Borides Part A: Basic Factors

BORIDES Part A: Basic Factors Bertil Aronsson Institute of Chemistry, University of Uppsala, Uppsala, Sweden

Page I. Introduction II. E l e m e n t a l

143

Boron

and

Its

Binary

Compounds

with

Oxygen,

Nitrogen,

Carbon, a n d Silicon

144

III. S o m e General A s p e c t s of t h e Preparation, Crystallography, a n d Properties of Borides

146

A. T h e Preparation of Borides

146

B. T h e Crystallography of Borides C. A

Survey

of

the

Properties

147

of

Borides

and

Their

Relationship

to

T h o s e of t h e SiHcides a n d Carbides

148

D . Theories of Borides

153

IV. A Systematic Treatment of Binary M e t a l - B o r o n S y s t e m s a n d Their Inter­ m e d i a t e Phases

154

A. Borides of t h e Alkali Metals, Beryllium, M a g n e s i u m , a n d A l u m i n u m .

154

B. Borides of t h e Alkaline Earths

(except

Beryllium a n d

Rare Earths, a n d A c t i n i d e s

Magnesium), 155

C. Borides of T i t a n i u m , Zirconium, a n d H a f n i u m

158

D . Borides of V a n a d i u m , N i o b i u m , a n d T a n t a l u m

160

E . Borides of C h r o m i u m , M o l y b d e n u m , a n d T u n g s t e n

167

F . Borides of t h e S e v e n t h a n d E i g h t h G r o u p Metals

171

V. Ternary Systems of T w o Transition Metals a n d Boron

175

A. T h e Quasi-Binary S y s t e m s of MeBg Phases

176

B. Additional Information A b o u t t h e M e i - M e 2 - B S y s t e m s

177

VI. Ternary S y s t e m s M e - B - X ( X = 0 , N , C , S i ) A. M e - B - O S y s t e m s B. M e - B - N

178 178

Systems

178

C. M e - B - C S y s t e m s

179

D . M e - B - S i Systems

179

VII. Conclusions

180

References

181

I. Introduction Since the pioneering work of Moissan and others before the first Vi^orld War, very little attention has been paid to the borides (i.e., com­ binations of boron with less electronegative elements) for several decades. The increasing demand for high temperature-resisting materials has called forth a renewed interest in this class of compounds, and since 143

144

BERTIL ARONSSON

1945 they have been very intensively studied. Much of the recent research has been devoted to the fundamental properties of borides, which are still incompletely known and understood. This chapter will give an account of the basic properties of the borides and will follow this plan: The second section covers elemental boron and its binary compounds with oxygen, nitrogen, carbon, and silicon. Although none of these substances should be called a boride (except possibly silicon-boron compounds), they are briefly presented here because of their importance in high temperature reactions of borides. In the next two sections the binary borides are treated. Section III con­ tains some general aspects of their preparation, crystallography, and properties. In particular, the trends in the properties of the transition metal diborides are pointed out as well as their relationship to other "hard metals." The main part of the chapter consists of a systematic ac­ count of the borides arranged according to the position of the metal in the periodic table (Section IV). The ternary systems Μ θ ι - Μ θ 2 - Β and Me-B-X (X = 0,N,C,Si) are presented in Sections V and VI. As far as possible the author has tried to estimate the reliability of the available information and to explain inconsistencies among the various results. A comprehensive bibliography will make it easy for the reader to find original papers. The number of publications in the field of borides is now very large, and because of this fact only the most important of older papers have been cited. The older literature is more thoroughly covered in two review articles (J, 2 ) . II. Elemental Boron and Its Binary Compounds v/ith Oxygen, Nitrogen, Carbon, and Silicon* Elemental boron of reasonable purity was first prepared by Moissan (3,4) who developed the method of reducing B2O3 with magnesium to give boron of more than 98% purity. Two other methods for preparing boron should be mentioned: namely, the electrolytic method, developed by Andrieux (5), and the decomposition of boron halides on a hot fila­ ment in the presence of hydrogen ( 6 ) . The last method produces fairly pure boron and has been used for the preparation of single crystals. Many modifications of crystalline boron have been reported. The simplest one is rhombohedral (a = 5.06 A , a = 58.4°; X-ray density = 2.46 gm/cm^) and was prepared by decomposing BI3 at 800°-1000°C (7). This a- modification is not stable at 1200°, and above 1500°C it transforms to the β - rhombohedral modification which has the lattice * A c o m p r e h e n s i v e account of boron, boron oxides, boron carbide, a n d b o r o n nitride is f o u n d in Gmehn's h a n d b o o k (36) w h i c h covers hterature u p t o 1 9 5 0 .

BORIDES: BASIC FACTORS

145

parameters a = 10.12 A, α = 65°28', and an X-ray density of 2.35 gm/cm^ (S). This second rhombohedral modification is the easiest one to prepare (9), but its crystal structure is complicated and has not yet been solved. A tetragonal modification {a — 8.73 Ä, c = 5.03 Ä, experimental density 2.33 gm/cm^) has been extensively studied by Hoard and co-workers (9-11), who recently solved its detailed crystal structure. The various boron modifications were recently reviewed and discussed by Hoard and Newkirk (IIa). The melting point of boron is not accurately known, and the reported value of 2075°C (12) is probably too low (13). The heat of sublimation is 139 ± 4 kcal/mole. Boron has a considerable vapor pressure at 2500°C, and the gas is chiefly monoatomic (13). Elemental boron is as hard as boron carbide and is the second hardest element—inferior only to diamond in that respect. The electrical properties of boron are those of a semiconductor (14,15), Among the chemical properties of boron which were already de­ scribed by Moissan (16), the high aflBnity of boron for oxygen should be mentioned in particular. The heat of formation (AHSQSK) of B2O3 is — 306 ± 4.5 kcal/mole (17). [A recent determination gave the value of —304.6 kcal/mole (18),] The volatile oxide B2O2 is also quite stable ( Δ Η 2 9 8 Κ — —107 kcal/mole) and is easily formed when boron and nonvolatile oxides are heated in vacuo (13). Boron nitride is a high melting compound but has a considerable vapor pressure (23 Torr) already at 1695°C and sublimes at 2730°C (19,20), Its normal modification is hexagonal [a ^ 2.5040 k, c = 6.6612 Ä, X-ray density = 2.27 gm/cm^ (21)] and is closely related to graphite. Like boron carbide, boron nitride is rather resistant to acid and alkaline solutions at room temperature but is easily oxidized at elevated tempera­ tures. The heat formation (Δί/298κ ) of BN is 60.7 =b 2.5 kcal/mole (17), A cubic modification, Tjorazon," has the (cubic) zincblende structure (a = 3.615 A) and is formed only at very high pressures (22), The boron-carbon system was recently discussed by Samsonov (23) who drew a tentative phase diagram. There are two eutectic points—at 1900°C (2 wt. % carbon) and at 2150°C (30-^1% carbon). According to a number of authors (20,24,25) only one intermediate phase (with the ideal composition B4C) exists between the eutectic points. This phase has a melting point maximum at about 2450° (20,24). Glaser et al. (25) found B4C to be homogeneous from 4 to 28 atomic % carbon, but these values have been criticized, and the carbon content of B4C is not likely to exceed 20 atomic % (20), Samsonov (23) has concluded that in reality two boron-carbon phases exist in the boron-rich part of the system. One has the ideal composition B13C2 and is probably identical with the "old"

146

BERTIL ARONSSON

B4C, while the other has the ideal composition B4C and is peritectically formed at 2250°C. The crystal structure of B4C (and also Β 13^2 ) is trigonal (26,27). For a compound with the composition B4C the lattice parameters are α = 5.598 A, c = 12.12 A (28), and the experimental density is 2.51 gm/cm'^ (20). Boron carbide is very hard and has a great chemical stability but has a poor oxidation resistance at elevated tem­ peratures (20,24). [According to Samsonov (23) B13C2 is harder and more stable than B4C.] B4C reacts vigorously with many metals (25) and is an important starting material in the preparation of borides. The heat of formation of B4C ( A H s s s k ) is —14.0 ±: 6.5 kcal/mole (17). The information about the boron-silicon system is controversial. There is a small substitutional solubility of boron in sihcon, accompanied by increasing density and decreasing cube edge of the parent silicon lattice (29). Moissan and Stock (30) claimed the existence of two intermediate phases, SiBs and SiBg. Samsonov and Latysheva (31) also found a phase with the composition SiBs, but their results have been doubted (32), and SiBs has not been reported in any other recent study of the Si-B system. Nor has cubic SiBo (with CaBe structure) (33) been verified. Recently two independent papers (34,35) on orthorhombic SiBe have appeared ( a = 14.39 A, Z? = 18.27 A, c = 9.855 A, experimental density 2.45 gm/cm^) (34), and the existence of this sihcon boride seems to be firmly established. Adamsky (34) claims the existence of at least two more silicon borides, but their compositions are unknown. III. Some General Aspects of the Preparation, Crystallography, and Properties of Borides A. THE PREPARATION OF BORMES

The simplest way to prepare borides is by direct combination of elemental boron with the relevant metal (or metal hydride) by melting or sintering. This method permits a good control of the composition of the end product, and is widely used when the cost of the starting ma­ terials is unimportant. Very little information about the mechanism of the reaction between boron and the transition metals is available. The only studies of this topic are those of Samsonov and Latysheva (37,38) who determined the diffusion coefiBcient of boron in titanium, zirconium, niobium, tantalum, molybdenum, and tungsten at various temperatures. CrB (39,40), MnB2 (41), and WB2 (42) have been prepared through the reduction of the metal oxide by aluminum in the presence of boron (thermite process), but this method gives very impure products and has not attracted much interest during the last years. McKenna (43) introduced the idea of simultaneous reduction of

