Effect of low temperature aging on microstructure and ...

Vol . 13 　No . 5 　　　 　　　　 J. CENT. SOUTH UNIV. TECHNOL. 　　　　　　　Oct . 2006 Article ID : 1005

9784 ( 2006 ) 05

0461

07

Effect of low temperature aging on microstructure and ① mechanical properties of super2high strength aluminum alloy F EN G Chun ( 冯 　春) 1 , L IU Zhi2yi ( 刘志义) 1 , N IN G Ai2ling ( 宁爱林) 1 , ZEN G Su2min ( 曾苏民) 1 , 2 ( 1. School of Material s Science and Engineering , Cent ral So ut h U niversit y , Changsha 410083 , China ; 2. So ut hwestern Aluminum ( Gro up ) Co It d , Cho ngqing 401326 , China) Abstract : The effect s of heat t reatment o n t he micro st ruct ure and mechanical p roperties of two alloys , namely Al2 12. 2 %Zn22. 48 %Cu22. 0 %Mg20. 15 %Zr20. 166 %Ag ( alloy 1) , and Al29. 99 %Zn21. 72 %Cu22. 5 %Mg20. 13 %Zr ( al2 loy 2) were investigated. The result s show t hat low temperat ure aging after p romotive solutio n t reat ment can in2 crease elongation wit hout t he lo ss of st rengt h fo r t he st udied alloys. The optimum aging t reat ment ( T6) fo r alloy 1 and alloy 2 is 100 ℃/ 80 h and 100 ℃/ 48 h , respectively. Compared with ot her heat t reat ment alloys , alloy 1 and al2 loy 2 show super2high tensile st rengt h up to 753 MPa and 788 MPa , remaining 9. 3 % and 9. 7 % elo ngatio n under T6 co ndition , respectively. During aging , t race addition of Ag enhances t he fo rmations of GP zone and metastable p hase , and stabilizes GP zo ne and metastable p hase to a higher temperat ure. Trace additio n of Ag p rolongs t he aging time of reaching t he peak st rengt h and delays over2aging conditio n of t he alloy. However , t race additio n of Ag p ro2 motes t he formatio n of coarse co nstit uent in t he alloy and co nsumes hardening alloying element s of Zn and Mg. Mo2 reover , t he additio n of t he t ransition element Zr in 7000 series super2high alloy fo rms inco herent Al 3 Zr dispersoid which can serve as nucleatio n sites for no nuniform p recipitation of η p hase during aging p rocess. The higher t he ag2 ing temperat ure , t he greater the tendency fo r nonunifo rm p recipitatio n of η p hase. Key words : Al2Zn2Mg2Cu alloy ; low temperat ure aging ; micro st ruct ure ; mechanical p roperties ; silver CLC number : T G146. 2 Document code : A

1 　INTROD UCTION Over t he year s , Al2Zn2Mg2Cu series age2 hardening super2high st rengt h aluminum alloys were widely used as aircraf t st ruct ural material s due to t heir low densit y , high st rengt h [ 1 ] . Fro m 7075 ( 75S) to 7055 , t he co ntent of hardening allo2 ying element s is higher and higher up to 11 % 13 % , and t he p urit y of t he alloy al so increases. For t he super2high st rengt h Al2Zn2Mg2Cu alloy , t he mass ratio of Zn to Mg has a p ro no unced effect o n t he mechanical p roperties t hat is ascribed to it s influence o n t he formatio n of p hase and kinetics of p recipitatio n in alloy. Increasing t he co ntent of Zn and Mg ( wit hin t he solubilit y limit ) can elevate t he st rengt h of alloy. However , wit h t he increase of alloying element s , it is difficult to cast ingot by using normal ingot metallurgy casting due to t he segregatio n of alloying element s. Meanwhile many coar ser co nstit uent s are formed because of interac2 tio n of t he alloying element s wit h imp urities such as Fe and Si , and t he t ransformatio n of no nequilib2 rium p hase during casting. These coar ser particles are sites for st ress co ncent ratio n and microcrack in2 itiatio n during ho mogenizatio n , deformatio n , and

