Properties and Microstructure of Aluminum- Copper-Magnesium-Lithium Alloys

DONALD WEBSTER

The m i c r o s t r u c t u r e and mechanica l p rope r t i e s of A1-Cu-Mg-Li alloy ex t rus ions p r epa red f rom both rapidly cooled powder f lakes and cast ingots have been de- t e r m i n e d . E m b r i t t l e m e n t of the a l loys by l i thium was observed in both powder source and ingot source m a t e r i a l . F r a c t u r e in al l the powder source al loys was par t ly t r a n s g r a n u l a r and par t ly along the boundar ies of the or ig ina l powder f lakes . In the ingot source A1 2024 the f r ac tu re was t r a n s g r a n u l a r but the addi- t ion of l i th ium changed the f r ac tu re path to p redominan t ly i n t e r g r a n u l a r . The fatigue c rack growth ra te of powder source A1 2024 with and without l i th ium was found to be five to ten t imes s lower than the growth ra te of ingot source A1 2024.

A L u M I N U M - l i t h i u m al loys have two advantages over convent ional a l u m i n u m al loys : f i r s t , they show an in - c r ea sed e las t ic modulus, ~ and secondly, they have a lower dens i ty . ~'2 The i r c o m m e r c i a l appl icat ion has been slowed by diff icul t ies in producing sound c a s t - ings and by the low toughness of the alloys.~ The cas t - ing p rob l ems can be overcome at some addit ional cost by the use of a powder meta l lu rgy p r o c e s s . This p r o - g r a m was des igned to m e a s u r e the mechanica l p rop- e r t i e s of a l u m i n u m - l i t h i u m powder al loys and, in p a r - t i cu la r , to de t e rmine whether the toughness p rob lems could be overcome by the use of rapidly cooled powder possess ing both a fine g r a in s ize and re la t ive f reedom f rom the large i n t e rme ta l l i c compounds that form in cas t ings .

EXPERIMENTAL TECHNIQUE

The powders were produced by mel t spinning 3 in the f o r m of f lakes about 1 • 1 m m • 50 pm at the Batel le Columbus L a b o r a t o r i e s . This p roce s s uses a r e s e r v o i r of mol ten meta l in a graphi te c ruc ib le which is forced under the p r e s s u r e of an i ne r t gas through a sma l l o r i - f ice onto the sur face of a spinning copper wheel de- s igned to rapid ly chil l the mol ten meta l and b r e a k it up into a s e r i e s of f lakes . Because of the reac t iv i ty of the mol ten a l u m i n u m - l i t h i u m al loys the jet of molten meta l was protec ted f rom oxidation by a s t r e a m of argon gas . The a l loys examined in this p r o g r a m were nomina l ly A1 2024, A1 2024 + 1.5 Li, and A1 2024 + 3 Li . The chemi - cal composi t ions of the hot p r e s s e d and extruded s a m - ples are given in Table I.

A s e r i e s of cast a l loys of s i m i l a r composi t ions were made in the fo rm of 5 cm d iam cy l ind r i ca l ingots and used for compar i son purposes . The composi t ions of these al loys a re a lso g iven in Table I. The powders were consol idated by vacuum hot p r e s s i n g to full den- s i ty at 755 K and then extruded at 730 K with a 10:1 reduct ion ra t io to fo rm a flat ex t rus ion about 5 mm thick and 4 cm wide. The cas t ingots were a lso ex- t ruded under the same condi t ions . T r a n s m i s s i o n e l e c - t ron mic roscopy was pe r fo rmed on the f lakes without r e s o r t i n g to e lec t ropol i sh ing at some a r ea s of the a s -

r ece ived flake which were thin enough to t r a n s m i t e l ec - t rons even though the average th ickness was about 50 #m. T r a n s m i s s i o n mic roscopy on the ex t rus ions was accompl i shed by e lec t ropol i sh ing in the convent ional m a n n e r .

Fat igue c rack growth r a t e s were measu red on com- pact t ens ion spec imens formed f rom heat t r ea ted ex- t r u s i o n s . Specimens were tes ted in l abora to ry a i r with an R of 0.1 at a f requency of 50 Hz. The spec imens were machined so that the c rack grew in a d i rec t ion pe rpend icu la r to the ex t rus ion axis .

T e n s i l e tes t ing was conducted on flat sheet spec i - mens that were 75 mm long with a 12 m m gage length in the longi tudinal d i rec t ion and 40 mm long with a 6 m m gage length in the t r a n s v e r s e d i r ec t ion . Modulus and yield s t reng th de t e rmina t i ons were made f rom the output of two s t r a in gages on opposite s ides of the spec imen .

