Method of making an ordered alloy
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of making a substantially ordered alloy by inter-diffusing the constituents of the alloy wherein at least one of the constituents is in the form of a thin film. 
     2. Description of the Prior Art 
     In normal substitutional solid solutions, the different kinds of atoms are arranged at random on a common lattice. There are some cases, however, where alloys which at high temperatures consist of random substitutional solid solutions undergo atomic rearrangement on slow cooling or annealing at a low temperature. This rearrangement produces an ordered structure in which the different kinds of atoms take up regular positions in the lattice. In copper-gold alloys, for example, an alloy containing 25 atomic percent gold (Cu 3  Au) at high temperatures consists of a face-centered cubic structure with a random distribution of the two kinds of atoms. Upon annealing at a low temperature, the alloy is rearranged so that the gold atoms occupy the corners of the cube and the copper atoms occupy the centers of the faces. This type of structure is known as an ordered structure or sometimes as a superlattice. 
     When the atomic arrangement of alloys is transformed from the disordered phase to the ordered phase, various physical characteristics may be changed. Among the properties which may be changed as the alloy becomes more ordered are its electrical resistance, its Hall mobility, its magnetic properties, its Young&#39;s modulus, its volume, its mechanical hardness, its resistance to abrasion, its resistance to corrosion, and the like. Accordingly, ordered alloys can sometimes be used as magnetic recording media, in magnetic heads, for electrical contacts, ornaments and the like. 
     According to one prior art method of making a substantially ordered alloy, the metallic elements which form the desired alloy are melted with predetermined atomic percentages to prepare an alloy ingot which is then subjected to a suitable heat treatment. With this type of prior art method, it is difficult to make a desired ordered alloy, and it is virtually impossible to make such an alloy as a thin layer. 
     It has also been proposed in the prior art to make an ordered alloy by contacting metal plates made up of elements which will form the ordered alloy, and diffusing them by means of heat treatment. In such cases, the diffusion should be carried out at a temperature lower than the order-disorder transformation temperature of the ordered alloy, so that the diffusion requires a long period of time and is commercially impractical. If the diffusion is carried out at a temperature higher than the order-disorder transformation temperature, the concentration of the diffused element in one of the metal components is decreased from the surface to the interior exponentially and the diffusion portion which has the proper concentration ratio necessary to form the ordered alloy becomes extremely thin. 
     SUMMARY OF THE INVENTION 
     The present invention is based upon the discovery that if a layer consisting of one of the elements in the desired ordered phase is deposited on a metal base consisting of the remaining element or elements by electroplating, electroless plating, vapor deposition, chemical vapor deposition, sputtering, or the like, and thereafter the base and the deposited layer are heated at a temperature lower than the order-disorder transformation temperature of the alloy, a substantially ordered alloy layer integral with the metal base is produced. 
     The ordered alloy referred to in the specification and claims does not necessarily mean that the atomic arrangement of the alloy is completely in the ordered form, but that the alloy is substantially ordered so that the atomic arrangement is predominantly in the ordered phase. 
     The temperature involved in the thermal diffusion between the two layers varies in accordance with the time, the nature of the element or elements in the deposited layer, and the nature of the element or elements in the metal base. Consequently, it is difficult to provide threshold values of the thermal diffusion temperature even though a particular species of an alloy is involved. Generally speaking, however, if the diffusion of the two elements is not completed in a period of 1 day, it is uneconomical from an industrial point of view. Thus, if a source of radiation such as a gamma ray or the like is irradiated on the metal layer before or during the thermal diffusion process, the speed of diffusion increases even though a relatively low temperature is involved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other objects, features and advantages of the invention will be readily apparent from the following description of a preferred embodiment thereof, taken in conjunction with the accompanying drawing, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which: 
     FIGS. 1A and 1B are cross-sectional views on a greatly enlarged scale illustrating the steps involved in one example of a method of making a substantially ordered alloy according to this invention; 
     FIG. 2 is a graph showing the concentration of platinum in an ordered alloy of NiPt in the thickness direction from the surface of the platinum; and 
     FIGS. 3A and 3B and FIGS. 4A and 4B are cross-sectional views on a greatly enlarged scale illustrating, respectively, the steps involved in other examples of the methods of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An example of the methods involved for making a binary alloy having an ordered phase according to this invention will now be described with reference to FIGS. 1A and 1B. When the desired binary alloy is, for example, a copper-gold binary alloy, the metal base 1 may be provided composed of copper. On the metal base 1 there is deposited the other element, gold, as a gold layer 2 with an arbitrary thickness, without quantification, by plating, vapor deposition, sputtering or the like. Then, if the copper base 1 with the gold layer 2 is heat treated at a temperature lower than the order-disorder transformation temperature which is 380° C in the case of the alloy CuAu, a substantially ordered alloy 3 of a predetermined thickness is formed on the surface of the copper base 1 as shown in FIG. 1B. 
