EP0313484B1 - Tungsten-nickel-iron high-density alloys with very high mechanical properties, and process for manufacturing these alloys - Google Patents

Abstract

These alloys are characterised in that the alpha  phase of tungsten is in the shape of butterfly wings with dislocation cells between 0.01 and 1  mu m in size and the gamma  phase of the binder has a mean free path of less than 15  mu m. <??>The process consists in subjecting the sintered and annealed product to at least three cycles of operations consisting, in each case in following the puddling by a heat treatment. <??>The invention finds its application in the production of alloys which have a tensile strength of between 1300 and 2000 MPa and intended especially for use at very high stresses. <IMAGE>

Description

The invention relates to heavy tungsten-nickel-iron alloys with very high mechanical characteristics and to a process for manufacturing said alloys.

Those skilled in the art know that the materials intended for making balance weights, vibration absorption screens and X, α, ß, γ radiations, of projectiles having a large perforation capacity must have a relatively large specific mass.

This is why recourse is made to their manufacture to so-called "heavy" alloys containing mainly tungsten distributed homogeneously in a metallic matrix generally formed by connecting elements such as nickel and iron. These alloys most often have a tungsten content by weight of between 90 and 98% and a density of 15.6 to 18. They are obtained essentially by powder metallurgy, that is to say that their components are put in the pulverulent state, compressed to give them the appropriate shape, sintered and stabilized to give them mechanical strength and possibly subjected to a wrinkling and heat treatment operation so that they acquire mechanical characteristics: strength, elongation and hardness suitable for the use that will be made of it.

• on prépare un mélange homogène de poudres contenant en poids 85-96 % W, le reste étant du nickel et du fer dans un rapport pondéral Ni/Fe de 5,5 à 8,2
• on comprime le mélange sous forme de compactés
• on fritte les compactés dans une atmosphère réductrice à une température d'au moins 1200°C et au-dessous de la température d'apparition d'une phase liquide pendant une durée suffisante pour obtenir un produit ayant une densité d'au moins 95 % de la densité théorique
• on chauffe le produit à une température comprise entre 0,1 et 20°C au-dessus de la température d'apparition d'une phase liquide pendant le temps suffisant pour faire apparaître une phase liquide mais insuffisante pour obtenir la déformation du produit.
• on recuit sous vide le produit entre 700 et 1420°C pendant un temps suffisant pour le dégazer
• on l'usine aux dimensions souhaitées, opération qui peut être précédée par au moins une passe de corroyage pour en augmenter la résistance.

The teaching of such alloys is given for example by USP 3,979,234 which describes a process for manufacturing W-Ni-Fe alloys in which:

• a homogeneous mixture of powders containing by weight 85-96% W is prepared, the rest being nickel and iron in a Ni / Fe weight ratio of 5.5 to 8.2
• the mixture is compressed in the form of compacts
• the compacts are sintered in a reducing atmosphere at a temperature of at least 1200 ° C. and below the temperature of appearance of a liquid phase for a sufficient time to obtain a product having a density of at least 95% theoretical density
• the product is heated to a temperature between 0.1 and 20 ° C above the temperature of appearance of a liquid phase during sufficient time to reveal a liquid phase but insufficient to obtain the deformation of the product.
• the product is annealed under vacuum between 700 and 1420 ° C. for a time sufficient to degas it
• it is factory at the desired dimensions, an operation which can be preceded by at least one wrought pass to increase its resistance.

Under these conditions, one obtains, for example, a product having, after a wrinkling leading to a surface reduction of 31%, a tensile strength RM of 1220 MPa, an elastic limit R 0.2 of 1180 MPa, an elongation A of 7.8% and a Rockwell C: HRc hardness of 41.

These characteristics are sufficient for certain uses but, for applications of higher stress, they prove to be very clearly insufficient because levels of resistance to rupture higher than 1600 MPa and being able to go up to 2000 MPa are now sought.

The subject of the present invention is heavy alloys with very high mechanical characteristics, with a density between 15.6 and 18, containing between 80 and 99% by weight of tungsten in the form of nodules constituting the α phase, as well as nickel and iron. in a Ni / Fe weight ratio greater than or equal to 2 playing the role of binder and constituting the γ phase and optionally molybdenum, titanium, aluminum, manganese, cobalt, rhenium, characterized in that, according to a direction transverse to the working, the α phase has the form of ellipsoids joined two by two in the vicinity of one of the ends of their major axis so as to form between said axes an acute angle, with dislocation cells of dimensions included between 0.01 and 1 µm, and the γ phase of the binder has an average free path, defined as the average of the distances which in a given direction separates two successive zones of γ phase, less than 15 µm.

It is known to those skilled in the art that the tungsten-nickel-iron alloys have a structure formed of pure tungsten nodules more or less spherodized during sintering constituting the α phase, these nodules being surrounded by a γ phase composed of the three elements of the alloy which acts as a binder between said nodules. However, the applicant has found that, in order to develop very high mechanical characteristics, the tungsten alloys must have a particular structure.

