Alloys of refractory metals suitable for transformation into homogeneous and pure ingots

An alloy comprising at least two refractory metals having melting temperatures differing by at least 200.degree. C., and being present in proportions by weight such that solidification begins at a temperature at least 150.degree. C. less than the solidification temperature of the metal having the highest melting point. The alloy is produced by coelectrodeposition, and is in the form of conglomerates of dimensions between 0.2 and 30 mm of crystals of size 0.1 to 1 mm, in which the refractory metals are in a solid solution state.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to alloys of refractory metals capable of 
being transformed into homogeneous and pure ingots, and to processes for 
producing these alloys. 
More particularly, it relates to the alloys made from refractory metals 
having melting temperatures that differ by at least 200.degree. C., such 
as hafnium-zirconium, hafnium-titanium, niobium-titanium, 
niobium-zirconium, tantalum-titanium, tantalum-zirconium, 
tantalum-niobium, niobium-tantalum-titanium, and 
niobium-titanium-aluminum. 
More precisely, these alloys have compositions by weight such that the 
temperature at which solidification begins is less by at least 150.degree. 
C. than the solidification temperature of the least meltable metal. 
These alloys, initially produced in more or less divided form, are then 
subjected to at least one melting operation in order to convert them into 
ingots. The ingots may then be rolled in the form of sheets intended for 
manufacturing containers for reprocessing nuclear fuel, in the case of the 
Hf-Zr alloys, or neutron moderators in the case of Hf-Ti alloys, or 
superconductor compounds or aeronautical superalloys in the case of the 
Nb-Ti alloy. 
2. Description of Related Art 
It is known that such alloys can be produced by various processes, such as: 
coaluminothermics of oxides, which has the disadvantage of producing alloys 
that are contaminated with aluminum and oxygen and are in the form of 
solid blocks that must be crushed before being purified by electronic 
bombardment and converted into ingots; 
coreduction of chlorides by a metal such as sodium or magnesium, which 
leads to the formation of sponges that are highly polluted with the 
reducing agent and with iron and chlorine ions, and that also must be 
crushed; 
codeposition from the vapor phase, which produces highly pyrophoric 
whiskers that are hard to handle and must be compacted prior to melting; 
mechanosynthesis or cogrinding of the metals to be alloyed, which leads to 
particles of relatively coarse particle size that are also highly 
polluted, and which when melted result in a heterogeneous product because 
of the presence of unmelted residues of the least meltable metal. 
SUMMARY OF THE INVENTION 
The object of the invention is to produce alloys that have a homogeneous 
structure at the level of the elementary crystal, improved purity over 
that of the products of the prior art, and a suitable particle size so 
that they can be integrally melted and converted into ingots in which this 
homogeneity of structure and purity is maintained. 
The invention thus relates to alloys of refractory metals capable of being 
converted into homogeneous ingots with a purity near 99.9%, formed from 
metals whose melting temperatures differ by at least 200.degree. C. and 
whose proportions by weight are such that for each alloy, the temperature 
at which solidification begins is at least 150.degree. C. less than the 
temperature of solidification of the least meltable metal. The alloys are 
in the form of conglomerates of dimensions between 0.2 and 30 mm and 
comprising crystals having a specific surface area of between 0.005 and 
0.2 m.sup.2 /g, having a size of 0.1 to 1 mm, and in which the metals are 
in the solid solution state. 
Accordingly, the alloys of the invention are characterized by crystals 
where the metals are in solid solution, that is, they are homogeneous on 
the atomic scale and at the very most have a relative compositional 
spacing of 20% with respect to the mean composition of the alloy, such 
that this homogeneity persists during the melting and lends the resultant 
ingots properties that are identical in every respect. 
This is a major difference from the alloys made by melting of a mixture of 
constituents, where the unmelted residues form macroscopic zones that may 
reach several millimeters in size, and where major settling occurs, 
particularly when metals of quite different densities are involved. 