①

solutio n heat t reat ment which may do harm to t he f ract ure to ughness , resistance to fatigue and st ress corro sio n cracking [ 2 3 ] . Many new low2densit y and high st rengt h powder metallurgy aluminum alloys have been developed , such as PM/ 7090 , PM7091 , CW67. All t he alloys have t he similar st rengt h to IM/ 70752 T6 and similar resistance to st ress corro2 sio n cracking to IM/ 70752 T73. But t heir high co st limit s t heir applicatio n in aero space [ 4 ] . In t he past 10 years , t he develop ment of elect rical magnetic casting ( EMC) has made it po ssible to cast ingot by co nventio nal metallurgy casting. EMC can refine grain size , imp rove t he ingot surface , reduce t he segregatio n of alloying element s , and increase t he solubilit y of alloying element s as well [ 5 ] . It is fo und t hat t he high2zinc super2high st rengt h aluminum alloy p repared by low f requency elect ric metallurgical casting can acquire high st rengt h after aging at 120 ℃[ 6 ] . However , t he elo ngatio n appear s to decrease o bvio usly. In order to resolve t his p ro blem , t he influence of low tem2 perat ure aging o n t he mechanical p roperties and t he evolutio n of micro st ruct ure of t he st udied alloy were investigated in t his paper.

Foundation item : Project (2001AA332030) supported by t he National High Technology Research and Develop ment Program of China Received date : 2005

12

26 ; Accepted date : 2006

03

10

Correspondence :L IU Zhi2yi , Professor ; Tel : + 86273128836927 ; E2mail : liuzhiyi @mail . csu. edu. cn

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ly liquid nit rogen was used to cool t he specimen. 2 　EXPERIMENTAL The cast ingot s of t wo employed alloys in t he p resent st udy were made by low f requency elect ro 2 magnetic casting and t heir chemical co mpo sitio ns are listed in Table 1. The ingot s were ho mogenized at 430 ℃ for 6 h , 455 ℃ for 16 h and t hen hot squeezed into cylindrical ro d of 12 mm in diameter . The ext rusio n ratio was 17. 5. According to t he p revio us research result s[ 6 ] , p ro motive solutio n t reat ment s were applied to t he t wo alloys as 450 ℃/ 2 h + 470 ℃/ 1 h. Subsequently alloy 1 was quenched in cold water and aged at 120 ℃, 110 ℃, 100 ℃ for vario us time. According to t he test re2 sult of mechanical p roperties of alloy 1 , it was de2 termined t hat alloy 2 were quenched in cold water and aged at 100 ℃ for vario us time. The test of mechanical p ropert y was performed o n a univer sal tensile testing machine of CSS 44100 t ype. J SM 5600L V scanning elect ro n micro scope wit h energy dispersive analyzer were used for t he micro st ruc2 t ure inspectio n and t he determinatio n of t he co n2 stit uent element s in undissolved p hases. Transmis2 sio n elect ro n micro scop y ( T EM ) was performed o n a Tecnai G2 20 micro scope at 200 kV . The T EM specimens were p repared by mechanically grinding each side of t he sample to abo ut 10 μm in t hick2 ness , f urt her t hinning t he sample to a certain t hickness and t hen t he elect ro n t ransparency was carried o ut using accelerating voltage of 12 15 V and current of 70 90 mA , respectively , and final2

Table 1 　Chemical co mpo sitio ns of ingot made by low f requency elect ro magnetic casting ( mass f ractio n , %) Sample

Zn

Mg

Cu

Ag

Zr

Al

Alloy 1

12. 20

2. 48

2. 00

0. 166

0. 15

Bal

Alloy 2

9. 99

2. 50

1. 72

-

0. 13

Bal

3 　RESUL TS AND D ISCUSSION The optical micrograp hs of ally 1 and alloy 2 after t he hot2ext rusio n and p ro motive solutio n t reat ment are shown in Fig. 1. Co mpared wit h hot2 ext rusio n , bot h alloys display partially recrystal2 lized st ruct ures wit h elo ngated recrystallized grains ( light regio ns ) aligning alo ng t he ext rusio n direc2 tio n after p ro motive solutio n t reat ment . The coar se particles in t he alloy al so align alo ng t he ex2 t rusio n directio n af ter hot2ext rusio n. A nd co m2 pared wit h t he dist ributio n and morp hology of t he undissolved p hases of bot h alloys af ter p ro motive solutio n t reat ment , t he silver2bearing alloy has a larger number of co nstit uent particles t han t he sil2 ver2f ree o ne. By tensile test s , t he mechanical p roperties of alloy 1 af ter being aged at : 120 ℃, 110 ℃ and 100 ℃ are shown in Fig. 2. The ultimate tensile st rengt h of t he as2quenched alloy 1 is relatively low ( 579 M Pa) . At t he initial stage of aging ( 1 h) , t he

