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A note on the intermetallic chemistry of the later transition elements
I.52
JOURNALOFTHE
LESS-COMMON METALS
A NOTE ON THE INTERMETALLIC TRANSITION
CHEMISTRY
OF THE LATER
ELEMENTS
W. HUME-ROTHERY Department of Metallurgy,
University of Oxford, Oxford (Great Britain)
(Received
January zznd,
1964)
SUMMARY The equilibrium diagrams of the later transition metals of the Second and Third Long Periods are examined systematically. It is shown that the bodycentred cubic, close-packed hexagonal, and face-centred cubic phases have composition ranges whose boundaries appear to be controlled mainly by the Average Group Numbers of the alloys. Group Numbers of 6, 7. 8, g, and IO are allotted to the elements of Groups VIA, VIIA, VIIIA, B, and C respectively, so that an equiatomic alloy of MO and Ru has an AGN value of 7.0. Relations with the elements of the first transition series are discussed.
I. INTRODUCTION
In the alloys of the B sub-Group elements with copper and silver, it is known’ that electron compounds tend to occur at characteristic electron :atom ratios (electron concentrations), and that some of the Cu-rich and Ag-rich parts of the equilibrium diagrams are determined mainly by electron concentration, and are roughly superposed if the diagrams are drawn in terms of electron concentrations instead of atomic percentages. Deviations from an exact electron concentration principle have been explained2 by considering the lattice distortions which exist in different systems at a given electron concentration. In alloys of transition metals, the uncertainty regarding the exact valencies makes it better to discuss analogous effects in terms of average group numbers* (AGN) of the alloys, since these can readily be interpreted in terms of the different valency schemes. Thus, an equiatomic alloy of MO (Group VI) and Ru (Group VIII) has AGN 7.0, and will have an electron concentration of 6 if the later PAULING scheme3 is accepted, or of 7 if all the outer electrons are regarded as contributing. In the transition series the atomic diameters are such that, in any one series, the size factors are extremely favourable for combinations of elements in Groups VIA, VIIA, VIIIA, VIIIB, and VIIIC, whilst the element of Group VA although still within the zone of favourable size factor has a considerably larger atom. It is wellknown that valency effects are shown most clearly when size factors are favourable, * For this purpose values of 8, g, and IO are assigned to the elements of Groups VIII A, B, and C respectively. J. Less-Common
Metals, 7 (1964) 152-158
INTERMETALLIC
CHEMISTRY
OF SOME TRANSITION
153
ELEMENTS
and when solvent and solute are in the same Period. It has been established6 that there is a tendency for u-phases and for hexagonal close-packed or E phases to occur at roughly constant AGN values, but the variation from one system to another is considerable, and the equilibrium diagrams do not appear to have been examined in detail. In the present note we describe some characteristics of the equilibrium diagrams of the later Transition metals of the Second and Third Long Periods. The sources of the data are summarised on p. 158.
Dealing with the elements of Groups VA-VIIIC, the simplest types of equilibrium diagram are shown by the dotted lines in Figs. r(a) and r(b). Where the diagrams are more complicated, the general types of these two figures can be recognized, even
A
B
Fig.
B
I
though several intermediate phases are present. In many cases only the solidus and not the liquidus curves are known. In the present note, we examine the solidus curves and maximum solid solution limits of the primary solid solutions and intermediate phases, and, in the later figures, we show the parts of the equilibrium diagram indicated by the full lines in Fig. I, since there are very few systems in which the equilibrium solid-solubility curves are known at any but the highest temperatures. The later metals of the first transition series behave differently from those of the second and third transition series, and the present note is concerned almost entirely with the latter. In these series it appears that the transition metals divide themselves into 3 main types: (a) Those at the extreme ends of the series in Groups VIIIC and perhaps VIIIB. In these, there is a tendency to fill the d-bands, as is shown by the magnetic and electrical properties. Some of the outer electrons have become non- or anti-bonding; this means that some of the d-orbitals are no longer available for bonding, and so these metals are not widely soluble in elements of Groups VA and VIA where the very stable body-centred cubic structures involve a high proportion of d-functions in their bonding hybrids. (b) Those at the beginning of the transition series in Groups IIIA, IVA, and perhaps VA. These act as normal metals and their electrons tend to fill the d-shells of the metals in (a) above. J. Less-Common
Metals,
7 (1964) I j2-158
154
W. HUME-ROTHERY
(c) The intermediate elements of Groups VI, VII, VIIIA, and perhaps also Groups VA and VIIIB. It is these metals which show Group Number effects most simply, because they have no marked tendency either to absorb or donate electrons. III
Figure z refers to the alloys of elements of Groups VI, VII, and VIIIA, B, and C, and deals with the parts of the equilibrium diagrams referring to the body-centred cubic and o-phases; the horizontal scale is of AGN values, in place of the more usual atomic percentages. b.c.c.
