neutron guide; (c) multidetector (behind).

to 2500°C nominally. Moreover, due to the presence of various phases, this investigation requires a large rea&or number of measurements at many temperatures, with !. ' radiusof curvature27000m the possibility of observing a kinetic evolution of the (J I ~; sample at a fixed temperature. I~l i~guide tube H 22 In addition to the above, because of the difficulties DIA~I!I ~ ...... te of preparation to obtain exact and homogeneous B4 C epoxy6 rnm compositions (see Section 2.1), the volume of powder polyethylene150mm material for each sample was small. ,~,~ ~lead 100mm LaSt, it was important to have the largest number '?; \ ' % \ • ...... .rum.for Ola of Bragg peaks measured. \\\..re0.,t.o,, . . . . . ,u. For these reasons, the unique possibility was to use %\ ,-.°.,°-., the D1B powder diffractometer of the Institut Laue ~ * ~ . c:graphitefitters ;|t Langevin: this instrument has a good compromise flux/resolution, the precision in intensity lacing due to the high flux of the reactor and to the simultaneous ;'5] ~ "~-~\ f . . . . itor measurement on a large range of angles (80 °) by using a large Position Sensitive Detector (PSD). 2.3.1. Description of the two-axis powder '.,7,1 \~\~ I-'~'~ \ \ diffractometer with multidetector D I B [27]. This instrument (see Fig. 6) is installed on the H22 guide "l'~?'~detectorah/ielding; ~ mm B4 C'~xy tube of the high flux reactor at ILL (thermal neutrons). The take-off angle of the monochromator is fixed: 20=--44.22 °. Two monochromators are available: a Fig. 6. Schematic view of the D1B diffractometer (from Ref. [27]). pyrolytic graphite (0002), giving a wavelength of

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DUBOIS et al.: OVERVIEW NO. 69--I 2.52 A (the 2/2 being removed with a graphit~iit~r)i and a flux of 1.6.106 neutrons'cm2"s -~ at the sample position; a germanium (311), giving a wavelength of 1.28A and a flux of 0.4.106 neutrons.cm2.s-L The maximum beam size at the sample position is 5 x 2 cm 2. The PSD, or multidetector, is curved with a radius of 1.5 m; it is filled with a gaseous mixture of 3He/Xe. It contains a matrix system of electrodes giving 400 active elements (or cells) covering 20 = 80 °. The dispersion of efficiency of the cells is 5% at the maximum but the stability is much better than 0.5% over a period of 2 months. The PSD is calibrated with a vanadium rod at the beginning of every reactor cycle (44 days). The angular resolution is good between 20 = 0 and about 80 °, because of the small take-off angle of the monochromator [27]. D1B is controlled by a SOLAR 16/40 computer for the data acquisition and control of the instrument; the data are automatically transferred to the central DEC-10 computer of the Institute. Different programs of data acquisition (with a monitor or time pre-selection mode) are available, the dead-time between two measurements being about 2 s. Data reduction is done either at the instrument computer in order to drive the experiment (Tektronix display, integration of intensities, determination of peak positions . . . . ), or at the DEC-10 computer with much more powerful programs of data analysis (an example of application of one of the programs used in this study will be given in part II). 2.3.2. Determination o f the experimental conditions. In the choice of the experimental conditions (such as wavelength, angular domain in 20, and more generally the strategy of measurement), we were faced with a rather complex situation due to nature of our samples and their experimental requirements, e.g. • many phases exist, which means that a lot of diagrams have to be recorded at different temperatures; • different phases may be present simultaneously; • a structure determination requires that a maximum number of Bragg peaks, with an acceptable precision, are obtained; • at the highest temperatures, one might anticipate a possible chemical evolution of the samples, which is undesirable: thus, short measuring times are required; • the container and the furnace contribute to the background intensity of the diagrams, especially through their Bragg peaks. It was then obvious that we had to use a fixed position of the PSD, because of the large number of diagrams to record (displacing the multidetector is time-consuming), and owing to the necessity of having good reproducibility of the angular position and of the background conditions. Preliminary measurements at room temperature

