Abstract:
Dispersion strengthened aluminum-cerium-manganese alloys containing from about 0.05 to about 23.0 weight percent cerium and about 0.03 to about 9.5 weight percent manganese exhibit mechanical properties that make them useful alloys as a result of age hardening for extended periods at temperatures between 350° C. (662° F.) and 450° C. (842° F.).

Description:
BACKGROUND 
       [0001]    Aluminum alloys are constantly being considered for fatigue critical applications in the aeropropulsion industry. Alloys such as 6061, 2024 or 7075 are well established and have been used for low temperature applications in both automotive and aerostructural applications for a long time. However, the useful temperature range for these materials is at or below 200° F. Attempts have been made to develop higher temperature aluminum based alloys including Al—Fe—Mo—V, Al—Fe—Si—V, and Al—Fe—Ce (hereafter referred to as “conventional dispersion strengthened materials”). These alloys have microstructures resulting in a good balance of properties at the subscale level. Unfortunately, their transition to a production scale resulted in a reduction of strength properties. This result was due to a number of factors, but was primarily driven by the need to go to higher temperatures during primary extrusion of consolidated precursor powder billets. The high temperatures required for primary extrusion of the conventional dispersion strengthened materials are a consequence of the fact that the strengthening second phase size is finest in the unextruded powder resulting in the material having the highest strength at that point. By going to higher temperatures, the strength can be lowered to allow commercial scale extrusion, but the higher temperatures can drive undesirable phase transformations and microstructural coarsening that lowers strength. Even when such phases do not transform, the longer heat up and soak times required for larger scale material production lead to coarsening of the strengthening phases and a concomitant lowering of the strength. 
       SUMMARY 
       [0002]    Unlike the conventional dispersion strengthened materials, aluminum-cerium-manganese alloys containing from about 0.05 to 23.0 weight percent cerium and about 0.03 to about 9.5 weight percent manganese exhibit mechanical properties that make them useful alloys as a result of age hardening. That is, rather than starting out hard (or strong) as with conventional dispersion strengthened materials, these alloys start out soft, and then are aged, like heat treatable alloys, to have the desired strength properties. 
         [0003]    In an embodiment, an age hardenable aluminum-cerium-manganese alloy, after gas-atomization, includes an aluminum solid solution containing a dispersion of the Al 20 CeMn 2  phases. After aging, these alloys contain an aluminum solid solution plus Al 11 Ce 3  and Al 12 Mn. 
         [0004]    These alloys exhibit an aging response after soaking at temperatures between 350° C. (662° F.) and 450° C. (842° F.). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a 500° C. (932° F.) isothermal section of the aluminum-cerium-chromium ternary phase diagram. 
           [0006]      FIG. 2  is a 500° C. (932° F.) isothermal section of the aluminum-cerium-manganese ternary phase diagram. 
           [0007]      FIG. 3  is a 450° C. (842° F.) isothermal section of the aluminum-cerium-manganese ternary phase diagram. 
           [0008]      FIG. 4  shows aging curves showing hardness as a function of time at various temperatures for an aluminum-cerium-manganese alloy of the invention. 
           [0009]      FIG. 5  is a plot showing the volume fraction of microstructural features formed during the aging treatments in each sample shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The present disclosure relates to developing a class of aluminum alloys that are soft in powder form and are therefore easily extruded at low temperatures, but which can be aged to have higher elevated temperature strength after extrusion, or in the final product form after all hot working operations are complete. 
         [0011]    The invention is based on a consideration of equilibrium phase diagrams for the aluminum-cerium-chromium and aluminum-cerium-manganese systems. A 500° C. isothermal section (isotherm) of the aluminum-cerium-chromium system is shown in  FIG. 1 . It is apparent that the aluminum rich corner of the aluminum-cerium-chromium diagram contains two three-phase regions, namely the Al—Al 45 Cr 7 —Al 20 CeCr 2  region and the Al—Al 4  Ce—Al 20 CeCr 2  region. This system is interesting from a microstructural design standpoint in that very little solute (Ce and Cr additions) is needed to obtain a high volume fraction of a second phase. With reference to the pseudo-binary between Al and Al 20 CeCr 2 , it is apparent that a low atomic percentage of solute is needed to obtain a high atomic fraction (and therefore volume fraction) of Al 20 CeCr 2 . 
         [0012]    The aluminum-cerium-manganese system of interest for the present invention is shown in  FIG. 2 . The system has useful similarities to the aluminum-cerium-chromium system as will be shown. 
         [0013]    With reference to  FIG. 2 , it can be seen that the aluminum rich corner of the aluminum-cerium-manganese diagram also has two three-phase regions; namely, the Al—Al 6 Mn—Al 8 CeMn 4  region and Al—Al 11 Ce 3 —Al 8 CeMn 4  region. The Al 8 CeMn 4  phase is not as close to the aluminum corner as the Al 20 CeCr 2  phase in  FIG. 1 . It should also be noted that the Al 20 CeMn 2  phase does not appear as an equilibrium phase on the aluminum-cerium-manganese phase diagram in  FIG. 2 . However, as mentioned above, this phase is the only phase present after atomization, and this is likely due to the similarities between Cr and Mn and the rapid solidification of the melt. Hence, the phase would not be obtained unless this family of alloys are rapidly solidified. This then, sets the stage for the phase transformations described further below. 
         [0014]    In an embodiment, an experimental Al—2.0Ce-5.0Mn (atomic percent) alloy close to the aluminum corner of the ternary diagram was prepared. A 450° C. (842° F.) isotherm of the aluminum-cerium-manganese ternary diagram is shown in  FIG. 3 . The composition of the inventive alloy is indicated by C. As discussed below, equilibrium Al 11 Ce 3  and metastable Al 12 Mn are phases that play prominent roles in the invention. The alloy was prepared using gas atomization, powder consolidation and extrusion to form a billet. The billet was sectioned into samples that were then subjected to aging anneals at temperatures up to 500° C. (932° F.). 
         [0015]    A preferred method of making the alloy of the present invention is discussed below. 
         [0000]    Step 1. Gas atomization of powder. Materials may be placed in a crucible and atomized to form powder particles. The cooling rate is preferably greater than 10 3 ° C. per second. Atomization may be preferably conducted at a pressure of at least 120-150 psi, and preferably at least 200 psi. One may use a gas content of 85 percent He-15 percent argon or other inert gas. An ideal gas content is 100 percent helium.
 