     If the coated base is subjected to further heat treatment, for a longer period of time, the layer 3 becomes a layer of ordered alloy Cu 3  Au. When heat treatment is carried out at a temperature of 500° C which is higher than the order-disorder transformation temperature, no ordered alloy is formed. 
     In this case and in succeeding cases, it is preferred that the thickness of the deposited layer should not exceed about 10 microns because if the thickness is larger than 10 microns, the adhesion of the deposited layer to the metal base may be deteriorated by the stress of the deposited layer so that the deposited layer may be peeled off from the metal base during the heat treatment. 
     In FIG. 2 there is shown a graph of the relationship between the depth of a platinum layer from the surface of an NiPt ordered alloy made by the method of the invention described in connection with FIGS. 1A and 1B. In the graph of FIG. 2, the platinum concentration is plotted against the depth of the platinum layer from the surface of the ordered alloy. When a platinum layer is deposited on a nickel base with a thickness of 1 micron, the platinum concentration is shown in FIG. 2 by the broken line. After the platinum layer and the nickel base are heat treated for 3 hours at a temperature of 550° C, the platinum concentration becomes as shown by curve A in FIG. 2 resulting from the diffusion of platinum into the nickel base, and diffusion of the nickel base into the platinum layer. From FIG. 2 it will be seen that there is a substantially uniform composition NiPt ordered layer formed over a range of approximately 1.3 microns and that the platinum concentration is decreased exponentially as the depth becomes greater than 1.3 microns from the surface. 
     When the same material is further heated for two hours at the same temperature as previously mentioned, the NiPt ordered alloy layer becomes thinner, on the order of about 0.7 microns as shown in FIG. 2 by curve B but the platinum concentration in this range is not changed. 
     It may be apparent from a description of FIG. 2 that when the platinum layer of a suitable thickness is deposited on a nickel base and when they are heat treated only, an ordered alloy layer is produced which has a thickness according to the heat treatment time. Accordingly, with the method of this invention, it is no longer necessary to employ predetermined compositional ratios as required in the prior art method of making an alloy ingot. 
     With the method of the present invention, the substantially ordered alloy layer is formed integrally with the metal base by thermal diffusion, so that the adhesion of the ordered layer to the metal base is very strong and the ordered alloy layer is not readily peeled off from the metal base. Furthermore, if only the substantially ordered alloy formed on the metal base is desired, the metal base can be removed, for example, by etching after the ordered alloy layer is formed on the surface of the base. 
     FIGS. 3A and 3B illustrate processes for making a ternary ordered alloy according to this invention. The ternary alloy Pt 4  Ni 3  Co will be used as an example. In this case, an alloy base containing two elements of the desired ternary alloy, for example, an alloy base 4 made of Ni 3  Co is provided and a metal layer 5 composed of platinum is deposited on the surface of the alloy base 4 without quantification by plating, vapor deposition, sputtering or the like with a predetermined thickness as shown in FIG. 3A. Thereafter, the alloy base 4 with the overlying metal layer 5 is heat treated at a temperature lower than the order-disorder transformation temperature of the Pt 4  Ni 3  Co alloy but higher than the thermal diffusion temperature of Ni 3  Co and Pt. In this manner, an ordered alloy layer 6 of Pt 4  Ni 3  Co with a uniform thickness can be formed on the surface of the alloy base 4 as shown in FIG. 3B. 
