EP0570308B1 - Alloys of metals with high melting points, suitable for transformation into homogeneous and pure ingost, and preparation process of these alloys - Google Patents

Abstract

These alloys are made up of crystals of the said metals in solid solution which have a particular size and specific surface. They are obtained by coelectrodeposition of the metals in a molten salt bath. In the case where the metals exhibit a difference smaller than 0.5 volt in their deposition potential, the metals are delivered into the bath in the form of chlorides. In the case where this difference is greater than 0.5 volt, the least electronegative metal is delivered in the form of a chloride whereas the most electronegative metal is delivered by means of a soluble electrode. The invention finds its application especially in the production of Nb-Ti and Hf-Zr ingots.

Description

TECHNICAL FIELD OF THE INVENTION.-

The present invention relates to alloys of refractory metals suitable for transformation into homogeneous and pure ingots and to processes for obtaining said alloys.

It relates more particularly to alloys formed from refractory metals having different melting temperatures of at least 200 ° C., such as for example the hafnium-zirconium, hafnium-titanium, niobium-titanium, niobiumzirconium, tantalum-titanium, tantalum alloys -zirconium, tantalum-niobium, niobium-tantalum-titanium and niobium-titanium-aluminum.

More specifically, these alloys have weight compositions such that their starting solidification temperature is at least 150 ° C. lower than the solidification temperature of the least fusible metal.

These alloys obtained initially in more or less divided form are then subjected to at least one operation of fusion so as to transform them into ingots.

These ingots can then be rolled in the form of sheets intended for the manufacture of nuclear fuel reprocessing containers in the case of Hf-Zr alloys or of neutron retarders in the case of Hf-Ti alloys or else of superconductive compounds or superalloys aeronautics in the case of the Nb-Ti alloy.

PRIOR ART

• la coaluminothermie des oxydes qui a l'inconvénient de conduire à des alliages souillés par de l'aluminium et de l'oxygène et qui se présentent sous forme de blocs massifs qu'il faut concasser avant de les purifier par bombardement électronique et de les transformer en lingots.
• La coréduction des chlorures par un métal comme le sodium ou le magnésium qui donne lieu à la formation d'éponges fortement polluées par l'agent réducteur ainsi que par du fer et des ions chlorures et qu'il faut également concasser.
  Pour améliorer la pureté et l'homogénéité de ce type d'alliage avant transformation en lingot, il est possible de procéder à un affinage électrolytique en milieu halogénure fondu par dépôt cathodique dudit alliage à l'état particulaire utilisé comme anode soluble. Ainsi selon FR 1216255 on peut obtenir par dépôt sur une cathode inerte un alliage de titane contenant de 1 à 10% de vanadium, sous forme de cristaux enchevêtrés avec moins de 0,03% d'oxygène, par électrolyse en milieu NaCI fondu d'une anode constituée par une éponge broyée de titane vanadium résultant de la coréduction par le sodium des chlorures métalliques correspondants.
• La codéposition en phase vapeur qui fournit des whiskers très pyrophoriques, difficiles à manipuler qu'il faut compacter avant fusion.
• La mécano-synthèse ou cobroyage des métaux à allier qui aboutit à des particules de granulométrie relativement grossière et fortement polluées et à l'obtention lors de la fusion d'un produit hétérogène dû à la présence d'infondus du métal le moins fusible.

It is known that such alloys can be obtained by different methods such as:

• the coaluminothermy of oxides which has the disadvantage of leading to alloys contaminated by aluminum and oxygen and which are in the form of massive blocks which must be crushed before being purified by electron bombardment and transforming them in ingots.
• The co-reduction of chlorides by a metal such as sodium or magnesium which gives rise to the formation of sponges highly polluted by the reducing agent as well as by iron and chloride ions and which must also be crushed.
  To improve the purity and homogeneity of this type of alloy before transformation into an ingot, it is possible to carry out an electrolytic refining in a molten halide medium by cathodic deposition of said alloy in the particulate state used as soluble anode. Thus, according to FR 1216255, a titanium alloy containing 1 to 10% vanadium, in the form of crystals entangled with less than 0.03% oxygen, can be obtained by deposition on an inert cathode, by electrolysis in a molten NaCl medium of an anode consisting of a ground sponge of titanium vanadium resulting from the co-reduction by sodium of the corresponding metal chlorides.
• Codeposition in the vapor phase which provides very pyrophoric whiskeys, difficult to handle which must be compacted before melting.
• Mechanical synthesis or co-grinding of the metals to be alloyed, which results in relatively coarse-grained and highly polluted particles and on obtaining during the melting of a heterogeneous product due to the presence of fuses of the least fusible metal.

