Patent Publication Number: US-2015059934-A1

Title: High-strength aluminum alloy thin extruded shape and method for producing the same

Description:
TECHNICAL FIELD 
     The invention relates to a high-strength aluminum alloy thin extruded shape and a method for producing the same. More specifically, the invention relates to an Al—Zn—Mg—Cu-based high-strength aluminum alloy thin extruded shape that may suitably be used for transport machines (e.g., airplane) and sporting goods (e.g., bat), and a method for producing the same. 
     BACKGROUND ART 
     A high-strength aluminum alloy (particularly an Al—Zn—Mg—Cu-based aluminum alloy) has been widely used as a material for transport machines (e.g., airplane, helicopter, and motorcycle) and sporting goods (e.g., bat), and development of an Al—Zn—Mg—Cu-based aluminum alloy thin extruded shape having a yield strength of 700 MPa or more has been desired in order to implement a further reduction in weight. 
     A method that produces an aluminum alloy by consolidating a rapidly solidified powder obtained using an atomization method has been proposed in order to improve the strength of an Al—Zn—Mg—Cu-based aluminum alloy extruded shape. For example, it has been known that the tensile strength can be increased to about 900 MPa by performing a T6 treatment on a formed body produced by a powder metallurgical process using an Al alloy rapidly solidified powder that includes 5 to 11% of Zn, 2 to 4.5% of Mg, 0.5 to 2% of Cu, and 0.01 to 0.5% of Ag, with the balance substantially being Al. 
     However, the industrial production process becomes complex, and the production cost increases when using a rapidly solidified powder. Therefore, a wrought material produced by rolling or extrusion has been used at the expense of strength. A round bar-like extruded shape for which high strength can be easily obtained may have a yield strength of 700 MPa or more. However, it has been difficult to obtain a high-strength thin extruded shape having a yield strength of 700 MPa or more. 
     RELATED-ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP-A-7-316601 
       