Fig. 1 　Optical micrograp hs of alloys after ext rusio n and p ro motive solutio n t reat ment (a) —Alloy 1 after ext rusio n ; (b) —Alloy 1 after p ro motive solution t reat ment ; ( c) —Alloy 2 after ext rusion ; ( d) —Alloy 2 after p romotive solutio n t reat ment

FEN G Chun , et al : Effect of low temperat ure aging on micro st ruct ure and mechanical p roperties of aluminum alloy

Fig. 2 　Mechanical properties curves of alloy 1 after aging treatments at different temperatures (a) —Tensile strength curve ; ( b) —Elongation curve 1 —100 ℃; 2 —110 ℃; 3 —120 ℃;

tensile st rengt h increases up to 710 M Pa ( aging at 120 ℃) rapidly , which show s t hat t he alloy has st ro ng age2hardening abilit y. Aging hardening re2 spo nse rate is in sequence f ro m fast to slow as fol2 low s : 120 ℃, 110 ℃, 100 ℃. When being aged at 120 ℃, t he peak st rengt h appear s af ter being aged for 8 h , up to 743 M Pa. A nd t he aging hardening curve does not show o bvio us decreasing t rend until after being aged fo r 60 h. This behavior may be due to t he t race additio n of Ag which p rolo ngs t he p rocess of t he alloy to rep resent t he overage co ndi2 tio n. Aging at 110 ℃, t he peak st rengt h appear s after aging for 36 h , and t he co rrespo nding tensile st rengt h is up to 748 M Pa. The peak st rengt h is fo und after being aged for 80 h at 100 ℃, to be up to 753 M Pa. A st rengt h platform can be seen after extending aging time to 16 h , lasting until 96 h no o bvio us overage characterizatio n appear s. Co m2 pared wit h aging at 120 ℃ and 110 ℃, t he elo nga2 tio n is significantly imp roved after aging at 100 ℃. These result s imply t hat low temperat ure aging can enhance t he elo ngatio n wit ho ut lo ssing st rengt h of t he st udied alloy. Polmear [ 7 ] reported t hat microalloying for ad2 ditio ns of silver had t he unique effect o n t he en2

