Patent Publication Number: US-4927469-A

Title: Alloy toughening method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. patent application Ser. No. 860,546 filed May 7, 1986, abandoned which is a continuation-in-part of U.S. patent application Ser. No. 735,567 filed May 17, 1985, abandoned. 
     BACKGROUND OF THE INVENTION 
     Mettalurgical objects produced from rapidly cooled metal have been burdened by low toughness. the cause of this low toughness was not known. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for toughening metallurgical objects produced from rapidly cooled metal components. 
     We have discovered that metastable, featureless regions in rapidly cooled metal adversely affect toughness. 
     We achieve this as well as other objects which will become apparent from the discussion that follows, according to the present invention, by providing: a method of treating a metallurgical object containing metastable featureless regions adversely affecting fracture toughness, comprising heating the object for transforming the regions at least sufficiently out of their metastable state to improve fracture toughness; and, a method of treating metal particles containing metastable featureless regions which adversely affect fracture toughness when the particles are bonded together to form a metallurgical object, comprising heating the particles for transferring the regions at least sufficiently out of their metastable state to improve fracture toughness in metallurgical objects formed by bonding the particles together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1, composed of FIGS. 1a to 1d, are photomicrographs of a powder used in the invention. 
     FIGS. 2 to 4 are plots of data. 
    
    
     DETAILED DESCRIPTION 
     Featureless Regions 
     The present invention concerns a treatment of metallurgical objects containing certain metastable, featureless regions. The treatment improves fracture toughness. 
     Instances in the literature where the term &#34;featureless&#34; is used to refer to these regions are as follows: 
     
         ______________________________________                                    
Location in Reference                                                     
             Citation of Reference                                        
______________________________________                                    
Col. 4, line 21                                                           
             U.S. Pat. No. 3,899,820, 8/19/85                             
E.g. lines 7&amp;8, abstract                                                  
             RapidlyQu&#39;dMetalsIII,1,73-84,1978                            
E.g., the title                                                           
             Met.Trans.A,V.15A,1/84,pp29-31                               
Intro.,2nd.para.,line2                                                    
             Scrip.Met&#39;ica,V18,1984,pp905-9                               
Intro.,2nd.para.,line6                                                    
             Scrip.Met&#39;ica,V18,1984,pp911-6                               
E.g., page 26                                                             
             MatResSocSympProc,V28,1984,pp21-7                            
Pg. 148, top left col.                                                    
             Mat.Sci.&amp;Eng.,V65,1984,pp145-56                              
3rd.para.,line2                                                           
             43rdAnMt&#39;gElecM&#39;scopSoc,&#39;85,pp32-3                           
______________________________________                                    
 