    
     Non-Patent Document 
     
         
         Non-patent Document 1: Journal of Japan Institute of Light Metals, Vol. 60, p. 75 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     When producing an Al—Zn—Mg—Cu-based aluminum alloy thin extruded shape having a thickness of 5 mm or less, the brass orientation tends to be predominant in the extrusion direction, and it is difficult to obtain a high strength of 700 MPa or more. Therefore, a high-strength thin extruded shape has been produced by extruding a round bar shape or a thick shape in which the P orientation is predominant and for which high strength can be obtained, and machining the resulting extruded shape. 
     The invention was conceived in view of the problems relating to an Al—Zn—Mg—Cu-based high-strength aluminum alloy thin extruded shape having a thickness of 5 mm or less. An object of the invention is to provide an Al—Zn—Mg—Cu-based high-strength aluminum alloy thin extruded shape having a yield strength of 700 MPa or more, and a method for producing the same. 
     Solution to Problem 
     According to a first aspect of the invention, a high-strength aluminum alloy thin extruded shape includes 9.0 to 13.0 mass % (hereinafter may be referred to as “%”) of Zn, 2.0 to 3.0% of Mg, 1.0 to 2.0% of Cu, and 0.05 to 0.3% of Zr, with the balance being Al and unavoidable impurities, fine precipitates having a circle equivalent diameter of 5 to 20 nm being dispersed in a crystal grain of the extruded shape in a number of 4000 to 6000 per μm 2 . 
     The high-strength aluminum alloy thin extruded shape according to the first aspect of the invention may have a yield strength of 700 MPa or more and an elongation of 9% or more. 
     According to a second aspect of the invention, a method for producing a high-strength aluminum alloy thin extruded shape includes casting an aluminum alloy to obtain an ingot, subjecting the ingot to a homogenization treatment and hot extrusion to obtain a hot-extruded shape, and subjecting the hot-extruded shape to a solution treatment, a quenching treatment, and an aging treatment, the aluminum alloy including 9.0 to 13.0% of Zn, 2.0 to 3.0% of Mg, 1.0 to 2.0% of Cu, and 0.05 to 0.3% of Zr, with the balance being Al and unavoidable impurities, and the aging treatment including a first-stage aging treatment, a second-stage aging treatment, and a third-stage aging treatment, the first-stage aging treatment holding the hot-extruded shape at 100 to 130° C. for 6 to 48 hours, and cooling the hot-extruded shape to room temperature, the second-stage aging treatment heating the hot-extruded shape to 160 to 180° C. at a temperature increase rate of 0.5° C./sec or more, holding the hot-extruded shape at 160 to 180° C. for 10 X  to 10 Y  minutes, and cooling the hot-extruded shape to room temperature at a cooling rate of 0.02° C./sec or more, and the third-stage aging treatment holding the hot-extruded shape at 100 to 130° C. for 6 to 48 hours, and cooling the hot-extruded shape to room temperature, provided that X=−0.03×holding temperature+5.11, and Y=−0.03×holding temperature+7.07. 
     Advantageous Effects of the Invention 
     The aspects of the invention thus provide an Al—Zn—Mg—Cu-based high-strength aluminum alloy thin extruded shape that make it possible to obtain a structure in which fine precipitates having a circle equivalent diameter of 5 to 20 nm are dispersed in the crystal grain in a number of 4000 to 6000 per μm 2 , and achieve a yield strength of 700 MPa or more and an elongation of 9% or more by performing the three-step aging treatment even when producing a thin extruded shape having a thickness of 5 mm or less in which the brass orientation tends to be predominant in the extrusion direction. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     The effects of each alloy component (element) of the aluminum alloy that forms the high-strength aluminum alloy thin extruded shape, and the reasons for which the content range of each alloy component is limited as described above, are described below. Zn forms an η′ phase and MgZn 2  together with Mg, and improves the strength of the aluminum alloy. The Zn content is preferably 9.0 to 13.0%. If the Zn content is less than 9.0%, the aluminum alloy may exhibit insufficient strength. If the Zn content exceeds 13.0%, a decrease in ductility may occur. 
     Mg forms an η′ phase and MgZn 2  together with Zn, and improves the strength of the aluminum alloy. The Mg content is preferably 2.0 to 3.0%. If the Mg content is less than 2.0%, the aluminum alloy may exhibit insufficient strength. If the Mg content exceeds 3.0%, a decrease in ductility may occur. 
     Cu improves the strength of the aluminum alloy. The Cu content is preferably 1.0 to 2.0%. If the Cu content is less than 1.0%, the aluminum alloy may exhibit insufficient strength. If the Cu content exceeds 2.0%, a decrease in ductility may occur. 
     Zr precipitates as Al 3 Zr, and suppresses recrystallization. Zr forms a fibrous structure, and improves the strength of the aluminum alloy. The Zr content is preferably 0.05 to 0.3%. If the Zr content is less than 0.05%, a decrease in strength may occur. If the Zr content exceeds 0.3%, coarse crystallized products may be produced during casting, and a decrease in ductility may occur. 
     Note that the aluminum alloy may include 0.30% or less of Si, 0.30% or less of Fe, and the like as unavoidable impurities. The aluminum alloy may include 0.05% or less of Ti and 0.01% or less of B so that the cast structure is refined. 
     The high-strength aluminum alloy thin extruded shape is produced by casting an aluminum alloy having the above composition (preferably by semi-continuous casting) to obtain an extrusion billet, subjecting the billet to a homogenization treatment (using a normal method) and hot extrusion to obtain a hot-extruded shape, and subjecting the hot-extruded shape to a solution treatment, a quenching treatment, and an aging treatment. The aging treatment includes a first-stage aging treatment, a second-stage aging treatment, and a third-stage aging treatment. 
     The first-stage aging treatment holds the hot-extruded shape at 100 to 130° C. for 6 to 48 hours, and cools the hot-extruded shape to room temperature. Sufficient precipitation occurs during the first-stage aging treatment. If the first-stage aging temperature is less than 100° C., sufficient precipitation may not occur. If the first-stage aging temperature exceeds 130° C., an η phase may precipitate, and a decrease in strength may occur. Note that the cooling rate when cooling the hot-extruded shape to room temperature does not affect the advantageous effects of the invention, and is not particularly limited. 
