METHOD OF MANUFACTURING HARD-TO-WORK ALLOY ARTICLES

Abstract: A method for manufacturing a hard-to-work alloy article comprises the steps of: preparing a laminate member comprising a plurality of layers of different metals in a stoichiometric ratio for the intended alloy composition; dividing the laminate member into pieces; combining the peices to a body for plastic deformation; subjecting the body to plastic deformation under pressure to form an article having a given shape and dimensions; and applying a thermal diffusion treatment to the article.

Description: The present invention relates to a method for manufacturing a shaped article having a uniform alloy composition from hard-to-work alloys which at least partly comprise intermetallic compounds, to fabricate articles having stable qualities on an industrial scale. Intermetallic compounds have the general tendency that their metallic nature is lost to some extent with a degradation in workability and it is extremely difficult to form them into desired shapes. Recently, a large number of new types of materials, such as superconducting materials and shape-memory alloys, have been discovered, some of which are made of intermetallic compounds. Thus, there is a need for a method which can form intermetallic compounds into desired shapes. Among superconducting materials, Nb₃Sn, V₃Ga, Nb₃(Al 0.8 Ge 0.2 ), Nb₃Ge, Nb₃Al are known to have high values of critical magnetic field, critical current density and critical temperature. Alloys such as Ni-Ti, Ag-Cd, Cu-Al-Ni, Cu-Au-Zn, Cu-Sn, Cu-Zn, Cu-Zn-X (X = Si, Sn, Al), Cu-Zn-Y (Y = Ca, Al), Ni-Al, In-Tl, Fe-Pt, Fe-Pd, and Mn-Cu alloys are also known to have the ability to remember an initial shape and to return to that shape; i.e. they are called shape memory alloys. Intermetallic compounds, such as Zr₃Al, are used for making sheaths for fuel pellets in light-water reactors. Intermetallic compounds such as Ni₃Al, TiAl, Co₃Ti, Tl₃Al, Mo₃Si, and Ni₂AlTi are also useful as heat-resistant structural materials. FeTi and LaNi₅ are useful as hydrogen storage alloys, and FeCo and Fe₃Al are employed as magnetic materials. As already mentioned, none of these materials is easy to work, and it is quite difficult to form them into a fine wire, for example. In fact, in some cases it is impossible to perform mechanical working on such materials. Thus, in the past the following processes have been employed in order to manufacture elongated products such as plate, rod, and wire. In the case of intermetallic compounds such as TiNi and Ni₃Aℓ, it is difficult to carry out the plastic deformation of these materials. However, as they are plastically deformable to some extent, the same manufacturing processes as for conventional metallic materials have been applied. First, a melt of the material is prepared and is poured into a mold to form castings. Then, repeated steps of annealing and plastic deformation are performed to form articles in the shape of plates, rods, wires and the like. However, these manufacturing processes are expensive. Furthermore, there is an inevitable fluctuation in alloy composition caused by yielding of the alloying elements in preparing them into a melt, and also segregation of alloying elements which takes place during solidification after casting, resulting in a degradation in the properties of the material. On the other hand, in the case of intermetallic compounds such as Nb₃Aℓ which are hard to work and are not plastically deformable, the following processes have been proposed to form a wire, such as a Nb₃Aℓ wire. a) As shown in Figure 10, a mixture of Nb powder and Aℓ powder is subjected to extrusion to form a billet, which is then further processed by drawing, for example, to make a fine wire. The resulting wire is then subjected to a thermal diffusion treatment to form a wire comprising Nb₃Aℓ. This is called the powder metallurgy process. b) As shown in Figure 11, powdery Nb is subjected to isostatic pressure to form a compact which is then sintered. The sintered body is immersed in an Aℓ molten bath in which molten aluminum penetrates into the body to prepare a composite body, which is then extruded and rolled with a grooved roll or drawn to form a wire. The wire is subjected to a thermal diffusion treatment to form a wire comprising Nb₃ Aℓ. This process is called the infiltration method. c) As shown in Figure 12, a Nb plate and an Aℓ plate are laminated in a stoichiometrical ratio necessary to form them into an intermetallic compound. The laminated plate is then tightly wound in the form of a billet, which is subjected to drawing and a thermal diffusion treatment so as to obtain a Nb₃Al wire. This process is called the jelly-roll method (cf. R. Bruzzese et al., IEEE Transactions on Magnetics, Vol. MAG-23, No. 2 (March 1987), 653-656). d) As shown in Figure 13, an Aℓ rod is inserted into a Nb tube to fabricate a tubular assembly, which is then extruded and drawn to form a wire. The resulting wire is subjected to a thermal diffusion treatment to form a Nb₃Aℓ wire. This process is called the "Tube Process". These methods have the following drawbacks: I) In the methods described in sections a) and b), since it is necessary