    
     These featureless regions are crystalline. This is evident alone in the title of the second-listed reference, &#34;Rapidly Quenched Crystalline Alloys&#34;. It is also evident from what is believed to be the pioneer article on these regions, entitled &#34;Observations on a Structural Transition in Aluminum Alloys Hardened by Rapid Solidification&#34; by H. Jones, Mater. Sci. Eng., 5 (1969/70), pp. 1-18. Thus, in the Summary of the article by Jones, reference is to X-ray diffraction alpha-Al line broadening, and shift, in zone A regions (&#34;zone A regions&#34; is synonymous to &#34;featureless regions&#34;, as can be observed, for instance, in the references antedating Jones, as cited in the preceding paragraph), such indicating that discussion is of crystalline material. 
     The featureless regions result from rapid cooling. FIG. 1 illustrates the phenomenon of featureless regions. In FIG. 1a, taken using optical microscopy, the featureless regions appear white as compared to the other regions which have a texture that appears to be black specks on a gray background. Note that the smaller particles tend to be completely featureless, an effect of the higher cooling rate experienced by the smaller particles. The scanning electron microscopy photographs of FIGS. 1b-1d further illustrate the featureless regions, which appear uniformly gray as compared to the remaining, dendritically textured regions. FIGS. 1b and 1d show again the smaller, completely featureless regions. FIG. 1c shows in particularly good detail that the particle has a featureless half-moon region on its lower side. This is an aspect which also shows in FIGS. 1aand 1b, namely that higher cooling rates in some parts of a particle versus slower cooling rates in other parts can lead to a situation where the particle will be featureless in the rapidly cooled parts and textured in the slower cooled parts. 
     Alloys 
     In general, any alloy containing featureless regions can be treated according to the invention. 
     A preferred Al alloy consists essentially of 4 to 12% Fe, 2 to 14% Ce, remainder Al. Fe combines with Al to form intermetallic dispersoids and precipitates providing strength at room temperature and elevated temperature. Ce combines with Fe and Al to form intermetallic dispersoids which provide strength, thermal stability and corrosion resistance. Further information concerning this alloy is contained in U.S. Pat. Nos. 4,379,719 and 4,464,199. 
     Uniformizing 
     With respect to strength, such as yield or tensile strength, our uniformizing heat treatment, within the featureless regions, represents an overaging. 
     This heating step of the invention for the above preferred Al alloy will generally be in the range 750°-950° F. for 10 seconds to 4 hours. However, at lower temperatures, longer time may be suitable. This could be of advantage in the case of large billets, in order obtain temperature uniformity. 
     Fast heating appears to be best (via induction heating), since this will prevent coarsening, for instance dispersoid coarsening. 
     Deformation 
     In the heating to effect the uniformizing of the invention, the featureless particles are stabilized and they become deformable. Deformation after the uniformizing treatment, for instance deformation in the form of compaction, extrusion or rolling, will provide a more uniform microstructure, with improved bonding between powder particles. Improved interparticle powder bonding further increases toughness and resistance to crack propagation. 
     Illustration 
     The following Table A illustrates results achieved by procedure according to the present invention (with heat treatment, i.e. 1 to 3 minutes at 900° F. followed by cooling to 725° F. extrusion temperature) compared to results without heat treatment (i.e. the billet was heated directly to the 725° F. extrusion temperature and then extruded). Processing in going from extruded bar to sheet was the same in both instances. 
     
                       TABLE A                                                     
______________________________________                                    
Comparative Examples                                                      
       With Heat Treatment.sup.a                                          
                    Without Heat Treatment                                
       Toughness.sup.b                                                    
               Strength.sup.b                                             
                        Toughness.sup.b                                   
                                  Strength.sup.b                          
______________________________________                                    
Extrusions                                                                
         21.4      50.9     13.7    55.1                                  
Sheet    720.sup.c 70.2     405.sup.c                                     
                                    73.7                                  
______________________________________                                    
 .sup.a 1 min at 900° F.                                           
 .sup.b Toughness = Ksi · in .sup.1/2, Strength = Ksi            
 .sup.c Sheet toughness given in unit propagation energy (UPE)            
 inlb/in.sup.2                                                            
 
    
     In the case of the extrusion, there was a 56% increase in toughness for an 8% decrease in yield strength. For the sheet, toughness was increased 78% for an 5% decrease in yield strength. 
     Advantages 
     The invention improves toughness and thermal stability in metallurgical objects based on rapid solidification processes. It is expected that creep behavior will also be improved. 
     Further illustrative of the invention are the following examples. 
     Example I 
     Rapidly solidified aluminum alloy powder of composition 8.4% Fe, 4.0% Ce, rest essentially aluminum, had featureless regions resulting from rapid cooling during formation of the powder. To make the powder, a pot of such composition was alloyed by adding high purity alloying elements to high purity aluminum. The melt was passed through a filter and atomized using high temperature flue gas to minimize the oxidation of the alloying elements. During atomization, the powder was continuously passed through a cyclone to separate the particles from the high velocity air stream. The majority of powder particles had diameters between 5and 40 micrometers. Powder was screened to retain only less than 74 micrometers size powder and fed directly into a drum. Besides Fe, Ce, and Al, the powder had the following percentages of impurities: Si 0.14, Cu 0.02, Mn 0.04, Cr 0.01, Ni 0.02, Zn 0.02, Ti 0.01. The powder was found to have featureless regions in about the same quantity and distribution as shown in FIG. 1. The particle size distribution of the powder was 4.4% in the range 44 to 74 micrometers and 95.4% smaller than 44 micrometers. Average particle diameter was 15.5 microns as determined on a Fisher Subsieve Sizer. 
     Billet was made from this powder by cold isostatic pressing to approximately 75% of theoretical density. Each 66 kg (145 lb) cold isostatic compact was encapsulated in an aluminum container with an evacuation tube on one end. The canned compacts were placed in a 658 K (725° F.) furnace and continuously degassed for six hours, attaining a vacuum level below 40 microns. Degassed and sealed compacts were then hot pressed at 725° F. to 100 percent density using an average pressure of 469.2 MPa (68 ksi). 
     A cylindrical extrusion charge measuring 15 cm (6.125 in.) diameter ×30.5 cm (12 in.) length was machined from the billet and subjected to a uniformizing treatments of 1 minute at 850° F. and 1 minute at 900° F. Heating was done using an induction furnace operating at 60 H z . Temperature was measured by a thermocouple placed at an axial location about 1.2 cm (0.5 in.) from the end. It took about 10 minutes to heat the extrusion charge from room temperature to 850° F. or 900° F. at which point temperature was controlled at 850° F. and 900° F. for the 1 minute holding time. 
     The extrusion charge was then air-cooled to 725° F. and extruded as a bar of 5 cm (2 inches) ×10 cm (4 inches) cross section. 
     Another Al-Fe-Ce alloy having the composition Al-8.4% Fe-7.0% Ce was also uniformized at 900° F. for 1 min. 
     Properties for both alloys are recorded in Table I. Results from Table I are shown graphically in FIG. 2. Note the strength toughness relation for the two different alloys. 
     