     The second-stage aging treatment heats the hot-extruded shape to 160 to 180° C. at a temperature increase rate of 0.5° C./sec or more, holds the hot-extruded shape at 160 to 180° C. for 10 X  to 10 Y  minutes, and cools the hot-extruded shape to room temperature at a cooling rate of 0.02° C./sec or more. The second-stage aging treatment is performed in order to redissolve the intragranular precipitates in the matrix. Note that X=−0.03×holding temperature+5.11, and Y=−0.03×holding temperature+7.07. If the holding time is less than 10 X  minutes, the intragranular precipitates may not be sufficiently redissolved, and a decrease in strength may occur. If the holding time exceeds 10 Y  minutes, a coarse η phase may precipitate, and a decrease in strength and ductility may occur. 
     If the second-stage aging temperature is less than 160° C., the precipitates may not be sufficiently dissolved. If the second-stage aging temperature exceeds 180° C., the heat treatment time may decrease, and industrial production may be difficult. If the temperature increase rate when heating the hot-extruded shape to 160 to 180° C. is less than 0.5° C./sec, an η phase may precipitate during heating, and a decrease in strength and ductility may occur. If the cooling rate when cooling the hot-extruded shape to room temperature is less than 0.02° C./sec, the precipitates may grow during cooling, and a decrease in strength and ductility may occur. 
     The third-stage aging treatment holds the hot-extruded shape at 100 to 130° C. for 6 to 48 hours, and cools the hot-extruded shape to room temperature. The η′ phase remains at the grain boundaries during the second-stage aging treatment, and the intragranular precipitates are almost completely dissolved to obtain a single matrix phase. The third-stage aging treatment is performed in order to cause reprecipitation of the η′ phase by heating the matrix phase to improve the strength of the aluminum alloy. If the third-stage aging temperature is less than 100° C., sufficient precipitation may not occur. If the third-stage aging temperature exceeds 130° C., an η phase may precipitate, and a decrease in strength may occur. Note that the cooling rate when cooling the hot-extruded shape to room temperature does not affect the advantageous effects of the invention, and is not particularly limited. 
     The high-strength aluminum alloy thin extruded shape has a 0.2% yield strength specified by ASTM E9 of 700 MPa or more. The high-strength aluminum alloy thin extruded shape exhibits a strength necessary for a reduction in weight even when the brass orientation is predominant in the extrusion direction. 
     EXAMPLES 
     The invention is further described below by way of examples and comparative examples to demonstrate the advantageous effects of the invention. Note that the following examples are provided for illustration purposes only, and the invention is not limited to the following examples. 
     Example 1 and Comparative Example 1 
     An aluminum alloy having the composition shown in Table 1 was melted, and cast using a semi-continuous casting method to obtain an extrusion billet having a diameter of 90 mm. The billet was homogenized at 470° C. for 10 hours, cooled from 470° C. to 250° C. in 48 minutes (average cooling rate: 250° C./h), and cooled to room temperature. The billet was heated to 420° C. in 5 minutes in an induction furnace, held for 1 minute, and hot-extruded to produce a sheet-like extruded shape having a thickness of 4 mm and a width of 60 mm. The exit-side extrusion speed during extrusion was set to 1 m/min. 
     The extruded shape was heated to 470° C. at a temperature increase rate of 50° C./h, held at 470° C. for 60 minutes, quenched in water at 20 to 30° C., held at 120° C. for 24 hours, and air-cooled to room temperature (cooling rate: 25° C./sec) (first-stage aging treatment). The extruded shape was then heated to 160° C. at a temperature increase rate of 3° C./sec, held at 160° C. for 120 minutes, and air-cooled to room temperature (cooling rate: 25° C./sec) (second-stage aging treatment). The extruded shape was held at 120° C. for 24 hours, and air-cooled to room temperature (cooling rate: 25° C./sec) (third-stage aging treatment) to obtain a specimen (Specimens 1 to 16). The holding time at the holding temperature during the second-stage aging treatment falls within 10 X  to 10 Y  minutes (X=−0.03×holding temperature+5.11, and Y=−0.03×holding temperature+7.07). In Table 1, the values that fall outside the scope of the invention are underlined. 
     The tensile properties and the number of fine precipitates were measured as described below using Specimens 1 to 16. The measurement results are shown in Table 2. In Table 2, the number of fine precipitates that falls outside the scope of the invention is underlined. The tensile properties that fall outside the scope of the invention are also underlined. 
     Measurement of Tensile Properties 
     A tensile specimen was prepared from the specimen in accordance with ASTM E9, and the tensile strength, the yield strength, and the elongation were measured. A case where the yield strength was 700 MPa or more and the elongation was 9% or more was evaluated as “Acceptable”. 
     Measurement of Number of Fine Precipitates 
     The center area (2 mm in the thickness direction, and 30 mm in the widthwise direction) of the cross section (perpendicular to the extrusion direction) of the specimen was observed using a TEM (“JEM-2010” manufactured by JEOL Ltd.) (magnification: 50,000). The number (density) (per μm 2 ) of fine precipitates having a circle equivalent diameter of 5 to 20 nm that were observed as a dark contrast in a bright-field image was calculated. The specimen was observed in three fields of view (18*10 4  nm 2 /field of view), and the average value was employed. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Alloy 
                 Zn 
                 Mg 
                 Cu 
                 Zr 
                 Al 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 A 
                 9.0 
                 2.37 
                 1.47 
                 0.16 
                 Bal 
               
