Patent Publication Number: US-5833774-A

Title: High strength silver palladium alloy

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to precious metal alloys, and, more particularly, to such alloys which are especially adapted for electrical contact applications. 
     As is well known, precious metal alloys have been favored for a number of electrical contact applications where low contact resistance and/or noise is desired over extended periods of time. This is particularly true when the electrical products incorporating such elements may be exposed to relatively high temperatures, high humidity, sulfurous or other corrosive atmospheres, etc. Alloys with high gold and platinum contents were early favored for such applications, but the cost of such alloys became prohibitive for many applications and militated against more widespread use. This brought about efforts to develop alloys based upon less costly metals. As a result, palladium alloys became widely utilized in an effort to provide such desirable properties as corrosion and tarnish resistance at a lower cost. However, palladium alloys are also relatively expensive, and this cost has militated against still wider use. 
     U.S. Pat. No. 5,484,569 discloses a silver/palladium alloy having high oxidation and tarnish resistance, due to relatively low levels of copper and relatively high levels of nickel (at least 1%, and preferably greater amounts). That alloy relies on a synergistic effect of nickel and zinc to provide oxidation and tarnish resistance. However, that alloy does not exhibit the desired strength and hardness values for certain applications. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a novel silver/palladium alloy which exhibits a high strength and high hardness and provides good electrical properties. 
     It is also an object to provide such an alloy which is relatively low in cost in comparison to alloys having higher contents of noble metals. 
     Another object is to provide electrical components fabricated from such alloys and which exhibit desirable hardness and strength values as well as controllable formability for contact applications. 
     SUMMARY OF THE INVENTION 
     It has now been found that the foregoing and related objects may be readily attained in a silver/palladium alloy for electrical applications which comprises, on a percent by weight basis, 20-50 silver, 20-50 palladium, 20-40 copper, 0-0.5 nickel, 0.1-5 zinc, 0.01-0.3 boron, and up to 1 of modifying elements selected from the group consisting of rhenium, ruthenium, platinum and gold. 
     Preferably, silver comprises 28-45 percent by weight and palladium comprises 29-40 percent by weight. Copper preferably comprises 25-30 percent by weight, zinc preferably comprises 1 percent by weight, nickel preferably comprises 0 percent by weight, and boron preferably comprises about 0.15 percent by weight. 
     The alloy is formed into metal components which exhibit an elastic modulus of at least 14×10 6  p.s.i., and are capable of achieving hardness levels of greater than 350 Knoop with the appropriate thermal mechanical treatment. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As indicated hereinbefore, the combination of silver with palladium provides most of the present alloy. The combination of zinc and boron in the desired ranges, along with reduction in nickel content to substantially zero, provides an alloy having superior strength and hardness while maintaining the amount of palladium and/or copper. Indeed, the presence of zinc and boron in combination in the alloy provides superior strength while permitting the amounts of both copper and palladium to be reduced relative to their levels in prior art silver/palladium alloys. Moreover, by proper combination of the elements described hereinafter in detail, the alloy exhibits good processing characteristics and mechanical strength together with a desirable modulus to provide the desired flexibility for moving contact applications. 
     The alloys of the present invention essentially contain palladium, silver, copper, zinc and boron and desirably contain small amounts of modifiers selected from the group consisting of rhenium, ruthenium, platinum and gold. The level of nickel in these alloys is reduced to substantially negligible amounts (less than 1 percent). 
     The silver content may range from as little as 20 percent to as much as 50 percent, and is preferably in the range of 28-45 percent. 
     Palladium is provided in the range of 20-50 percent, and preferably in the range of about 29-40 percent. 
     Copper is the next largest component of the alloy and is provided in the range of 20-40 percent, and preferably 25-30 percent. In these alloys, the palladium reacts with the copper component to provide a basis for the age hardening reaction to provide physical/mechanical properties of desirable characteristics. Moreover, it also increases the modulus. 
     Zinc is provided in the range of 0.1-5 percent, and preferably in the range of 0.5-1.5 percent. It participates in the second phase reaction which the alloy undergoes. It also serves as a deoxidant for the alloy during the initial casting into ingots. 
     Boron is provided in the range of 0.01 to 0.30 percent, and preferably about 0.15 percent. The boron is believed to participate synergistically with zinc in hardening and providing other desirable physical properties. For optimum effect of the zinc/boron enhancement, the amount of nickel must be kept to less than 1 percent, and preferably from 0-0.5 percent. 
     Small amounts of the modifying elements rhenium, ruthenium, platinum and gold in the range of 0-1.0 percent total do contribute some improvement in properties. Platinum provides nobility, and rhenium and ruthenium appear to act as grain refiners in this alloy. Gold contributes to oxidation resistance. 
     Two highly desirable commercial alloys of the present invention have the following compositions, respectively: 
     