‘F ‘\
W-Re
‘.
.\ CJ ‘\.
--
. . . . . . . .;:
W-lr
:
. . . . : . :
: : :.
. :. ..
. .
.
.
.
w-05
:a; :. :.
‘%, l.
:
JUT-.:: A..
Ii :: :to ::
6.5
7.0
Fig. 2.
In the W alloys, the solidus curves and maximum solid-solubilities for the W-rich body-centred cubic solid solutions in the systems W-Re and W-OS are very nearly superposed in terms of AGN values. For the corresponding a-phases, the solid-solubility limits on the high AGN side are in good agreement with an AGN principle, whilst the solidus curves and the phase boundary on the low AGN side are in approximate agreement. It seems clear, therefore, that in these two systems, the equilibrium diagram is determined primarily by the AGN, and that two atoms of Re are equivalent to one atom of 0s. J. Less-Common
Metals,
7 (1964)
152-158
INTERMETALLIC
CHEMISTRY
OF SOME TRANSITION
ELEMENTS
I55
In the system W-Ir, the solidus curve in terms of AGN falls more steeply, but the maximum solubility of Ir in W is only slightly less than that expected from an AGN principle. The solubility limit of the W’-Ir o-phase on the high AGN side is at almost exactly the same AGN value as those for o W-OS and o W-Re, but the composition range is less. For the Mo-alloys, no data exist for MO-Tc, but the solidus curves and maximum solubilities of the body-centred cubic phases in the systems MO-Ru and Mo-Rh are almost superposed in terms of AGN values, and the composition of the MO-Ru o-phase lies in exactly the same region as the high AGN boundary of the W-Re and W-OS u-phases. In these alloys, therefore, an AGN principle again appears to control the equilibrium diagrams, and 3 atoms of Ru are equivalent to z atoms of Rh. In both W- and MO- series these simple principles break down on passing to W’-Pt and Mo-Pd for which the solid-solubilities in the body-centred cubic metal are smaller than would be expected from an AGN principle; the reason for this has been discussed previously (II above). IV
Figure ferring
3 shows, in terms of AGN values, the part of the equilibrium diagrams reto close-packed hexagonal structures. In the third transition series, Re and
3200
mo
OS
&z%$Os-lr
2-b
2500
y&-P,
>
Ir-W
2000
MO- Pa )- -
Ru-Pd
1500
1200
J.
Less-Common
Metals,
7
(1964)
152-158
156
W.