               
                   
                 B 
                 12.9 
                 2.36 
                 1.48 
                 0.16 
                 Bal 
               
               
                   
                 C 
                 10.4 
                 2.08 
                 1.52 
                 0.16 
                 Bal 
               
               
                   
                 D 
                 10.2 
                 2.98 
                 1.52 
                 0.16 
                 Bal 
               
               
                   
                 E 
                 10.1 
                 2.41 
                 1.03 
                 0.15 
                 Bal 
               
               
                   
                 F 
                 10.1 
                 2.42 
                 1.91 
                 0.15 
                 Bal 
               
               
                   
                 G 
                 9.8 
                 2.36 
                 1.32 
                 0.07 
                 Bal 
               
               
                   
                 H 
                 9.9 
                 2.36 
                 1.34 
                 0.28 
                 Bal 
               
               
                   
                 I 
                 8.8 
                 2.36 
                 1.50 
                 0.16 
                 Bal 
               
               
                   
                 J 
                 13.5 
                 2.40 
                 1.47 
                 0.16 
                 Bal 
               
               
                   
                 K 
                 9.9 
                 1.97 
                 1.46 
                 0.15 
                 Bal 
               
               
                   
                 L 
                 10.1 
                 3.04 
                 1.48 
                 0.15 
                 Bal 
               
               
                   
                 M 
                 9.7 
                 2.44 
                 0.98 
                 0.15 
                 Bal 
               
               
                   
                 N 
                 9.9 
                 2.46 
                 2.03 
                 0.15 
                 Bal 
               
               
                   
                 O 
                 9.7 
                 2.33 
                 1.37 
                 0.02 
                 Bal 
               
               
                   
                 P 
                 9.9 
                 2.31 
                 1.31 
                 0.35 
                 Bal 
               
               
                   
                   
               
               
                   
                 Note: 
               
               
                   
                 The unit for the content of each component is mass %. 
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Tensile 
                 Yield 
                   
                 Number of fine 
               
               
                   
                   
                 strength 
                 strength 
                 Elongation 
                 precipitates 
               
               
                 Specimen 
                 Alloy 
                 (MPa) 
                 (MPa) 
                 (%) 
                 (per μm 2 ) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 A 
                 731 
                 713 
                 10 
                 5246 
               
