Iron-boron solid solution alloys having high saturation magnetization and low magnetostriction
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to ferromagnetic alloys characterized by a high saturation magnetization, low or near-zero magnetostriction and, in particular, to iron-boron solid solution alloys having a body centered cubic (bcc) structure. 
     2. Description of the Prior Art 
     The equilibrium solid solubilities of boron in α-Fe (ferrite) and γ-Fe (austenite) are quite small, being less than 0.05 and 0.11 atom percent, respectively; see M. Hansen et al., Constitution of Binary Alloys, pp. 249-252, McGraw-Hill Book Co., Inc. (1958). Attempts have been made to increase the solubility of boron in iron by a splat-quenching technique, without success; see, e.g., R. C. Ruhl et al., Vol. 245, Transactions of the Metallurgical Society of AIME, pp. 253-257 (1969). The splat-quenching employed gun techniques and resulted only in the formation of ferrite and Fe 3  B, with no changes in the amount of austenitic phase. Compositions containing 1.6 and 3.2 weight percent (7.7 and 14.5 atom percent, respectively) boron were prepared. These splat-quenched materials, as well as equilibrium alloys which contain two phases, are very brittle and cannot easily be processed into thin ribbons or strips for use in commercial applications. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, iron-boron solid solution alloys having high saturation magnetization and low or near-zero magnetostriction are provided which consist essentially of about 1 to 9 atom percent boron, balance essentially iron plus incidental impurities. The alloys of the invention possess bcc structures in the range of about 1 to 9 atom percent of boron. 
     Also provided by the invention is a preferred grouping of iron-boron solid solution alloys wherein the boron constituent ranges from about 1 to less than 4 atom percent and the balance of the alloy consists essentially of iron plus incidental impurities. These alloys have a combination of high saturation induction with relatively low magnetostriction that makes them particularly well suited for use in transformer applications wherein minimal core size and weight are prerequisites. 
     The alloys of the invention are advantageously easily fabricated as continuous filament with good bend ductility by a process which comprises 
     (a) forming a melt of the material; 
     (b) depositing the melt on a rapidly rotating quench surface; and 
     (c) quenching the melt at a rate of about 10 4  ° to 10 6  ° C./sec to form the continuous filament. 
     The alloys of the invention possess moderately high hardness and strength, good corrosion resistance, high saturation magnetization, low or near-zero magnetostriction and high thermal stability. The alloys in the invention find use in, for example, magnetic cores requiring high saturation magnetization and low or near-zero magnetostriction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The compositions of alloys within the scope of the invention are listed in Table I, together with their equilibrium structures and the phases retained upon rapid quenching to room temperature. X-ray diffraction analysis reveals that a single metastable phase α-Fe(B) with bcc structure is retained in the chill cast ribbons. Table I also summarizes the change of lattice parameter and density with respect to boron concentration. It is clear that the lattice contracts with the addition of boron, thus indicating predominant dissolution of small boron atoms on the substitutional sites of the α-Fe lattice. It should be noted that neither the mixture of the equilibrium phases of α-Fe and Fe 2  B expected from the Fe-B phase diagram nor the orthorhombic Fe 3  B phase previously obtained by splat-quenching are formed by the alloys of the invention. 