HUME-ROTHERY
OS form continuous solid solutions, as would be expected from the fact that they are adjacent to one another in the Periodic Table, and that the size-factor is very favourable. In the systems Re-Ta and Re-W, the limits of the Re-based solid solutions are at roughly the same AGN value, but this alone is not of great significance, because the solubilities are small. It is, however, highly significant that the maximum solubility of W in OS is at about the same AGN value as those of Ta and W in Re, and it is even more significant that the solubility limit on the low AGN side of the intermediate close-packed hexagonal E Ir-W phase occurs in the same region of the AGN diagram. This same region also corresponds to the low AGN solubility limit of the intermediate close-packed hexagonal .sRu-MO phase, although the corresponding value for eMo-Rh is at a somewhat higher AGN. On the high AGN side, the solubility limits of the intermediate eMo-Rh and cIr-W phases, and of the solid solution of Pt in Re and of Pd in Ru, and also of the very imperfectly known solid solution of Ir in OS are all at roughly the same AGN value. The accepted value of the maximum solid-solubility of Ir in Re is at a lower AGN, but this system has not been examined in detail. In the system Mo-Rh, the close-packed hexagonal s-phase has its maximum melting point at an AGN of 8.0, and it is perhaps significant that in the system Mo-Pd the corresponding phase, which is stable only over a very limited range of composition and temperature, occurs at this AGN. The accepted equilibrium diagram of the system W-Pt does not show a corresponding intermediate phase, but we understand from KUBASCHEWSKI~ that tentative work in the Max-Planck Institute in Stuttgart has revealed the existence of an intermediate phase in this system. Taken as a whole, Fig. 3 suggests strongly that, whether present as a primary solid solution* in Re, Ru, or OS, or as an intermediate phase in alloys of adjacent elements, the close-packed hexagonal structure is bounded by AGN limits of about 6.8-8.4. V
Figure 4 shows, in terms of AGN, the parts of the equilibrium diagrams referring to the face-centred cubic phases. Continuous solid solutions are formed in the systems Pt-Ir and Pd-Rh, as would be expected from the atomic diameters, and the adjacent Group Numbers. On the side of low AGN, the limits of the face-centred cubic solid solutions in the systems Pt-Re, Ir-W, Ir-Ta, Pd-Mo, and Rh-Mo are at roughly the same AGN. The equilibrium diagram of the system Pd-Ru has been examined in details, and the solubility of Ru in Pd is undoubtedly smaller than would agree with an AGN principle, whilst the solubility of Re in Ir is somewhat greater, but this system is not known in detail. The very large solubility of W in Pt would be in complete contradiction to an AGN principle but, as explained above, intermediate phases have been discovered in this system, and the present diagram is highly suspect. It is not possible to include the curve for Pt-Ta because, if the accepted equilibrium diagram is correct, the solubility of Ta in Pt increases with decreasing temperature, and the slowness of diffusion prevents the maximum value from being determined. If the accepted Pd-Nb diagram is correct, the face-centred cubic solid solution extends back to AGN 8 which is lower than the other values in Fig. 4, but the Pd-Nb * There are no data for Tc alloys. J. Less-Common
Metals, 7 (1964) 152-158
INTERMETALLIC
CHEMISTRY
OF SOME TRANSITION
ELEMENTS
157
diagram is also suspect because it is shown as involving maxima in the liquidus and solidus curves which do not meet at a maximum point as they should do. For the face-centred cubic phases, therefore, the data are not completely in agreement with an AGN principle but, except for Pd-Ru, there is a clear suggestion that this phase extends back to AGN values of about 8.0, or about 8.2 if the doubtful Pd-Nb value is omitted - the Pt-W should clearly be ignored until more information is available. f.c.c.
3400
3Ooc --?,-
Ir-
Re
‘\\ ‘\\
. \ 1?h -
Pt- Ir
MO. l-
1-b
vlo A
\
I
Pd.
/
8.0
9.0
Fig.
I
4.
VI
Figure z shows that on passing backwards from the Third to the Second Long Period, the AGN value for the limit of the body-centred cubic phase increases from about 6.4 to 6.6 whilst the mean composition of the o-phase also moves to a higher AGN value. This tendency is maintained on passing back to the First Long Period where maximum solubilities of Mn and Fe in Cr are at AGN values of 6.71 and 8.0 (complete miscibility), whilst the a-phases occupy AGN ranges of 6.73-6.85 (Cr-Mn) and 7.0-7.12 (Cr-Fe). Figure 4 shows that, if the doubtful Pt-W curve is ignored, the face-centred cubic phases extend back to an AGN value of N 8.3-8.5. In alloys of the corresponding J.