to handle metal powders, and especially very active metal powders, the following disadvantages are inevitable. i) There is a tendency for the powder to absorb gas from the atmosphere, particularly oxygen. This absorption increases the content of impurities, resulting in a degradation of formability. Nb can easily absorb oxygen gasp and due to an increase in the oxygen content, plastic deformability and superconductive properties are markedly degraded. ii) When combining powders, local concentrations occur and it is difficult to obtain a uniform mixture of powders, resulting in a marked fluctuation in the alloy composition. iii) Since very active powders are handled, it is necessary to be extremely careful to avoid explosions. iv) Nb powder is harmful when it enters the human body. In particular, although it is necessary to make the starting powders as fine as possible so as to obtain a uniform mixture of the powders, the fineness of the powder imposes limitations on the dimensions of the shaped article since the finer the powder the greater the surface oxidation. II) When above-described method c) is used, the winding is apt to be carried out loosely at the core of the roll. Furthemore, when the material is subjected to drawing, the metal flow in the central area differs from that in peripheral areas (metal flow in the central area lags behind that in the perphery). Therefore, when the resulting assembly is shaped into wires, the central portion of the cross section does not have such a fine structure that is necessary to obtain the intended Nb₃Al composition after a thermal diffusion treatment is performed. When the above-described method d) is used, an extremely high reduction is required to obtain the fine Nb-Al structure which is necessary to get Nb₃Al by diffusion reaction. Thus, the prior art methods for producing shaped articles of hard-to-work metals, such as intermetallic compounds, are costly and not adequately reliable. Moreover, such articles have some limits with respect to the uniformity of their alloy composition as well as dimensions, so that it is quite difficult to produce them on an industrial scale. The object of the present invention is to provide an efficient and industrial method for producing shaped articles having a uniform alloy composition from hard-to-work alloys which at least partly comprise intermetallic compounds and particularly, shaped articles made of intermetallic compounds. Such articles will be referred to as "hard-to-work alloy articles". The method for manufacturing such a shaped article having a uniform alloy composition from hard-to-work alloys which at least partly comprise intermetallic compounds according to the present invention comprises the steps of: preparing a laminate member comprising a plurality of layers of different metals in a stoichiometric ratio for the intended alloy composition; dividing the laminate member into pieces; combining the pieces to a body for plastic deformation; subjecting the body to plastic deformation under pressure to form an article having a given shape and dimensions; and applying a thermal diffusion treatment to the article. Figure 1 is a schematic illustratlon of the process of manufacturing a hard-to-work material article in accordance with the present invention; Figure 2(a) and (b) are photographs showing the microstructure of a transverse cross section and a longitudinal cross section, respectively of a dense rod obtained by cold extrusion in a working example of the present invention; Figures 3(a), (b), and (c) are photographs showing the microstructure of dense rods subjected to different degrees of working and obtained by cold extrusion, rolling with grooved rolls, and drawing in a working example of the present invention; Figures 4 - 6 are graphs showing the critical current density (Jc) versus the applied magnetic field (B) for Nb₃Aℓ wires obtained in a working example of the present invention; Figure 7 is a graph showing the upper critical magnetic field (Hc₂) of a Nb₃Aℓ wire obtained in a working example of the present invention; Figures 8(a) and (b) are photographs showing the microstructures of the cross section of the dense rod obtained from disc pieces and randomly shaped pieces, respectively in a working example of the present invention; Figures 9(a) and 9(b) are photographs each showing the microstructure of dense rods subjected to different degrees of working which were obtained from randomly shaped pieces by cold extrusion, rolling, and drawing in a working example of the present invention; Figure 10 is an illustration of a conventional powder metallurgy process; Figure 11 is an,illustration of a conventional infiltration method; Figure 12 is an illustration of a conventional jelly-roll method; Figure 13 is an illustration of a conventional tube process. The present invention will now be described in conjunction with the accompanying drawings. Figure 1 schematically illustrates a process for manufacturing a hard-to-work metal article which at least partly comprises intermetallic compounds in accordance with the present invention. According to the illustrated process, metal components which constitute an intended alloy or an