BORIDES: BASIC FACTORS

147

boron oxide and a metal oxide with carbon. He succeeded in preparing ZrBs of a purity greater than 95% that contained as little as 1.09 wt. % carbon. Diborides of vanadium and titanium (44), alkaline-earth hexaborides (45), and solid solutions of transition metal diborides (46) have also been prepared in this way. Kieffer et al. {47) have studied the reactions between a metal (and/or metal oxide), boron carbide, and boron oxide in vacuo at high tempera­ tures and have prepared TiBa and ZrBs with quite low carbon content (down to 0.3% by weight) by this method. Still better results are ob­ tained when a metal oxide is reacted with boron carbide and carbon. Thus, Meerson and Samsonov {48) prepared T i B s , ZrBs, V B s , NbBs, and TaBa of approximately 99.5% purity (carbon content ^-'0.1%), while Baroch and Evans (49), who independently investigated the same reaction, obtained satisfactory results for TiBg, ZrBa, CrB, CoB, W2B5, F e B , and MnB2. An interesting study of the reaction between TÍO2, B4C and C was made by Samsonov (50) who found that carbon first reduces TÍO2 to TiO before B4C enters into the reaction. The preparation of borides by fused salt (igneous) electrolysis was developed by Andrieux (5). This author and various co-workers have isolated hexaborides of alkaline earths, rare earths, and thorium, TÍB2, VB2, & 3 Β 2 , MnB (5); NbB2, TaB2 (5J); & 4 Β , CrsB, CrsBa (52); M02B, MoB, WB (53); U B 4 (5) and UB12 {54). Nickel borides have also been prepared by igneous electrolysis (55). Norton et al. {44) obtained TÍB2, ZrB., NbB2, and T a B s of about 99% purity by this method but did not get satisfactory results with VBg. The simultaneous hydrogen reaction of a metal halide and boron hahde on a heated surface (or filament) was first described by Moers (56) who prepared borides of titanium, zirconium, hafnium, vanadium, tantalum, and tungsten. The vapor phase deposition of borides was studied more recently by Campbell et al. (57). A similar method is the pyrolysis of metal borohydrides. The two latter methods were discussed by Powell (58). In basic research the direct combination of boron and the relevant metal(s) is the most usual method for the preparation of borides. The other methods are greatly important for special purposes and in the commercial preparation of borides. They will be comprehensively treated from that point of view in Dr. Steinitzs article in this volume (p. 197flF), and, therefore, they will not be further discussed in this chapter. B. T H E CRYSTALLOGRAPHY OF BORIDES

The unique identification of the numerous known borides has been greatly facilitated by the use of X-ray diflFraction methods. The results

148

BERTIL ARONSSON

of X-ray diffraction work have also laid the foundation for a rational systematization of borides. Table I contains the most important data for all crystal structure types found among the borides. At present there is no permanent policy in alloy phase nomenclature, but the notations of "Strukturbericht" are widely used and are given in the table. The extension of this system suggested in the handbooks of Smithells ( 5 9 ) and Pearson ( 6 0 ) (given within parentheses in the table) has not been generally accepted, how­ ever, and therefore structures determined after 1940 will be noted with the first discovered representative. All lattice parameters are given in true Angstrom units. Following the ideas originating from Kiessling ( 7 7 ) , the borides for structural reasons are divided into six groups. The first group includes borides with the M n 4 B , D O n , T h T F c g , C 1 6 and B8 structures where the boron atoms are isolated. The second group contains phases in which boron pairs occur (CrsBg and U3SÍ2 types). In the third group the boron atoms form chains ( B 2 7 , CrB, and MoB types), in the fourth they form double chains (Ta3B4 type) and in the fifth, hexagonal nets ( C 3 2 , M02B5, and W2B5 types). The boron skeleton of the sixth group, finally, is three dimensional ( T h B 4 , CaBg, and UB12 types). The division among the various groups is not sharp. Thus, a tendency to form chains is revealed in the C 1 6 and B8 structures, while CrsBa also contains isolated boron atoms. C. A SURVEY OF THE PROPERTIES OF BORIDES AND THEIR RELATIONSHIP TO THOSE OF THE SiLICmES AND C A R B m E S

As one would expect from the position of boron in the periodic table, the borides resemble the silicides and carbides. Like the latter com­ pounds the borides are metallic. The intermediate position of the borides between the silicides (which show many similarities to proper alloy phases) and the typical "interstitial phases" found among carbides and nitrides makes a knowledge of their properties essential for the develop­ ment of a general theory of metallic compounds. Unfortunately, t h e available information about borides is scattered and rather poor. Only t h e diborides of t h e fourth, fifth, and sixth group metals are relatively well known, and therefore a general discussion of the properties of borides must be confined to these compounds. The MeaBg phases of molybdenum and tungsten are closely related to the diborides, and at high temperature M02B5 and possibly also W2B5 trans­ form into M e B z phases of the C 3 2 type (see Section IV,E). The melting points of the diborides (see Table II) are somewhat lower than those of the monocarbides, while these two classes of com-

α = 3.3O9,

c = 4.224

(71)

(69) a = 5.506, h = 2.952, c = 4.061 a = 2.969, b = 7.858, c = 2.932

a = 5.44, c = 10.07 α = 5.746, c = 3.032

a = 14.53, b = 7.293, c = 4.209 α = 4.389, = 5.211, c = 6.619 0 = 7.46^,0 = 4.713 (63) α = 4.67o α = 5.109, c = 4.249 (65)

Example Lattice parameters in Á Other representatives

PtB

(65) TiB, MnB, CoB (70) VB, NbB, TaB, β-ΜοΒΑ /ß-WBf, NiB

* V^SiB^, Mo^SiB,, W5(Si,B)3, Mn5SiB2, Fe^SiB^ Nb3B2, Ta3B2, M02NÍB2, M02C0B2

( 6 1 ) Cr.oBo.5-1 (62) C03B, (Fe,Si)3B Rh.Bs, R e B ^ (64) Ta^B, Cr^B, M02B, W2B, Mn^B, C02B, Ni^B

Ref.

MoB α = 3.110, c = 16.95 (72) WB TSL3, a = 3.29, b = 14.0, c = 3.13 (78) Nh^B,, CTS^, MUZB, AIB2 α = 3.009, c = 3.262 ( 74) MgBa, ScB^, YB2, UB2 (See Table II) M02B5 a = 7.190, α = 24° 10' (72) W2B5 α = 2.892, c = 13.17 (72) Ti3,,\ CTM ThB, a = 7.256, c = 4.113 ( 7 5 ) See Table III CaBe α = 4.415 (76) See Table IV UB12 a = 7.468 ( 7 6 ) ZrB^

RhB

V3B2 FeB CrB

CrSs

MmB NiS RuSs BcsB FeS

Formula

*The symmetry and structure of CrsBg was determined by Bertaut and Blum (66). Their value for the c-axis was probably al (68) are given erroneous, therefore the lattice parameters of Nowotny et al (67) which agree well with those of Epel'baum in the table. t Stable only at elevated temperatiures.

PQjmmc

Wamd Immm P6/mmm RSm PSs/mmc P4/mbm Pm3m Fm3m

anti-B8(AsNi)

PA/mhm Pnma Cmcm

14/mcm

Fddd Pbnm Pßzmc Fm3m M/mcm

Space group

ΜοΒ(Βί;) Ta3B4 ( Ό Ί , ) C32 (AIB2) M02B5 (D8i) W2B5(D8„) ThB4 ( D I E ) CaBe(D2a) UB12 (D2/)

B27 (FeB) CrB ( B j )

CrsB, ( D 8 1 ) U3SÍ2(D5A)

Mn4B ( D l f ) DOniFeaC) ThrFesiDlOs) CI (CaFa) C16(CuAL)

Structure type

TABLE I CRYSTAL STRUCTURE OF BORIDES

BORIDES: BASIC FACTORS

149

3.141 3.470 1.105 5.3 5.3 11.19 3250

3.169 3.530 1.115 5.6 5.5 6.082 3050 -39 2300 7-10 0.055 3.70 -20 5.8

3.028 3.228 1.066 5.6 6.6 4.478 2920 -36 3370 9-15 0.062 3.95 -24 9.7

==

These values refer to MoB 2 •

t AC == ampere gauss.

o

Thermal expansion X 106 parallel a parallel c X-ray density, gm/cm 3 Melting point, °C Heat of formation (AH 298K) kcal/ gm atom boron Microhardness (30-gm load), kg/mm 2 Resistivity (at 20°C), Ilohm-cm Thermal conductivity, cal/cm sec °C Work function (eV) Hall coefficient, 10-12 Vern/ACt Thermoelectric force (against Cu), ,uV/deg C

cia

c==

a==

-17 2.5

3.88 -1.1

10-12

3.8 15.9 5.012 2400

3.006 3.056 1.017

VB2

( -12) 2070 16-38

Narrow

Narrow

Narrow

Homogeneity range (atomic '% boron) Lattice parameters (in A)