PURPOSE OF THE INVENTION

The object of the invention is to obtain alloys having a homogeneous structure at the level of the elementary crystal, an improved purity compared to that of the products of the prior art and a suitable particle size so that they can be fully melted and transformed. in ingots in which this homogeneity of structure and purity is maintained.

DESCRIPTION OF THE INVENTION

The invention relates to alloys of refractory metals suitable for transformation into homogeneous ingots of purity greater than 99.9%, the melting temperatures of which differ from each other by at least 200 ° C. and whose weight proportions are such that for each alloy the starting solidification temperature is more than 150 ° C lower than the solidification temperature of the least meltable metal in the form of agglomerates, characterized in that these agglomerates of dimensions between 0.2 and 30mm are composed crystals with a specific surface of between 0.005 and 0.2 m ² / g, having a size of 0.1 to 1 mm and in which the metals are in the form of a solid solution, that is to say that they are homogeneous on the atomic scale and at most have a relative deviation of composition of 20% from the average composition of the alloy so that this homogeneity remains during the melting and gives the ingots obtained identical properties at all points.

This is an important difference with the alloys obtained by melting a mixture of the constituents where the undesirable forms macroscopic zones which can reach dimensions of several millimeters and where in particular when metals of very different densities are involved, significant settling occurs.

In addition, these crystals and their agglomerates have a size and a specific surface such that the problems of spontaneous oxidation which arise when this surface is too large or of shaping of the products before fusion when this size is too large are avoided. , and such that the dissolution in the liquid metal is promoted. Hence the absence of any pollution of these products, not oxygen and iron, in particular during grinding operations, and the possibility of obtaining ingots of high purity ingots.

Preferably, the crystals have a specific surface of between 0.01 and 0.05 m ² / g and the agglomerates have dimensions of between 1.5 and 12mm because it is within these ranges that the homogeneities and the maximum purities are obtained.

The invention also relates to methods for obtaining said alloys.

These methods are based on co-electrodeposition, that is to say the simultaneous electrolytic deposition of the elements forming the alloy.

However, the technique for obtaining the alloy varies as a function of the difference in deposition potential of each of the elements of the alloy.

A first technique is applicable to metals whose electrolytic deposition potentials are very little different from each other, that is to say when they have a difference between them less than 0.5 volt while a second technique relates to metals whose difference in deposition potentials is at least equal to this value.

In the first case, which relates more particularly to hafnium-zirconium alloys, the obtaining process consists in using an igneous electrolysis cell containing a bath of molten salts based on alkali chlorides and at least one fluoride ion in an amount between 1.5 and 5% by weight of the weight of the bath in which a measurement electrode is at least partially immersed in relation to a reference electrode which are used to measure a potential for controlling electrolysis, an anode assembly provided with a diaphragm based on carbon fibers and graphite, a cathode to which a difference in continuous potential is applied with respect to said assembly, an injector of material to be electrolyzed and of inert gas, characterized in that one introduces simultaneously in the injector the metals in the form of gaseous chlorides in proportions corresponding to those of said alloy and in an amount such as the molar ratio of fluorine contained in the bath at the quantity of metals introduced is between 2.5 and 15, the value of the control potential known as the reference potential is noted, the metals are deposited at the cathode in the form of an alloy while continuing to introduce the chlorides in desired proportion in the injector and in such quantity that the potential measured on the control electrode remains in absolute value close to the absolute value of the setpoint potential.

Thus, in the case where it is desired to obtain metal alloys whose deposition potentials differ from one another by less than 0.5 volts, the method consists in carrying out electrolysis in a cell equipped with a control device.