L~SS-COWZWZO~~ Mel&,
7 (1964)
I,jz-IsR
158
W. HUME-ROTHERY
elements in the first transition series, it has been shown5 that the face-centred cubic phases extend back as far as AGN values of N 7.8*. It will be seen, therefore, that on comparing the alloys of the first transition metals with those of the second and third, the general tendency is for the body-centred cubic and o-phase fields to move to the right, and for the face-centred cubic phase fields to move to the left for the first transition series, with the result that the close-packed hexagonal phases are removed. As explained previously, it seems fairly certain that the relatively unstable close-packed hexagonal modification of Co (which is always heavily faulted), is quite different from the very stable close-packed hexagonal metals Ru and OS which have low axial ratios and extremely strong bonding. We may conclude, therefore, that there is a strong suggestion that the composition limits of the body-centred cubic, close-packed hexagonal, and face-centred cubic phases of metals of the second and third transition series, and of the corresponding face-centred cubic phases in the first transition series, are controlled mainly by AGN principles. There is, however, no such relation for the body-centred cubic phases of the first transition series, We hope in a later communication to deal with the more complex relations which are found when the alloying metals are from different Periods. REFERENCES I W. HUME-ROTHERY AND G. V. RAYNOR, The Structure of Metals and Alloys, Institute 1962. 2 W. HUME-ROTHERY, J. Ifist. Metals, go (1961) 42. 3 L. PAULING, The Nature of the Chemical Bond, Cornell Universitv Press, 1960. 4 0. KUBASCHEWSKI, private communication. 5 W. HUME-ROTHERY, Phil. Msg., 6 (1961) 770. 6 C. W. HAWORTH AND W. HUME-ROTHERY, Phil. Mag., 3 (1958) 1013.
of Metals,
Sources of Data The equilibrium diagrams have been taken from the Defense Metals Information Centre Reports No. 152 and No. 183, and from M. HANSEN’S Constitzttiolz of Binary AZZoys, (McGraw-Hill, New York). The originals have been checked where this was possible. Ru-Pd. A. S. DARLING AND J. M. YORKE, Platinum Metals Rev., 4 (1960) 104. Ir-Re. M. A. TYLKINA, I. A. TSYGANOVA AND E. M. SAVITSKII, Zh. Neorgan. Khim., 7 (1962)
1917.
-* The behaviour of Mn is abnormal
and this element is excluded from the above generalisation.
J. Less-Common
Metals, 7 (1964) 152-158
JOURNALOFTHE
LESS-COMMON METALS
A NOTE ON THE INTERMETALLIC TRANSITION
CHEMISTRY
OF THE LATER
ELEMENTS
W. HUME-ROTHERY Department of Metallurgy,
University of Oxford, Oxford (Great Britain)
(Received
January zznd,
1964)
SUMMARY The equilibrium diagrams of the later transition metals of the Second and Third Long Periods are examined systematically. It is shown that the bodycentred cubic, close-packed hexagonal, and face-centred cubic phases have composition ranges whose boundaries appear to be controlled mainly by the Average Group Numbers of the alloys. Group Numbers of 6, 7. 8, g, and IO are allotted to the elements of Groups VIA, VIIA, VIIIA, B, and C respectively, so that an equiatomic alloy of MO and Ru has an AGN value of 7.0. Relations with the elements of the first transition series are discussed.
I. INTRODUCTION
In the alloys of the B sub-Group elements with copper and silver, it is known’ that electron compounds tend to occur at characteristic electron :atom ratios (electron concentrations), and that some of the Cu-rich and Ag-rich parts of the equilibrium diagrams are determined mainly by electron concentration, and are roughly superposed if the diagrams are drawn in terms of electron concentrations instead of atomic percentages. Deviations from an exact electron concentration principle have been explained2 by considering the lattice distortions which exist in different systems at a given electron concentration. In alloys of transition metals, the uncertainty regarding the exact valencies makes it better to discuss analogous effects in terms of average group numbers* (AGN) of the alloys, since these can readily be interpreted in terms of the different valency schemes. Thus, an equiatomic alloy of MO (Group VI) and Ru (Group VIII) has AGN 7.0, and will have an electron concentration of 6 if the later PAULING scheme3 is accepted, or of 7 if all the outer electrons are regarded as contributing. In the transition series the atomic diameters are such that, in any one series, the size factors are extremely favourable for combinations of elements in Groups VIA, VIIA, VIIIA, VIIIB, and VIIIC, whilst the element of Group VA although still within the zone of favourable size factor has a considerably larger atom. It is wellknown that valency effects are shown most clearly when size factors are favourable, * For this purpose values of 8, g, and IO are assigned to the elements of Groups VIII A, B, and C respectively. J. Less-Common
Metals, 7 (1964) 152-158
INTERMETALLIC
CHEMISTRY
OF SOME TRANSITION
153
ELEMENTS
and when solvent and solute are in the same Period. It has been established6 that there is a tendency for u-phases and for hexagonal close-packed or E phases to occur at roughly constant AGN values, but the variation from one system to another is considerable, and the equilibrium diagrams do not appear to have been examined in detail. In the present note we describe some characteristics of the equilibrium diagrams of the later Transition metals of the Second and Third Long Periods. The sources of the data are summarised on p. 158.