HfB2 NbB2

---

METALS~

3.085 3.31] 1.073

4.3

-18 2200 12-65 0.040 3.65 -1

5.9 8.4 6.95 (av) 3050

3.110 3.264 1.050

Extended

TABLE II THE TRANSITION

ZrB2

DmORIDES OF

TiB 2

THE

Property

PROPERTIES OF

3.057 3.291 1.076

5.0

-26 2500 14-68 0.030 2.89 -2.7

5.2 5.9 12.56 (av) 3200

3.099 3.224 1.040

64 to 72 at %B

TaB 2

10

3.36 -1.1

-15 1800 21-56

6.0 6.7 5.196 1900

0

3.38 +0.1

-10 2330 18-45

7.6 0 7.8 0 7.48 2100 0

3.011 20.93

Extended

Narrow 2.969 3.066 1.033

Mo2Bs

CrB 2

Mo 2Bs AND W 2B5

0

2.62 -1.6

-9 2660 21-56

13.1 2200

2.982 13.87

Extended

W 2Bs

BORIDES: BASIC FACTORS

151

" Unfortunately most of t h e m e a s u r e m e n t s h a v e b e e n m a d e on s p e c i m e n s w i t h i n c o m p l e t e l y specified purity and porosity; information about their h e a t treatment is often also limited. Therefore, t h e reliability of m a n y results are difficult to estimate and it is r e c o m m e n d e d that t h e critical reader c h e c k the values in T a b l e II ( w h i c h t h e author finds m o s t p r o b a b l e ) against t h e original papers. References are g i v e n b e l o w . T h e httice parameters (Ip.) of TiB,. are taken from papers b y N o r t o n et al. (44) a n d Baroch a n d E v a n s (49) a n d agree w i t h i n 0 . 1 - 0 . 1 % w i t h several others (48, 78, 79). T h e l.p. of ZrB2 g i v e n b y Kiessling {80) a n d E p e l ' b a u m a n d G u r e v i c h (81) also agree w e l l w i t h various other results ( 4 4 , 49, 79). S o m e w h a t l o w e r values for t h e l.p. of TÍB2 a n d ZrBz w e r e recently reported b y M e e r s o n et al. (82), b u t t h e s e are p r o b ­ ably c a u s e d b y t h e p r e s e n c e of iron ( ~ 1 % b y w e i g h t ) in their samples. T h e l.p. of HfBa d e r i v e from Glaser et al. (83). N o variations of t h e l.p. of TÍB2, ZrBz, a n d HfBz w i t h composition h a v e b e e n o b s e r v e d . T h i s indicates that t h e h o m o g e n e i t y ranges of these p h a s e s are narrow. T h e l.p. of VBo w e r e g i v e n b y M e e r s o n a n d S a m s o n o v (48) for a 9 9 . 8 % pure product. N o r t o n et al. (44) f o u n d t h e l.p. of VB2 in a 9 5 % alloy t o b e α = 2 . 9 9 β A, c — 3 . 0 5 7 A. T h e h o m o g e n e i t y r a n g e of VB2 is u n k n o w n . NbBz has an e x t e n d e d single p h a s e region. T h e l.p. values are t h o s e of B r e w e r et al. (79). T h e left v a l u e corresponds to t h e boron-poor limit, t h e right v a l u e to t h e boron-rich limit. V a l u e s b e t w e e n t h o s e g i v e n b y B r e w e r et al. (79) h a v e b e e n reported ( 4 4 , 48); Andersson a n d Kiessling (84) state t h e l.p. are a = 3 . 0 8 9 A, c = 3 . 3 0 3 A for a n alloy w i t h t h e stoichiometric composition. A c c o r d i n g to Kiessling ( 7 3 ) TaB2 is h o m o g e n e o u s from 6 4 t o 7 2 atomic % boron a n d his l.p. are s h o w n in t h e table. B r e w e r et al. (79) cor­ roborated t h e values of Kiessling w i t h i n 0 . 1 % . V a l u e s i n t e r m e d i a t e t o t h o s e of t h e table h a v e also b e e n reported ( 4 4 , 48). T h e l.p. of CrB2 are t h o s e of Kiessling (70). Rather similar values w e r e r e c e n t l y p u b ­ lished b y E p e l ' b a u m et al. (68). T h e values for M02B5 ( c o n t a i n i n g 7 0 a t o m i c % b o r o n ) a n d W 2 B 5 ( 6 7 - 6 8 a t o m i c % b o r o n ) are also t h o s e of Kiessling (72). N o information about l.p. variations w i t h c o m p o s i t i o n is available for t h e l a s t - m e n t i o n e d p h a s e s . T h e coefficients of linear thermal expansion were given by Binder and Moskowitz (85). A v e r a g e values of thermal expansion h a v e b e e n d e t e r m i n e d b y M e e r s o n et al. (82): 6.8, 8.4, a n d 9.8 X 10-' for TÍB2, ZrBs, a n d CrBz, respectively, w h e r e a s N e s h por a n d S a m s o n o v (86) report t h e v a l u e s 6.4, 6.8, a n d 1 1 . 1 X 10-*, r e s p e c t i v e l y . T h e melting points originate from t h e p a p e r b y Post et al. (87) a n d agree w i t h i n ± 1 0 0 ° w i t h t h o s e of Kieffer et al. (47). Glaser (88) reported a m u c h higher m e l t i n g point for W 2 B 5 , n a m e l y , 2 9 8 0 ° C . T h e heats of formation h a v e b e e n c o m p u t e d b y B r e w e r a n d Haraldsen (89) from studies of equilibria in M e - B - N a n d M e - B - C s y s t e m s . S a m s o n o v e s t i m a t e d t h e h e a t of formation of 1/2 VB2 to b e - 1 2 kcal ( 9 0 ) . Gilles a n d Pollock (91) d e t e r m i n e d the heat of formation of M0B2.33 to b e — 8 . 6 k c a l / g m a t o m boron, in g o o d a g r e e m e n t with t h e findings of Brewer a n d Haraldsen. T h e values of microhardness are taken from papers b y S a m s o n o v a n d co-workers (92,93). Kieffer et al (47) reported similar v a l u e s for TÍB2 a n d ZrB2. T h e values of electrical resistivity s h o w extreme scattering. T h e limits w i t h i n w h i c h t h e true values are likely to b e f o u n d ( 5 6 , 88, 94-97) are s h o w n in t h e table. S i n c e m o s t of t h e errors w i l l cause t h e values to b e too h i g h , values near t h e l o w e r limit are probably m o r e correct. T h e thermal conductivity of TÍB2 a n d ZrBo at 2 0 0 ° C w a s d e t e r m i n e d b y Norton et al. (44). T h e v a l u e for NbB2 is taken from t h e b o o k b y Kieffer a n d Schwarzkopf (1) ( n o reference to original p a p e r is g i v e n ) ; for TaBz Kieffer a n d Schwarzkopf ( 1 ) report 0 . 0 2 6 , a n d S a m s o n o v a n d Markovskii (2) report 0 . 0 3 3 c a l / c m s e c ° C . T h e work function of TaB2 is that g i v e n b y G o l d w a t e r a n d H a d d a d (98); all the other values are taken from a p a p e r b y S a m s o n o v et al. (96). T h e Hall coefficients w e r e d e t e r m i n e d b y Juretschke a n d Steinitz (94) o n single crystals, a n d t h e thermoelectric p o w e r against c o p p e r b y S a m s o n o v a n d Strel'nikova (99).

152

BERTIL ARONSSON

pounds have about the same hardness and, consequently, are consider­ ably harder than the silicides. Most of the borides are chemically very stable and are attacked only by strongly oxidizing chemicals or fused alkali (1,5,51-54), Evidently no boride shows the same resistance to oxidation at elevated temperatures as do some silicides (in particular, M0SÍ2).

From a crystal chemical point of view the borides of the fourth to sixth group metals show many similarities to the silicides. Nonmetal-rich phases (in particular of the type MeXa) are frequent in both these two classes of compounds. This points to a relatively great importance of nonmetal-nonmetal bonding, and a tendency to such bonding is also revealed in more metal-rich phases. The carbides and nitrides of the fourth to sixth group metals, on the contrary, never contain more than 50 atomic % nonmetal, and bonding between nonmetal atoms seems to play no important role in these phases. For the borides of the iron and platinum metals the situation is different—the metal-rich borides of these metals show many similarities to the carbides. Thus NÍ3B and C03B are isomorphous with Fe^C (62) and a large carbon-boron substitution has been observed in C03B (62) and in FcaC (100). RU7B3 and Rh7B3 are structurally closely related to the carbides of the CT^CS type (63). As seen in Table II, the diborides are excellent conductors of elec­ tricity and heat. The temperature coefficient of the resistivity is positive (95,97), and the Wiedemann-Franz ratio has the normal value for metals and alloys (44). Thus, there is no doubt that the borides should be classified as metallic phases. In general, the borides are better con­ ductors of electricity than the carbides, and the diborides of titanium, zirconium, and hafnium are comparable with the disilicides of the fifth group metals in this respect. None of the diborides is a superconductor—the only known super­ conducting borides are M02B, W2B, and NbB (101,102). The thermo­ electric power against copper is lower for the diborides than for the monocarbides (99). According to Juretschke and Steinitz (94), all diborides except ZrB2 are weakly paramagnetic. The susceptibility increases from the fourth to the fifth group metals. As is evident from the heats of formation and the melting points, there is a marked decrease in stability of the diborides with increasing group number of the metal. This tendency has also been observed for related compounds (103-105), but it becomes gradually less pronounced in the sequence: nitrides, carbides, borides, sihcides. Thus, while the carbides of the iron metals are unstable, some quite stable borides are formed by these metals. Silicon reacts even with copper to form well-defined phases.

BORIDES: BASIC FACTORS

153

while boron does not. The comparatively high stability of the borides of the eighth group metals indicates that the problem of "cementing" borides in order to get a reasonably ductile product will be more difficult to solve than it was for the carbides. D . THEORIES OF BONMES

In his paper on hydrides, borides, carbides, and nitrides of the transition elements, Hägg (106) pointed out that when f x / f M e < 0.59 the metal atoms of these compounds often form a simple close-packed lattice, in the interstices of which the nonmetals are accommodated. When the radius ratio exceeds 0.59, more complicated structures arise. Since ΓΒΜΙΘ is greater than 0.59 for most of the transition metals, it is consistent with the original ideas of Hägg that the borides are structurally related to the carbides of chromium, manganese, and iron (for which f c / ^ M e ^ 0.59) while they show few similarities to the carbides of the fourth to sixth group metals (except chromium) for which fc/rne < 0.59. Even with a favorable f ß / ^ M e ratio, however, it seems questionable whether any borides of the typical "interstitial" type are formed. The existence of cubic TiB and ZrB with the Bl(NaCl) structure (in which the metal atoms have a cubic close-packed arrangement) cannot be regarded as definitely proved (see Section IV,C), and no such phases have been reported for scandium and yttrium. Kiessling (77) ascribes the re­ luctance of boron to form typical interstitial phases to the importance of boron-boron bonds in the borides. The existence of such bonds is strongly supported by a study of the lattice parameters of a series of isomorphous borides. Thus, Kiessling pointed out that when a larger metal atom is substituted for a smaller one in borides of the MeB and Μ θ 3 Β 4 types, the lattice is expanded less in the direction of the boron chains than in the other directions. The same tendency is revealed by the diborides (Table I I ) . On passing from CrBa to ZrBg the a&-plane which contains the B-B contacts is less expanded than the c-axis. The question of electron transfer in borides has been discussed at some length. Pauling (107) has suggested that electrons are transferred from the iron atoms to the boron atoms in FeB, but Kiessling (77) believes the transfer is in the opposite direction. The idea that boron atoms act as electron donors has been further developed by Kiessling (108) and also by Robins (109), Samsonov (110) has discussed the importance of an incomplete d-shell of the metal to form a stable boride. In the borides the d-shells have been somewhat filled at the expense of boron electrons. He also uses the expression that the boron atom con­ tributes electrons to the electronic collective. According to Samsonov the electrons are more tightly bound to the nonmetal atoms in the nitrides

154

BERTIL ARONSSON

and carbides, since nitrogen and carbon have a higher ionization potential (i.e., are more electronegative) than boron. In order to get some idea of the band structure of the diborides, Juretschke and Steinitz (94) made a comprehensive study of the elec­ trical properties of these compounds (see Table I I ) . They concluded that the band structure of the metals is not drastically changed on the com­ bination with boron. One important difference is that the center of gravity of the d-band is lower in the diborides than in the corresponding metal. The 5-band is filled only to a very small extent in TiBg, ZrBg, and HfBs, and the additional metal electrons in the diborides of the fifth and sixth group metals are accommodated mainly in this band. This picture is consistent with the trends in electrical properties. As the density of states is low in the 5-band, it is reasonable that the cohesive energy decreases when additional electrons fill the band. Thus, the interpretation of Juretschke and Steinitz explains the decreasing stability as well as the decreasing value of the work function with increasing group number of the metal. IV. A Systematic Treatment of Binary Metal-Boron Systems and Their Intermediate Phases This section presents a critical survey of the present knowledge of metal-boron systems and of the properties of binary borides. It is ar­ ranged according to the position of the metal in the periodic table. A . BORIDES OF THE ALKALI METALS, BERYLLIUM, MAGNESIUM, AND ALUMINUM