Such a device has already been described in US Pat. No. 4,567,643. It allows, by the choice of a fluorine / metal ratio to be deposited, to measure with great sensitivity an electrical potential which is a function of the concentration of the ion bath of said metal. Thus, an optimum concentration having been determined, one can note the potential which corresponds to it. This potential will then serve as a reference and it then suffices to supply the cell with chloride so as to maintain this constant potential to be sure to have permanently in the bath the desired concentration of dissolved metal ions.
The merit of the applicant is to have found that this device was also usable in the case where it was desired to simultaneously measure the concentration of several types of ions.

In this process, an anode assembly is also used, provided with a particular diaphragm and as described in US Pat. No. 5,064,513.

This diaphragm is formed by carbon fibers embedded in a rigid graphite-based material and has the property of having a porosity of determined value which allows easier conduct of the electrolysis and obtaining a metallic deposit of regular structure.
Here again, the merit of the Applicant has been to show that these advantages are achieved when several types of ions are used simultaneously.

The method also uses an injector such as that described in French patent 2653139 and which has the effect of maintaining the weight concentration of the bath within a limited range and of adjusting it gradually and precisely. This has the advantage in the present case of being able to more easily control the conditions for obtaining a deposit where the proportions of the different metals must be included within narrow limits.

The combination of these different means allows the simultaneous electrolysis of several chlorides as well as the simultaneous deposition of the metals of the alloy in desired proportions and according to a structure corresponding to the characteristics described above.

However, this process is not applicable when the metals to be deposited have a deposit potential difference greater than or equal to them. 0.5 volts as for example in the case of niobium-titanium alloys because there is then a preferential deposit of the least electronegative metal and therefore an alloy in which the elements are not in desired proportion. It is therefore necessary to find another process for obtaining it.

From where the idea of the applicant which consists in putting the most electronegative metal in solution in the bath not by electrolysis of its halide but, by electrodissolution of the metal itself starting from a soluble anode.

Hence the process for obtaining where an igneous electrolysis cell is used containing a bath of molten salts based on alkali chlorides and at least one fluoride ion in an amount between 1 and 3% by weight of the bath in which are at least partially immersed a control electrode in relation to a reference electrode which are used to measure a control potential, an anode assembly provided with a diahragm based on carbon fibers and graphite, a cathode of deposit to which a continuous potential difference E1 is applied with respect to said assembly, an injector of material to be electrolyzed and of inert gas characterized in that a positively polarized electrode consisting of the most electronegative metal is introduced into the bath, and by the negatively polarized injector the halide of the most electropositive metal of the alloy to be deposited, a positive potential difference E2 is established between ladi te electrode and the injector so 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 fluorine contained in the bath to the quantity of metals present is between 2.5 and 15, the value of the control potential known as the reference potential is noted, the metals are deposited in the form of an alloy with the cathode while continuing to introduce chloride into the injector and maintain the potential difference E2 so that the potential measured on the control electrode remains in absolute value greater than the absolute value of the setpoint potential and that E2 corresponds to the passage of at least X / 2 Faraday per mole of MCl _X introduced into the bath where M is the least electronegative metal and X its valence and that E1 corresponds to the passage of at least 1/2 Faraday per mole of MCl _X.

Thus, as in the previous method, the invention consists in combining the teaching of the three patents mentioned above in the same igneous electrolysis cell, but it differs from it in that at the filing of at least one metal by electrolytic reduction of its halide is associated with a deposit obtained from metal ions resulting in part from an anodic dissolution.

It should be noted that since the metals have a large difference in their deposition potential, there is a strong chemical dissolution of the soluble anode in the bath. To have the desired concentration of ions in the bath, it is necessary to take account of this chemical effect and to polarize more or less this anode and at the same time regulate the prereduction of the halide in the injector.

Hence the need, unlike the previous process, to link the potential E2 between the soluble electrode and the injector to the quantity of chloride introduced. Thus, it is possible to control the proportion of ions dissolved in the bath and to obtain alloys having the desired composition.

This type of process is also applicable to the case of two metals having close deposition potentials but since the chemical dissolution is then relatively low, it is necessary to strongly polarize the soluble anode to obtain the suitable concentration in the bath.