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hancement of age hardening because of a p referred interactio n bet ween Ag ato ms (or in co njunctio n of Ag wit h Mg ato ms ) and vacancies , which stimu2 lates nucleatio n of t he intermediate p recipitate η′ . 32dimensio n ato mic p ro be ( 3DA P ) data indicates [ 7 ] t hat silver p ro motes t he formatio n of solute cluster and GP zo ne in t he early stage of aging. Ag ato ms p ro mote early co2clustering of Ag wit h Zn and Mg ato ms ( forming Ag2Zn , Ag2Mg co2clusters ) , fol2 lowed by Zn2Mg2Ag rich solute cluster s f ro m which t he intermediate p recipitate η′seems to be nucleated. Moreover t he number densit y of GP zo ne appear s to be higher in t he silver2bearing alloy and t he average size is slightly smaller t han t hat of t he GP zo ne in t he silver2bearing alloy. When t he co ntent of Ag is lower t han 0. 5 % ( mass f ractio n) , t he incubatio n time of t he GP zo ne decreases as t he Ag co ntent increases[ 8 9 ] . Therefo re t he effect of rapid aging respo nse of silver2bearing alloy 1 can be at t ributed to t he additio n of Ag which p ro motes t he forming of solute cluster s and GP zo nes in t he early stage of aging. Macchi et al [ 8 10 ] suggested t hat Ag increased t he stabilit y of G P zo nes and η′ p hase. The solvus of GP zo ne increased f ro m 135 ℃ to 160 180 ℃ due to t he additio n of 0. 1 % 0. 4 % Ag. They fo und t hat t race additio n of Ag reduced t he time to peak2aging , which is , however , totally co nt rary to o ur result ( as shown in Fig. 2) . It is suggested t hat t hese are clo sely related to t he low temperat ure ag2 ing ( 100 ℃) and high co ntent of zinc of alloy. It has been p ropo sed t hat a p referred interactio n be2 t ween Zn , Mg , Ag ato ms and vacancies accelerates t he respo nse to aging hardening. The co ntent of Ag in t he mat rix increases rapidly in t he early stage of aging. Large number s of super sat urated Zn ato ms in t he mat rix co mbine wit h Ag ato ms to form Zn2Ag clusters [ 11 ] , and t hen nucleate o n t hese cluster s , η′p hase forms and grow s. These p recipi2 tatio n may induce a dynamic imbalance of Zn be2 t ween mat rix and undissolved p hase. Wit h t he ex2 tensio n of aging p rocess , Zn ato ms in t he undis2 solved p hase co ntinue diff using into t he mat rix , and t he volume f ractio n of st rengt hening p hase in2 creases. So it p resent s a gradual rise of st rengt h aging f ro m 4 h to 16 h in t he aging hardening curve. The nat ure of aging p rocess is t he diff usio n of solute ato ms , and t he diff usio n is affected by t he aging temperat ure. Low temperat ure aging causes a relatively low diff usio n rate. Meanwhile , wit h t he increase of volume f ractio n of st rengt hen p hase in t he mat rix , t he super sat urated vacancy and t he co ntent of Zn , Mg etc , which are t he st rengt hened element s of alloy , are decreased in t he undissolved

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p hase. The diff usio n of Zn and Mg ato ms to t he mat rix t hen beco mes more difficult . These t hermo 2 dynamics and dynamics factor s , as a result , are caused by t he decrease of driving force of p recipita2 tio n. A s shown in Fig. 2 , t he hardening rate of t he alloy appears to decrease af ter aging for 16 h , t herefore a slow variatio n of st rengt h plate occurs. When aging fo r 80 h , t he volume f ractio n of st rengt hen p hase may be at t he maximum value. So t he alloy p resent s peak2aging co nditio n. Be2 cause Ag increases t he stabilit y of GP zo nes andη′ p hase [ 7 , 9 , 10 ] , and t he diff usio n of solve ato ms is relatively slow when aging at a low temperat ure , so t he t ransforming of GP zo nes and η′p hase to η p hase reduces. Taking t he above mentio ned factor s into co nsideratio n , we can see t hat t he volume f ractio n of η′p hase does not decrease serio usly af 2 ter aging for 96 h. So t he alloy does not p resent an o bvio us over2aging characterizatio n. Alloy 1 and alloy 2 were quenched in cold wa2 ter and aged at 100 ℃ for vario us time , t he me2 chanical p roperties of alloy 2 are shown in Fig. 3.

Journal CSUT 　Vol . 13 　No . 5 　2006

and alloy 2 is 100 ℃/ 80 h and 100 ℃/ 48 h , respec2 tively. Co mpared wit h t he ot her 7000 series heat t reat ment alloys , alloy 1 and alloy 2 show higher tensile st rengt h up to 753 M Pa and 788 M Pa , re2 maining 9. 3 % and 9. 7 % elo ngatio n under T6 co n2 ditio n , respectively. During aging at 100 ℃, t he silver2bearing alloy hardens more rapidly after quenching , alt ho ugh extending aging time to 96 h result s in similar st rengt h for bot h alloys. The ad2 ditio n of Ag causes an increase in t he time to peak st rengt h , and a slow variatio n of st rengt h plate2 form. Moreover t he peak st rengt h and correspo nd2 ing elo ngatio n of alloy 2 are higher t han t ho se of alloy 1. Fig. 4 show s t he scanning elect ro n micro scope ( SEM ) images of alloy 1 and alloy 2 af ter aging at 100 ℃/ 80 h and 100 ℃/ 48 h , respectively. The f ract ures of t he two alloys are bot h in t ransgranu2 lar/ intergranular co mbined mo de. And bot h alloys p resent fibro us ductile f ract ure , wit h many small dimples inside t he surface of fibro us f ract ure. Co mpared wit h alloy 1 , t here are more t ranscrys2 talline f ract ures , and less intergranular f ract ures caused by t he seco nd p hases wit hin t he dimples. The f ract ure behavior of alloy 2 is due to t he re2 ductio n of undissolved coar ser p hase and t he in2 crease of p recipitated p hase ( t he increase of yield st rengt h and f ract ure st rengt h) . The decrease of undissolved coar ser p hase al so causes t he smoot h2