intermetallic compound (hereunder referred to as constituent metals) are first assembled or laminated in the form of a plate to fabricate a blank which has a shape which is easy to work and which contains each of the components in a suitable ratio, e.g., in a stoichiometrical ratio in the case of intermetallic compounds. The total thickness of the assembled blank is preferably 0.01 - 5 mm. The thinner the blank the less degree of working is required. When the assembly is composed of easy-to-work metals or pure metals, and not of intermetallic compounds or hard-to-work metals, i.e., of metals in which thermal diffusion has not yet to takes place, the blank is deformable and can easily be worked even with a high working ratio. Figure 1 shows a process which employs a roll-clad method for assembling a blank. Other methods, such as plating, vapor-depositing, up-setting, explosive, molten metal spraying, cementating and ion-plating methods may also be employed for this purpose. The laminated blank need not be in the shape of a plate, and any other shapes such as rod and bar may be employed. Furthermore, in laminating the constituting metal members, an active metal member is preferably placed in an inner position as a core to form a sandwich-like laminate. The thus-prepared laminate blank is then chopped or clipped into pieces. The blank may be stamped to give disc shaped pieces. The chopped pieces may be rectangular as shown in Figure 1, circular, elliptical, triangular, polygonal, star-shaped, or of any other desired shape. The maximum dimension (diameter, side, or diagonal of the pieces is preferably of 1 - 50 mm. The dimensions of the pieces can be selected in accordance with the capacity of a pressing machine, for example, which is used in a subsequent step to compact the pieces to form an article. That is, the size of the pieces is selected so that the pieces can be packed closely enough upon working for the constituent metals to diffuse into each other when the resulting compact body is subjected to thermal diffusion. The thus-prepared pieces are combined and formed into a desired shape by extrusion, rolling, and other methods. When the working ratio is suitably selected, the resulting extruded or rolled article has an intended composition throughout with respect to constituent metals and is substantially free of segregation. In accordance with one embodiment of the present invention it is preferable to apply extrusion, since during extrusion the chopped pieces are further strongly combined with each other due to additional shearing deformation and are oriented in the direction of extrusion. A worked article has a fine and uniform structure. Before being subjected to thermal diffusion, none of the constituent metals are reacted, so the compact body can easily be subjected to further working. It is desirable that the pieces are first packed into a sheath, such as hollow tube. The packed sheath can be charged into a container for extrusion and then extruded into a rod or plate. The combined pieces may also be charged into the container directly and then extruded. The specific material (a metal or alloy, for example) for making the sheath can be selected on the basis of whether it has a deformation resistance on the same level as the chopped pieces, and whether it has chemical properties different from those of the chopped pieces, since it must be easily removable, upon need, by a chemical process (corrosion, evaporation, etc. ). Furthemore, if necessary, the resulting dense bodies can be collected and then deformed into an article having a desired shape and dimensions. Since the collected bodies have not yet been subjected to thermal diffusion, and since each dense body is in mixture with others, it is rather easy to carry out plastic working on the collected bodies. The deformed dense article having a final shape and dimensions is then subjected to thermal diffusion as a final stage of processing. When heat treatment is applied, each of the constituent metals diffuses to make the alloy composition of the body uniform, resulting in formation of intermetallic compounds (alloys) in a desired proportion. Conditions under which the thermal diffusion is carried out are not restricted to specific ones so far as an intermetallic compound is formed. The thermal diffusion treatment may be carried out in two stages, first at a high temperature for a short time and second at a low teperature for a long time. Thus, according to the present invention, an article having a desired shape and dimensions which is at least partly made of hard-to-work alloys can be easily and efficiently manufactured. Some of the advantages of the present invention are as follows. 1) There is no segregation, which usually occurs in conventional processes including melting and casting. 2) Not so many times of annealing treatment as in conventional processes are requires while plastic deformation is being performed. 3) There is no substantial deviation of the alloy composition from the intended composition which occurs in a conventional process due to yielding of alloying elements during melting. 