In addition, these crystals and their conglomerates have a size and a 
specific surface area such that the problems of spontaneous oxidation, 
which occurs when this surface area is too large, or of shaping the 
products before melting when the size is too large are avoided, and such 
that dissolution in the molten metal is promoted. 
Hence, these products are free of pollution with oxygen and iron, 
particularly during grinding operations, and it is possible to produce 
ingots of high purity by melting. Preferably, the crystals have a specific 
surface area of between 0.01 and 0.05 m.sup.2 /g and the agglomerates have 
a dimension of between 1.5 and 12 mm, because it is within these ranges 
that the maximum homogeneities and purities are produced. 
The invention also relates to processes for producing these alloys which 
are based on coelectrodeposition, that is, on the simultaneous 
electrolytic deposit of the elements forming the alloy. However, the 
technique of producing the alloy varies as a function of the potential 
difference in the deposit of each of the elements of the alloy. 
A first technique is applicable to metals whose electrolytic deposit 
potentials differ very slightly from one another, that is, by less than 
0.5 V, while a second technique relates to metals whose difference in 
deposit potentials is at least equal to 0.5 V. 
In the first case, which more particularly relates to hafnium-zirconium 
alloys, the production process comprises using a pyrogenic electrolytic 
cell containing a bath of molten salts based on alkaline chlorides and at 
least one fluoride ion in a quantity by weight of between 1.5 and 5% of 
the weight of the bath, in which the following components are at least 
partly submerged: an electrode for measurement connected to a reference 
electrode, these electrodes serving to measure a monitoring potential for 
the electrolysis, an anodic assembly provided with a diaphragm based on 
carbon fibers and graphite, a cathode to which a continuous potential 
difference with respect to this assembly is applied, and an injector for 
injecting material to be electrolyzed along with inert gas. The process is 
characterized in that the metals are introduced simultaneously into the 
injector, in the form of gaseous chlorides in proportions corresponding to 
those of said alloy and in a quantity such that the molar ratio of the 
fluoride contained in the bath to the quantity of metals introduced will 
be between 2.5 and 15; the value of the monitoring potential, called the 
set-point potential, is noted; the metals are deposited onto the cathode 
in the form of an alloy while chlorides continue to be introduced into the 
injector in the desired proportion and in a quantity such that the 
potential measured at the monitoring electrode remains, in terms of 
absolute value, near the absolute value of the set-point potential. 
Hence in the case where one seeks to produce alloys of metals whose deposit 
potentials differ by less than 0.5 V, the process consists in performing 
electrolysis in a cell equipped with a monitoring device. 
Such a device has already been described in U.S. Pat. No. 4,567,643. By the 
choice of a ratio of fluoride to metal to be deposited, it is possible 
with great sensitivity to measure an electrical potential, which is a 
function of the concentration of the bath in terms of ions of this metal. 
Hence, once an optimum concentration has been determined, the potential 
that corresponds to it can be noted. This potential will then serve as a 
reference, and it suffices then to supply chloride to the cell in such a 
way as to keep this potential constant, in order to be sure of permanently 
having the desired concentration of dissolved metal ions in the bath. 
The achievement of the invention is the discovery that this device can also 
be used in the case where one wishes to measure the concentration of a 
plurality of types of ions simultaneously. In this process, an anodic 
assembly is also used, provided with a particular diaphragm, as described 
in U.S. Pat. No. 5,064,513. 
This diaphragm is constituted by carbon fibers embedded in a rigid 
graphite-based material, and it has the property of having a porosity of a 
predetermined value, which makes it easier to carry out the electrolysis 
and to produce a metal deposit with a regular structure. 
Once again, the achievement of the invention is to demonstrate that these 
advantages are attained when a plurality of types of ions are used 
simultaneously. 
The process also relates to an injector of the kind described in French 
Patent No. 2653139, whose effect is to keep the concentration by weight of 
the bath within a limited range and to adjust it progressively and 
precisely. This has the advantage in the present case of enabling easier 
control of the conditions under which a deposit is produced, where the 
proportions of the various metals must be within narrow limits. 