                                           TABLE I                                 
__________________________________________________________________________
Room Temperature Tensile and Fracture Toughness Test Results of           
Extrusions                                                                
                 Uniformizing                                             
                 Treatment                                                
                         Yield Strength                                   
                                 Tensile                                  
                 Temp.                                                    
                      Time                                                
                         0.2% Offset                                      
                                 Strength                                 
                                        Elongation                        
                                              Fracture Toughness          
Sample No..sup.a                                                          
       Alloy     °F.                                               
                      Min.                                                
                         MPa (Ksi)                                        
                                 MPa                                      
                                    (Ksi)                                 
                                        (%)   MPa · m.sup.1/2    
                                                   (Ksi ·        
                                                   in.sup.1/2)            
__________________________________________________________________________
514295-1B                                                                 
       Al--8.4 Fe--4.0 Ce                                                 
                 Control 388 (56.2)                                       
                                 497                                      
                                    (72.0)                                
                                        12.5  14.7 (13.4)                 
514282-1                                                                  
       Al--8.4 Fe--4.0 Ce                                                 
                 Control 380 (55.1)                                       
                                 469                                      
                                    (68.0)                                
                                         9.6  15.1 (13.7)                 
514412-T                                                                  
       Al--8.4 Fe--4.0 Ce                                                 
                 850  1  366 (53.0)                                       
                                 449                                      
                                    (65.0)                                
                                        17.8  19.6 (17.8)                 
514413-1B                                                                 
       Al--8.4 Fe--4.0 Ce                                                 
                 900  1  351 (50.9)                                       
                                 425                                      
                                    (61.6)                                
                                        16.7  23.5 (21.4)                 
514398-2T                                                                 
       Al--8.4 Fe--7.0 Ce                                                 
                 Control 426 (61.7)                                       
                                 530                                      
                                    (76.8)                                
                                        11.0  9.35 (8.5).sup.c            
514416-2T                                                                 
       Al--8.4 Fe--7.0 Ce                                                 
                 900  1  373 (54)                                         
                                 450                                      
                                    (65.2)                                
                                        16.0  27.8 (25.3)                 
__________________________________________________________________________
 Notes:                                                                   
 Values are averages from duplicate tests. Yield and tensile strengths wer
 measured in the longitudinal (L) direction using 0.907 cm (0.357&#34;)       
 diameter specimens machined from the extruded product. Elongation was    
 measured in a 3.56 cm (1.40&#34;) gauge length. Tensile properties were      
 obtained according to ASTM B557. Fracture toughness was measured in the L
 orientation using compact tension specimens of size 1.90 cm (0.75&#34;) thick
 × 3.81 cm (1.50 m) × 4.57 cm (1.80&#34;).                        
 .sup.a Product size: 5.1 cm × 10.2 cm (2.0 in. × 4.0 in.)    
 .sup.b Values are Kic per ASTM E399.                                     
 .sup.c This value was not a valid Kic but a meaningful value per ASTM B64
                                                                          
 
    