1899

(with the samples in a vanadmm tube) and at high temperatures have been made using both wavelengths available on DIB; it was shown that at 2 = 1.28 A, we needed to count 10 times longer than at 2 = 2.52 A in order to obtain the same quality in intensity: the flux is lower (about 4 times), as are the scattering power of the samples and the detection efficiency of the PSD. Of course, the advantage of 2 = 1.28 A is to obtain many more Bragg peaks within the angular aperture of 80°; nevertheless, the factor of 10 in intensity, combined with problems of overlapping of peaks, have led us to choose 2 = 2.52A systematically for the high temperature measurements; room temperature spectra were however recorded with 1.28 A neutrons, since one can collect a lot more crystallographic information with this smallest wavelength. During these preliminary measurements, we have also observed that a basic counting of 15 min was sufficient to obtain an acceptable statistic for the intensity estimations; generally, the measurements for each temperature were repeated 2-3 times, and much longer measurements were performed when a structural evolution as a function of time was expected (or studied). As we had chosen to set the PSD at a fixed position, we had to determine the most convenient angular domain to explore: therefore, we have made simulations of the expected diagrams for all the possible structures mentioned in the literature for the M2C compounds which are shown in Fig. 1; these simulations were performed in the case of W2C, with a Debye-Waller factor equal to 0.5A 2 for both atomic species, for the wavelength which was retained for the high temperature study, i.e. 2.52A. The results are shown in Fig. 7, which also reports the diagrams of pure tungsten and tungsten monocarbide WC, which is present in small quantity in the W2CI _ x samples (see Section 2.1). The last problem to be solved concerned the reduction of the background contribution due to the scattering by air and by the various materials of the furnace which are in the neutron beam: because of its geometry, the large PSD is widely open to a parasitic background around the sample: sample holder, heating element and thermal screens (tungsten made components), aluminium walls of the vacuum vessel of the furnace (see Fig. 4). As the Soller slits of a classical powder diffractometer cannot be used, two solutions remain possible. • firstly, the Radial Oscillating Collimator (ROC), described in Fig. 8(a), which is available for the high temperature furnace that we were using [28]: the radial absorbing blades suppress completely the parasitic background beyond a certain distance from the sample. They are oscillating to average the deformation of the peaks profile which is caused by the position of the blades and

1900

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by their local defects. U p to now, the existing R O C has not a very homogeneous transmission as a function of the angle, and the intensities of Bragg peaks are rapidly modified if the sample position moves, which should not be very surprising in a very high temperature furnace. 400 ¢~t PSD

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• A second solution consists in adjusting carefully cadmium screens outside the furnace in order to suppress the aluminium lines of the vessel as depicted in Fig. 8(b).

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(i) the wavelength of 2.52 A gives only a few a l u m i n i u m lines which are easily suppressed by the cadmium screens [see Fig. 9(a, b)]; (ii) the angular domain of measurements (20 ,-, 25 to 105 °, determined on the basis of the simulations shown in Fig. 7) was not affected by the cadmium screens; (iii) it was preferred to perform measurements with a good intensity, even with a higher, but stable background.

DUBOIS et al.: OVERVIEW NO. 69--1 (O) o

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pov,~der diffractorneter with multidetector D1B for conducting this study, using an optimized high temperature device. The results of the investigations on M o : C ] _ x and W2Cm_, are presented in the second part of this paper. Acknowledgements--The authors wish to thank the ~echnical staff of "High Temperature Group" of the ILL for the preparation of the furnace used in this study (Mr P. Andant and R. Serve).