     FIGS. 4A and 4B illustrate a further embodiment of the invention. As shown in FIG. 4A a nickel base 7 is provided and then a Pt 4  Co alloy layer 8 which contains platinum and cobalt at the atomic ratio 4:1 is deposited on the surface of the nickel base 7 by plating, vapor deposition, sputtering or the like. Alternatively, a platinum layer and then a cobalt layer at an atomic ratio of 4:1 is sequentially formed on the surface of the nickel base 7. Thereafter, the coated base is subjected to a heat treatment above the diffusion temperature but below the order-disorder transformation temperature to provide an ordered alloy layer 9 of Pt 4  Ni 3  Co. 
     In the last-named example, the platinum and cobalt must be quantified, but at least the surface layer of the nickel base 7 can be provided with an ordered alloy layer 9 of Pt 4  Ni 3  Co. In this case, there is no possibility that an alloy layer of a composition different from that of the ordered alloy Pt 4  Ni 3  Co can be formed and also the adhesion of the ordered alloy layer 9 to the nickel base 7 is strong. 
     The following examples illustrate specific embodiments of the invention. 
     EXAMPLE 1 
     A gold layer was deposited on a copper base plate with a thickness of one micron by vapor deposition in vacuum and the plated base was subjected to a heat treatment for three hours in hydrogen at a temperature of 340° C. This resulted in a single phase of an ordered layer of AuCu. This ordered layer has excellent anti-abrasion characteristics and anti-corrosion characteristics as well as a low electrical resistance so that the alloy thus made is suitable for use as an electrical contact. When the same material is subjected to heat treatment under the same conditions for 15 hours, an ordered alloy layer of Cu 3  Au was formed. 
     EXAMPLE 2 
     A layer of platinum was deposited on a copper base by electroplating with a thickness of 1 micron, and the coated base heat treated in a hydrogen atmosphere at a temperature of 340° C for 3 hours. It was confirmed that the resulting structure had a platinum layer, an ordered alloy layer of PtCu, an ordered alloy layer of PtCu 3  and a copper layer in order successively from the top surface of the platinum layer. 
     EXAMPLE 3 
     The same material as in Example 2 was subjected to a heat treatment at a temperature of 400° C for 1 hour. In this case, a single phase of an ordered alloy PtCu 3  was formed on the surface. This ordered alloy had excellent anti-abrasive characteristics and anti-corrosion characteristics and was low in electrical resistance, so that the ordered alloy could be used as an electrical contact. 
     EXAMPLE 4 
     The same material as used in Example 2 was heat treated under a temperature of 500° C for 1 hour. In this case an ordered alloy of PtCu 3  was formed on the copper base of the single phase. 
     EXAMPLE 5 
     Cobalt was plated on the surface of a zinc base to a thickness up to 10 microns and then platinum was plated thereon to a thickness of 2 microns. Thereafter, the zinc base was selectively etched away with an alkali etchant. Next, the laminated cobalt and platinum were heat treated in a hydrogen gas flow at a temperature of 550° C for 4 hours. An ordered alloy of PtCo and a cobalt phase were formed. Thereafter, the cobalt phase layer was dissolved away by hydrochloric acid solution to provide an ordered alloy layer of PtCo. The magnetic characteristics of the resulting ordered alloy layer of PtCo were measured as follows: 
     Coercive force H c  = 4500 oersteds 
     Squareness ratio Br/B S  = 0.80 
     From the above measurements, it is apparent that the ordered layer of PtCo can be used as a permanent magnet material. 
     EXAMPLE 6 
     On an alloy base of 75% nickel and 25% cobalt, there was plated a platinum layer with a thickness of 2 microns and the resulting material was heat treated with a hydrogen gas flow at a temperature of 550° C for 2 hours. An ordered alloy of Pt 4  Ni 3  Co was formed on the Ni 3  Co base as a single phase. After the heat treatment, the Ni 3  Co base was dissolved away with hydrochloric acid solution. The magnetic characteristics of the thus formed ordered layer of Pt 4  Ni 3  Co were measured as follows: 
     Coercive force H c  = 1900 oersteds 
     Squareness ratio Br/B S  = 0.80 
     Curie temperature T c  = 120° C 
     The above magnetic characteristics are substantially the same as those of the ordered alloy material Pt 4  Ni 3  Co formed by the prior art alloy ingot method, and so the ordered alloy of Example 6  could be used as an intermediate recording medium in thermal and magnetic printing. 