Impact tes t ing was conducted at room t e m p e r a t u r e on a Manlabs 33-J impact machine using Charpy spec i - mens that were s tandard except for the th ickness which was only 6 mm.

RESULTS

M i c r o s t r u c t u r e

Powder Alloys. The m i c r o s t r u c t u r e of the rapidly cooled f lakes containing 0, 1.3 and 2.5 pct Li is shown in Figs . 1 to 3. The s t r u c t u r e cons i s t s of equiaxed reg ions 1 to 2 ~tm in d i a m e t e r which appear f rom e lec t ron dif f ract ion to be subgra ins r a the r than g r a i n s . At the boundar ies of the subgra ins are r e - gions of divorced eutec t ic . The re is l i t t le s t r u c t u r a l change due to the l i th ium.

Targe t Initial Composition Form Li Cu Mg

Table I, Chemical Composition of Alloys in Extruded Condition

Wt Pct ppm

DONALD WEBSTER is Staff Engineer , Lockheed Missiles and Space Co., Sunnyvale , CA 94086 .

Manuscr ip t submi t t ed Apri l 3, 1979.

ISSN 0360-2133/79! 1211-1913500.75/0 M E T A L L U R G I C A L TRANSACTIONS A

Mn Si

A1 2024 Powder 0 4.24 1.24 0.57 0.1.5 A1 2024+ 1,5Li Powder 1.3 4.25 1.42 0.44 0.14 A1 2024 + 3 Li Powder 2.5 3.05 1.72 0.57 0.15 AI 2024 Ingot 0.0007 4.45 1.52 0.53 0.10 AI 2024 + 1,5 Li Ingot 1.6 3.6 2.3 0.52 0.14 A1 2024+ 3 Li Ingot 2.5 4.2 2.2 0.61 0.18

Na K

0.2 0.1 1 0.1

27 0.1 9 <1

13 2 8 2

�9 1979 AMERICAN SOCIETY FOR METALS AND VOLUME 10A, DECEMBER 1 9 7 9 - 1 9 1 3 THE METALLURGICAL SOCIETY OF AIME

T r a n s m i s s i o n e l e c t i o n m i c r o s c o p y of a n u m b e r of f l a k e s r e v e a l s s m a l l v a r i a t i o n s in s u b g r a i n s i z e a n d b o u n d a r y p h a s e m o r p h o l o g y , bu t t h e s e a p p e a r to b e r e l a t e d m o r e to t he n o r m a l s t r u c t u r a l v a r i a t i o n a c r o s s e a c h g r a i n t h a n to t h e i n f l u e n c e of l i t h i u m . F i g u r e 3 c l e a r l y s h o w s t he s u b g r a i n b o u n d a r i e s a n d r e v e a l s t h a t e v e n in t h e a l l o y c o n t a i n i n g 2 .5 p c t L i t h e r e i s n o v i s i - b l e p r e c i p i t a t i o n w i t h i n e a c h s u b g r a i n .

There is very little change in the optical m ic ro - s t ructure during hot press ing (Fig. 4) with any of the alloy powders, although the electron microscope shows that extensive general precipitation is occurring (Fig. 5). As can be seen f rom Fig. 4, the fine equiaxed sub- grains are maintained during hot press ing together

Fig. I -Transmission electron micrograph of A l 2024 rapidly cooled powder particle, as-received. Subgrains 1 to 2 tam diam have divorced copper rich eutectic at their boundaries. Magnification 13,680 times.

Fig. 2-Transmission electron micrograph of A 1 2024 + 1.3 pct Li, rapidly cooled powder particle, as-received. Subgrains 2/~m in diam have divorced copper rich eutectic along their boundaries. Magnifi- cation 13,440 times.

Fig. 3-Transmission electron micrograph of AI 2024 + 2.5 pct Li. Divorced copper rich eutectic on subgrain boundary. Magnification 40,320 times.

Fig. 4-Optical micrograph of AI 2024 + 2.5 Li, as hot pressed. The long, straight boundaries are the surfaces of the original powder flakes. The variation in subgrain size across each flake which existed in the original powder is maintained in the hot pressing. Magnifica- tion 385 times.

1914-VOLUME 10A, DECEMBER 1979 METALLURGICAL TRANSACTIONS A

with the graduation in subgrain size within each flake resul t ing f rom the difference in cooling ra te between the side of the flake cooled by d i rec t contact with the rotat ing copper wheel and the side facing away from

the wheel. The la t te r side cools s lower and t h e r e - fore h a s a c o a r s e r s u b s t r u c t u r e .