         ______________________________________                                    
              Percent by Weight                                           
Element         A (PE253)                                                 
                         B (PE260)                                        
______________________________________                                    
Silver          44.85    28.9                                             
Palladium       29       40                                               
Copper          25       30                                               
Zinc            1        1                                                
Boron            0.15     0.10                                            
Nickel          0        0                                                
Re, Ru, Pt, Au  --       --                                               
______________________________________                                    
 
    
     Hardness values for these alloys are, respectively, 395 Knoop at 100 grams for alloy A (PE253), and &gt;500 Knoop at 100 grams for alloy B (PE260). To achieve these hardness levels, the alloys were rolled in strip form with a 60-70% reduction in area after a solution anneal followed by a precipitation hardening heat treatment. Alloy A was heat treated for 60 minutes at 700° F. and alloy B was heat treated for 60 minutes at 800° F. 
     These alloys are rolled into sheets which can be utilized to fabricate various electrical products including stationary contacts or connectors. Other uses include contacts in non-stationary applications such as commutators, potentiometers, and slip rings. The material can be drawn or otherwise formed into various types of electronic components because of its physical properties. 
     Processability is another significant property since the cast bar stock must be rolled into relatively thin strip or sheet. Accordingly, an alloy which evidences cracking at less than a 50 percent reduction is generally considered to have poor processing characteristics. The alloy can be used as wrought, and it may or may not be heat treated depending upon the intended application. 
     The following non-limiting examples are presented: 
    
    
     EXAMPLE I 
     An alloy comprising 35 weight percent silver, 35 weight percent palladium, and 30 weight percent copper was heat-treated at 700° F. for 45 minutes and 90 minutes. Alloy A, described above, was similarly heat-treated. The following hardness values (Knoop at 100 grams) were reported: 
     
         ______________________________________                                    
               HT 700F  HT 700F                                           
Alloy          45 mins  90 mins                                           
______________________________________                                    
35-35-30       348      358                                               
A              363      379                                               
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     Alloy A contains a reduced amount of copper relative to the 35-35-30 alloy, and zinc and boron in combination. In Alloy A and its variants, hardness is provided by a PdCu x  -type precipitate, in which x is typically 3. One would expect the hardness of the alloy to depend on the size and volume percent of the precipitated phase and to increase with increased copper and/or Pd, dependent on which element is stoichiometrically deficient. However, the addition of zinc and boron in small but effective amounts provides superior hardness with decreased amounts of copper and palladium. 
     EXAMPLE II 
     The following alloys were provided and heat treated at 60, 90 and 120 minutes from a 68 percent cold-worked state and from an annealed state. Hardness values are given in Knoop for a 100 gram load. See Tables I and II for comparative data. 
     The difference in hardness values for the alloys with negligible nickel (PE-260 and PE-261) relative to those with 1 percent nickel (PE-224 and PE-262) is significant: 30-60 Knoop points in the heat-treated from cold-worked state and 5-100 +   Knoop points in the heat-treated from annealed state. 
     Thus, it can be seen from the foregoing detailed specification that the present invention provides a novel silver/palladium alloy for electrical applications which exhibits good contact properties and superior hardness and strength values. The alloy can be fabricated relatively easily into metal sheet, or wire as a precursor to stamping or forming into the final geometry. As with many precipitation hardenable alloys, it is possible to achieve the desired hardness by properly heat treating solutionized (annealed) material. This allows for greater formability prior to heat treatment. 
     