Fig. 3 　Mechanical p roperties curves of alloy 2 after aging t reat ment at 100 ℃ 1 —Tensile st rengt h ; 2 —Yield st rengt h ; 3 —Elo ngatio n

The curve of mechanical p roperties of alloy 2 after being aged for vario us time at 100 ℃ shapes as a parabola form. The ultimate tensile st rengt h of t he as2quenched alloy 2 is relatively low ( 613 M Pa ) . Wit h t he increase of aging time , t he st rengt h elevates gradually in t he early stage of ag2 ing. The peak st rengt h appear s af ter aging for 48 h and t he co rrespo nding tensile st rengt h is up to 788 M Pa. After aging for 60 h , t he characterizatio n of overaging beco mes more o bvio us. There is a slight decrease of t he elo ngatio n in t he early stage of ag2 ing. Af ter t hat t he elo ngatio n keep s a relatively high value wit ho ut o bvio us variatio n during aging p rocess. The op timum aging t reat ment ( T6 ) for alloy 1

Fig. 4 　SEM images of alloy 1 and alloy 2 af ter aging t reat ment ( a) —Alloy 1 at 100 ℃/ 80 h ; ( b) —Alloy 2 at 100 ℃/ 48 h

FEN G Chun , et al : Effect of low temperat ure aging on micro st ruct ure and mechanical p roperties of aluminum alloy

ness of crystalline grain f ract ure. The backscat tered elect ro n images of alloy 1 and alloy 2 af ter as2quench t reat ment are shown in Fig. 5. Co rrespo nding to energy dispersive spec2 t rum ( EDS) analysis of t he undissolved p hases ( see Table 2) , it is fo und t hat t he undissolved p hases in alloy 1 are M ( Al ZnMgCu ( Ag) ) co mpo unds and M′( Al FeCuZnMg ( Ag ) ) co mpo unds. The undis2 solved p hases in alloy 2 are mainly M ( Al ZnMgCu) co mpo unds. It is said t hat t he quaternary co m2 po und may co nsist of so me ternary co mpo unds [ 12 ] . In co nt rast to t he mat rix , co ntent s of Zn and Mg increases in M″( Al ZnMgCu) of alloy 1.

Fig. 5 　Backscat tered elect ro n images of alloy 1 and alloy 2 af ter as2quench t reat ment ( a) —Alloy 1 ; ( b) —Alloy 2

Table 2 　EDS analysis of alloy 1 and alloy 2 af ter as2quenched t reat ment ( mass f ractio n , %) Sample

Alloy 1

Alloy 2

Zn

Cu

Mg

Ag

Fe

Al

-

31. 50

Undissolved phase A

38. 73 15. 73 4. 36 9. 67

Undissolved phase B

25. 51 10. 24 7. 78 5. 54 10. 52 40. 41

Mat rix

17. 16 1. 24 3. 39

-

-

78. 20

Undissolved phase

14. 15 15. 18 1. 31

-

-

69. 35

Mat rix

15. 12 0. 28 2. 82

-

-

81. 78

　　The generally accepted p recipitatio n sequences for 7000 series alloys are as follow s[ 13 ] : supersat u2 rated solid solutio n ( SSS ) co herent stable GP zo nes semi2co herent intermediate p hase η′