Both methods lead to the formation on the cathode of a deposit of easily detachable crystals where the elements are in solid solution and have the physical characteristics defined above.

After separation of the cathode, said crystals are washed with water to remove the salts present in the bath and then formed into ingots by fusion using a suitable means such as, for example, an arc furnace , induction, electron bombardment, inductive plasma or arc plasma.

DRAWINGS.-

• figure 1 : la vue suivant une coupe transversale d'une cellule d'électrolyse mise en oeuvre dans le cas de l'obtention d'alliages dont les éléments ont un écart de leur potentiel de dépôt < 0,5 volt.
• figure 2 : la vue suivant une coupe verticale d'une cellule d'électrolyse mise en oeuvre dans le cas de l'obtention d'alliages dont les éléments ont un écart de leur potentiel de dépôt supérieur ou égal à 0,5 volt.
• figure 3 : une macrographie d'un alliage Zr₇₀Hf₃₀ obtenu suivant l'art antérieur à partir d'éponge de zirconium et de cristaux électrolytiques de hafnium.
• figure 4 : une micrographie au grandissement 100 de l'alliage précédent.
• figure 5 : une micrographie au grandissement 3 du même alliage obtenu suivant l'art antérieur à partir d'éponge de zirconium et de copeaux de hafnium.
• figure 6 : une micrographie au grandissement 500 du même alliage obtenu suivant l'invention.
• figure 7 : une photographie de la section d'un lingot d'un alliage de Nb₅₃Ti47 obtenu suivant l'art antérieur à partir d'éponge de titane et de copeaux de niobium.
• figure 8 : une photographie de la section d'un lingot du même alliage que celui de l'exemple précédent obtenu suivant l'invention.

The invention will be better understood using the attached figures which represent:

• Figure 1: the view in cross section of an electrolysis cell used in the case of obtaining alloys whose elements have a difference in their deposition potential <0.5 volt.
• Figure 2: the view in a vertical section of an electrolysis cell used in the case of obtaining alloys whose elements have a difference in their deposition potential greater than or equal to 0.5 volts.
• FIG. 3: a macrography of a Zr ₇₀ Hf ₃₀ alloy obtained according to the prior art from zirconium sponge and hafnium electrolytic crystals.
• Figure 4: a magnification micrograph 100 of the previous alloy.
• Figure 5: a magnification micrograph 3 of the same alloy obtained according to the prior art from zirconium sponge and hafnium chips.
• FIG. 6: a micrograph at 500 magnification of the same alloy obtained according to the invention.
• Figure 7: a photograph of the section of an ingot of an alloy of Nb ₅₃ Ti47 obtained according to the prior art from titanium sponge and niobium chips.
• Figure 8: a photograph of the section of an ingot of the same alloy as that of the previous example obtained according to the invention.

• une anode 5 en carbone entourée par un diaphragme 6 équipé d'une tubulure 7 par laquelle s'échappe l'halogène gazeux formé lors de l'électrolyse des halogénures et reliée au pôle positif d'une source de courant continu,
• un dispositif 8 d'alimentation en halogénures gazeux qui sont introduits dans le bain suivant le sens des flèches 9,
• une cathode 10 en acier sur laquelle se dépose l'alliage 11 et reliée au pôle négatif de la source de courant alimentant l'anode.
• une électrode 12 de mesure reliée à une électrode de référence non représentée.

In FIG. 1, there is a tank 1 containing a bath of molten salts 2 closed by a cover 3 pierced with openings through which pass by means of rings 4 insulating from electricity to partially plunge into the bath:

• a carbon anode 5 surrounded by a diaphragm 6 fitted with a tube 7 through which escapes the gaseous halogen formed during the electrolysis of the halides and connected to the positive pole of a direct current source,
• a device 8 for supplying gaseous halides which are introduced into the bath in the direction of the arrows 9,
• a steel cathode 10 on which the alloy 11 is deposited and connected to the negative pole of the current source supplying the anode.
• a measuring electrode 12 connected to a reference electrode not shown.