               
                 2 
                 B 
                 740 
                 729 
                 9 
                 5363 
               
               
                 3 
                 C 
                 720 
                 705 
                 13 
                 5187 
               
               
                 4 
                 D 
                 744 
                 731 
                 11 
                 5378 
               
               
                 5 
                 E 
                 724 
                 716 
                 12 
                 5231 
               
               
                 6 
                 F 
                 718 
                 710 
                 13 
                 5224 
               
               
                 7 
                 G 
                 711 
                 707 
                 13 
                 5202 
               
               
                 8 
                 H 
                 721 
                 711 
                 9 
                 5231 
               
               
                 9 
                 I 
                 704 
                 680 
                 15 
                 3400 
               
               
                 10 
                 J 
                 744 
                 734 
                 4 
                 6166 
               
               
                 11 
                 K 
                 723 
                 690 
                 14 
                 3450 
               
               
                 12 
                 L 
                 728 
                 720 
                 2 
                 6048 
               
               
                 13 
                 M 
                 703 
                 682 
                 12 
                 3410 
               
               
                 14 
                 N 
                 737 
                 730 
                 5 
                 6132 
               
               
                 15 
                 O 
                 701 
                 627 
                 15 
                 3135 
               
               
                 16 
                 P 
                 731 
                 719 
                 6 
                 6040 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, Specimens 1 to 8 that fall within the scope of the invention had a structure in which fine precipitates having a circle equivalent diameter of 5 to 20 nm were dispersed in the crystal grains in a number of 4000 to 6000 per μm 2 . Specimens 1 to 8 had a yield strength of 700 MPa or more and an elongation of 9% or more (i.e., exhibited excellent strength and ductility). 
     On the other hand, Specimens 9 to 16 that fall outside the scope of the invention had a yield strength of less than 700 MPa or an elongation of less than 9%. Specimen 9 had inferior yield strength since the Zn content was too low, and a sufficient strength improvement effect could not be obtained. Specimen 10 showed insufficient elongation since the Zn content was too high, and grain boundary precipitation occurred. Specimen 11 had inferior yield strength since the Mg content was too low, and a sufficient strength improvement effect could not be obtained. Specimen 12 showed insufficient elongation since the Mg content was too high, and grain boundary precipitation occurred. 
     Specimen 13 had inferior yield strength since the Cu content was too low, and a sufficient strength improvement effect could not be obtained. Specimen 14 showed insufficient elongation since the Cu content was too high, and grain boundary precipitation occurred. Specimen 15 had inferior yield strength since the Zr content was too low (i.e., a recrystallized structure was formed), and a sufficient strength improvement effect could not be obtained. Specimen 16 showed insufficient elongation since the Zr content was too high, and a decrease in ductility occurred due to coarse crystallized products. 
     Example 2 
     An aluminum alloy having the composition shown in Table 3 was melted, and cast using a semi-continuous casting method to obtain an extrusion billet having a diameter of 90 mm. The billet was homogenized at 470° C. for 10 hours, cooled from 470° C. to 250° C. in 48 minutes (average cooling rate: 250° C./h), and cooled to room temperature. The billet was heated to 420° C. in 5 minutes in an induction furnace, held for 1 minute, and hot-extruded to produce a sheet-like extruded shape having a thickness of 4 mm and a width of 60 mm. The exit-side extrusion speed during extrusion was set to 1 m/min. 
     The extruded shape was heated to 470° C. at a temperature increase rate of 50° C./h, held at 470° C. for 60 minutes, quenched in water at 20 to 30° C., and subjected to the first-stage aging treatment, the second-stage aging treatment, and the third-stage aging treatment under the conditions (a1 to a13) shown in Table 4 to obtain a specimen (Specimens 17 to 29). In the first-stage aging treatment, the extruded shape was air-cooled from the holding temperature to room temperature (cooling rate: 25° C./sec). In the third-stage aging treatment, the extruded shape was air-cooled from the holding temperature to room temperature (cooling rate: 25° C./sec). The holding time at the holding temperature during the second-stage aging treatment falls within 10 X  to 10 Y  minutes (X=−0.03×holding temperature+5.11, and Y=−0.03×holding temperature+7.07). 
     The tensile properties and the number of fine precipitates were measured in the same manner as in Example 1 using Specimens 17 to 29. The measurement results are shown in Table 5. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Zn 
                 Mg 
                 Cu 
                 Zr 
                 Al 
               
               
                   
               
             
            
               
                 9.74 
                 2.30 
                 1.34 
                 0.16 
                 Bal 
               
               
                   
               
               
                 Note: 
               
               
                 The unit for the content of each component is mass %. 
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 First-stage  
                   
                   
               
               
                   
                 aging treatment 
                 Second-stage aging treatment 
                 Third-stage aging treatment 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Holding 
                   
                 Temperature 
                 Holding 
                 Holding time 
                   
                 Holding 
                   
               
               
                 Aging treatment 
                 temperature 
                 Holding time 
                 increase rate 
                 temperature 
                 (min) 
                 Cooling rate 
                 temperature 
                 Holding time 
               
               
                 conditions 
                 (° C.) 
                 (h) 
                 (° C./sec) 
                 (° C.) 
                 *(sec) 
                 (° C./sec) 
                 (° C.) 
                 (h) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 a1 
                 120 
                 24 
                 3 
                 160 
                  2 
                 25 
                 120 
                 24 
               
               
                 a2 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 a3 
                 120 
                 24 
                 3 
                 160 
                 180 
                 25 
                 120 
                 24 
               
               
                 a4 
                 120 
                 24 
                 3 
                 170 
                  1 
                 25 
                 120 
                 24 
               