     Example II 
     Extruded bar of Example I was rolled at 600° F. to sheet of final thickness equalling 1.60 mm (0.063 inch). 
     Prior to rolling, the extrusion was sawed to approximately 25 cm (10 in.) lengths. Surface roughness, caused by pickup of aluminum on the extrusion dies, was eliminated by machining the extrusions to the thicknesses listed in Table III. Also listed are process parameters used to roll the Al-Fe-Ce 1.60 mm (0.063 in.) sheet. 
     Each piece was cross rolled until the desired width, greater than 41 cm (16 inches), was obtained, followed by straight rolling to the desired thickness, 1.60 mm (0.063 inch). 
     1.27 cm (0.5 in.) width ×5.08 cm (2.0 in.) gage length tensile specimens were prepared and tested to give results as shown in Table II. Sheet tensile strength was determined per ASTM E8 and E23. The Alcoa-Kahn tear test (see &#34;Fracture Characteristics of Aluminum Alloys, &#34; J. G. Kaufman, Marshall Holt, Alcoa Research Laboratories, Technical Paper No. 18, pp. 10-18, 1965) and fracture toughness K c  per ASTM B646 and E561 were used to compare sheet toughness. These results are shown in Table II. FIG. 3 shows the graphic representation of the strength/fracture toughness, K c , relationships for representative samples of Table II, while FIG. 4 provides a corresponding presentation from Table II in the form of toughness indicator, or unit propagation energy, against yield strength. The superiority of sheet treated according to the present invention compared to the ingot metallurgy representatives is apparent. 
     It is to be noted that for a given alloy, the tradeoff between strength loss and toughness improvement is a function of time and temperature during the uniformizing treatment. 
     
                                           TABLE II                                
__________________________________________________________________________
Room Temperature Tensile and Fracture Toughness 1.60 mm (0.063 in.)       
Sheet                                                                     
 Sample No..sup.a                                                         
       Alloy                                                              
                 ##STR1##                                                 
                         ##STR2##                                         
                               ##STR3##                                   
                                    Elon- gation %                        
                                         ##STR4##                         
                                                 ##STR5##                 
__________________________________________________________________________
514295-2B                                                                 
      Al--8.3Fe--4.0Ce                                                    
                Control 508                                               
                           73.7                                           
                              546                                         
                                 79.1                                     
                                    6.8 70.9                              
                                            405.sup.b                     
                                                122.7                     
                                                     111.7 Yes            
554314                                                                    
      Al--8.3Fe--4.0Ce                                                    
                Control 523                                               
                           75.8                                           
                              575                                         
                                 83.4                                     
                                    10.0                                  
                                        68.9                              
                                            395                           
514388-2                                                                  
      Al--8.3Fe--4.0Ce                                                    
                Control 524                                               
                           76.0                                           
                              561                                         
                                 81.3                                     
                                    6.5 69.2                              
                                            395.sup.f                     
514412-BR                                                                 
      Al--8.3Fe--4.0Ce                                                    
                850  10 477                                               
                           69.2                                           
                              513                                         
                                 74.3                                     
                                    5.8 125.6                             
                                            715.sup.c                     
                                                180.8                     
                                                     164.5 No             
514413-1BR                                                                
      Al--8.3Fe--4.0Ce                                                    
                900  1  484                                               
                           70.2                                           
                              518                                         
                                 75.1                                     
                                    6.0 125.7                             
                                            720.sup.d                     
                                                191.2                     
                                                     174.0 No             
514408-2BR                                                                
      Al--8.3Fe--4.0Ce                                                    
                900  10 424                                               
                           61.6                                           
                              460                                         
                                 66.7                                     
                                    8.0 135.5                             
                                            775 168.1                     
                                                     153.0 No             
554311                                                                    
      Al--8.3Fe--4.0Ce                                                    
                850  60 432                                               
                           62.6                                           
                              483                                         
                                 70.0                                     
                                    10.0                                  
                                        135.5                             
                                            775 214.5                     
                                                     195.0 No             
514398-2T                                                                 
      Al--8.4Fe--7.0Ce                                                    
                Control 579                                               
                           84.1                                           
                              622                                         
                                 90.2                                     
                                    6.5 0   0.sup.g                       
514416-2TR                                                                
      Al--8.4Fe--7.0Ce                                                    
                900  1  519                                               
                           75.4                                           
                              549                                         
                                 79.6                                     
                                    8.2 117.3                             
                                            670.sup.e                     
                                                98.9 90.0  Yes            
      7075-T6   --   -- 517                                               
                           74.9                                           
                              568                                         
                                 82.3                                     
                                    11.2                                  
                                        50.7                              
                                            290 70.8 64.4  Yes            
      7075-T73  --   -- 416                                               
                           60.3                                           
                              494                                         
                                 71.6                                     
                                    10.6                                  
                                        89.2                              
                                            510 --   --                   
      2024-T81  --   -- 482                                               
                           69.8                                           
                              512                                         
                                 74.2                                     
                                    6.6 29.7                              
                                            170 --   --                   
      2024-T6   --   -- 367                                               
                           53.2                                           
                              464                                         
                                 67.2                                     
                                    9.2 48.1                              
                                            275 --   --                   
__________________________________________________________________________
 NOTES:                                                                   
 .sup.a All tests were done in the LT orientation. Sheet thickness varies 
 from 1.60 to 1.78 mm (0.063&#34; to 0.070&#34;) except 554311 which has a nominal
 thickness of 1.42 mm (0.056&#34;). Al--Fe--Ce tensile and tear test results  
 are averages of duplicate tests, Kc results are single tests. 7075 and   
 2024 results are averages of 2-10 tests.                                 
 .sup.b One of the duplicates underwent rapid &amp; diagonal fracture (UPE may
 be estimated and slightly high; included in average).                    
 .sup.c Both tests: diagonal fracture (tear strength and UPE may be       
 slightly high; included in average).                                     
 .sup.d One of the duplicates underwent diagonal fracture (tear strength  
 and UPE may be slightly high; included in average).                      
 .sup.e One of the duplicates underwent rapid fracture (UPE was estimated,
 but not included in average shown).                                      
 .sup.f One test: rapid and diagonal fracture  curve not reliable (energy 
 near zero; not included in average shown).                               
 .sup.g Crack growth was unstable.                                        
 .sup.h Invalidities are due to specimen size, i.e., specimen was not larg
 enough to provide enough recoverable elastic energy to produce unstable  
 crack growth in an elasticstress field.                                  
 Specimen Sizes:                                                          
 Tensile: Sheet thickness × 1.27 cm (0.5&#34;) wide specimen. Elongation
 was measured in 5.08 cm (2.0&#34;) gauge length.                             
 Tear Test: Kahntype, sheet thickness × 3.65 cm (1.44&#34;) × 5.72
 cm (2.25&#34;).                                                              
 Fracture Toughness: Centercrack, sheet thickness × 40.6 cm (16.0&#34;) 
 × 111.8 cm (44.0&#34;).                                                
 