• une anode 25 en carbone entourée par un diaphragme 26 équipé d'une tubulure 27 par laquelle s'échappe l'halogène gazeux qui se dégage au cours de l'électrolyse et reliée au pôle positif d'une source de courant continu,
• une électrode 28 consommable constituée par le métal le plus électronégatif de l'alliage à déposer et reliée au pôle positif d'une source de courant continu,
• un dispositif 29 d'alimentation en halogénure du métal le moins électronégatif de l'alliage à déposer et qui est introduit dans le bain à l'état gazeux suivant les flèches 30 ; ce dispositif est relié au pôle négatif de la source de courant alimentant l'électrode consommable,
• une cathode 31 sur laquelle se dépose l'alliage 32 à fabriquer et qui est reliée au pôle négatif de la source de courant alimentant l'anode 25.
• une électrode 33 de mesure reliée à une électrode de référence non représentée.

In FIG. 2, there is an electrolysis cell 21 containing a bath 22 of molten salts closed by a cover 23 pierced with openings through which pass through rings 24 of insulating material to partially plunge into the bath:

• a carbon anode 25 surrounded by a diaphragm 26 fitted with a tube 27 through which escapes the halogen gas which is released during the electrolysis and connected to the positive pole of a direct current source,
• a consumable electrode 28 consisting of the most electronegative metal of the alloy to be deposited and connected to the positive pole of a direct current source,
• a device 29 for supplying the least electronegative metal of the alloy to be deposited with halide and which is introduced into the bath in the gaseous state according to the arrows 30; this device is connected to the negative pole of the current source supplying the consumable electrode,
• a cathode 31 on 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 Figure 3, there are, indicated by arrows, black areas corresponding to pieces of unfounded hafnium.

In FIG. 4, a distinction is made between white areas corresponding to the same unfounded.

In Figure 5, we see white lines representing the remains of unfounded hafnium chips.

In FIG. 6, the structure of the alloy is perfectly homogeneous.

In Figure 7, we distinguish in black the unfused niobium shavings.

In FIG. 8, we do not note any presence of unfunded.

APPLICATION EXAMPLES.

The invention can be illustrated with the aid of the following application examples:

Example 1.-

• une anode en graphite entourée d'un diaphragme en fibres de carbone noyées dans du graphite suivant la technique décrite dans le brevet US 5064513,
• un dispositif d'alimentation en halogénures du type décrit dans le brevet français 2653139,
• une cathode de dépôt en acier,
• un dispositif de contrôle du potentiel par rapport à une électrode de référence du type décrit dans le brevet US 4657643,

on a fait passer un courant de 1500 Ampères ( soit une intensité cathodique de 75 mA/cm² ) entre l'anode et la cathode tout en introduisant par le dispositif d'alimentation un mélange de ZrCl₄ et de HfCl₄ de manière à avoir en poids 66,2% de Hf et 33,8% de Zr et une quantité telle dans le bain que le rapport molaire du fluor à la quantité de métaux introduits soit égal à 5, on a noté le potentiel de référence mesuré sur le dispositif de contrôle. Puis, on a alimenté la cellule en continu pendant 10 heures à la fois en courant électrique et en chlorures de manière que le potentiel mesuré reste en valeur absolue voisin de la valeur absolue du potentiel de consigne.In an Inconel 600 electrolysis cell containing a bath of molten salts formed from an equimolecular mixture of NaCl-KCl + 3.5% by weight of NaF brought to 720 ° C., said cell being equipped with:

• a graphite anode surrounded by a carbon fiber diaphragm embedded in graphite according to the technique described in patent US 5064513,
• a halide supply device of the type described in French patent 2,653,139,
• a steel deposition cathode,
• a device for checking the potential with respect to a reference electrode of the type described in US Pat. No. 4,676,643,

a current of 1500 amperes (a cathode intensity of 75 mA / cm ² ) was passed between the anode and the cathode while introducing a mixture of ZrCl ₄ and HfCl ₄ through the supply device so as to have by weight 66.2% of Hf and 33.8% of Zr and a quantity such in the bath that the molar ratio of fluorine to the quantity of metals introduced is equal to 5, the reference potential measured on the device was noted control. Then, the cell was supplied continuously for 10 hours with both electric current and chlorides so that the measured potential remains in absolute value close to the absolute value of the setpoint potential.