               
                 a5 
                 120 
                 24 
                 3 
                 170 
                  30 
                 25 
                 120 
                 24 
               
               
                 a6 
                 120 
                 24 
                 3 
                 170 
                  90 
                 25 
                 120 
                 24 
               
               
                 a7 
                 120 
                 24 
                 3 
                 180 
                  31* 
                 25 
                 120 
                 24 
               
               
                 a8 
                 120 
                 24 
                 3 
                 180 
                  15 
                 25 
                 120 
                 24 
               
               
                 a9 
                 120 
                 24 
                 3 
                 180 
                  30 
                 25 
                 120 
                 24 
               
               
                 a10 
                 100 
                 6 
                 3 
                 160 
                 120 
                 25 
                 100 
                 6 
               
               
                 a11 
                 100 
                 6 
                 3 
                 160 
                 120 
                 25 
                 130 
                 48 
               
               
                 a12 
                 130 
                 48 
                 3 
                 160 
                 120 
                 25 
                 100 
                 6 
               
               
                 a13 
                 130 
                 48 
                 3 
                 160 
                 120 
                 25 
                 130 
                 48 
               
               
                   
               
               
                 Note: 
               
               
                 The second-stage aging holding time under the aging treatment conditions a7 was 31 seconds. 
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Aging 
                 Tensile 
                 Yield 
                   
                 Number of fine 
               
               
                   
                 treatment 
                 strength 
                 strength 
                 Elongation 
                 precipitates 
               
               
                 Specimen 
                 conditions 
                 (MPa) 
                 (MPa) 
                 (%) 
                 (per μm 2 ) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 17 
                 a1 
                 735 
                 703 
                 13 
                 5202 
               
               
                 18 
                 a2 
                 730 
                 711 
                 12 
                 5231 
               
               
                 19 
                 a3 
                 734 
                 724 
                 10 
                 5358 
               
               
                 20 
                 a4 
                 739 
                 708 
                 13 
                 5239 
               
               
                 21 
                 a5 
                 729 
                 710 
                 11 
                 5181 
               
               
                 22 
                 a6 
                 726 
                 719 
                 11 
                 5321 
               
               
                 23 
                 a7 
                 734 
                 708 
                 12 
                 5239 
               
               
                 24 
                 a8 
                 730 
                 714 
                 10 
                 5212 
               
               
                 25 
                 a9 
                 735 
                 726 
                 9 
                 5227 
               
               
                 26 
                  a10 
                 721 
                 708 
                 11 
                 5168 
               
               
                 27 
                  a11 
                 731 
                 720 
                 10 
                 5328 
               
               
                 28 
                  a12 
                 719 
                 703 
                 10 
                 5132 
               
               
                 29 
                  a13 
                 723 
                 714 
                 9 
                 5141 
               
               
                   
               
            
           
         
       
     
     As shown in Table 5, Specimens 17 to 29 that fall within the scope of the invention had a structure in which fine precipitates having a circle equivalent diameter of 5 to 20 nm were dispersed in the crystal grains in a number of 4000 to 6000 per μm 2 . Specimens 17 to 29 exhibited a yield strength of 700 MPa or more and an elongation of 9% or more (i.e., exhibited excellent strength and ductility). 
     Comparative Example 2 
     A sheet-like extruded shape having a thickness of 4 mm and a width of 60 mm was produced in the same manner as in Example 2 using an aluminum alloy having the composition shown in Table 3. The extruded shape was heated to 470° C. at a temperature increase rate of 50° C./h, held at 470° C. for 60 minutes, quenched in water at 20 to 30° C., and subjected to the first-stage aging treatment, the second-stage aging treatment, and the third-stage aging treatment under the conditions (b1 to b26) shown in Table 6 to obtain a specimen (Specimens 30 to 55). In the first-stage aging treatment, the extruded shape was air-cooled from the holding temperature to room temperature (cooling rate: 25° C./sec). In the third-stage aging treatment, the extruded shape was air-cooled from the holding temperature to room temperature (cooling rate: 25° C./sec). In Table 6, the values that fall outside the scope of the invention are underlined. 
     The tensile properties and the number of fine precipitates were measured in the same manner as in Example 1 using Specimens 30 to 55. The measurement results are shown in Table 7. In Table 7, the number of fine precipitates that falls outside the scope of the invention is underlined. The tensile properties that fall outside the scope of the invention are also underlined. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                   
                 First-stage  
                   