     EXAMPLE 7 
     The material used in Example 6 was heat treated at a temperature of 500° C for 2 hours. An alloy of Pt 6  Ni 3  Co was formed on the surface, with an ordered alloy of Pt 4  Ni 3  Co formed therebelow, just above the alloy base of Ni 3  Co. 
     When the conditions for heat treatment of Example 6 were varied to employ a temperature of 450° C for 2 hours, it was confirmed that a platinum layer was formed on the outermost surface, a Pt 16  Ni 3  Co layer was formed therebeneath, a Pt 6  Ni 3  Co layer was formed thereunder, just over the Ni 3  Co base. 
     When the materials used in Example 6 were heat treated at a temperature of 600° C for 2 hours, it was confirmed that a Pt 2  Ni 3  Co alloy was formed on the surface of the Ni 3  Co base. 
     In order to understand the characteristics of the ordered alloy layer produced by this invention, a comparison of the characteristics of the ordered alloy with those of a disordered alloy of the same composition was carried out with respect to the anti-corrosion properties, anti-abrasion properties and electrical resistivity at room temperature. 
     An alloy layer of Pt 4  Ni 3  Co, a disordered alloy layer, was made by plating the same directly on a copper base plate with a thickness of 2 microns. This alloy layer was dissolved in a 4 normal hydrochloric acid solution in about 2 or 3 days. In contrast, an ordered alloy layer of Pt 4  Ni 3  Co produced according to the present invention by diffusion of platinum into an Ni 3  Co base was not corroded by a 4 normal hydrochloric acid solution. 
     An alloy layer of AuCu, a disordered alloy layer, was made by plating CuAu directly on a copper base plate with a thickness of 2 microns. This alloy layer disappeared in a 4 normal hydrochloric acid solution in 2 or 3 days. In contrast, an alloy layer of AuCu in the form of an ordered alloy layer of the present invention formed by heat diffusion of gold into a copper base plate was not corroded by 4 normal hydrochloric acid solution. 
     The anti-abrasion characteristics of the materials were determined as follows. The alloys were used to form tape guides of a tape recorder and an ordinary magnetic tape was transported in contact with the tape guides under tape tensions of 50 and 100g at a speed of 19cm/sec. The abrasion of the tape guides was measured, resulting in the following measurements: 
     
                       Table 1                                                     
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            Amount of Abrasion                                            
Specimen      Tape Tension 50g                                            
                           Tape Tension 100 g                             
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Disordered AuCu Alloy                                                     
Material      0.2  micron/ 0.57 mic-                                      
              hour         ron hour                                       
Ordered AuCu Alloy                                                        
Material (made by prior                                                   
              0.05 micron/ 0.14 mic-                                      
art)          hour         ron/hour                                       
AuCu Alloy Material                                                       
of the Invention                                                          
              0.06 micron/ 0.13 mic-                                      
              hour         ron/hour                                       
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                       Table II                                                    
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Specimen               Resistivity                                        
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Disordered AuCu Alloy Material                                            
                       14 μΩ cm                                  
Ordered AuCu Alloy Material                                               
                        4 μΩ cm                                  
Disordered AuCu.sub.3 Alloy Material                                      
                       12μΩ cm                                   
Ordered AuCu.sub.3 Alloy Material                                         
                        4 μΩ cm                                  
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     The method of the present invention is applicable to production of any type of ordered alloy between two or more metals. Various binary alloys of the ordered type to which the invention is applicable, and their order-disorder transformation temperatures are summarized in Table III. 