After extrusion, the mic ros t ruc tu re of a l l three powder al loys is g rea t ly refined with a gra in size, as

Fig. 5-Transmission electron micrograph of A1 2024 + 1.3 pct Li in the hot pressed condition. A large volume fraction of spherical pre- cipitates 0.03 to 0.06 tam diam have precipitated inside the subgrains during hot pressing. Magnification 17,700 times.

Fig. 6-Transmission electron micrograph of A1 2024 + 2.5 pct Li in the extruded condition. Recrystallized grains about 1 gm diam have formed during extrusion. Magnification 32,400 times.

Fig. 7-Transmission electron micrograph of A 1 2024 extrusion formed from rapidly cooled powder and heat treated to the T6 con- dition (16 h, 460 K). The structure shows fine recrystallized grains in the 1 to 3 #m range with a Widmanstatten precipitation of copper rich hardening precipitate. Magnification 15,900 times.

Fig. 8-Transmission electron micrograph of A1 2024 + 2.5 pct Li extrusion formed from rapidly cooled powder and heat treated to the T6 condition (16 h, 460 K). Recrystallized grains about 2 #m in diam do not show the Widmanstatten precipitate that forms in the lithium free alloys. The A 13 Li hardening precipitate is not resolved at this magnification. Magnification 13,920 times.

METALLURGICAL TRANSACTIONS A VOLUME 10A, DECEMBER 1979-1915

determined by the mean l inear in tercept of 200 gra ins , of l e ss than 1 /~m (Fig. 6). Genera l a r e a e lect ron dif - f ract ion pat terns f rom the extrus ions produce ring pat terns which indicate that many of the fine gra ins existing at this stage are t rue gra ins produced by r e - c rys ta l l i za t ion ra the r than the subgrains that were presen t in the or iginal powder flake.

There is no pronounced effect of lithium on the e l ec - t ron mic ros t ruc tu re until the ma te r i a l s a re solution

Fig. 9-Dark-field transmission electron micrograph of A1 2024 + 2.5 pct Li heat treated to T6 condition (16 h, 460 K). Spherical A 13 Li precipitates, 0.010 to 0.015 #m dram, inside one grain are illumhaated. Magnification 63,600 times.

Fig. 10-Scanning election micrograph of fracture in powder source A1 2024 after solution treatment at 770 K and aging at 339 K for 16 h. Fracture shows mainly ductile dimples with some fracture along the boundaries of the original flakes (at A). Magnification 2000 times.

Fig. 11 -Scanning election micrograph of fracture in powder source A1 2024 + 2.5 pct Li after solution treatment at 770 K and aging at 339 K for 16 h. Fracture shows a high density of ductile dimples. Magnification 2000 times.

Fig. 12-Scanning election micrograph of powder source A1 2024 + 2.5 pct Li after solution treating at 770 K and aging for 16 h at 464 K (maximum strength condition). Fracture shows ductile dimples similar to appearance after aging at 339 K. Magnification 2000 times.

1916 VOLUME 10A, DECEMBER 1979 METALLURGICAL TRANSACTIONS A

t reated and aged. Figure 7 shows a t ransmission elec- tron micrograph of the A1 2024 powder alloy Contain- ing 0 pct Li after extrusion and heat treatment to the T6 condition. The grain size has grown considerably compared to the extruded condition, and the grains are now in the 1 to 3 #m range. Widmanstatten needles of the copper rich hardening phase 0.003 to 0.006 ~zm thick have precipitated within each grain. The 2.5 pct Li alloy in the same condition and at a similar magni- fication (Fig. 8) does not show an acicular hardening

Table II. Modulus and Density Values of Extrusions Formed from Rapidly Cooled Powders of Aluminum-Lithium Alloys

Aging Density, Modulus, Alloy Treatment Kg/m 3 X 10 3 GN/m 2

A1 2024 339 K 16 h 2.80 73.0 A] 2024 + 1.3 Li 464 K 16 h 2.72 - AI 2024 + 2.5 Li 464 K 16 h 2.59 83.5

phase. The hardening phase in the lithium containing alloy consists of spheres 0.010 to 0.015 #m in diam and is resolved at higher magnifications (Fig. 9) pref- erably by the use of superlattice reflections to form a