                   
               
               
                   
                 aging treatment 
                 Second-stage aging treatment 
                 Third-stage aging treatment 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Holding 
                   
                 Temperature 
                 Holding 
                   
                   
                 Holding 
                   
               
               
                 Aging treatment 
                 temperature 
                 Holding time 
                 increase rate 
                 temperature 
                 Holding time 
                 Cooling rate 
                 temperature 
                 Holding time 
               
               
                 conditions 
                 (° C.) 
                 (h) 
                 (° C./sec) 
                 (° C.) 
                 (min) 
                 (° C./sec) 
                 (° C.) 
                 (h) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 b1 
                 120 
                 24 
                 3 
                 
                   150 
                 
                 30 
                 25 
                 120 
                 24 
               
               
                 b2 
                 120 
                 24 
                 3 
                 
                   150 
                 
                 90 
                 25 
                 120 
                 24 
               
               
                 b3 
                 120 
                 24 
                 3 
                 160 
                   1   
                 25 
                 120 
                 24 
               
               
                 b4 
                 120 
                 24 
                 3 
                 160 
                 
                   240 
                 
                 25 
                 120 
                 24 
               
               
                 b5 
                 120 
                 24 
                 3 
                 170 
                     0.5   
                 25 
                 120 
                 24 
               
               
                 b6 
                 120 
                 24 
                 3 
                 170 
                 
                   120 
                 
                 25 
                 120 
                 24 
               
               
                 b7 
                 120 
                 24 
                 3 
                 180 
                     0.20   
                 25 
                 120 
                 24 
               
               
                 b8 
                 120 
                 24 
                 3 
                 180 
                   50   
                 25 
                 120 
                 24 
               
               
                 b9 
                 120 
                 24 
                 
                   0.4 
                 
                 170 
                  10 
                 25 
                 120 
                 24 
               
               
                 b10 
                 120 
                 24 
                 3 
                 170 
                  10 
                     0.01   
                 120 
                 24 
               
               
                 b11 
                 100 
                   5   
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b12 
                 100 
                 
                   72 
                 
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b13 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                 100 
                   5   
               
               
                 b14 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                 100 
                 
                   72 
                 
               
               
                 b15 
                 130 
                   5   
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b16 
                 130 
                 
                   52 
                 
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b17 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                 130 
                   5   
               
               
                 b18 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                 130 
                 
                   52 
                 
               
               
                 b19 
                   90   
                  6 
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b20 
                   90   
                 48 
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b21 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                   90   
                  6 
               
               
                 b22 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                   90   
                 48 
               
               
                 b23 
                 
                   140 
                 
                  6 
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b24 
                 
                   140 
                 
                 48 
                 3 
                 160 
                 120 
                 25 
                 120 
                 24 
               
               
                 b25 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                 
                   140 
                 
                  6 
               
               
                 b26 
                 120 
                 24 
                 3 
                 160 
                 120 
                 25 
                 
                   140 
                 
                 48 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                 Aging 
                 Tensile 
                 Yield 
                   
                 Number of fine 
               
               
                   
                 treatment 
                 strength 
                 strength 
                 Elongation 
                 precipitates 
               
               
                 Specimen 
                 conditions 
                 (MPa) 
                 (MPa) 
                 (%) 
                 (per μm 2 ) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 30 
                 b1  
                 715 
                 688 
                 11 
                 2752 
               
               
                 31 
                 b2  
                 720 
                 692 
                 10 
                 3114 
               
               
                 32 
                 b3  
                 720 
                 690 
                 12 
                 3450 
               
               
                 33 
                 b4  
                 720 
                 695 
                 7 
                 6090 
               
               
                 34 
                 b5  
                 730 
                 697 
                 14 
                 3485 
               
               
                 35 
                 b6  
                 709 
                 694 
                 8 
                 6038 
               
               
                 36 
                 b7  
                 722 
                 696 
                 12 
                 3480 
               
               
                 37 
                 b8  
                 723 
                 692 
                 7 
                 3555 
               
               
                 38 
                 b9  
                 719 
                 688 
                 8 
                 3440 
               
               
                 39 
                 b10 
                 721 
                 685 
                 7 
                 3425 
               
               
                 40 
                 b11 
                 707 
                 692 
                 7 
                 3460 
               