4) There is no contamination by gases such as oxygen, which sometimes occurs when metallic powder is used as starting material. Such contamination with gas results in a change in alloy composition and degradation in workability. 5) A uniform composition is attainable. When a mixing step for metallic powder is employed, a local concentration of some constituent is inevitable, resulting in the alloy having a non-uniform composition. 6) There is no need for powdery materials, which must be handled with care to avoid explosions and inhalation of the powders by humans. 7) The processing steps are very simple, and manufacturing costs can be reduced markedly. 8) A composite article can be manufactured when the sheath is made of a different metal from the constituent metals of the body and remains on the body after working. 9) An elongated article having a variety of deformed sections can be manufactured by extrusion using appropriate dies. 10) Since the chopped pieces, which are used as starting material, may have a desired alloy composition, fine wires and foils can be formed by deformation under pressure such as extrusion by placing separating or releasing members between each of the assembled bodies. 11) Chopped or clipped pieces can be pre-assembled, and continuous production of lengthy articles can be performed. 12) It is possible to produce Nb₃Aℓ fine wire and Nb₃(Aℓ,Ge) fine wire efficiently on an industrial scale. The method of the present invention can obtain a superconducting wire having a critical current density with respect to external magnetic fields which is much higher than that obtained through a powder metallurgy process. The present invention will next be described in further detail in conjunction with a mumber of working examples. Examples Example 1 In this example, a Nb₃Aℓ wire was produced. As shown in Figure 1, a Nb plate having a thickness of 1.015 mm was sandwiched between two Aℓ plates each measuring 0.165 mm thick. The resulting assembly was rolled to form an Aℓ-Nb-Aℓ clad plate having a thickness of 0.20 mm (the atomic ratio Nb:Aℓ ≒ 3:1). The clad plate was then chopped into pieces of about 10 x 10 mm. 138 g of the pieces were packed into an 8-2 brass cyrindrical container (40 mm in diameter) to make a billet for extrusion. The compacting was carried out using compacting loads of 25 t, 20 t, and 10 t, respectively, to prepare three different billets. The billets were subjected to extrusion using an extrusion ratio of 5 at maximum extrusion load of 185 t. Dense rods measuring 18 mm in diameter were obtained. Transverse and longitudinal cross sections of the extruded rods were observed. Photomicrographs of the cross sections are shown in Figures 2(a) and 2(b), respectively. The extruded rods were then rolled with grooved rolls and then drawing to reduce the rod diameter from 18 mm to 6.2 mm (including the sheath thickness). After removing the sheath of 8-2 brass, another sheath of S35C steel (steel containing 0.35% of carbon which has been subjected to a patenting treatment) was used to carry out drawing. The sheath had an initial outer diameter of 9.5 mm. This was reduced by drawing to 0.2 mm, while the rod diameter was decreased from 4.7 mm to 0.1 mm to form a wire. Photomicrographs of the cross section of the resulting wires which were subjected to different degrees of working are shown in Figures 3(a) - 3(c). The degree of wording of the wires in Figures 3(a) - 3(c) is the ratio of the initial cross sectional area to the cross sectional area after working. It is necessary to refine the structure in order to form the intended intermetallic compound of Nb₃Aℓ efficiently. It is said that the intermetallic compound of Nb₃Aℓ can be easily and efficiently formed when the Nb portion thereof is less than 1 µm in size. After removing the sheath, a final product in the form of Nb-Aℓ wire was obtained. It was then subjected to a variety of heat treatments to form a Nb₃Aℓ intermetallic compound. The heat treatment was carried out in two stages. The first stage was treatment at a high temperature for a short time, and the second stage was treatment at a low temperature for a long time. The purpose of the first stage was to react Nb with Aℓ to form Nb₃Aℓ of the short range A15 type compound. The second stage was carried out to achieve the ordering of the short range compound to form Nb₃Aℓ of the long range A15 type compound, which has much improved superconducting properties. Superconductivity was measured for the resulting intermetallic compound wires. The results are summarized in Figures 4 - 7. Figures 4 - 6 show the critical current density (Jc) and Figure 7 shows the upper critical magnetic field (Hc₂) which was found using the Kramer · Plotting method. Using the values shown in Figures 4 - 7, the properties of a typical Nb₃Aℓ wire which was obtained by the process of the present invention can be described quantitatively as follows: Thermal treatment carried out at 1000°C for 1 minute and then at 750 °C for 4 days: Critical Temperature (Tc): 17 K (-256°C) Critical Current Density (Jc): 3 × 10⁴ A/cm² [External Magnetic Field 15 Tesla, at 4.2 K] Upper Critical Magnetic Field (Hc₂): about 25 Tesla [Extrapolated Value of Kramer · Plotting] These properties are comparable with those of