87.6 kg of alloys in which the metal proportions were as follows were collected during five successive operations and with an average Faraday yield of 92%. 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%

• oxygène : 620 ppm
• carbone : < 10 ppm
• azote : < 10 ppm
• chlore : < 50 ppm
• fer : < 20 ppm
• chrome : < 10 ppm
• nickel : < 10 ppm.

soit une pureté (Zr + Hf) supérieure à 99,9%.These alloys are in the form of agglomerates having an average size of 10 mm and composed of crystals of 3 mm in average diameter having a specific surface of 0.03 m ² / g in which the metals were in solid solution.
From the purity point of view, these alloys had for composition:

• oxygen: 620 ppm
• carbon: <10 ppm
• nitrogen: <10 ppm
• chlorine: <50 ppm
• iron: <20 ppm
• chromium: <10 ppm
• nickel: <10 ppm.

or a purity (Zr + Hf) greater than 99.9%.

Example 2.-

• une teneur en NaF de 2,5%,
• une température du bain de 725°C,
• la présence d'une électrode consommable en titane polarisée positivement et en relation électrique avec l'injecteur de chlorure polarisé négativement,

on a fabriqué un alliage niobium-titane équimolaire. Pour cela, on a alimenté l'injecteur avec du chlorure de niobium et fait passer un courant de 100 Ampères entre l'anode et la cathode et un courant de 20 Ampères entre l'électrode consommable et l'injecteur de manière à obtenir dans le bain une concentration en ions métalliques telle que le rapport de la quantité de fluor à la quantité de métal dissous dans le bain soit égal à 6. On a alors noté le potentiel de référence indiqué par le dispositif de contrôle. Puis, on a règlé la polarisation de l'injecteur de manière à faire passer 5 Faraday par mole de NbCl₅ introduit et celle de la cathode de manière à faire passer 2 Faraday par mole également, tout en contrôlant le potentiel du dispositif de contrôle qui reste compris entre - 1,85 et - 1,95 volt.
On a constaté que la concentration en ions Ti dans le bain se maintenait entre 1,5 et 2,5% en poids avec une valence moyenne comprise entre 2 et 2,3 tandis que la concentration en ions Nb variait entre 0,1 et 0,15%en poids pour une valence moyenne comprise entre 3,4 et 3,7.In an electrolysis cell having the same characteristics as those of Example 1 except:

• an NaF content of 2.5%,
• a bath temperature of 725 ° C.,
• the presence of a positively polarized titanium consumable electrode and in electrical connection with the negatively polarized chloride injector,

an equimolar niobium-titanium alloy was made. For this, the injector was supplied with niobium chloride and a current of 100 amperes passed between the anode and the cathode and a current of 20 amperes between the consumable electrode and the injector so as to obtain in the bath a concentration of metal ions such that the ratio of the quantity of fluorine to the quantity of metal dissolved in the bath is equal to 6. The reference potential indicated by the control device is then noted. Then, the polarization of the injector was adjusted so as to pass 5 Faraday per mole of NbCl ₅ introduced and that of the cathode so as to pass 2 Faraday per mole also, while controlling the potential of the control device which remains between - 1.85 and - 1.95 volts.
It was found that the concentration of Ti ions in the bath was maintained between 1.5 and 2.5% by weight with an average valence of between 2 and 2.3 while the concentration of Nb ions varied between 0.1 and 0 , 15% by weight for an average valence of between 3.4 and 3.7.

For a valence of titanium equal to 2.15, the material balance highlights a chemical attack complementary to the electrochemical attack and generally represented by

2 Nb ⁵⁺ + 2e ^- = 2 Nb ⁴⁺



Ti = Ti ²⁺ + 2 e ^-

then,

2 Nb ⁴⁺ + Ti ²⁺ = 2Nb ^{(4-x) +} + Ti ^{(2 + 2x) +} .

Under these conditions, there was obtained with a chlorine yield of 95% and metal of 90% at a flow rate of 473 g / h an alloy containing crystals of Nb-Ti in solid solution at 50% + or - 10% atomic having a average dimension of 0.5 mm and a specific surface of 0.02 m ² / g in the form of agglomerates of 10 mm having the following composition:
oxygen: 500 ppm; carbon: 20 ppm; nitrogen: <20 ppm; iron: <20 ppm; chromium <10 ppm; nickel <10 ppm; chlorine <100 ppm; fluorine <10 ppm; sodium: <10 ppm; potassium: <10 ppm; balance niobium and titanium.