     
                       TABLE I                                                     
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Results of X-ray Analysis and Density Measurements on                     
Fe(B) Chill Cast Ribbons                                                  
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Alloy Composition (atom %)                                                
        Fe.sub.99 B.sub.1                                                 
                 Fe.sub.98 B.sub.2                                        
                          Fe.sub.97 B.sub.3                               
                                 Fe.sub.96 B.sub.4                        
                                        Fe.sub.95 B.sub.5                 
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Equil-  --Fe +   --Fe +   --Fe + --Fe + --Fe +                            
ibrium  Fe.sub.2 B                                                        
                 Fe.sub.2 B                                               
                          Fe.sub.2 B                                      
                                 Fe.sub.2 B                               
                                        Fe.sub.2 B                        
Phases                                                                    
at Room                                                                   
Temp..sup. c                                                              
Phases  --Fe     --Fe     --Fe   --Fe   --Fe                              
Present (B)      (B)      (B)    (B)    (B)                               
after   solid    solid    solid  solid  solid                             
Chill   soln..sup.b                                                       
                 soln..sup.b                                              
                          soln..sup.b                                     
                                 soln..sup.b                              
                                        soln..sup.b                       
Casting                                                                   
Average 7.87     7.84     7.82   7.79   7.78                              
Density,                                                                  
g/cm.sup.3                                                                
Lattice --       --       --     2.864  --                                
Para-                                                                     
meter                                                                     
(A).sup.a                                                                 
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       Alloy Composition (atom %)                                         
         Fe.sub.94 B.sub.6                                                
                  Fe.sub.93 B.sub.7                                       
                             Fe.sub.92 B.sub.8                            
                                    Fe.sub.91 B.sub.9                     
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Equil-   --Fe +   --Fe +     --Fe + --Fe +                                
ibrium   Fe.sub.2 B                                                       
                  Fe.sub.2 B Fe.sub.2 B                                   
                                    Fe.sub.2 B                            
Phases                                                                    
at Room                                                                   
Temp..sup. c                                                              
Phases   --Fe     --Fe       --Fe   --Fe                                  
Present  (B)      (B)        (B)    (B)                                   
after    s.s      s.s        s.s    s.s                                   
Chill                                                                     
Casting                                                                   
Average  7.74     7.73       7.70   7.68                                  
Density,                                                                  
g/cm.sup.3                                                                
Lattice  2.863    --         2.861  --                                    
Para-                                                                     
meter                                                                     
(A)                                                                       
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 .sup.a Estimated maximum fractional error = ±.001 A.                  
 .sup.b Metastable solid solutions α-Fe(B) is of the WA2 type.      
 .sup.c Hansen et al., Constitution of Binary Alloys.                     
 
    
     The amount of boron in the compositions of the invention is constrained by two considerations. The upper limit of about 9 atom percent is dictated by the cooling rate and the requirement that the filament be ductile. At the cooling rates employed herein of about 10 4  ° to 10 6  ° C./sec, compositions containing more than about 12 atom percent (7.6 weight percent) boron are formed in a substantially glassy phase, rather than the bcc solid solution phase obtained for compositions of the invention. The lower limit of about 1 atom percent is dictated by the fluidity of the molten composition. Compositions containing less than about 1 atom percent (0.8 weight percent) boron do not have the requisite fluidity for melt spinning into filaments. The presence of boron increases the fluidity of the melt and hence the fabricability of filaments. 
     Table II lists the hardness, the ultimate tensile strength and the temperature at which the metastable alloy transforms into a stable crystalline state. Over the range of 4 to 8 atom percent boron, the hardness ranges from 425 to 698 kg/mm 2 , the ultimate tensile strength ranges from 206 to 280 ksi and the transformation temperature ranges from 820 to 880 K. 
     
                       TABLE II                                                    
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Mechanical Properties of Melt                                             
Spun Fe(B) bcc Solid Solution Ribbon                                      
                      Ultimate                                            
Alloy                 Tensile  Transformation                             
Composition                                                               
          Hardness    Strength Temperature                                
(atom percent)                                                            
          (kg/mm.sup.2)                                                   
                      (ksi)    (K.)                                       
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Fe.sub.96 B.sub.4                                                         
          425         206      880                                        
Fe.sub.94 B.sub.6                                                         
          557         242      860                                        
Fe.sub.92 B.sub.8                                                         
          698         280      820                                        
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     At the transformation temperature, a progressive transformation to a mixture of stable phases, substantially pure -Fe and tetragonal Fe 2  B, occurs. The high transformation temperatures of the alloys of the invention are indicative of their high thermal stability. 
     Magnetic properties of the alloys of the invention are listed in Table III. These include the saturation magnetization (B s ) and magnetostriction (λ) both at room temperature and the Curie temperatures (θ f ). For comparison, the room temperature saturation magnetization of pure iron (α-Fe) is 2.16 Tesla and its Curie temperature is 1043 K. 
     
                       TABLE III                                                   
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Results of Magnetic Measurements on Crystalline                           
Fe.sub.100-x B.sub.x Alloys of the Invention                              
         Room Tem-    Room Tem-                                           
         perature     perature                                            
         Saturation   Saturation Curie                                    
Boron    Magneti-     Magneto-   Temper-                                  
Content  zation       striction  ature                                    
x (at. %)                                                                 
         (Tesla)      (10.sup.-6)                                         
                                 θ.sub.f (K.)                       