     
                                           Table III                               
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      Order-Disorder Order-Disorder Order-Disorder                        
      Transformation Transformation Transformation                        
Alloy Temperature ° C                                              
               Alloy Temperature ° C                               
                              Alloy Temperature ° C                
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CuAu  380      Cu.sub.3 Au                                                
                     390      Mg.sub.3 Cd                                 
                                    150                                   
CoPt  825      Au.sub.3 Li                                                
                     ˜600                                           
                              Cd.sub.3 Mg                                 
                                    80                                    
NiPt  ˜645                                                          
               Mg.sub.3 In                                                
                     ˜350                                           
                              Ni.sub.3 Sn                                 
                                    850˜                            
                                    920                                   
FePt  ˜1300                                                         
               Cu.sub.3 +Pt                                               
                     ˜645                                           
                              Au.sub.4 Cr                                 
                                    ˜400                            
FePd  ˜700                                                          
               Ag.sub.3 Pt                                                
                     785      Ir.sub.3 Mo                                 
                                    &gt;1600                                 
InMg  330      Au.sub.3 Pd                                                
                     ˜850                                           
                              Rh.sub.3 W                                  
                                    &gt;1200                                 
NiMn  ˜750                                                          
               Cu.sub.3 Pd(α&#39;)                                      
                     ˜500                                           
                              Ni.sub.3 V                                  
                                    1045                                  
CuZn  468      Ni.sub.3 Fe                                                
                     500      Pd.sub.3 V                                  
                                    815                                   
FeCo  730      Ni.sub.3 Mn                                                
                     510      Pd.sub.3 Nb                                 
                                    ˜1200                           
CuPd  600      Mn.sub.3 Pt                                                
                     ˜1050                                          
                              Ni.sub.2 V                                  
                                    920                                   
AuMn  615      Fe.sub.3 Pt                                                
                     835      Ni.sub.2 Cr                                 
                                    580                                   
AgZn  ˜130                                                          
               Ni.sub.3 Pt                                                
                     580      Pd.sub.2 V                                  
                                    905                                   
AgCd  235      In.sub.3 Mg                                                
                     ˜110                                           
                              Pt.sub.2 V                                  
                                    over                                  
                                    1100                                  
CoAl  &gt;740     Pd.sub.3 Au                                                
                     ˜800                                           
                              Cr.sub.2 Al                                 
                                    ˜860                            
MgCd  250      Fe.sub.3 Ni                                                
                     ˜800                                           
                              U.sub.2 Mo                                  
                                    ˜600                            
MoRh  950˜                                                          
               Pt.sub.3 Mn                                                
                     ˜1000                                          
      1200                                                                
MoIr  1570˜                                                         
               Pt.sub.3 Co                                                
                     ˜750                                           
      1650                                                                
WIr   &gt;1200    Pd.sub.3 Fe                                                
                     ˜800                                           
                              Ni.sub.4 Mo                                 
                                    ˜860                            
Fe.sub.3 Al                                                               
      550      Ag.sub.3 Mg                                                
                     393      Ni.sub.4 W                                  
                                    ˜970                            
Fe.sub.3 Si                                                               
      ˜1200                                                         
               Au.sub.3 Cd                                                
                     412      Au.sub.4 V                                  
                                    ˜565                            
Cu.sub.3 +Pd                                                              
      ˜480                                                          
               Cu.sub.3 Pt                                                
                     ˜600                                           
                              Au.sub.4 Mn                                 
                                    ˜420                            
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     Many of the noted alloys of the present invention can be produced by simply plating the components onto a base containing the other constituents. As examples of such platable alloys as gold, copper, platinum, palladium, cobalt, nickel, zinc, silver, cadmium, rhodium, iridium, chromium, and tin. 
     Ordered alloys such as CoPt, FePt, Pt 4  Ni 3  Co and the like can be utilized as magnetic recording media. Ordered alloys of CuPt, CuAu, FePt, AgPt, CuPd and the like can be employed as electrical contact materials. The ordered alloys CuPt, FeNi, FeAl, FePd, CuAu, Fe 3  Pt and the like can be used as anti-corrosion materials. Ni 3  Mn alloy can be used as a magnetic head material. 
     As described in the foregoing, the ordered alloy of the present invention can be made without quantifying the amount of metals which form the ordered alloys, the surface layer of a metal base can be provided with an ordered alloy layer of a constant thickness, and ordered layers of predetermined characteristics can be prepared. 
     In the above examples of the invention, it is preferable that the metal base and the deposited layer contain no impurities, but impurities can be tolerated in such an amount as not to prevent the formation of the ordered phase. 
     It will be apparent that many modifications or variations can be effected by one skilled in the art through the described embodiments without departing from the spirit or scope of the invention.