The invention finds its application in obtaining alloys of refractory metals of very high purity having very good homogeneity on the microscopic scale.

Claims (5)

1. Refractory metal alloys capable of being formed into homogeneous ingots with a purity greater than 99.9 % which consist of refractory metals, the melting points of which differ from each other by at least 200°C and the mass proportions of which are such that for each alloy the temperature at which solidification starts is more than 150°C lower than the solidification temperature of the least fusible metal, being in the form of agglomerates, characterized in that these agglomerates with a size of 0.2 to 30 mm consist of crystals with a specific surface area of between 0.005 and 0.2 m²/g having a size of 0.1 to 1 mm and in which the metals are in a solid solution state, that is to say they are homogeneous on the atomic scale and have a composition which differs by no more than 20 % from the average composition of the alloy and that they are free from macroscopic zones of a mixture of unfused constituents.
2. Alloys according to claim 1, characterized in that the crystals have a specific surface area of between 0.01 and 0.05 m²/g.
3. Alloys according to claim 1, characterized in that the agglomerates are between 1.5 and 12 mm in size.
4. Process for obtaining alloys according to claim 1, wherein the metals have deposition potentials which differ from each other by less than 0.5 volts and where a hot electrolytic cell (1) is used containing a bath (2) of molten salts based on alkali metal 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 are immersed at least partially a measuring electrode (12) in conjunction with a reference electrode which serve to measure an electrolysis control potential, an anode assembly (5) provided with a diaphragm (6) based on carbon and graphite fibres, a cathode (10) to which a continuous potential difference is applied with respect to the said assembly, an injector (8) for the material to be electrolysed and for inert gas, characterized in that metals in the form of gaseous chlorides are injected simultaneously into the injector in proportions corresponding to those of the said alloy and in a quantity such that the molar ratio of the fluorine contained in the bath to the quantity of metals introduced is between 2.5 and 15, the value of the control potential, the so-called index potential, is noted, metals are deposited at the cathode in the form of an alloy while continuing to introduce chlorides in the desired proportion into the injector and in a quantity such that the potential measured on the control electrode remains in absolute value close to the absolute value of the index potential.
5. Process for obtaining alloys according to claim 1 wherein the metals have deposition potentials which differ from each other by at least 0.5 volts and where a hot electrolytic cell (21) is used containing a bath (22) of molten salts based on alkali metal 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 are immersed at least partially a control electrode (33) in conjunction with a reference electrode which serve to measure a control potential, an anode assembly (25) provided with a diaphragm (26) based on carbon and graphite fibres, a deposition cathode (31) to which a continuous potential difference El is applied with respect to the said assembly, an injector (29) for the material to be electrolysed and for inert gas, characterized in that a positively polarized electrode (28) consisting of the most electronegative metal of the alloy to be deposited is introduced into the bath and the halide of the most electropositive metal of the alloy to be deposited is introduced by means of the negatively polarized injector (29) and that a positive potential difference E2 is established between the said electrode and the injector such that the metal of the electrode (28) passes into solution in the bath, the concentrations of metal ions in the bath are adjusted so as to have a proportion with respect to that of the desired alloy and a quantity such that the molar ratio of fluorine contained in the bath to the quantity of metals present is between 2.5 and 15, the value of the control potential, the so-called index potential, is noted, the metals are deposited in the form of an alloy at the cathode while chloride continues to be introduced into the injector and the potential difference E2 continues to be maintained so that the potential measured on the control electrode remains in absolute value close to the absolute value of the index potential and that E2 corresponds to the passage of at least X/2 Faraday per mole of MCl_x introduced into the bath, where M is the least electronegative metal and X its valence and that El corresponds to the passage of at least ½ Faraday per mole of MCl_x.

Images