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1        2.11         -4.7       1023                                     
2        2.09         -3.8       1013                                     
3        2.06         -3.2       --                                       
4        2.05         -1.5       978                                      
5        2.03         -1.1       --                                       
6        2.00         -0.1       964                                      
7        1.97         +0.7       --                                       
8        1.92         +1.5       944                                      
9        1.90         +2.3       920                                      
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     Alloys consisting essentially of about 4 to 8 atom percent boron, balance iron, have B s  values ranging between 1.92 T and 2.05 T comparable to the grain-oriented Fe-Si transformer alloys having about 8 atom percent (B s  =19.7 kGauss). More importantly, the value of the magnetostriction is rather small and ranges between -1.5×10 -6  for Fe 96  B 4  and +1.5×10 -6  for Fe 92  B 8  passing through the zero or near-zero magnetostriction point at about Fe 94  B 6  composition. 
     The zero or near-zero magnetostriction point possessed by the Fe 94  B 6  alloy makes it especially well suited for use in transformer applications wherein low core loss is essential. Since low core loss is essential for many transformer applications, an alloy that contains about 94 atom percent iron and about 6 atom percent boron is especially preferred. These values should be compared with that (about 5×10 -6 ) of a Fe-Si transformer alloy having about 8 atom percent Si. The combination of a high saturation magnetization and low or near-zero magnetostriction is often required in various magnetic devices including transformers. Further, alloys in this range are ductile. Thus, these alloys are useful in transformer cores and are accordingly preferred. 
     The alloys of the invention are advantageously fabricated as continuous ductile filaments. The term &#34;filament&#34; as used herein includes any slender body whose transverse dimensions are much smaller than its length, examples of which include ribbon, wire, strip, sheet and the like having a regular or irregular cross-section. By ductile is meant that the filament can be bent to a round radius as small as ten times the foil thickness without fracture. 
     The alloys of the invention are formed by cooling an alloy melt of the appropriate composition at a rate of about 10 4  ° to 10 6  ° C./sec. Cooling rates less than about 10 4  ° C./sec result in mixtures of well-known equilibrium phases of α-Fe and Fe 2  B. Cooling rates greater than about 10 6  ° C./sec result in the metastable Fe 3  B phase. The Fe 3  B phase, if present, forms a portion of the matrix of the bcc Fe(B) phase, as in the order of up to about 20 percent thereof. The presence of the Fe 3  B phase tends to increase the overall magnetostriction by up to about 2×10 -6 , thus shifting the near zero magnetostriction composition to near Fe 95  B 5 . Cooling rates of at least about 10 5  ° C./sec easily provide the bcc solid solution phase and are accordingly preferred. 
     A variety of techniques are available for fabricating rapidly quenched continuous ribbon, wire, sheet, etc. Typically, a particular composition is selected, powders of the requisite elements in the desired proportions are melted and homogenized and the molten alloy is rapidly quenched by depositing the melt on a chill surface such as a rapidly rotating cylinder. The melt may be deposited by a variety of methods, exemplary of which include melt spinning processes, such as taught in U.S. Pat. No. 3,862,658, melt drag processes, such as taught in U.S. Pat. No. 3,522,836, and melt extraction processes, such as taught in U.S. Pat. No. 3,863,700, and the like. The alloys may be formed in air or in moderate vacuum. Other atmospheric conditions such as inert gases may also be employed. 
     EXAMPLES 
     Alloys were prepared from constituent elements (purity higher than 99.9%) and were rapidly quenched from the melt in the form of continuous ribbons. Typical cross-sectional dimensions of the ribbons were 1.5 mm by 4 μm. Densities were determined by comparing the specimen weight in air and toluene (density=0.8669 g/cm 3  at 20° C.) at room temperature. X-ray diffraction patterns were taken with filtered copper radiation in a Norelco diffractometer. The spectrometer was calibrated to a silicon standard with the maximum error in lattice parameter estimated to be ±0.001 A. The thermomagnetization data were taken by a vibrating sample magnetometer in the temperature range between 4.2 and 1050 K. The room temperature saturation magnetostriction was measured by a bridge technique. Hardness was measured by the diamond pyramid technique, using a Vickers-type indenter consisting of a diamond in the form of a square-based pyramid with an included angle of 136° between opposite faces. Loads of 100 g were applied. The results of the measurements are summarized in Tables I, II and III.