Dealing with the elements of Groups VA-VIIIC, the simplest types of equilibrium diagram are shown by the dotted lines in Figs. r(a) and r(b). Where the diagrams are more complicated, the general types of these two figures can be recognized, even
A
B
Fig.
B
I
though several intermediate phases are present. In many cases only the solidus and not the liquidus curves are known. In the present note, we examine the solidus curves and maximum solid solution limits of the primary solid solutions and intermediate phases, and, in the later figures, we show the parts of the equilibrium diagram indicated by the full lines in Fig. I, since there are very few systems in which the equilibrium solid-solubility curves are known at any but the highest temperatures. The later metals of the first transition series behave differently from those of the second and third transition series, and the present note is concerned almost entirely with the latter. In these series it appears that the transition metals divide themselves into 3 main types: (a) Those at the extreme ends of the series in Groups VIIIC and perhaps VIIIB. In these, there is a tendency to fill the d-bands, as is shown by the magnetic and electrical properties. Some of the outer electrons have become non- or anti-bonding; this means that some of the d-orbitals are no longer available for bonding, and so these metals are not widely soluble in elements of Groups VA and VIA where the very stable body-centred cubic structures involve a high proportion of d-functions in their bonding hybrids. (b) Those at the beginning of the transition series in Groups IIIA, IVA, and perhaps VA. These act as normal metals and their electrons tend to fill the d-shells of the metals in (a) above. J. Less-Common
Metals,
7 (1964) I j2-158
154
W. HUME-ROTHERY
(c) The intermediate elements of Groups VI, VII, VIIIA, and perhaps also Groups VA and VIIIB. It is these metals which show Group Number effects most simply, because they have no marked tendency either to absorb or donate electrons. III
Figure z refers to the alloys of elements of Groups VI, VII, and VIIIA, B, and C, and deals with the parts of the equilibrium diagrams referring to the body-centred cubic and o-phases; the horizontal scale is of AGN values, in place of the more usual atomic percentages. b.c.c.
‘F ‘\
W-Re
‘.
.\ CJ ‘\.
--
. . . . . . . .;:
W-lr
:
. . . . : . :
: : :.
. :. ..
. .
.
.
.
w-05
:a; :. :.
‘%, l.
:
JUT-.:: A..
Ii :: :to ::
6.5
7.0
Fig. 2.