The alkali metals do not form any well-defined borides, but sodium atoms are able to substitute for 77% of the thorium atoms in ThBg, and sodium is possibly also dissolved by other hexaborides (76). According to Markovskii and Kondrashev (64) who recently reviewed the borides of the first and second group elements, beryllium forms three borides, BcoB, BeB2(?), and BeBc(?). BcsB crystallizes in the CI (CaFa) structure ( a = 4.67o A) and thus is isomorphous with B c g C . Βθ2Β is easily decomposed by dilute acids, whereas BeB2(?) and BeBrX?) are chemically much more resistant (64), The existence of magnesium diboride, MgBo, has been ascertained ( I I I , 112). The crystal structure of MgB^ is the C32 type with a = 3.084 A, c = 3.522 Ä and has an observed density of 2.667 gm/cm^ (HI) - Like BcgB, MgBo is easily dissolved by dilute HCl, while a more boron-rich phase MgB4 (111) is not attacked by dilute acids. Two more "insoluble" magnesium borides with the compositions MgB,j(?) and MgBi2(?) have also been reported (64), The aluminum borides were recently comprehensively reviewed by

BORIDES: BASIC FACTORS

155

Kühn et ah {113), Four aluminum borides with narrow homogeneity ranges exist: namely, AIB2, AlBio, o:-AlBi2, and ^-AlBi2. The earlier reported monoclinic AIB12 is identical with β- rhombohedral (II) elemen­ tal boron (113,114), According to Hofman and Jänicke (115) the solid solubility of boron in aluminum is very small. These authors also estab­ lished the existence of AIB2 and determined its crystal structure which has later been found among a great number of diborides. The most recent values of the lattice parameters of AIB2 are a = 3.009 A, c = 3.262 A (74). AlBio has a melting point of 2000-2100°C and crystallizes in an orthorhombic structure (a = 8.881 A, = 9.100 A, c = 5.690 A), a-AlBi2 (red-transmitting) has a tetragonal structure (a = 10.28, c = 14.30 A), whereas ß-AlBi^ (amber-colored) is orthorhombic (a = 12.34 A, fc = 12.631 A, c = 10.161 ) (113). AlB^o (113) and βΆ\Β^2 (116) are harder than silicon carbide. The semiconducting properties of boronaluminum compounds (AIB12) were studied by Lagrenaudie (117). B. T H E BORIDES OF THE ALKALINE EARTHS (EXCEPT BERYLLIUM AND MAGNESIUM), RARE EARTHS, AND A c T i N m E S

The borides of the alkaline earths, rare earths, and actinides are closely related and are best treated in the same paragraph. Borides of the ThB4 and CaBe types are common. Attempted preparations pf borides richer in boron than the hexaborides were without success (118), and very few borides containing less than 80 atomic % boron have been reported. The borides of the rare earths were recently reviewed by Samsonov (118a). 1, Borides of the ThB^ Type The lattice parameters of known MeB4 phases are given in Table III. T h B 4 and U B 4 probably have narrow homogeneity ranges (71). Varying parameters (118,119) have been reported for GdB4 and these variations may depend upon an extended homogeneity range of this phase. The most stable MeB4 phases are formed by the actinides. T h B 4 and U B 4 are the highest melting compounds of the Th-B and U-B sys­ tems (79), The tetraborides of the rare earths, on the contrary, seem to have lower melting points than the corresponding hexaborides [it is certainly so for cerium (79)] and the alkaline earths do not form any tetraborides at all. No information about the physical properties of the MeB4 borides is available. 2. Borides of the CaBe Type Some properties of these hard and high-melting compounds are col­ lected in Table IV. According to LaflFerty (124), who made a very com­ prehensive study of the properties of LaBe, the MeBß phases may lose

156

BERTIL ARONSSON TABLE I I I LATTICE PARAMETERS (IN A ) OF MEBÍ BORIDES

YB4

LaB.

CeB4 PrB4 NdB4 SmB4 GdB4 TbB4 DyB. H0B4 ErB^ YbB4

LuB4 ThB4 UB4

PuB4

α-Axis

c-Axis

Ref.

7.09 7.30 7.205 7.20 7.219 7.12 7.12 7.118 7.23 7.15 7.12 7.01 7.15 7.256 7.075 7.10

4.01 4.17 4.090 4.11 4.1020 4.07 4.05 4.028e 4.09 4.10 3.98 4.00 4.045 4.113 3.975 4.014

(131) {118b) (75) (118) (118c) (118) (118) (118c) (119) (119) (119) (118) (119) (75) (75) (118d)

metal atoms while retaining the structure. The existence of extended homogeneity ranges might explain the discrepancies found among the various lattice parameter determinations. The alkaline-earth and rareearth hexaborides have higher melting points and greater hardness than ThBö. The MeBe phases are good conductors of electricity. The thermal coefficient of resistivity is positive for LaBe {124) and ThBg {123) and this is presumably true also for the other representatives. Especially interesting are the thermoemission properties which make these com­ pounds useful as cathode materials. The hexaborides are chemically very stable but are attacked by warm oxidizing acids and fused alkali (5). Europium and ytterbium form hexaborides with larger unit cells than the neighboring elements (see Table IV). It has been pointed out that this fact is consistent with the "anomalous" radii of these metals {118,119,121), Neshpor and Samsonov {119) assume that europium and ytterbium are divalent in the MeBe borides, whereas the other rare earths are trivalent. The trivalency of cerium, neodymium, and gadolin­ ium but not the divalency of ytterbium is supported by results of mag­ netic measurements {129). The connection between the electronic struc­ ture of the rare-earth hexaborides and their thermoemission properties has recently been discussed {128), The preparation and properties of YBg {122), EuBe {121), and ThBe {122) have been studied at some length by Samsonov and co-workers.

YbBe LuBe ThBe

7.088

5.552

5.55x

2195

1740

37 2.92

3.0

3.37

0.5

3.13 0.36

9.9

(-2.5)

* Most of the lattice parameters (l.p.) are taken from the paper by Blum and Bertaut (76) whose values agree well with the results of Post et al (118). The l.p. of LaBe, PrB«, and ErBe are due to von Stackelberg and Neuman (120), those of EuBe (121) as well as of DyBe, HoBe and LuBe (119) derive from papers by Samsonov and co-workers. Lattice parameters very different from those given in the table have been reported for YBe (a = 4.128 A) (122) and ThBe (a = 4.326 A) (123), The melting points, except that of YBe (122), originate from Lafferty (124). Samsonov and Zorina (123) found the melting point of ThB« to be 2150°C. The coefficients of linear thermal expansion are taken from the papers of Zhdanov et al. (125) (left value) and Samsonov and Grodstein (J26) (right value). Kudint'seva et at, (122) reported the value for YB«. The thermoemission constants have been determined by Lafferty (124) (CaBe, SrBe, BaB«, LaB«, CeBe, and ThBe) Samsonov and Grodstein (126) (work func­ tions of CaBe, BaBe, LaBe, and CeBe are in good agreement with the values of Lafferty), and Kudint'seva and Tsarev (127,128). The values of microhardness and resistivity are taken from papers by Samsonov and co-workers (122,123,126). Lafferty (124) found the resistivity of LaBe to be 27 /lohm-cm at room temperature. t This value, quoted by Samsonov (118a), seems improbable.

4.110

4.144 4.11 5.738 4.113

ErBe

Boride CaBˇ SrBe BaBe ScBe YBe LaBe CeBe PrBe NdBe SmBe EuBe GdBe DyBe HoBe

Thermal Resistivity Thermoemission constants Cube edge X-ray denMelting expansion, Microhard/iohm-cm (inÁ) sity(gmW) point, °C xW ness, kg/mm"^ (20°C) ^ev A/cm'(°C) 4M5 2.44e 2235 5.2r-6.5 2740 123.5 2^86 2^6 4.198 3.424 2235 0.67 0.14 4.268 4.31e 2270 6.8-6.1 3000 306 3.45 16 4.435t 2.094t 4.113 3.67i 2300 5.85 3260 2.2 4.153 4.725 2210 6.4-4.9 2770 17.4 2.66-2.74 29 4.141 4.79. 2290 7.3-6.2 3140 60.5 2.59-2.93 3.6 4.129 4.855 3.12^.46 -300 4.128 4.938 3.97-4.57 '^420 4.13 5.07e 4.4 4.167 4.97« 4.9 4.112 5.29e 2.05 0.84 4.13 5.35« 3,53 25.1 4.13 5.45T 3.42 13.9

TABLE IV.* PROPERTIES OF MEBO BORIDES

BORIDES: BASIC FACTORS

157

158

BERTIL ARONSSON

In addition to the phases in Table IV, recent studies have shown that also the following hexaborides with the CaBe structure exist: TbBe (a = 4.102 A) {118c); TmBg {129a); and PuB« (α - 4.115 - 4.140 A) {118d). 3, Other Borides Only the hexaborides are known for calcium, strontium, and barium. Borides of the C32 ( A I B 2 ) type have been reported for scandium {130) and yttrium {131). The lattice parameters of SCB2 are α = 3.146 A, c — 3.517 A, and the experimental density is 3.67 gm/cm^ {130), Binder {131) found a tetragonal YB^. {x <6) phase which is isomorphous with LaB;r, PrB^, GdB^ and ΥΒ,^ reported by Post et al. {118), Brewer et al, (79) did not find any borides richer in cerium than CeB^, but they dis­ covered a ThXg phase in the Th-B system. They supposed this phase to be identical with a thorium boride reported earlier by Hägg {132) and Andersson and Kiessling (84), but believed that it contained a con­ siderable amount of oxygen. Uranium forms the borides U B 2 (C32 type, a = 3.136 A, c = 3.998 A) (79), U B 4 (see Table ΠΙ) and UB12 (UB12 type, A = 7.468 A). The coefficients of thermal expansion of U B 2 (20°-205°C) are 9 X 10"^ (in the basal plane) and 8 X 10"^ along the c-axis {133). According to McDonald and Stuart {118d) the plutonium borides PuB {Bl structure; a = 4.92 A) and PuBs (C32 structure; a = 3.18 Ä, c = 3.90 Ä) exist. C.