Step 2. Vacuum hot pressing of powder into billet. The powder is poured into an aluminum container and the container evacuated. The container may be heated to a temperature of 300 to 400° C. (572 to 752° F.). Pressure may be applied in the range of 10 ksi to 100 ksi.
 
Step 3. Extrude billet into bar stock. The billet from Step 2 may be extruded into bar stock at a temperature of 350 to 500° C. (662 to 932° F.). The extrusion ratio may be preferably greater than 10:1 for better material behavior and preferably from 10:1 to 25:1. For the aging study, samples were cut from the billet and aged for up to 48 hours at temperatures up to 500° C. (932° F.). Vickers hardness measurements were made on samples soaked for 1, 2, 8, 24, and 48 hours. The results are shown as hardness versus aging time in  FIG. 4 . The initial hardness is indicated to the left of the figure by the letter H. The aging temperatures and corresponding number for each curve in  FIG. 4  are as follows:
 
         [0000]                                            Curve Number   Aging Temperature                           20   300° C. (572° F.)           30   350° C. (662° F.)           40   400° C. (752° F.)           50   450° C. (842° F.)           60   500° C. (932° F.)                        
Samples aged at temperatures at or greater than 350° C. (662° F.) showed aging and a resulting increase in hardness. Hardnesses reached a peak and leveled off after about 10 hours at 400° C. (752° F.) and 450° C. (842° F.). A 500° C. (932° F.) aging temperature softened the alloy.
 
         [0016]    To provide insight into what is causing the increase in hardness, a plot of the volume fraction for each phase present after processing, and after 48 hours at each aging temperature is shown in  FIG. 5 . The aging curve numbers and corresponding phases are as follows: 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 Curve Number 
                 Phase 
               
               
                   
               
             
             
               
                 110 
                 Al 
               
               
                 120 
                 Al 20 CeMn 2   
               
               
                 130 
                 Al 6 Mn 
               
               
                 140 
                 Al 11 Ce 3   
               
               
                 150 
                 Al 12 Mn 
               
               
                   
               
             
          
         
       
     
         [0017]    The phase content at the different stages shown in  FIG. 5  is as follows: 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 As Formed Powder 
                 Al 20 CeMn 2   
               
               
                 Extruded billet 
                 Al 20 CeMn 2 , Al 6 Mn 
               
               
                 300° C. (572° F.) aged billet 
                 Al 20 CeMn 2 , Al 6 Mn, Al 11 Ce 3 , Al 12 Mn 
               
               
                 400° C. (752° F.) aged billet 
                 Al 11 Ce 3 , Al 12 Mn 
               
               
                   
               
             
          
         
       
     