In the W alloys, the solidus curves and maximum solid-solubilities for the W-rich body-centred cubic solid solutions in the systems W-Re and W-OS are very nearly superposed in terms of AGN values. For the corresponding a-phases, the solid-solubility limits on the high AGN side are in good agreement with an AGN principle, whilst the solidus curves and the phase boundary on the low AGN side are in approximate agreement. It seems clear, therefore, that in these two systems, the equilibrium diagram is determined primarily by the AGN, and that two atoms of Re are equivalent to one atom of 0s. J. Less-Common
Metals,
7 (1964)
152-158
INTERMETALLIC
CHEMISTRY
OF SOME TRANSITION
ELEMENTS
I55
In the system W-Ir, the solidus curve in terms of AGN falls more steeply, but the maximum solubility of Ir in W is only slightly less than that expected from an AGN principle. The solubility limit of the W’-Ir o-phase on the high AGN side is at almost exactly the same AGN value as those for o W-OS and o W-Re, but the composition range is less. For the Mo-alloys, no data exist for MO-Tc, but the solidus curves and maximum solubilities of the body-centred cubic phases in the systems MO-Ru and Mo-Rh are almost superposed in terms of AGN values, and the composition of the MO-Ru o-phase lies in exactly the same region as the high AGN boundary of the W-Re and W-OS u-phases. In these alloys, therefore, an AGN principle again appears to control the equilibrium diagrams, and 3 atoms of Ru are equivalent to z atoms of Rh. In both W- and MO- series these simple principles break down on passing to W’-Pt and Mo-Pd for which the solid-solubilities in the body-centred cubic metal are smaller than would be expected from an AGN principle; the reason for this has been discussed previously (II above). IV
Figure ferring
3 shows, in terms of AGN values, the part of the equilibrium diagrams reto close-packed hexagonal structures. In the third transition series, Re and
3200
mo
OS
&z%$Os-lr
2-b
2500
y&-P,
>
Ir-W
2000
MO- Pa )- -
Ru-Pd
1500
1200
J.
Less-Common
Metals,
7
(1964)
152-158
156
W.
HUME-ROTHERY
OS form continuous solid solutions, as would be expected from the fact that they are adjacent to one another in the Periodic Table, and that the size-factor is very favourable. In the systems Re-Ta and Re-W, the limits of the Re-based solid solutions are at roughly the same AGN value, but this alone is not of great significance, because the solubilities are small. It is, however, highly significant that the maximum solubility of W in OS is at about the same AGN value as those of Ta and W in Re, and it is even more significant that the solubility limit on the low AGN side of the intermediate close-packed hexagonal E Ir-W phase occurs in the same region of the AGN diagram. This same region also corresponds to the low AGN solubility limit of the intermediate close-packed hexagonal .sRu-MO phase, although the corresponding value for eMo-Rh is at a somewhat higher AGN. On the high AGN side, the solubility limits of the intermediate eMo-Rh and cIr-W phases, and of the solid solution of Pt in Re and of Pd in Ru, and also of the very imperfectly known solid solution of Ir in OS are all at roughly the same AGN value. The accepted value of the maximum solid-solubility of Ir in Re is at a lower AGN, but this system has not been examined in detail. In the system Mo-Rh, the close-packed hexagonal s-phase has its maximum melting point at an AGN of 8.0, and it is perhaps significant that in the system Mo-Pd the corresponding phase, which is stable only over a very limited range of composition and temperature, occurs at this AGN. The accepted equilibrium diagram of the system W-Pt does not show a corresponding intermediate phase, but we understand from KUBASCHEWSKI~ that tentative work in the Max-Planck Institute in Stuttgart has revealed the existence of an intermediate phase in this system. Taken as a whole, Fig. 3 suggests strongly that, whether present as a primary solid solution* in Re, Ru, or OS, or as an intermediate phase in alloys of adjacent elements, the close-packed hexagonal structure is bounded by AGN limits of about 6.8-8.4. V
Figure 4 shows, in terms of AGN, the parts of the equilibrium diagrams referring to the face-centred cubic phases. Continuous solid solutions are formed in the systems Pt-Ir and Pd-Rh, as would be expected from the atomic diameters, and the adjacent Group Numbers. On the side of low AGN, the limits of the face-centred cubic solid solutions in the systems Pt-Re, Ir-W, Ir-Ta, Pd-Mo, and Rh-Mo are at roughly the same AGN. The equilibrium diagram of the system Pd-Ru has been examined in details, and the solubility of Ru in Pd is undoubtedly smaller than would agree with an AGN principle, whilst the solubility of Re in Ir is somewhat greater, but this system is not known in detail. The very large solubility of W in Pt would be in complete contradiction to an AGN principle but, as explained above, intermediate phases have been discovered in this system, and the present diagram is highly suspect. It is not possible to include the curve for Pt-Ta because, if the accepted equilibrium diagram is correct, the solubility of Ta in Pt increases with decreasing temperature, and the slowness of diffusion prevents the maximum value from being determined. If the accepted Pd-Nb diagram is correct, the face-centred cubic solid solution extends back to AGN 8 which is lower than the other values in Fig. 4, but the Pd-Nb * There are no data for Tc alloys. J. Less-Common
Metals, 7 (1964) 152-158
INTERMETALLIC
CHEMISTRY
OF SOME TRANSITION
ELEMENTS
157
diagram is also suspect because it is shown as involving maxima in the liquidus and solidus curves which do not meet at a maximum point as they should do. For the face-centred cubic phases, therefore, the data are not completely in agreement with an AGN principle but, except for Pd-Ru, there is a clear suggestion that this phase extends back to AGN values of about 8.0, or about 8.2 if the doubtful Pd-Nb value is omitted - the Pt-W should clearly be ignored until more information is available. f.c.c.