BoRmEs OF TITANIUM, ZIRCONIUM, AND HAFNIUM

1. The Ti-B System Although many investigations have been devoted to this system, it still has points which need clarifying. Ehrlich (78) and Brewer et al (79) prepared alloys by sintering the elements in vacuo, while Palty et al. {134) used arc melting techniques. The phase diagram of Palty et al {134) (omitting TÍ2B, see below) is shown in Fig. 1. The solid solubility of boron in titanium is small, probably below 0.1% by weight {134,135), Ehrlich (78) observed "superstructure lines" in the powder photographs of alloys in the composition range TiBo.iTiBo.8. It has been suggested that these superstructure lines are derived from a hexagonal intermediate phase with the approximate composition TÍ2B {136). TÍ2B has been reported in more recent papers {88,134, 137) but is there claimed to possess a tetragonal structure. The published X-ray data for "tetragonal TizB" show that this phase is very likely identical with orthorhombic TiB {138), and thus no phase with the ideal composition TisB seems to exist.

159

BORIDES: BASIC FACTORS

leU

2600

31.2

36.1

weighty^

A

/

2200-\

ΙΘΟΟλ TiB

1^00

λ

IODO

TiB 600

Ti

?

20

60

ΘΟ

ATOMIC PER CENT

FIG. 1. T h e T i - B system, m a i n l y according to Palty et al American Society F o r M e t a l s . )

(134).

( C o u r t e s y of

Two phases of the composition TiB have been claimed to exist. Orthorhombic TiB (with B27 structure) seems to be firmly established (134,188). A cubic TiB phase (a = 4.21 A) was first reported by Ehr­ lich (78) who supposed it to have the B3 (zincblende) structure. An­ dersson and Kiessling (84) emphasized that the Bl (NaCl) structure (in which the metal atoms have the same arrangement) is much more probable—a point of view accepted by subsequent investigators (88, 134,137). The existence of a binary cubic TiB phase has been doubted by Brewer et al. (79) who believe Ehrhch's "TiB" to be TiN. Glaser (88) found cubic TiB (a = 4.26 A) in samples with very low nitrogen and carbon contents and concluded that binary cubic TiB did exist. No analyses for oxygen were carried out, however, and the small cube edge of Glaser's TiB suggests that it might be a T i ( 0 , B) phase with an atomic ratio B/O of about 0.5. This would also explain why Ehrlich and Palty et al. observed three phases when "TiB" was present. The question of whether cubic TiB exists or not cannot be definitely answered before the Ti-B system has been critically re-examined by use of completely analyzed alloys.

160

BERTIL ARONSSON

TÍB2 [first discovered by Andrieux (5)] has a narrow homogeneity range and a great chemical stabihty (5). TÍ2B5 was reported by Glaser (8S), who was unable to verify the existence of a more boron-rich titanium boride (139). The properties of the titanium borides are collected in Table V [see also Table II (TÍB2)].

2. The Zr-B System The phase diagram of Glaser and Post (140) is shown in Fig. 2 (ZrB has been omitted, see below). A small solid solubility of boron in zirconium has been reported (80,141), While pure zirconium has the lattice parameters a = 3.2312 A, c = 5.1477 A (60), the lattice parameters of zirconium saturated with boron were found to be fl 3.249 A, c = 5.203 A (80) or a = 3.253 A, c = 5.191 A (1900°C) (141), Much greater lattice parameters of boron-saturated zirconium, α = 3.37 A, c = 5.57 A (137), have been doubted (141) and do not seem very probable. It must be mentioned that the oxygen content of the samples used in the quoted investigations are unknown, and the enlarged unit cell of zirconium could also be caused by the presence of this element. Cubic ZrB (a = 4.65 A) was found by Post and Glaser (137,140), but its existence has not been verified (79,80,102). The small cube edge suggests that the reported ZrB phase might have contained a consider­ able amount of oxygen (or carbon and nitrogen). The most stable zirconium boride, ZrBa, was already described by McKenna (43) and is fairly resistant against nonoxidizing acids ( Í ) . ZrBi2, finally, was discovered and studied by Post and Glaser (140, 142). It has approximately the same hardness as ZrB2 (142). The properties of zirconium borides are collected in Table V (for ZrB2, see also Table I I ) . 3. The Hf-B

System

Glaser et al. (83) made some Hf-B alloys by hot pressing hafnium (97%) and boron (96.5%). They found HfB2 (C32 type) and HfB (Bl type) but were unable to prepare HfBi2. The oxygen content of HfB was not determined but may have been important for the occurrence of this phase. D . BORIDES OF VANADIUM, NIOBIUM, AND TANTALUM

Kieffer and Benesovsky (143) in collaboration with Nowotny, Fin­ deisen, and Piegger recently drew tentative phase diagrams (Figs. 3-5) of the V-B, Nb-B, and Ta-B systems based on their own studies of

C32 UB12 C32

ZrBo ZrBa2 HfBa

a = 3.168, c = 3.532 α = 7.408 fl = 3.141, c = 3.470

4.63

2920

2200 (?) f

4.48 6.09 3050 3.6 2680 11.2 3250

4.56

60 10-12

7-10

9-15

/xohm-cm point, ° C

(20°C)

Resistivity,

* References: For references to values for TÍB2, ZrBs and HfBs, see Table II. The l.p. of TiB derive from Decker and Kasper (J38), those of TÍ2B5 from Post and Glaser (137) and that of ZrBu from Glaser (142). The melting point of TiB is taken from the diagram of Palty et al. (134), that of ZrBi2 from Glaser and Post (140). The resistivity of ZrBi2 was given by Post and Glaser (142). f Melts incongruently.

W2B5

TÍ0B5

c = 13.98

a = 3,028, c = 3.228

a = 2.98,

a ~ 6.12, h = 3.06, c = 4,56

X-ray density Melting Lattice parameters (in A ) (gm/cm')

TÍB2

Structure type

TiB (= "TÍ2B") Β2Ί

C32

TABLE V *

SOME PROPERTIES OF THE BORIDES OF TITANIUM, ZIRCONIUM, AND HAFNIUM

BORIDES: BASIC FACTORS

161

162

BERTIL ARONSSON

26eo±ioo

AO 60 ATOMIC PER CENT

FIG. 2. T h e Zr-B s y s t e m , a c c o r d i n g to Glaser a n d Post {140). ( C o u r t e s y of American Institute of M i n i n g , Metallurgical, a n d P e t r o l e u m E n g i n e e r s . )

pressure-sintered and arc-melted alloys (prepared from metal hydride and elemental boron) and available literature data (see below). More recently the quoted authors have published slightly modified phase diagrams of the same systems {143a, 14Sb). 1. The V-B

System

The sohd solubility of boron in vanadium is probably below 0.25% by weight (144). When the composition of the system was approxi­ mately VgB, Hardy and Hülm (102) observed two phases. In arc-melted samples only one of these was present, but in specimens annealed at 1500°-1700°C both appeared in about equal amounts. Kudielka et al {145) who prepared alloys by hot pressing the elements at 1600°C and subsequent annealing at 2000°C observed only one phase around the composition V2B. This phase [identical with one of the "V2B-phases" of Hardy and Hülm {102)] was later shown to have the U3SÍ2 structure (69). VB was first reported by Blumenthal {146) who found the crystal structure to be the CrB type. This structure has been corroborated in more recent papers {102,145). It has been suggested that VB is formed

163

BORIDES: BASIC FACTORS

I2U

17^.5 22.1

29.-hj<.7

weighty^

2^00

2100A

ΪΘΟΟ

/500-I

1200

900 <0 60 ATOMIC PER CENT

FIG. 3 . T h e V - B system, according to KieflFer a n d B e n e s o v s k y {143). of P o w d e r Metallurgy.)

(Courtesy

by a peritectic reaction VB2 + liquid = VB {144), but according to KieflFer and Benesovsky {143) it has a melting point maximum. V3B4 has the Ta3B4 structure {145,147), VB2, which was first prepared by fused salt electrolysis, is only slowly attacked by nonoxidizing acids but rapidly dissolves in concentrated H N O 3 , H2O2, and fused alkah ( 5 ) . V2B5 with the W o B r , structure has also been reported {148), but ac­ cording to Nowotny et al. {143a) its existence seems very doubtful. Some properties of vanadium borides are found in Table VI [see also Table II ( V B 2 ) ] . 2. The Nb'B System Systematic investigations of the Nb-B system have been performed by Andersson and Kiessling {84), Brewer et al. (79), Kudielka et al, {145), and KieflFer and Benesovsky {143), The tentative phase diagram of KieflFer and Benesovsky is shown in Fig. 4. No significant sohd solu­ bility of boron in niobium has been observed. Three phases are reported to exist in the range Nb-NbB. The cubic j8'-phase of Andersson and Kiessling {84) was actually NbO as shown by Kudielka et al, {145),

7.71-8.00

a = 3.298, b = 8.724, c ~ 3.166 7.563 a = 3.305, b = 14.08, c = 3.137 7.325 (See Table II) 6.95 0 = 5.778, c = 4.864 a = 6.I84, c = 3.28, a = 3.276, b = 8.669, c = 3.157 14.29 a = 3.29, b = 14.0, c = 3.13 13.60 (See Table II) 12.56

0 = 6.185, c = 3.28i

α = 5.74β, c = 3.032 5.46-5.83 a = 3.058, b = 8.026, c 2.971 5.626 a = 3.030, b = 13.18, c = 2.986 5.461 a = 3.006, c = 3.056 5,012

Structure type Lattice parameters (in A)

2700t

2300f

3050 15.24 14.72-15.01 2400 2650t 3200

2300

1850t

2400

2040 2250

X-ray density (gm/cm^)

14-68

2100t -TOO

12-65

64

16-38

35-40

Melting point, °C

/żohm-cm (20°C)

Resistivity,

* References: For references to VB2, NbBz, and TaBa, see Table II. The l.p. of V3B2, Nb3B2, and Ta3B2 are those reported by Nowotny and Wittmann (69), those of VB by Hardy and Hülm (102), those of V3B4 by Moskowitz (147); while those of NbB, Nb3B4 (84), and of Ta2B, TaB and TaS^ (73) were all determined by Kiessling. Melting points are taken from the diagrams of Kieffer and Benesovsky (143), and all the values of resistivity were reported by Glaser (88). t Melts incongruently.