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( MgZn2 ) inco herent stable η ( MgZn2 ) or T p hase ( Al ZnMgCu ) . The mo rp hology , size and t he degree of co herence wit h t he mat rix will influ2 ence t he p roperties of alloy. The co nt ributio n of semi2co herent intermediate , η′p hase ( MgZn2 ) to t he st rengt h is greater t han GP zo ne. The inco her2 ent stable η p hase ( MgZn2 ) co nt ributes less t han GP zo ne and η′p hase. Fig. 2 and Fig. 5 show t hat t here are more undissolved coar ser p hases in t he al2 loy 1 co mpared wit h alloy 2 af ter p ro motive solu2 tio n t reat ment . Co rrespo nding to EDS analysis of t he undissolved p hases above , t he undissolved p ha2 ses in alloy 1 are Zn rich M ( Al ZnMgCu ( Ag ) ) co mpo unds. However , t he co ntent of Zn , Mg and Ag are lower in mat rix t han t ho se in M co m2 po unds. That may be due to t he t race additio n of Ag causing t he formatio n of many coar ser undis2 solved p hases which co nsume t he hardening ele2 ment s in t he mat rix. A nd t he co nsump tio n of t he hardening element s in t he mat rix will reduce t he volume f ractio n of p recipitates during aging. Mo2 reover , t he binding fo rce bet ween undissolved p hase inside t he grain bo undary and t he surface of t he mat rix is weak and t he undissolved p hase is brit tle , so t hat it beco mes sites for st ress co ncen2 t ratio n and microcrack initiatio n during plastic de2 formation which reduces the strength of the alloy[ 14 ] . T EM images of alloy 1 af ter different aging t reat ment are shown in Fig. 6. Fig. 6 ( a ) shows t he bright2field T EM image of alloy 1 af ter aging at 120 ℃for 8 h. Correspo nding select area elect ro nic diff ractio n ( SA ED ) indicates t hat t he p recipitates are mainly GP zo ne and η′ p hase. There are so me no nuniform dist ributio n of coarse η p hases. These η p hases nucleate and grow o n t he inco herent Al 3 Zr dispersoids because of t he high interfacial energy of inco herent Al 3 Zr/ mat rix [ 15 ] . The dislo2 catio n t ransit s t hese no nunifo rm η p hases t hro ugh Orowan mechanism during plastic deformatio n. Local st ress co ncent ratio ns are generated mo re eas2 ily aro und t hese equilibrium p hases which decrease t he elo ngatio n of t he alloy. Fro m Fig. 6 ( b) , it can be seen t hat t he degree of no nuniform dist ributio n of η p hase seems to decrease. That indicates wit h t he decrease of aging temperat ure , t he no nuniform dist ributio n of η p hase which may nucleate o n t he inco herent Al 3 Zr disper soids reduces. Figs. 6 ( c ) ( e ) show t he bright2field T EM images of alloy 1 after aging at 100 ℃ for 36 h , 80 h and 96 h , re2 spectively. Co mpared wit h aging at 120 ℃ and 110 ℃, t he degree of no nuniform dist ributio n of η p hase decreases o bvio usly after aging at 100 ℃, and t here is a ho mogeneo us dist ributio n of fine p recipitates in t he mat rix. This micro st ruct ure re2 sult s in a superior co mbinatio n of st rengt h and elo ngatio n.

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Fig. 6 　TEM images of alloy 1 af ter different aging t reat ment s ( a) —120 ℃/ 8 h ; ( b) —110 ℃/ 36 h ; ( c) —100 ℃/ 36 h ; ( d) —100 ℃/ 80 h ; ( e) —100 ℃/ 96 h ;

　　T EM images of alloy 2 after aging at 100 ℃ for 24 h and 48 h ( T6) are shown in Fig. 7. There is a ho mo geneo us dist ributio n of fine p recipitates wit h inho mogeno us dist ributio n of η p hase in t he mat rix. Correspo nding SA ED show s t hat t he p re2 cipitates are mainly GP zo ne and η′p hase. After aging for 48 h , t he volume f ractio n of η′p recipi2 tates is larger t han t hat af ter aging for 24 h. Kanno [ 15 ] suggested t hat 70752Cr alloy co n2 tainning E ( Al 18 Cr 2 Mg3 ) disper soids may decrease