• la phase α n'a plus une forme sphérodisée mais plutôt celle d'ellipsoïdes accolés deux à deux au voisinage de l'une des extrémités de leur grand axe de manière à former entre lesdits axes un angle aigu, disposition plus communément appelée "ailes de papillon"
• la phase γ de liant a un libre parcours moyen qui décroît au fur et à mesure que la résistance à la rupture, notamment, augmente. Ainsi, en-dessous de 15 µm, on atteint des valeurs supérieures à 1600 MPa. Par ailleurs, l'accroissement des caractéristiques mécaniques au-délà de 1500 MPa passe par une polygonisation de la phase α . Complémentairement, se développe un réseau de précipitation de la phase γ dans le domaine de contiguité des nodules de la phase α .

Thus, from the morphological point of view, if one examines on a test piece obtained from these alloys a surface transverse to the direction of working, one notes that:

• the α phase no longer has a spherodized shape but rather that of ellipsoids joined two by two in the vicinity of one of the ends of their major axis so as to form between said axes an acute angle, an arrangement more commonly called "wings of butterfly"
• the binder γ phase has an average free path which decreases as the breaking strength, in particular, increases. Thus, below 15 µm, values greater than 1600 MPa are reached. Furthermore, the increase in mechanical characteristics beyond 1500 MPa requires a polygonization of the α phase. In addition, a γ phase precipitation network develops in the domain of contiguity of the nodules of the α phase.

From the microstructure point of view, by sampling thin sections, we note the presence in the α phase of dislocation cells of dimensions between 0.01 and 1 μm which will decrease as the mechanical characteristics increase. Following this increase, there is also a disorientation of these cells relative to each other. These cells are thought to give these alloys the plasticity necessary for their deformation. In addition, the examination on a test piece of the surface parallel to the direction of wrought reveals a fibrous texture all the more pronounced as the mechanical characteristics are high. These fibers are characterized by a particular orientation corresponding, according to the Miller indices, to the direction <100> for the poles {110} in the central part of the test-tube.

The invention also relates to a method for manufacturing alloys having such a structure and in which the value of the desired mechanical characteristics can be adjusted at will and in particular reaching a tensile strength close to 2000 MPa.

To achieve this, it has developed a treatment for alloys allowing to favor the plastic deformation of the α phase knowing that this is normally fragile but that it has a high elastic limit.

• mettre en oeuvre des poudres de chaque élément de l'alliage, chacune d'elles ayant un diamètre FISHER compris entre 1 et 15 µm
• mélanger lesdites poudres dans des proportions correspondant à la composition de l'alliage souhaité
• comprimer lesdites poudres sous forme de compactés
• fritter les compactés entre 1490 et 1650°C pendant 2 à 5 heures
• traiter les compactés frittés sous vide entre 1000 et 1300°C
• soumettre les compactés ainsi obtenus à au moins une passe de corroyage.

This process includes the steps already known and consisting in:

• use powders of each element of the alloy, each of them having a FISHER diameter of between 1 and 15 μm
• mixing said powders in proportions corresponding to the composition of the desired alloy
• compress said powders in the form of compacts
• sinter the compacts between 1490 and 1650 ° C for 2 to 5 hours
• treat sintered compacts under vacuum between 1000 and 1300 ° C
• subject the compacts thus obtained to at least one wrought pass.

But what characterizes it is that the compacts are subjected after vacuum treatment to at least three cycles of operations each comprising a working followed by a heat treatment.

Thus, the invention consists of a succession of cycles which are all the more numerous when it is desired to reach structures corresponding to the highest values of the mechanical characteristics.
Thus, three cycles make it possible to reach a breaking strength of between 1400 and 1450 MPa, while at the end of four cycles, it is close to values of 1850 MPa.
Each of these cycles comprises in order a step of wrought-out carried out by hammering, for example, so as to develop a certain reduction rate of surface of the sintered compact between 10 and 50% followed by an annealing treatment by passage in an oven heated to a temperature below 1300 ° C under an inert atmosphere for 4 to 20 hours.

Preferably, during the first two cycles, the wrought rates are lower and the temperatures higher than during the cycles later.
During the fourth cycle, the suitable degree of wrought work is achieved by making at least two successive passes in the hammer hammer, for example, before carrying out the heat treatment.

• fig. 1,2,3, les structures sous un grossissement de 200 de coupes transversales d'éprouvettes ayant respectivement une résistance à la rupture de 1100, 1540 et 1850 MPa
• fig. 4,5,6, des microstructures de faciès de rupture en traction obtenues à partir des mêmes éprouvettes sous des grossissements respectifs de 1000-1000-2600.
• fig. 7,8,9, des microstructures obtenues par observation au microscope électronique de lames minces sous des grossissements respectifs de 35.000, 30.000 et 60.000, mettant en évidence l'état spécifique de la phase α permettant d'atteindre les caractéristiques souhaitées.