C32 C16 U3SÍ2 CrB Ta3B4 C32

NbB2 Ta2B Ta3Bi-2 TaB TasB* TaBo

CrB

Ta3B4

Nb3B4

NbB

U3SÍ2

CrB TasB* C32

VB V3B4 VBo

NbsBi-z

U3SÍ2

V3B1-2

TABLE VI *

SOME PROPERTIES OF THE BORIDES OF VANADIUM, NIOBIUM, AND TANTALUM

164 BERTIL ARONSSOK

BORIDES; BASIC FACTORS

165

1000

<0

60

ATOMIC PCR CENT

FIG. 4 . T h e N b - B system, according to Kieffer a n d B e n e s o v s k y (143). of P o w d e r M e t a l l u r g y . )

(Courtesy

The NbB^ phase of Brewer et al. (79) and the /?"-phase of Andersson and Kiesshng (84) is very probably identical with N b g B s (145), the crystal structure of which is the U3SÍ2 type (69). The composition and equilibriums of the j5-phase (84) and of N b B n (79) are unknown, and these phases are not indicated in Fig. 4. NbB and N b s B * were first established by Andersson and Kiessling (84) who also corroborated the existence of the earlier reported NbB2 (44,51). NbB2 has an extended range of homogeneity (79,84). NbB is a superconductor with a transition temperature of 8.25°Κ (101). Some other properties of the niobium borides are collected in Tables VI and II (NbB2). NbBa is resistant to oxidizing acids but is attacked by molten alkah (51). 3. The Ta-B System Kiessling (73), Brewer et al. (79), and KiefiEer and Benesovsky (143) have systematically studied this system. A tentative diagram was recently published (143) and is shown in Fig. 5.

166

BERTIL ARONSSON

3000

2500

2000 λ

1500λ

1000 λ

500 <0

60

ATOMIC PER CENT

FIG. 5. T h e T a - B system, according to KieflFer and B e n e s o v s k y {143). of P o w d e r M e t a l l u r g y . )

(Courtesy

When anneahng a Ta-B alloy containing 10 atomic % boron in silica tubes, Kiessling (73) observed that the cube edge of tantalum increased with increasing annealing temperature and concluded that there is a small solid solubility of boron in tantalum. However, the increasing cube edge of tantalum might, to a considerable extent, have been caused by the presence of oxygen. This would explain why Kiessling did not get reproducible results when he attempted to determine the solubility limit. TaaB (C16 structure) was first reported by Kiessling (73), and its existence has been corroborated (79,145). TagB has never been pre­ pared in a pure state but has always been obtained together with Ta or Ta + TaB. Kiessling (73) and Brewer et al. (79) ascribe this effect to nonequihbrium conditions, but a possible explanation is also that these authors, in reality, studied an oxygen-poor (0.5 wt. %) section of the Ta-B-O system. Kieffer and Benesovsky have omitted Ta^B from their diagram but in a more recent paper on the Ta-B system (148a), TasB is included and the existence of the two-phase regions Ta + TagB and TaoB + TasB^ is estabhshed. Kiessling (73) identified TaB and T a 3 B 4 (narrow homogeneity

167

BORIDES: BASIC FACTORS

ranges) by determining their crystal structures and showed that TaBg had an extended homogeneity range. These findings were confirmed by Brewer et al (79). Like NbBg, TaBs is not even attacked by oxidizing acids, but it dis­ solves in molten alkali (51). Some properties of the tantalum borides are shown in Tables VI and II (TaBs). E.

BORIDES OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN

1. The Cr-B System Much interest has been devoted to the Cr-B system, but unfortu­ nately, the results of the various studies are often inconsistent. The tentative diagram of Kieffer and Benesovsky {143), based on their own and literature data, is shown in Fig. 6. I

I

I

I

a < //./ 172 22.1 29.2^3<2

-^O

1

weighty^

60

ATOMIC PCR CENT

FIG. 6. T h e Cr-B system, according to Kieffer a n d B e n e s o v s k y {143). of P o w d e r M e t a l l u r g y . )

(Courtesy

The solid solubility of boron in chromium is small (70,149). EpeFbaum et al. (149) observed hexagonal ''ß-Ci' in chromium-rich alloys annealed at 1300°C in purified argon. In the absence of analyses for

168

BERTIL ARONSSON

nitrogen, the possibihty that the "ß-Ci' of these authors was, in reality, CrzN (perhaps with dissolved boron) cannot be excluded. This would explain why they observed three phases ( a - C r , "/ßCr," and CrgB) in some of their samples. There is general agreement that the most chromium-rich intermediate phase crystallizes in the Mn4B structure (66, 67,150,151). [Kiessling first believed his "CrgB phase" to possess a structure of lower symmetry (84) but revised his opinion in a later paper (J50)]. Kiessling (70), who chemically analyzed selected alloys, and Epel'baum et al. (6S), who made chemical and metallographic analyses, found the composition to be near the ideal one, that is CTZB. [In the analogous manganese phase only half of the boron positions are occupied, and therefore Kiessling gave the formula Mn4B (61).] Bertaut and Blum (66) and Nowotny et al. (67) believe the composition to be around Cr4B but give no analytical data to support this proposition. It seems probable that the composition of the most chromium-rich boride of the Cr-B system is around CvJB. Another compound of this composition was isolated by fused salt electrolysis (52) and was found to have the C16 structure (66). This phase was corroborated by Nowotny et al. (67) but was not seen in other studies (68,70). The conditions for the occurrence of this phase need clarifying. The ideal composition of CrgBs was given by Bertaut and Blum (66) who solved its crystal structure. Kiessling verified the existence of CrB and, in addition, discovered Cr3B4 and CrBa (70,84). CrB was independently reported and described by Sindeband (40) and Frueh (152). According to Schwarzkopf and Glaser (148), CrgBs with W 2 B 5 structure exists above 1400°C. No variations of the lattice parameters with composition have been observed for any chromium boride. This has been taken to indicate that the homogeneity ranges are narrow. Epel'baum et al. (68), however, concluded from chemical and metallographic studies of their alloys (annealed at 1300-1350''C in argon) that the following homogeneity ranges exist: CrB0.41-CrB0.51 (CrsB); CrBo 59-CrBo.r.3 (Cr.Ba); CrBo.9CrBi.15 (CrB); CrB1.5-CrB1.c5 ( C r 3 B 4 ) ; and C r B i . 9 o - ( ? ) (CrB^). It must be mentioned that the presence of impurities (which may cause the formation of B 2 O 3 , Cr2N, BN, etc.) have a considerable influence on the amounts of chromium and boron that actually participate in the chromium-boron equilibriums. In the author's opinion the results of Epel'baum et al. (68), that extended homogeneity ranges exist without any significant lattice parameter variations, should be retested by use of pure and completely analyzed alloys.

169

BORIDES: BASIC FACTORS

The chromium borides are not attacked by nitric acid but dissolve in perchloric acid, warm sulfuric and hydrochloric acid, and fused alkali (5,70). Physical properties are collected in Tables VII and II. 2. The Mo-B System Kiessling (72) did not observe any solid solubility of boron in molyb­ denum at 1200°C, but at 2180°C a small solubility with decreasing unit volume of the parent metal lattice has been reported (136) (compare the W-B system). Kiessling solved the crystal structures of M 0 2 B , MoB and M 0 2 B 5 (72). Steinitz et al. (Í53) confirmed the existence of these borides and, in addition found M 0 3 B 2 , ß-MoB, and M 0 B 2 all of which are only stable at high temperatures. The M O 2 B - M 0 B region of the tentative phase dia­ gram of Steinitz et al. (Fig. 7) was verified by Gilles and Pollock ,{153a)

-ref.153

5.37.0 m n,

10.2

ISA 2 2 . 0

I

2500

2250

weighfVo

'21Qd' , \MoB 2100" 2ooa

2000

1750

/goo° Lf1500-

1250

1000

20

^o

so

ATOMIC PCR CCNT

FIG. 7. T h e M o - B system. ( C o u r t e s y lurgical, a n d Petroleum E n g i n e e r s . )

eo

of A m e r i c a n Institute of Mining,

Metal­

who report somewhat higher temperatures, however. While M 0 2 B and M 0 3 B 2 have narrow homogeneity ranges, those of the other molybdenum borides are extended. The single phase region of MoB includes the

C32 15.7 13.1

16.72

1900f 16.0 ^^^^ 2200 21-56

7.77 2100

0 = 5.564, c = 4.740 α = 3.115, c = 16.93 0 = 3.19, Z? = 8.40, c = 3.07 a = 2.982, c = 13.87

= 3.06, c = 3.l0

fl

40

2660

2100-3700

2420

1200

2500 2250f 2350 2500 2330

kg/mm"

Micro-

* References: For references to CrB2, M02B5 and W2B5, see Table II. The l.p. of Cr2Bo.5-i were determined by Aronsson and Aselius (151). Rather similar values were reported by Andersson and Kiessling (84) and Bertaut and Blum (66). The l.p. of CroB derive from the paper by Bertraut and Blum (66), those of Cr5B3 from Nowotny et al. (67). The l.p. of CrB (70), CrS^ (84), M02B, a-MoB, W2B, a-WB (72) are all quoted from Kiessling. The l.p. of ß-MoB and M02B5 are taken from the paper by Steinitz et al. (153), those of ß-WB were found by Post and Glaser (137). The melting points of the chromium and timgsten borides derive from KiefiFer and Benesovsky (143), those of M02B, M03B2 and a-MoB from Gilles and Pollock (153a). The melting points of ^-MoB, M0B2 and the decomposition temperature of M02B5 were determined by Steinitz et al. (153) who report '--150°C lower values for M02B, M03B2 and a-MoB than do Gilles and Pollock (153a). The resistivity of CrB was given by Glaser (88), and the resistivities of the molybdenum borides by Steinitz et al. (153). The values of microhardness were determined by Epel'baum et al. (68) (chromium borides), Steinitz et al. (153) (molybdenum borides), and Samsonov (154) (tungsten borides). f Decomposes in the solid state or melts incongruently.

W2B C16 a-WB WB ^-WB CrB W2B5 W2B5

2120

8.77 2350 45 8.4e 2180 25 7.48 1600t 18-45

9.31

= 3.110, c = 16.95 α = 3.16, Z? = 8.61, c = 3.08 M02B5 a = 3.011, c = 20.93

fl

a = 5.543, c = 4.735

Resistivity, hardness, (20°C)

BEBTIL

M0B2

M02B C16 M03B2 a-MoB MoB ß-UoB CrB M02B5

CraB C16 CrsBa CrSs CrB CrB Cr3B4 TdL,B, CrB2 C32

Cr2Bo.5-i

X-ray density Melting /xohm-cm (gm/cm"^) point, ''C Lattice parameters (in Á) a —14.71, h ~ 7.41, c = 4.25 äsT 167üt 0 = 5.185, c = 4.316 6.57 1849t a = 5.44, c = 10.07 6.52 IQOOf α = 2.969, Z? = 7.858, c = 2.932 6.11 2060 64 1250 0 = 2.984, Z? = 13.02, c = 2.953 5.76 19601 1450 0 = 2.969, c = 3.066 5.20 1900 21-56 1800

Structure type

TABLE VIL* SOME PROPERTIES OF THE BORIDES OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN

170 ARONSSON

BORIDES: BASIC FACTORS

171

stoichiometric composition, whereas the boron content of M02B5 does not attain 71.4 atomic % and that of M 0 B 2 is always greater than 66.7 atomic % (72,153). KiessHng reports a more narrow homogeneity range (around 70 atomic % boron) for M02B5, than do Steinitz and co-workers. This discrepancy may depend on the various temperatures used. The supposition that M03B2 has the & 3 Β 2 structure (153) can hardly be correct since no crystal structure has been reported for a phase of the composition Cr3B2.