t he st rengt h of t he alloy due to t he no nuniform dis2 t ributio n of η p hase which nucleates o n t he E p hase. The additio n of Ag in 2000 series alumium alloy causes high interfacial energy of Mg3 Ag/ ma2 t rix due to t he p recipitatio n of inco herent Mg3 Ag [ 16 ] . Co mpared wit h aging at low tempera2 t ure , it is easier for incoherent θ phase to precipitate on Mg3 Ag/α misfit interface when aging at a relative2 ly high temperat ure. Therefore t he elo ngatio n may decrease because of t he no nuniform dist ributio n of θp hase which nucleates o n Mg3 Ag p recipitates[ 17 ] . The t race additio n of Ag in 7000 series alloys p ro2 motes Zn2Mg2Ag co2clusters rat her t han t he p re2 cipitatio n of Mg3 Ag in t he early stage of aging [ 8 ] . After aging at a relatively high temperat ure , it is easier to p recipitate no nunifo rmly t he coar se η p hase nucleating o n Zn2Mg2Ag co2cluster s , which reduces t he elo ngatio n. 4 　CONCL USIONS

Fig. 7 　T EM images of alloy 2 af ter aging t reat ment s at 100 ℃ ( a) —24 h ; ( b) —48 h

1 ) Low temperat ure aging can enhance elo nga2 tio n and keep t he high st rengt h of t he st udied high zinc super2high st rengt h aluminum alloy. The opti2 mum aging regimes of alloy 1 and alloy 2 are 100 ℃/ 80 h and 100 ℃/ 48 h wit h tensile st rengt h up to 753 M Pa and 788 M Pa , elo ngatio n 9. 3 % and 9. 7 % , respectively. 2 ) Trace additio n of Ag p ro motes a larger amo unt of undissolved p hase which co nsumes t he co ntent of st rengt hen element s of Zn and Mg. 3) Due to t he effect of silver additio n , silver2 bearing alloy has st ro nger age2hardening abilit y t han silver2f ree alloy at t he early stage of aging , t he tensile st rengt h of silver2bearing alloy 1 after aging for 1 h rapidly increases to 700 M Pa , while

FEN G Chun , et al : Effect of low temperat ure aging on micro st ruct ure and mechanical p roperties of aluminum alloy

t hat of alloy 2 is o nly abo ut 650 M Pa. 4 ) During aging p rocess , t here is a slowly in2 creasing and deceasing st rengt h platform in t he sil2 ver2bearing alloy. Trace additio n of Ag p rolo ngs t he time of peak2aging and delays t he time of over2 aging. REFERENCES [ 1 ] 　Heinz A , Haszler A , Keidel C. Recent develop ment in aluminium alloys for aero space applications[J ] . Mate2 rials Science and Engineering , 2000 , A280 ( 1 ) : 102 107. [ 2 ] 　Mondolfo L F. Micro st ruct ure and p roperty in alumi2 num alloy[ M ]. WAN G Zhu2tang. Beijing : Metallurgi2 cal Indust ry Press , 1976. [ 3 ] 　Pickens J R. Aluminum powder metallurgy technology fo r high2st rengt h applications[J ]. J Mater Sci , 1981 , 16 (6) : 1437 1442. [ 4 ] 　Osamura K. Mesoscopic structure of super2high strength P/ M Al2Zn2Mg2Cu alloys [J ]. Mater Sci Forum , 1996 , 217 222 (3) : 1829 1834. [ 5 ] 　ZHAN G Qing , CU I Jian2zhong , L U Gui2min. Experi2 mental investigation on surface meniscus shape under effect of elect romagnetic field wit h low f requency of 7075 aluminum alloy p roduced by CR EM p rocess [J ] . The Chinese Jo urnal of Nonferrous Metals , 2002 , 12 ( s) : 109 116. (in Chinese) [ 6 ] 　ZHAN G Kun , L IU Zhi2yi , F EN G Chun. The effect of small addition of silver o n t he micro st ruct ure and me2 chanical p roperties of a high2zinc super2high st rengt h aluminum alloy [J ]. The Chinese Journal of Nonfer2 ro us Metals , 2005 , 15 ( 1) : 116 122. (in Chinese) [ 7 ] 　Polmear I J . A t race element effect in alloys based o n t he aluminum2zinc2magnesium system [ J ] . Nat ure , 1960 ( 186) : 303 304.

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( Edited by YANG Hua)