The combination of these different means makes it possible to carry out the 
simultaneous electrolysis of a plurality of chlorides, and the 
simultaneous deposit of the metals of the alloy, in the desired 
proportions and in accordance with a structure meeting the characteristics 
of the invention. 
However, this process is not applicable when the metals to be deposited 
have a difference in deposit potential near or equal to 0.5 V, as is the 
case, for example, with niobium-titanium alloys, because the process would 
produce a preferential deposit of the least electronegative metal and 
hence produces an alloy in which the elements are not in the desired 
proportions. It is accordingly necessary to utilize a variation of the 
process for producing these alloys. 
Applicants have discovered that placing the most electronegative metal into 
solution in the bath not by means of electrolysis of its halide but rather 
by electrodissolution of the metal itself, using a sacrificial anode, 
achieves the desired result. 
This leads to a process of producing alloys where a pyrogenic electrolytic 
cell is used containing a bath of molten salts based on alkaline chlorides 
and at least one fluoride ion in a quantity by weight of between 1 and 3% 
of the weight of the bath, in which the following elements are at least 
partly submerged: a monitoring electrode connected to a reference 
electrode, the electrodes serving to measure a monitoring potential, an 
anodic assembly provided with a diaphragm based on carbon fibers and 
graphite, a deposit cathode onto which a continuous potential difference 
E1 with respect to this assembly is applied, and an injector of material 
to be electrolyzed and inert gas. The process is characterized in that an 
electrode comprising the most electronegative metal of the alloy to be 
deposited and, via the injector, the halide of the most electropositive 
metal of the alloy to be deposited are introduced into the bath; a 
positive potential difference E2 is established between the sacrificial 
electrode and the injector, such that the metal of the electrode goes into 
solution in the bath; the concentrations of metal ions in the bath are 
adjusted so as to have a proportion in relation to that of the desired 
alloy and a quantity such that the molar ratio of the fluoride contained 
in the bath to the quantity of metals present is between 2.5 and 15; the 
value of the monitoring potential, known as the set-point potential, is 
noted; the metals are deposited in the form of an alloy on the cathode 
while continuing to introduce the chloride into the injector and 
maintaining the potential difference E2 in such a manner that the 
potential measured at the monitoring electrode remains in terms of 
absolute value near the absolute value of the set-point potential; and E2 
corresponds to the passage of at least X/2 Faradays per mole of MCl.sub.x 
introduced into the bath, where M is the least electronegative metal and X 
is its valence, and that E1 corresponds to the passage of at least 1/2 
Faraday per mole of MCl.sub.x. 
Hence as in the previous process, the invention includes elements of the 
three patents referred to above but all in the same pyrogenic electrolytic 
cell, and it is distinguished from those patents by the fact that a 
deposit made from metal ions originating in part from anodic dissolution 
is associated with the deposit of at least one metal by electrolytic 
reduction of its halide. 
It should be noted that since the metals vary widely in terms of their 
deposit potential, a strong chemical dissolution of the soluble anode 
takes place in the bath. In order to have the desired concentration of 
ions in the bath, it is necessary to take this chemical effect into 
account and to polarize this anode more or less and at the same time to 
regulate the prereduction of the halide in the injector. 
Hence the invention, unlike the previous process, entails the necessity of 
linking the potential E2 between the sacrificial electrode and the 
injector with the quantity of chloride introduced. This makes it possible 
to control the proportion of ions dissolved in the bath and to produce 
alloys having the desired composition. 
This type of process is also applicable to the case of two metals having 
deposit potentials that are close to one another, but since the chemical 
dissolution is then relatively slight, it is then necessary to polarize 
the soluble anode strongly in order to produce the appropriate 
concentration in the bath. 
The two processes lead to the formation on the cathode of a deposit of 
readily detachable crystals, where the elements are in solid solution and 
have the physical characteristics according to the invention. 