Nb₃Aℓ wire obtained by the conventional infiltration method shown by ▴ in Figure 4. Thus, it is apparent that according to the present invention, it is possible continuously to produce an extremely fine wire of a hard-to-work material, i.e., Nb₃Aℓ intermetallic compound in an easy and efficient manner on an industrial scale. Example 2 In this example, a NiTi wire was manufactured. A Ti plate was sandwiched between two Ni plates to prepare a laminate assembly, and the assembly was rolled to form a Ni-Ti-Ni laminate with a thickness of 0.3 mm (atomic ratio of Ni:ti=50:50). From this laminate, discs (10 mm in diameter) were stamped out, and the remaining laminate with stamped holes was cut into pieces having random shapes. These shapes were packed into an 8-2 brass cylindrical container (40 mm in diameter). After packing, the compact was subjected to a pressure of 15 tons for 3 minutes to prepare a billet for extrusion. Cold extrusion was carried out using an extrusion ratio of 5 to obtain a dense rod measuring 18 mm in diameter. Figures 8(a) and 8(b) are photomicrographs of a cross section of the resulting dense rods. Figure 8(a) shows a rod prepared from discs, and Figure 8(b) shows a rod prepared from the pieces with random shapes. It is apparent from these figures that the structure depends on the shape of the pieces. From the pieces with random shapes, it is possible to form a shape memory alloy having a uniform structure having improved shape memory properties. When working is performed repeatedly, it is possible to make the structure more uniform. This is apparent from Figure 9(a) and 9(b) which show photomicrographs of cross section of alloy rods which were manufactured from the randomly shaped pieces using two different working ratios. When the working ratio is high, it is possible to manufacture a uniform alloy. The thus-obtained rods were then subjected to rolling with grooved rolls to form a wire measuring 3 mm in diameter. After annealing (450 °C × 15 minutes), the sheath of 8-2 brass was removed, and the rod was inserted into another sheath of carbon steel having a diameter of 9.4 mm. The rod inside the sheath was then subjected to rolling with grooved rolls while drawing was performed to form a wire measuring 0.2 mm in diameter. After the drawing, the sheath was removed using nitric acid. The remaining Ti-Ni wire was subjected in vacuum to a thermal diffusion treatment at 850 °C for 80 minutes to form a TiNi intermetallic compound, and it was then subjected in vacuum to a shape memory treatment at 400 °C for 40 minutes. The shape memory properties of the resulting TiNi wire were measured. The shape memory properties of the alloy were satisfactory, and its shape memory temperature, i.e., transformation temperature was determined to be 65 °C . Thus, it is possible to manufacture an article made of hard-to-work materials, such as a TiNi intermetallic compound in an easy and efficient manner on an industrial scale. Although the present invention has been described with preferred embodiments it is to be understood that variations and modifications may be employed without departing from the concept of the present invention as defined in the following claims.

Claims: 1. A method for manufacturing a shaped article having a uniform alloy composition from hard-to-work alloys which at least partly comprise intermetallic compounds comprising the steps of: preparing a laminate member comprising a plurality of layers of different metals in a stoichiometric ratio for the intended alloy composition; dividing the laminate member into pieces; combining the pieces to a body for plastic deformation; subjecting the body to plastic deformation under pressure to form an article having a given shape and dimensions; and applying a thermal diffusion treatment to the article. 2. The method as set forth in claim 1, wherein the laminate member is in the form of a plate. 3. The method as set forth in claim 2, wherein the laminate plate is prepared by rolling an assembly of a plurality of plates. 4. The method as set forth in claim 2, wherein the laminate comprises an Al plate-Nb plate-Al plate with an atomic ratio of Nb : Al = 3 : 1. 5. The method as set forth in claim 2, wherein the laminate plate comprises a Ni plate-Ti plate-Ni plate with an atomic ratio of Ni : Ti = 50 : 50. 6. The method as set forth in claim 2, wherein the laminate member is divided into pieces by cutting. 7. The method as set forth in claim 1, wherein the laminate member is divided into pieces by stamping. 8. The method as set forth in claim 1, wherein the pieces are rectangular, circular, elliptical, triangular, polygonal, or star-shaped. 9. The method as set forth in claim 1, wherein the plastic deformation is carried out by means of extrusion or rolling or drawing or a combination thereof. 10. The method as set forth in claim 1, wherein the thermal diffusion is carried out under conditions sufficient to form an intermetallic compound.

Inventors: HANADA, SHUJI, HOSHI, HIRONOBU, IKEDA, SENRI, SAITO, SAKAE
Applicants: IKEDA, SENRI, NIPPON STAINLESS STEEL CO., LTD., SAITO, SAKAE
Application year: 1989
Main classification: C22C1
IPC codes: C22C1, B30B11, B23D31, H01L39