3400
3Ooc --?,-
Ir-
Re
‘\\ ‘\\
. \ 1?h -
Pt- Ir
MO. l-
1-b
vlo A
\
I
Pd.
/
8.0
9.0
Fig.
I
4.
VI
Figure z shows that on passing backwards from the Third to the Second Long Period, the AGN value for the limit of the body-centred cubic phase increases from about 6.4 to 6.6 whilst the mean composition of the o-phase also moves to a higher AGN value. This tendency is maintained on passing back to the First Long Period where maximum solubilities of Mn and Fe in Cr are at AGN values of 6.71 and 8.0 (complete miscibility), whilst the a-phases occupy AGN ranges of 6.73-6.85 (Cr-Mn) and 7.0-7.12 (Cr-Fe). Figure 4 shows that, if the doubtful Pt-W curve is ignored, the face-centred cubic phases extend back to an AGN value of N 8.3-8.5. In alloys of the corresponding J.
L~SS-COWZWZO~~ Mel&,
7 (1964)
I,jz-IsR
158
W. HUME-ROTHERY
elements in the first transition series, it has been shown5 that the face-centred cubic phases extend back as far as AGN values of N 7.8*. It will be seen, therefore, that on comparing the alloys of the first transition metals with those of the second and third, the general tendency is for the body-centred cubic and o-phase fields to move to the right, and for the face-centred cubic phase fields to move to the left for the first transition series, with the result that the close-packed hexagonal phases are removed. As explained previously, it seems fairly certain that the relatively unstable close-packed hexagonal modification of Co (which is always heavily faulted), is quite different from the very stable close-packed hexagonal metals Ru and OS which have low axial ratios and extremely strong bonding. We may conclude, therefore, that there is a strong suggestion that the composition limits of the body-centred cubic, close-packed hexagonal, and face-centred cubic phases of metals of the second and third transition series, and of the corresponding face-centred cubic phases in the first transition series, are controlled mainly by AGN principles. There is, however, no such relation for the body-centred cubic phases of the first transition series, We hope in a later communication to deal with the more complex relations which are found when the alloying metals are from different Periods. REFERENCES I W. HUME-ROTHERY AND G. V. RAYNOR, The Structure of Metals and Alloys, Institute 1962. 2 W. HUME-ROTHERY, J. Ifist. Metals, go (1961) 42. 3 L. PAULING, The Nature of the Chemical Bond, Cornell Universitv Press, 1960. 4 0. KUBASCHEWSKI, private communication. 5 W. HUME-ROTHERY, Phil. Msg., 6 (1961) 770. 6 C. W. HAWORTH AND W. HUME-ROTHERY, Phil. Mag., 3 (1958) 1013.
of Metals,
Sources of Data The equilibrium diagrams have been taken from the Defense Metals Information Centre Reports No. 152 and No. 183, and from M. HANSEN’S Constitzttiolz of Binary AZZoys, (McGraw-Hill, New York). The originals have been checked where this was possible. Ru-Pd. A. S. DARLING AND J. M. YORKE, Platinum Metals Rev., 4 (1960) 104. Ir-Re. M. A. TYLKINA, I. A. TSYGANOVA AND E. M. SAVITSKII, Zh. Neorgan. Khim., 7 (1962)
1917.
-* The behaviour of Mn is abnormal
and this element is excluded from the above generalisation.
J. Less-Common
Metals, 7 (1964) 152-158