After separation from the cathode, these crystals are washed in water to 
eliminate the salt present in the bath, and are then converted to ingots 
by melting with an appropriate apparatus, such as an arc furnace, an 
induction furnace, electron bombardment furnace, inductive plasma furnace, 
or arc plasma furnace.

DETAILED DESCRIPTION OF THE DRAWINGS 
In FIG. 1, a vessel 1 contains a bath of molten salts 2, and is closed by a 
lid 3 that is pierced with openings through which the following elements 
pass, via electrical insulation rings 4, in order to pass partway into the 
bath: 
a carbon anode 5, surrounded by a diaphragm 6 and equipped with a tube 7 by 
which gaseous halogen formed in the electrolysis of the halides escapes, 
anode 5 connected to the positive pole of a source of direct current; 
a device 8 for feeding gaseous halides which are introduced into the bath 
in the direction of arrows 9; 
a steel cathode 10 onto which alloy 11 is deposited and which is connected 
to the negative pole of the current source supplying the anode; 
a measuring electrode 12 connected to a reference electrode, not shown. 
In FIG. 2, an electrolytic cell 21 contains a bath 22 of molten salts, and 
is closed by a lid 23 pierced with openings through which the following 
elements pass, by way of rings 24 of insulating material, in order to pass 
partway into the bath: 
an anode 25 of carbon, surrounded by a diaphragm 26, and equipped with a 
tube 27 through which the gaseous halogen that is produced in the course 
of the electrolysis escapes, anode 5 connected to the positive pole of a 
source of a direct current; 
an expendable electrode 28 constituted by the most electronegative metal of 
the alloy to be deposited and connected to the positive pole of a source 
of direct current; 
a device 29 for feeding the halide of the least electronegative metal of 
the alloy to be deposited, which is introduced into the bath in the 
gaseous state as indicated by the arrows 30, this device being connected 
to the negative pole of the current source supplying the expendable 
electrode; 
a cathode 31 onto which the alloy 32 to be produced is deposited and which 
is connected to the negative pole of the current source supplying the 
anode 25; 
a measuring electrode 33, connected to a reference electrode, not shown. 
In FIG. 3, black zones indicated by arrows can be seen corresponding to 
pieces of unmelted hafnium. 
In FIG. 4, white zones can be seen, corresponding to the same unmelted 
residues. 
In FIG. 5, white lines can be seen which represent residues of unmelted 
hafnium chips. 
In FIG. 6, the structure of the alloy, produced according to the invention, 
is completely homogeneous. 
In FIG. 7, the unmelted niobium chips can be seen in black. 
In FIG. 8, in the alloy according to the invention, no presence whatever of 
unmelted residues can be found. 
EXAMPLES 
The invention may be illustrated with the aid of the following examples: 
Example 1 
An electrolytic cell of Inconel 600 containing a bath of molten salts 
formed of an equimolar mixture of NaCl and KCl plus 3.5% by weight of NaF, 
heated to 720.degree. C., is equipped with: 
a graphite anode surrounded by a diaphragm of carbon fibers embedded in 
graphite, by the technique described in U.S. Pat. No. 5,064,513; 
a device for feeding halides, of the type described in French Patent No. 
2653139; 
a steel deposit cathode; 
a device for monitoring the potential with respect to a reference 
electrode, of the type described in U.S. Pat. No. 4,657,643. 
A current of 1,500 amperes (hence a current density of 75 mA/cm.sup.2) is 
passed between the anode and the cathode, while a mixture of ZrCl.sub.4 
and HfCl.sub.4 is introduced, by the feeding device, in such a manner as 
to provide 66.2 wt. % Hf and 33.8 wt. % Zr and a quantity in the bath such 
that the molar ratio of fluoride to the quantity of metals introduced 
equals 5, and the reference potential measured at the monitoring device is 
noted. 