    
     
                                           TABLE III                               
__________________________________________________________________________
Process Parameters Used To Roll 1.60 mm (0.063 in.) Al--Fe--Ce Sheet      
                    Extrusion                                             
        Rolling Temperature                                               
                    Thickness                                             
                            Sheet Thickness                               
Sample No.                                                                
        K.    F.    cm in.  mm  in.                                       
__________________________________________________________________________
514295-2B                                                                 
        589   600   4.72                                                  
                       1.86 1.59                                          
                                 0.0625                                   
554314  616/589                                                           
              650/600*                                                    
                    4.45                                                  
                       1.75 1.55                                          
                                0.061                                     
514388-2                                                                  
        589   600   2.51                                                  
                        0.988                                             
                            1.65                                          
                                0.065                                     
514412-BR                                                                 
        589   600   5.08                                                  
                       2.0  1.68                                          
                                0.066                                     
514413-1BR                                                                
        589   600   5.08                                                  
                       2.0  1.69                                          
                                 0.0665                                   
514408-2BR                                                                
        589   600   5.08                                                  
                       2.0  1.70                                          
                                0.067                                     
554311  616/589                                                           
              650/600*                                                    
                    4.45                                                  
                       1.75 1.37                                          
                                0.054                                     
514398-2T                                                                 
        589   600   4.65                                                  
                       1.83 1.54                                          
                                 0.0605                                   
514416-2TR                                                                
        589   600   4.76                                                  
                        1.875                                             
                            1.60                                          
                                0.063                                     
__________________________________________________________________________
 *Extrusions were heated to 616° K. (650° F.) for the first 
 rolling reductions and 589° K. (600° F.) for subsequent    
 reductions.                                                              
 
    
     Unless noted otherwise, percentages herein are on a weight basis. 
     While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.