In addition to the properties of molybdenum borides shown in Tables VII and II, the heats of formation (AHSOSK ) of the following molybdenum borides are known (91): M 0 2 B (—26 kcal/mole); M03B2 (—42 kcal/mole), MOBO.ge (—16 kcal/mole), M O B I . o e (—17 kcal/mole), M0B2.14 (—20 kcal/mole), and M0B2.33 (—20 kcal/mole). The estimated accuracy is ±: 8 kcal/mole. M 0 2 B is superconducting below 4.74°K (102). The molybdenum borides are not attacked by hydrochloric acid but are easily dissolved in nitric acid, warm concentrated sulfuric acid, and fused alkali (53). 3. The W-B System Kiessling (72) did not detect any solid solubility of boron in tung­ sten at 1200°C, but Samsonov (154) did observe a small solubility at 1900°C with decreasing lattice parameter and increasing microhardness of the metal phase. The crystal structures of W 2 B , W B , and W2B5 are well established (72,79,154). W 2 B has a narrow single phase region, while this is not true for W B and W 2 B 5 (72,154). Kiessling (72) gave the homogeneity ranges 48.0-50.5 and 66.7-68 atomic % boron (at 1200°C ?), respectively, while Samsonov (154) found W B to be ho­ mogeneous from 44.4 to 50-55 atomic % boron and W 2 B 5 from 68 to 75 atomic % (1900°C). A high temperature modification of W B (stable above 1850°C) has been discovered (137), and it has also been suggested that W 2 B 5 (similarly to M 0 2 B 5 ) transforms to W B 2 near the melting point (87). W 2 B is superconducting below 3.10°K (102). W B is chemically more stable than the molybdenum borides. Thus, it is only slowly attacked by cold nitric and sulfuric acids, and fused alkali dissolves it only with difficulty, whereas the reaction with molten nitrates is rapid (53). F . BORIDES OF THE SEVENTH AND EIGHTH GROUP METALS

The borides of the seventh and eighth groups metals are less explored than those of the preceding transition groups A considerable amount of

172

BERTIL ARONSSON

crystallographic data is available, however, and is collected in Table VIII. TABLE V I I I CONSTANTS OF BORIDES OF THE SEVENTH AND EIGHTH GROUP METALS

CRYSTALLOGRAPfflC

Structure type Mn4B

Mn^B MnB Mn3B4

RetBa Fe2B FeB Co.B CoS CoB

Μη,Β C16 B27 TaaB^ THTFCS

C16 B27

Ni^B NiB

C16 B27 DOn C16 CrB

RU7B3

RU7B3

RuB Rh.Ba RhB OsB PtB

Cubic, u n k n o w n ThFe3 ß8 Cubic, u n k n o w n B8

NÍ3B

Lattice parameters ( i n A ) a = a = a = a = a = a = a = a = 0 = a = a = fl = a = a = a = a =

14.53, 5.148, 5.560, 3.032, 7.504, 5.109, 5.506, 4.408, 5.016, 5.253, 4.389, 4.990, 2.925, 7.467, 6.97 4.475,

b = c = b = b = c = c = b = b = c = b = b = c = b = c =

a =

3.3O9,

c =

7.293, 4.208 2.977, 12.86, 4.882 4.249 2.952, 5.225, 4.220 3.043, 5.211, 4.245 7.396, 4.713

c = 4.778 4.224

a = 7.03 fl = 3.358, c ==4.058

c = 4.209 c = 4.145 c == 2 . 9 6 0

c = 4.061 c = 6.629 c = 3.956 c = 6.619 c = 2.966

Ref. (61) (61) (61) (61) (71) (65) (65) (62) (163) (163) (62) (163) (171) (63) (173) (63) (71) (173) (71)

1. The Mn-B System Kiessling (61) prepared manganese borides by sintering the elements in evacuated silica tubes at 1100°C. He determined the crystal struc­ tures of four intermediate phases (see Table VIII). According to Binder and Post {148a) manganese also forms a diboride, MnBg, which crystal­ lizes in the C32 structure with a = 3.007 Ä and c = 3.037 Ä. 2. The Re-B System Neshpor et al (155) sintered boron and rhenium powders at IQOO'^C in vacuo. They found a tetragonal phase around 30 atomic % boron with a = 5.47 A, c — 4.73 A (C16 type ?) and another phase that contained more than 40 atomic % boron. In arc-melted Re-B alloys three inter­ mediate phases with the following approximate compositions were recently identified (71): RcsB (orthorhombic), RotBS ( T h 7 F e 3 struc­ ture), and ReBa (hexagonal).

173

BORIDES: BASIC FACTORS

3. The Fe-B System The equilibrium diagram of Wever and Müller (156), who critically discussed earlier diagrams, is shown in Fig. 8. McBride et al. {157) \^ B i g ht

7o

FeB

20

<0

(High)

eo

ATOMIC PER CENT

FIG. 8. T h e F e - B system, mainly according t o W e v e r a n d Müller

(156).

found the eutectic temperature to be 1149°C which is 25° lower than the original value of Wever and Müller. The transformation temperatures of iron are only slightly changed with the addition of boron {156). There is a small soHd solubihty of boron in iron. The maximum solubility in γ-iron is about 0.02 wt. % {158, 159), and the boron atoms are probably interstitially dissolved {157, 160). The solution in «-iron [bout 0.005 wt. % {157,158,159)] has been presumed to be substitutional {157,160) as well as interstitial {161). Two iron borides with narrow homogeneity ranges have been identi­ fied: namely, FcsB {65,156,162) and FeB {65,163). Wever and Müller (156) claim that FeB transforms to a high temperature modification at 1131° C. 4. The Co-B System Equilibrium diagrams (Fig. 9) have been outlined by Köster and Mulfinger {164) and Chizhevskii and Shmalev {165). In addition to

174

BERTIL ARONSSON

1100

700

Co

20

^ir;

prtn

<0

r^NT

FIG. 9. T h e C o - B system.

C02B and CoB [first identified by X-ray methods (163)] which have nar­ row homogeneity ranges, a more cobalt-rich boride C03B has also been reported (62). CooB is ferromagnetic with a Curie point at 510°C (164). Kolomytsev {164a) recently reinvestigated the Co-B system and confirmed the existence of C03B which he found to be formed peritectically at 1100°C. 5. The Ni-B

System

The nickel-rich part of the Ni-B system was recently studied by Hoppin {166), whose diagram is shown in Fig. 10. According to recent work at the University of Uppsala, the diagram of Giebelshausen (167) which covers compositions up to 20 wt. % boron is not reliable and there­ fore is not reproduced. Many nickel borides have been reported. The existence of NÍ3B {62,84,168) and Ni^B {84,163,167) are firmly established. The re­ ported NÍ3B2 phase {100,167) might have been NÍ6SÍ2B (169) which is easily formed when nickel and boron are sintered in silicon-containing ceramics. Rundqvist (170) has discovered two phases with the approxi­ mate composition NÍ4B3 and with complicated crystal structures. NiB is well known (171), but the existence of NÍ2B3 (55,167) seems dubious. NÍ3B is resistant to dilute sulfuric acid but is dissolved by concen­ trated acids and strong oxidizers (168).

175

BORIDES: BASIC FACTORS

x^e/ghf 7o

5Θ0ΘΑΑ

/500

J300

noo

900

700

Ni

AO

20 ATOMIC PER CENT

FIG. 10. T h e N i - B system, according to H o p p i n (166). Welding Society).

6. Borides of the Phtinum

( C o u r t e s y of American

Metals

Boron is a serious platinum poison and forms low melting eutectics (Table IX) with the platinum metals (172). The platinum metals form a great number of borides (173). In addition to the phases given in Table VII, Buddery and Welch (173) reported the following borides: R U 2 B 3 , RuBs, RhoB, RhB2, PdsB^, OsB^, OSB2+, IrgBg, IrB, IrBs, Pt3B2. From studies of the crystallographic constants of Rh2B (174) and Pd3B2

EuTECTic

TABLE I X TEMPERATURES IN SYSTEMS OF PLATINUM METALS WITH BORON

Temperature,

°C.:

Ru

Rh

Pd

Os

1370

1131

743

>1500

(172) Pt

1046

^800

(173) it has been concluded that these phases might be RhgSi and Pd2Si formed by reaction with the silica tubes (71). V. Ternary Systems of Tv/o Transition Metals and Boron The first part of this section will be devoted to the quasi-binary systems of the diborides of the fourth to sixth group metals. Additional

176

BERTIL ARONSSON

information on Moi-Mos-B systems, which is very incomplete and scat­ tered, is presented in the second part. A. T H E QUASI-BINARY SYSTEMS OF MEBS PHASES

Solid solutions of the diborides have usually been obtained by sinter­ ing mixtures of pure diborides, but have also been prepared by direct reduction of two oxides and B2O3 with carbon (46). The mutual solu­ bility of MeB2 phases has been extensively studied by Post et al. (87). Most of the data in Table X derives from these authors. The solubility TABLE X MUTUAL SOLID SOLUBILITY OF MEB2 ( M E 2 B 5 ) PHASES

TÍB2 ZrBo HfB2 VB2 NbB2 TaB2 CrB2 M02B5

CrB2

M02B5

W2B5

ZrB2

RÍB2

VB2

NbB^

TaB^

C S .

C S . C S .

C S . L.S. _

C S . C S . _ -

C S . C S . C S . L.S. C S . L.S. C S . _ L.S. C S . C S . C S . C S . C S . C S . -

KEY: C S . = complete solid solubility; L . S . == limited solid solubility ( < 1 Q % ) .

limits in the T Í B 2 - W 2 B 5 system are those of Meerson et al. (82) who confirmed the observations of Post et al, that there is a limited solid solubility in the ZrBs-CrBg system and a complete solubility between T i B s and ZrB2. The data for the M0B2-VB2 and TaB2-NbB2 systems are taken from the paper of Blumenthal (46), who did not find a complete solid solution in the ZrBa-MoBg system. The resistivity has been studied for the following systems: T i B a - Z r B s (95,175), TÍB2-VB2 (94), TiB2-NbB2 (95,176), TiB2-CrB2 (82,94), ZrB2-NbB2 (94), ZrB2-TaB2 (94,95). As is usual in cases of solid solu­ tions of metallic phases, the resistivity has a maximum about the equimolar composition. 1.