The cell is supplied continuously for 10 hours with both electric current 
and chlorides, in such a manner that the potential measured remains close 
in absolute value to the absolute value of the set-point potential. 
In the course of five successive operations and with a mean Faraday yield 
of 92%, 87.6 kg of alloys were collected, in which the proportions of 
metals were as follows: 
1 Hf: 60.5%, Zr: 39.5% 
2 Hf: 67%, Zr: 33% 
3 Hf: 66%, Zr: 34% 
4 Hf: 66.5%, Zr: 33.5% 
5 Hf: 67%, Zr: 33% 
These alloys are present in the form of conglomerates having a mean size of 
10 mm and composed of crystals 3 mm in mean diameter, having a specific 
surface area of 0.03 m.sup.2 /g, in which the metals are in solid 
solution. 
From the standpoint of purity, the composition of these alloys was: 
oxygen: 620 ppm 
carbon: &lt;10 ppm 
nitrogen: &lt;10 ppm 
chlorine: &lt;50 ppm 
iron: &lt;20 ppm 
chromium: &lt;10 ppm 
nickel: &lt;10 ppm 
hence a purity (Zr+Hf) of near 99.9%. 
Example 2 
An electrolytic cell was used having the same characteristics as example 1, 
except: 
an NaF content of 2.5%, 
a bath temperature of 725.degree. C., and 
the presence of a sacrificial titanium electrode polarized positively and 
electrically connected to the chloride injector polarized negatively. 
An equimolar niobium-titanium alloy was made, by feeding the injector with 
niobium chloride, passing a current of 100 amperes between the anode and 
the cathode, and passing a current of 20 amperes between the sacrificial 
electrode and the injector, in such a way as to produce a concentration of 
metal ions in the bath such that the ratio between the quantity of 
fluoride and the quantity of dissolved metal in the bath was equal to 6. 
The reference potential indicated by the monitoring device was then noted. 
The polarization of the injector was adjusted in such a manner as to pass 
5 Faradays per mole of NbCl.sub.5 introduced, and that of the cathode was 
adjusted in such a manner as to pass 2 Faradays per mole of NbCl.sub.5, 
while monitoring the potential of the monitoring device, which remains 
between -1.85 and -1.95 V. 
The concentration of Ti ions in the bath was demonstrated to be kept 
between 1.5 and 2.5% by weight, with a mean valence of between 2 and 2.3, 
while the concentration of Nb ions varied between 0.1 and 0.75% by weight 
for a mean valence of between 3.4 and 3.7. 
For a titanium valence equal to 2.15, the material balance is evidence of a 
chemical attack complimentary to the electrochemical attack and globally 
represented by the equations: 
EQU 2 Nb.sup.5+ +2e.sup.- .fwdarw.2 Nb.sup.4+ 
EQU Ti.fwdarw.Ti.sup.2+ +2e.sup.-, hence 
EQU 2 Nb.sup.4+ +Ti.sup.2+ .fwdarw.2 Nb.sup.(4-x)+ +Ti.sup.(2+2x)+. 
Under these conditions, with a chloride yield of 95% and a metal yield of 
90% for a discharge of 473 g/h, an alloy was produced containing crystals 
of Nb-Ti in solid solution at an atomic concentration of 50%.+-.10%, the 
crystals having a mean size of 0.5 mm and a specific surface area of 0.02 
m.sup.2 /g, in the form of 10 mm conglomerates having the following 
composition: 
oxygen: 500 ppm; carbon: 20 ppm; nitrogen: &lt;20 ppm; iron: &lt;20 ppm; 
chromium: &lt;70 ppm; nickel: &lt;10 ppm; chlorine: &lt;10 ppm; fluorine: &lt;10 ppm; 
sodium: &lt;10 ppm; potassium: &lt;10 ppm; the remainder being niobium and 
titanium. 
The invention is applicable to the production of alloys of refractory metal 
of very high purity, which have very good homogeneity on the microscopic 
scale.