The invention can be illustrated with the aid of the attached drawing plates which, for an alloy containing by weight 93% of tungsten, 5% of nickel and 2% of iron:

• fig. 1,2,3, the structures under a magnification of 200 of transverse sections of test pieces having respectively a breaking strength of 1100, 1540 and 1850 MPa
• fig. 4,5,6, microstructures of tensile fracture facies obtained from the same test pieces under respective magnifications of 1000-1000-2600.
• fig. 7,8,9, microstructures obtained by observation with an electron microscope of thin sections under respective magnifications of 35,000, 30,000 and 60,000, highlighting the specific state of the α phase making it possible to achieve the desired characteristics.

In FIG. 1, the nodular structure of the α phase of tungsten and the γ phase of binder, the mean free path of which is close to 20 μm, are observed in white.
In Figure 2, we see the formation of butterfly wings while the mean free path drops to around 10 to 14 µm.
In FIG. 3, the trend observed in FIG. 2 is accentuated and the mean free path is in the range 3 to 7 μm.
In FIG. 4, the rupture of the alloy is essentially internodular and cupular at the level of the γ phase.
In FIGS. 5 and 6, corresponding to test pieces with characteristics superior to those of FIG. 4, it can be seen that the global failure mode becomes transnodular with rare internodular rupture initiations. At the level of the microstructure of the α phase, states of substructures are developed.
In FIG. 7, there is a restoration structure with rearranged cells of size 0.4 to 0.8 μm.
In Figure 8, we observe the polygonized step, step necessary for the transition to the highest characteristics.

In Figure 9, we see a typical structure of the highest characteristics with development of dislocation microcells from 0.05 to 0.01 µm.

The invention can be illustrated using the following example of application:

Elementary powders with a FISHER diameter of between 1.4 and 10 μm were mixed so as to obtain a product having the following composition by weight: W 93% - Ni 5% - Fe 2%.

After isostatic compression under a pressure of 230 MPa, the compacts with a diameter of 90 mm and a length of 500 mm were sintered in a passing oven at a temperature of 1490 ° C. for 5 hours then kept under vacuum for 25 hours in a heated oven. between 1000 and 1300 ° C.

The products thus obtained were then treated according to the invention. The specific conditions under which the cycles were carried out as well as the mechanical characteristics Rm (resistance to rupture), Ro, 2 (resistance to 0.2% elongation), A (elongation), HV30 (Vickers hardness) and HRc (Rockwell hardness) obtained at the different treatment cycles have been collated in the following table:

It is therefore found that the breaking strength increases sharply when the number of cycles is increased and that the elongation remains sufficient to allow the transformation of the alloy.

Claims (7)

1. Heavy alloys with very high mechanical characteristics, of a specific gravity of between 15.6 and 18, containing between 80 and 99% by weight of tungsten in the form of nodules constituting the α phase as well as nickel and iron in a Ni/Fe ratio by weight which is higher than or equal to 2, performing the bonding function and constituting the γ phase and optionally elements such as molybdenum, titanium, aluminium, manganese, cobalt and rhenium, characterised in that in a direction transverse to the working direction, the α phase is in the form of ellipsoids joined two by two in the neighbourhood of one extremity of their major axes so as to form an acute angle between the said axes, with dislocation cells of dimensions from 0.01 to 1 µm, and the bonding γ phase has a mean free path, defined as the average of the distances which in a given direction separates two successive zones of γ phase, of less than 15 µm.

2. Alloys according to Claim 1 characterised in that the α phase in the working direction has a fibrous texture of a direction <110>.

3. Alloys according to Claim 1 characterised in that, for levels of breaking strength of higher than 1500 MPa, the α phase in the working direction is polygonised.

4. Alloys according to Claim 1 characterised in that the γ phase forms a precipitation network in the area of contiguity of the nodules of the α phase.

5. A process for the production of alloys according to Claim 1 comprising:

- using powders of each elements of a FISHER diameter of between 1 and 15 µm,

- mixing said powders in proportions corresponding to the composition of the desired alloy,

- compressing said powders in the form of compacted items,

- sintering the compacted items at a temperature of between 1490 and 1650°C for form 2 to 5 hours,

- treating the sintered compacted items under vacuum at between 1000 and 1300°C, and

- subjecting them to at least one working pass,

characterised by subjecting the compacted items after treatment under vacuum to at least three cycles of operations, each cycle comprising a working step followed by a heat treatment.

6. A process according to Claim 5 characterised in that, in the course of the first two cycles, the degrees of working are lower and the heat treatment temperatures higher than in the course of the subsequent cycles.

7. A process according to Claim 6 characterised in that, in the course of the fourth cycle, the working operation is effected in at least two passes.

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