                                           TABLE 1                                 
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Composition, weight %                                                     
                   68% C.W.                                               
                        Heat Treated From 68% C.W. - Hardness, HK (100)   
Alloy              Hardness                                               
                        700° F.                                    
                                  750° F.                          
                                            800° F.                
                                                      850° F.      
Code                                                                      
    Ag Pd                                                                 
         Cu                                                               
           Zn                                                             
             Ni                                                           
               Boron                                                      
                   HK (100)                                               
                        60 90 120 60 90 120 60 90 120 60 90 120           
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PE-260                                                                    
    28.9                                                                  
       40                                                                 
         30                                                               
           1 --                                                           
               0.1 333  464                                               
                           490                                            
                              490 488                                     
                                     477                                  
                                        483 504                           
                                               464                        
                                                  452 476                 
                                                         490              
                                                            437           
PE-224                                                                    
    27.9                                                                  
       40                                                                 
         30                                                               
           1 1 0.1 330  410                                               
                           415                                            
                              414 435                                     
                                     435                                  
                                        439 439                           
                                               414                        
                                                  405 426                 
                                                         432 387          
PE-261                                                                    
    28.85                                                                 
       40                                                                 
         30                                                               
           1 --                                                           
               0.15                                                       
                   345  479                                               
                           483                                            
                              488 482                                     
                                     489                                  
                                        482 499                           
                                               463                        
                                                  464 486                 
                                                         490 440          
PE-262                                                                    
    27.35                                                                 
       40                                                                 
         30                                                               
           1.5                                                            
             1 0.15                                                       
                   343  416                                               
                           410                                            
                              431 448                                     
                                     451                                  
                                        454 464                           
                                               434                        
                                                  436 443                 
                                                         443 414          
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                                           TABLE II                                
__________________________________________________________________________
Composition, weight %                                                     
                   Annealed                                               
                        Heat Treated From Annealed (1580° F./20    
                        min @ 68% C.W. HK (100)                           
Alloy              Hardness                                               
                        700° F.                                    
                                  750° F.                          
                                            800° F.                
                                                      850° F.      
Code                                                                      
    Ag Pd                                                                 
         Cu                                                               
           Zn                                                             
             Ni                                                           
               Boron                                                      
                   HK (100)                                               
                        60 90 120 60 90 120 60 90 120 60 90 120           
__________________________________________________________________________
PE-260                                                                    
    28.9                                                                  
       40                                                                 
         30                                                               
           1 --                                                           
               0.1 253  338                                               
                           345                                            
                              362 497                                     
                                     496                                  
                                        485 467                           
                                               464                        
                                                  464 465                 
                                                         457              
                                                            428           
PE-224                                                                    
    27.9                                                                  
       40                                                                 
         30                                                               
           1 1 0.1 244  340                                               
                           335                                            
                              339 362                                     
                                     367                                  
                                        358 412                           
                                               416                        
                                                  413 420                 
                                                         425 397          
PE-261                                                                    
    28.85                                                                 
       40                                                                 
         30                                                               
           1 --                                                           
               0.15                                                       
                   253  341                                               
                           347                                            
                              356 485                                     
                                     497                                  
                                        490 465                           
                                               473                        
                                                  469 471                 
                                                         454 494          
PE-262                                                                    
    27.35                                                                 
       40                                                                 
         30                                                               
           1.5                                                            
             1 0.15                                                       
                   247  335                                               
                           341                                            
                              345 383                                     
                                     390                                  
                                        389 428                           
                                               411                        
                                                  421 417                 
                                                         401 386          
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