TÍB2-ZTB2

Glaser and Ivanick (175) investigated the reaction rate for solid solution formation and found the activation energy for this reaction rate to be 42 kcal/mole. They observed very regular variations in the lattice parameters (Vegard's law was strictly obeyed) and in the melting points. According to Samsonov (177) the microhardness has a maximum of 4200 kg/mm^ at 90 mole % T i B g .

BORIDES: BASIC FACTORS

2.

177

TiB2'NbB2

The reaction rate for formation of sohd solutions of these borides has also been studied, and the activation energy for this reaction rate was about 26.8 kcal/mole {176). The microhardness has a maximum of 3900 kg/mm^ at 70 mole % TiBg {176,177), while the oxidation resist­ ance of the 50/50 (Ti, Nb)B2 alloy is greater than for the pure com­ ponents {178). 3.

TiB2-CrB2

The lattice parameters and the coefiBcient of thermal expansion vary smoothly with composition, whereas the microhardness has a maximum of 4200 kg/mm^ at 80 mole % TÍB2 (82). 4.

ZrB2-TaB2

The microhardness has a maximum (3370 kg/mm^) at 70 mole % TaB2 {95,177), and the magnetic susceptibility at the equimolar com­ position {179). 5.

TÍB2-W2B,

and ZrB^-CrB^

According to Meerson et al. {82) the small mutual solubility of the components changes their properties only slightly. The quasi-ternary system (Ti, Cr)B2 —ZrB2 (with the atomic ratio Ti/Cr = 50/50) has been recently studied {180). (Ti, Cr)B2 dissolves about 40 mole % ZrBs. The microhardness of this solid solution has a maximum (3900 kg/mm^) at 20 mole % ZrB2, and the resistivity at 10 mole % ZrB2. The solubility of (Ti, Cr)B2 in ZrB2 is small. B. ADDITIONAL INFORMATION ABOUT THE M E I - M E 2 - B SYSTEMS

In Section III it was pointed out that the stability of borides decreases with the increasing group number of the metal. Consistent with this observation, it has been found that in the Mci-Mcs-B systems the metal with the lower group number is concentrated in the boron-richest phase present. Thus, Hägg and Kiessling {181), who studied the MosB-MeB section of some Μθι-Μθ2-Β systems (Me = Cr, Μη, Fe, Co, Ni), showed that the lower atomic number metal was enriched in the MeB phase. Brewer et al. {79) observed that titanium, zirconium, and tantalum reacted with W2B to give tungsten and in the metal-rich part of the Cr-Fe-B system the chromium content of the (Cr, Fe)2B phase is much greater than that of the (Fe, Cr) phase {151). TÍB2 seems to be in equilibrium with iron, cobalt, and nickel and forms simple eutectic systems with these metals (182).

178

BERTIL ARONSSON

As in Moi-Mej-C systems, ternary phases also occur in the Mci-McaB systems. In Mo-Me'-B systems (Me' = Co, Ni), ternary phases of the U3SÍ2 (183,184) and Mo2Me'B4 (183) types occur, and in the Cr-Ni-B system an orthorhombic phase, Cr2NiB4, exists [a = 6.106 A, b = 12.67 Ä, c = 5.96 Ä (185)], In the Zr-Mo-B (184a) and Cr-Mo-B (185a) systems, ternary phases with the approximate compositions ZrMo2B2 and CraMoBi have been identified. Both these phases melt congruently. VI. Ternary Systems Me-B-X (X = 0,N,C,Si) A. M E - B - O SYSTEMS

Because of the great stability of B2O3 (see Section II) this phase will play a dominant role in Me-B-O systems. Nicholson (159) found that even small amounts of oxygen in iron-boron alloys quantitatively com­ bined with boron to form B2O3. In M e - B - O systems of metals with great affinity for oxygen (e.g., titanium, zirconium, and tantalum), the existence of equilibriums between solid solutions of oxygen in the metal or metal-rich oxides (eventually with dissolved boron) with metal borides cannot a priori be ruled out. B. M E - B - N SYSTEMS

The available information implies that the equilibriums in Me-B-N systems are very temperature sensitive. The treatment of borides of chromium, tungsten, and iron with streaming ammonia at temperatures between 350° and 1180°C resulted in the formation of metal nitrides and boron nitride (150). Thus, under these conditions none of the mentioned borides is in equilibrium with nitrogen. Brewer and Haraldsen (89), however, concluded from experiments at higher temperatures that above 1300°C all tungsten borides are stable in the presence of nitrogen as are all chromium borides above 1200°-1700°C. They also found that TiN was in equilibrium with TiB and TÍB2 and ZrN with ZrB2, while the nitrides of chromium and tungsten (which are much less stable than TiN and ZrN) were not in equilibrium with any boride. The observation of Schwarzkopf and Glaser (148), that at high temperatures the main product of a reaction between boron nitride and a metal of the fourth to sixth group was a boride (except for chromium), is consistent with the diagrams of Brewer and Haraldsen (89). The quasi-binary system TiBa-TiN was studied by Samsonov and Petrash (186) who observed a small solid solubility of TÍB2 in TiN and found that an alloy with 40 mole % TiN had the best oxidation resistance.

BORIDES: BASIC FACTORS

179

C. M E - B - C SYSTEMS

Studies on Me-B-C systems have been carried out by Brewer and Haraldsen (89) and by Glaser (88). The former authors outlined MeB-C systems for Me = Ce, Th, Ti, Zr, Ta, Mo, W. The Ti-B-C and MoB-C diagrams agree with the results of other experiments on these sys­ tems {187). Only the most boron-rich borides are in equilibrium with boron carbide. In the systems of titanium, zirconium, and tantalum (and probably also niobium and vanadium) only the diboride is stable in the presence of graphite. With increasing group number of the metal the situation is changed, and all the borides of chromium, molybdenum, and tungsten (except WoB) form two phase ranges with graphite. [According to Glaser (88) even WgB is stable in the presence of graphite.] As the borides of the seventh and eighth group metals are much more stable than are the carbides of these metals, most of them are presumably in equilibrium with graphite. No ternary compounds have been discovered in the mentioned MeB-C systems, but iron forms a ternary borocarbide Fe23(B, C)G with the CrssCc structure (188). Boron also substitutes to a great extent for carbon in cementite {100). D . M E - B - S I SYSTEMS

The main features of Me-B-Si systems with Me = Zr, V, Nb, Ta, Cr, Mo, and W have been determined by Nowotny, KieflFer, and co-workers {67,145,189). A review of these investigations has recently appeared {143). Some information about the Fe-B-Si, Co-B-Si {190), and NiB-Si {169) systems is also available. The Me-B-Si systems are considerably more complicated than the previously treated ternary system. The binary silicon-boron phase(s) (see Section II) is not very stable and appears only in the metal-poor parts of the systems. The borides and silicides evidently have about the same stability—equilibriums are established between binary phases which have about the same metal content. In general, boron and silicon substitute for each other only to a small extent. A characteristic feature is the presence of ternary phases (see Table XI). Two types of such phases are particularly common: namely, phases of the CrgBs type (in Me-B-Si systems with Me = V, Nb, Ta, Cr, Mo, W, Μη, Fe) and phases of the D8s (Mn.SU) type (in Me-B-Si systems with Me = Zr, V, Nb, Ta, Cr). In systems of the fourth to sixth group metals, all reported ternary phases are of these two types. CrsBs evidently exists as a binary phase, but there is some question whether the homogeneity ranges of the

180

BERTIL ARONSSON

TABLE X I APPROXIMATE COMPOSITION AND CRYSTALLOGRAPHIC DATA OF SOME M E - S I - B PHASES Structure

L a t t i c e parameters

type

(in A )

Composition

Ref.

2jr5SÍ3B'~0.45

D8s(MnSh)

α = 7.92,

0

= 5.56

{143,191)

V5SÍ3B-0.45

DSs

a = 7.18,

c = 4.89

{143,191)

V5SÍB2

CiS^

fl =

c = 10.79

Nb5SÍ3B-2

DSs

(D81)

5.81,

a = 7.54,

(145)

c = 5.25

{143,191) {143,191)

TasSiaB—a

DSs

fl = 7 . 4 7 ,

c=5.23

CrsSiaB

DSs

a = 7.06,

c = 4.73

M05SÍB2

CrsBa

α = 6.01,

c = 11.05

W5(Si,B)3

CrsBs

a ~ 6.047, c = 10.99

{189)

MnsSiBa

C1S3

a = 5.61,

c = 10.44

{190)

a = 5.54,

c = 10.32

{190)

a = 8.82,

c = 4.34

{190)

c = 4.25

{190)

FesSiBi Fe4.7SÍ2B

WSU{D8m)

C04.7SÍ2B

W5SÍ3

a = 8.62,

NieSÍ2B

C22(Fe2P)

a = 6.105, c = 2.875

{67) {189,192)

{169)

CrgBg type phases in the Nb-Si-B and Ta-Si-B systems include the binary composition McsSig (143), The 0 8 3 (MusSig) type phases have been discovered and studied by Nowotny and co-workers (67,191). The increasing unit cell dimensions of these phases with increasing boron content indicate that the boron atoms are (at least partially) interstitially dissolved ("AuflFüUung") (191,192). Fe4.7SÍ2B and C 0 4 . 7 S Í 2 B (see Table XI) have small homogeneity ranges which do not contain the stoichiometric composition Me5SÍ2B (190). A ternary phase with the D O u (FcgC) structure has also been found in the Fe-B-Si system (190). One ternary phase is reported to exist in the Ni-Si-B system: namely, NigSisB. It crystallizes in the revised C22 (Fe2P) structure (169). VII. Conclusions The basic research on borides is evidently far from being finished. The author would like to recommend that in future research more efforts are devoted to the preparation of pure borides and that, whenever pos­ sible, complete chemical analyses of the samples are given. This would not only make it easier to explain inconsistencies among various results but would also give much valuable information about the influence of minor impurities (such as oxygen and nitrogen) on metal-boron equi­ libriums. In view of their technical importance it is remarkable that

BORIDES: BASIC FACTORS

181

Me-B-O systems have been so Httle investigated. Systematic studies on such systems should also give results of great theoretical interest. As pointed out in this chapter, the knowledge of the basic properties of borides is very incomplete. Much research in this field is necessary before it will be possible to present a satisfactory theoretical treatment of the borides and their interesting relationship to intermetallic phases as well as to typical "interstitial" phases. ACKNOWLEDGMENTS

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