               
                 41 
                 b12 
                 718 
                 697 
                 9 
                 6309 
               
               
                 42 
                 b13 
                 709 
                 686 
                 9 
                 3499 
               
               
                 43 
                 b14 
                 701 
                 677 
                 10 
                 6228 
               
               
                 44 
                 b15 
                 699 
                 683 
                 9 
                 3620 
               
               
                 45 
                 b16 
                 712 
                 691 
                 9 
                 6288 
               
               
                 46 
                 b17 
                 703 
                 688 
                 10 
                 3715 
               
               
                 47 
                 b18 
                 704 
                 671 
                 9 
                 6240 
               
               
                 48 
                 b19 
                 690 
                 670 
                 11 
                 3424 
               
               
                 49 
                 b20 
                 705 
                 691 
                 10 
                 6634 
               
               
                 50 
                 b21 
                 693 
                 674 
                 10 
                 3444 
               
               
                 51 
                 b22 
                 708 
                 698 
                 9 
                 6701 
               
               
                 52 
                 b23 
                 693 
                 671 
                 10 
                 3429 
               
               
                 53 
                 b24 
                 672 
                 647 
                 9 
                 6230 
               
               
                 54 
                 b25 
                 679 
                 660 
                 10 
                 3373 
               
               
                 55 
                 b26 
                 681 
                 651 
                 9 
                 6250 
               
               
                   
               
            
           
         
       
     
     As shown in Table 7, Specimens 30 to 55 that fall outside the scope of the invention had a yield strength of less than 700 MPa and/or an elongation of less than 9%. Specimens 30 and 31 had inferior yield strength since the second-stage aging temperature was low (i.e., fine precipitates were not sufficiently redissolved), and sufficient precipitation hardening did not occur during the third-stage aging treatment. 
     Specimens 32, 34, and 36 had inferior yield strength since the holding time during the second-stage aging treatment was short (i.e., redissolution of the η′ phase did not proceed), and sufficient precipitation hardening did not occur during the third-stage aging treatment. Specimens 33, 35, and 37 had inferior ductility since the holding time during the second-stage aging treatment was long (i.e., precipitation of a coarse η phase occurred during heating), and had inferior yield strength since sufficient precipitation hardening did not occur during the third-stage aging treatment. 
     Specimen 38 had inferior ductility since the temperature increase rate during the second-stage aging treatment was low (i.e., precipitation of a coarse η phase occurred during heating), and had inferior yield strength since sufficient precipitation hardening did not occur during the third-stage aging treatment. Specimen 39 had inferior ductility since the cooling rate during the second-stage aging treatment was low (i.e., precipitation of a coarse η phase occurred during cooling), and had inferior yield strength since sufficient precipitation hardening did not occur during the third-stage aging treatment. 
     Specimen 40 had inferior yield strength since the holding time during the first-stage aging treatment was short, and sufficient precipitation hardening did not occur. Specimen 42 had inferior yield strength since the holding time during the first-stage aging treatment was long, and a coarse phase was formed. Specimen 42 had inferior yield strength since the holding time during the third-stage aging treatment was short, and sufficient precipitation hardening did not occur. Specimen 43 had inferior yield strength since the holding time during the third-stage aging treatment was long, and a coarse η phase was formed. 
     Specimen 44 had inferior yield strength since the holding time during the first-stage aging treatment was short, and sufficient precipitation hardening did not occur. Specimen 45 had inferior yield strength since the holding time during the first-stage aging treatment was long, and a coarse η phase was formed. Specimen 46 had inferior yield strength since the holding time during the third-stage aging treatment was short, and sufficient precipitation hardening did not occur. Specimen 47 had inferior yield strength since the holding time during the third-stage aging treatment was long, and a coarse η phase was formed. 
     Specimens 48 and 49 had inferior yield strength since the holding temperature during the first-stage aging treatment was low, and sufficient precipitation hardening did not occur. Specimens 50 and 51 had inferior yield strength since the holding temperature during the third-stage aging treatment was low, and sufficient precipitation hardening did not occur. Specimens 52 and 53 had inferior yield strength since the holding temperature during the first-stage aging treatment was high, and sufficient precipitation hardening did not occur. Specimens 54 and 55 had inferior yield strength since the holding temperature during the third-stage aging treatment was high, and sufficient precipitation hardening did not occur.