         [0018]    The study showed Al 20 CeMn 2  formed during the initial powder formation and was gone after a 48-hour heat treatment at 400° C. (752° F.). Al 6 Mn formed during the extrusion and was gone after 48 hours at 400° C. (752° F.). Al 11 Ce 3  and Al 12 Mn formed during the aging and were present after 48 hours at 400° C. (752° F.). The results indicate that the inventive alloy is age hardenable and that the strengthening of Al 12 Mn and Al 11 Ce 3  are stable at temperatures at and above (350° C.) 662° F. 
         [0019]    The above microstructural analysis shows Al 12 Mn and Al 11 Ce 3  as stable phases in the microstructure. This suggests use of the “metastable” phase diagram shown in  FIG. 3 . The diagram shows a 450° C. (842° F.) isotherm of the aluminum-cerium-manganese phase diagram. The three phase field in the aluminum rich corner of the phase diagram consists of Al—Al 11 Ce 3 —Al 12 Mn in quasi-equilibrium. As noted earlier, the proximity of Al 12 Mn and Al 11 Ce 3  to the aluminum corner allows large amounts of second phase to be formed with relatively small amounts of solute additions. As evidenced in  FIG. 5 , the Al 12 Mn is present in an amount of 70 volume percent. The inventive composition used for these studies is shown by point C in the diagram of  FIG. 3 . 
         [0020]    During the heat treatment, the Al 20 CeMn 2  dissolves and is almost gone after 48 hours at (350° C.) 662° F. Al 6 Mn in the extruded billet is also almost gone after 48 hours at the same temperature. 
         [0021]    Precipitation of the intermetallic compounds Al 12 Mn and Al 11 Ce 3  result in age hardening as shown in  FIG. 4 . The aging curves showing the Vickers hardness as a function of time at each aging temperature show the alloys of the present invention are age hardenable at temperatures greater than (350° C.) 662° F. after 10 hours, but less than (500° C.) 932° F., which results in an immediate loss of hardness. 
         [0022]    The composition range for the alloys of the present invention may be found on the aluminum-cerium-manganese phase diagram in  FIG. 3 . Converting the atomic percent in the phase diagram to weight percent, the cerium may be in amounts ranging from 0.05 to about 23.0 weight percent. Preferably, the cerium may be in amounts of from 0.10 to about 10.0 weight percent. The manganese may be in amounts ranging from 0.03 to about 9.5 weight percent. Preferably the manganese may be in amounts from about 0.05 to about 4.0 weight percent. 
         [0023]    In an embodiment, the manganese to cerium ratio (using atomic %) may range from about 0.1 to about 10.0. Preferably the ratio may be from about 1.0 to about 3.0. 
         [0024]    The aging heat treatment temperatures may be between about (350° C.) 662° F. to about 500° F. (932° C.). Preferably the heat treatment temperatures may be between about (350° C.) 662° F. and about (450° C.) 842° F. The aging times may vary between 1 and 100 hours. Preferably the times are between about 1 and 48 hours. 
       Discussion of Possible Embodiments 
       [0025]    The following are non-exclusive descriptions of possible embodiments of the present invention. 
         [0026]    An age hardenable aluminum-cerium-manganese alloy may comprise about 0.05 to about 23.0 weight percent cerium; about 0.03 to about 9.5 weight percent manganese; and the balance substantially aluminum. 
         [0027]    The system of the preceding paragraph can optionally include, additionally and/or alternatively any, one or more of the following features, configurations and/or additional components: 
         [0028]    About 0.1 to about 10.0 weight percent cerium; about 0.5 weight percent manganese to about 4.0 weight percent manganese; and the balance substantially aluminum. 
         [0029]    The manganese to cerium ratio may be between about 0.1 to about 10.0. 
         [0030]    The alloy may be formed by rapid solidification processing. 
         [0031]    The alloy may comprise an aluminum solid solution matrix containing a plurality of Al 12 Mn, Al 11 Ce 3 , A l6 Mn, and Al 20 CeMn 2  as dispersed second phases. 
         [0032]    The alloy may comprise an aluminum matrix containing a plurality of Al 12 Mn and Al 11 Ce 3  following a heat treatment. 
         [0033]    The aging temperatures may be from about 300° C. (572° F.) to about 500° C. (932° F.). 
         [0034]    The aging temperatures may be from about 350° C. (662° F.) to about 450° C. (842° F.). 
         [0035]    The aging times may be from about 1 hour to about 100 hours. 
         [0036]    The aging times may be from about 1 hour to about 48 hours. 
         [0037]    An age hardenable aluminum-cerium-manganese alloy may comprise aluminum solid solution; dispersed Al 11 Ce 3  second phase; and dispersed Al 12 Mn phase. 
         [0038]    The alloy of the preceding paragraph can optionally include, additionally and/or alternatively, any, one or more of the following features, configurations and/or additional components: 
         [0039]    The alloy may have an operating temperature of between room temperature and 450° C. (842° F.). The alloy may comprise about 0.05 to about 23.0 weight percent cerium; about 0.03 to about 9.5 weight percent manganese; and the balance substantially aluminum. 
         [0040]    The manganese to cerium ratio may be between about 0.1 to about 10.0. 
         [0041]    The Vickers hardness at 450° C. (842° F.) may be between 40 and 300. The alloy may be formed by rapid solidification. 
         [0042]    The aging temperatures may be from about 300° C. (662° F.) to about 500° C. (932° F.). 
         [0043]    The aging temperatures may be from about 350° C. (662° F.) to about 450° C. (842° F.). 
         [0044]    A method of forming an age hardenable aluminum-cerium-manganese alloy may comprise: gas atomization to form powder wherein cooling is greater than 10 3 ° C. per second; vacuum hot pressing powder to form billet; and extruding billet into bar stock. 
         [0045]    The method of the preceding paragraph can optionally include, additionally and/or alternatively, any, one or more of the following features, configurations and/or additional components: 
         [0046]    The age hardenable aluminum-cerium-manganese alloy composition, may comprise: about 0.05 to about 23.0 weight percent cerium; about 0.03 to about 9.5 weight percent manganese; and the balance substantially aluminum. 
         [0047]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.