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1. L. E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York (t971). 2. J. Dubois, G. Fantozzi, T. Epicier and C. Esnouf, Int. Phys. Conf. Ser. No. 75, Chapter 4, 2rid Int. Conf. Sci. Hard. Mater., Rhodes, p. 265, (1986). 3. E. Parthe and K. Yvon, Acta crystallogr. B26, 153 (1970). 4. L. N. Butorina and Z. G. Pinsker, Kristallografiya 5, 585 (1960). 5. N. Morton, B. W. James, G. H. Wostenholm and D. C. B. Hepburn, J. less-common. Metals 29, 423 (1972). 6. E. Rudy and St Windisch, J. Am. Ceram. Soc. 50, 272 (1967). 7. V. S. Telegus, E. I. Gladyshevskii and P. I. Kripyakevich, Soviet Phys. Crystallogr. 12, 813 (1968). 8. K. Yvon, H. Nowotny and F. Benesovsky, Monatsh. Chem. 99, 726 (1968). 9. A. L. Bowman and G. P. Arnold, Adv. High Temp. Chem. 4, 243 (1971). 10. T. Epicier, J. Dubois, C. Esnouf and G. Fantozzi, C.r. Acad. Sci., Paris t297, s6rie II, 215 (1983). 11. A. H~trsta, S. Rundqvist and J. O. Thomas, Acta chem. scand. A32, 891 (1978). 12. B. L6nnberg, T. Lundstrom and R. Tellgren, J. lesscommon. Metals 120, 239 (1986). 13. R. V. Sara, J. Am. Ceram. Soc. 48, 251 (1965). 14. E. Rudy, Ternary Phase Equilibria in Transition Metal-Boron-Carbon-Silicon Systems, part V, Compendium o f Phase Diagram Data, Tech. Rep. AFM.L,

The Fig. 9 illustrates the influence of the cadmium screens on the diagrams recorded at 2.52 A. 3. CONCLUSION We have shown in this first part that the comparison of the most representative works on the metallic hemicarbides W2C ~_x and Mo2C~_~ points out controversies concerning the various ordered phases which may exist in these compounds at different temperatures. In order to complete and clarify these literature data, in situ high temperature structural investigations are required, since the high temperature phases may be unretainable at room temperature by quenching. Since carbon atoms adopt various distributions in the mentioned superstructures, one has to use neutron diffraction, which is the most sensitive technique to use for this light element. The numerous technical and crystallographic requirements have led us to choose the two-axis

TR 65-2 (1969). 15. E. Parthe and V. Sadagopan, Acta crystallogr. 16, 202 (1963). 16. E. Rudy, St Windisch, A. J. Stosick and J. R. Hoffman, Trans. metall. Soc. A.I.M.E. 239, 1247 (1967). 17. A. N. Christensen, Acta chem. scand. A31, 509 (1977). 18. R. J. Fries and C. P. Kempter, Analyt. Chem. 32, 1898 (1960).

19. K. Yvon, H. Nowotny and R. Kieffer, Monatsh. Chem. 98, 34 (1967). 20. N. Terao, J. appl. Phys. Japan 3, 104 (1967). 21. E. Rudy and C. E. Brukl, J. Am. Ceram. Soc. 50, 265 (1967). 22. L. Koester and H. Rauch, Summary of Neutron Scattering Length, Contract 2517/RB, IAEA, Vienna (1981). 23. P. Aldebert, Rev. Phys. Appl. 19, 649 (1984). 24. D. Treheux, J. Dubois and G. Fantozzi, Ceram. Int. 7, 142 (1981). 25. S. A. Mersol, F. W. Vahldiek and C. T. Lynch, J. less common. Metals 10, 373 (1966). 26. P. Aldebert, thesis No. 928, Toulouse (F) (1980). 27, ILL Report, Neutron Research Facilities at the ILL High Flux Reactor, Institute Laue-Langevin, Grenoble (F) (1983). 28. A. F. Wright, M. Berneron and S. P. Heathman, Nucl. lnstrum. Meth. 180, 655 (1981).