Table III. Impact Properties and Hardness of Hot Pressed Powder Source Alloys

Impact Alloy Condition Value, J Hardness, 15T

Hot Pressed (HP) 11 71 A1 2024

HP + Annealed - 68

Hot Pressed 2.2 77 HP + Annealed 1.9 73

A1 2024 + 1.3 Li HP + Solution Treated and 3.8 87 Aged 60 h, 300 K

Hot Pressed 1.8 78 HP + Annealed 1.9 74

AI 2024 + 2.5 Li HP + Solution Treated and 6.4 83 Aged 60 h, 300 k

(a) (b) Fig. 13- (a ) Optical micrograph of fractured tensile specimen formed from ingot source A1 2024 extrusion in the T6 condition (16 h, 460 K). Frac- ture is transgranular. Magnification 500 times, (b) Optical micrograph of fractured tensile specimen formed from ingot source AI 2024 + 2.5 pct Li in the T6 condition (16 h 460 K). Fracture is mainly intergranular. Many large intermetallic particles are present and those near the fracture region have formed cracks transverse to the tensile axis. Magnification 500 times.

METALLURGICAL TRANSACTIONS A VOLUME 10A,DECEMBER 1979 1917

Fig. 14-Scanning election micrograph of the fracture surface of ingot source A1 2024 solution treated at 770 K and aged to the maximum strength condition (464 K 16 h). Fracture shows transgranular frac- ture with large ductile dimples originating at intermetallic particles. Magnification 1340 times.

Fig. 15-Scanning election micrograph of ingot source A1 2024 + 2.5 pct Li heat treated to the maximum strength condition (464 K for 16 h). Fracture shows a mixture of large and small ductile dimples with some intergranular fracture at A. Magnification 1260 times.

dark-field image. The hardening phase in the powder source al loys containing lithium appears to be ordered AlsLi as reported by other workers for cast A1-Li a l - loys .~'~

The fracture surfaces of the heat treated powder source al loys were examined by optical and scanning elect ion microscopy . Fracture in both A1 2024 and A1 2024 + 2.5 Li al loys was partly transgranular and partly along the elongated boundaries of the original powder f lakes. No evidence of fracture along the fine recrys ta l l i zed grains within each flake was observed. The fracture surface of powder source A1 2024, so lu- tion treated and aged at 339 K 16 h (maximum strength condition) shows ductile dimples (Fig. 10) with some

90

AGING TIME 16 HR

88 - - CAST PRODUCTS EXTRUDED 10:1 SOLUTION TREATED 770K AND AGED AS INDICATED E a'~

0 I �9 I

I i 8, - / / . a ~

I AI 2024+0% L i ' / L ~ o / J, ~.~

IT/ \1 85 - -

5Z t.) o

80--

AI 2024 + 1 6% Lr

'

300 400 500

AGtNO TEMPE~TURE ir Fig. 16-Aging response of ingot source A1 2024 with 0, 1.6, and 2.5 pct Li.

90

88

86

82

o

AGING TIME 16 HR

- - Al 2024 ~- 0% Li

.... + ,

dA12024 \ _ q,

\ \

POWDER PRODUCTS EXTRUDED 10:1

7 8 - SOLUTION TREATED AT 770K AND AGED AS INDICATED

76 I I I 300 400 500

AGING TEMPERATURE (K)

Fig. 17-Aging response of powder source A 1 2024 with 0, 1.3, and 2.5 pct Li.

fracture along the surface of the original flake boun- daries (at A). The fracture surface of A1 2024 + 2.5 Li after the same heat treatment (339 K 16 h) shows a greater density of ductile dimples in the range 1 to 4 tzm diem (Fig. 11). Many of these dimples can be seen at higher magnifications to be initiated by part ic les about half the dimple s i z e . When the AI 2024 + 2.5 Li

1918 VOLUME 10A, DECEMBER 1979 METALLURGICAL TRANSACTIONSA

500

1~ ..,,-Ik.. POWDER PRODUCTS \ \ , ;.I 41 ~,., EXTRUDEDIO:I

~ >r ~ UTS / " '~ . . ._ \ .~- '~, . al 2024 �9 YO S

~ _/ ~ . . . . 1 x AI 2024 + 1.3 LI �9 zX v 400 " 1 .~ "~ "13~ ' - \ AI 2024 + 2.5 Li �9 []

o . . . . . .

. / , ,

~ - - r.~ ".. / . . . . . . . . \ '/,

i /7,..~ ................... \ \ 'b

/ - A I 2024

0 "AI 2024 + 1.3 L] "~.,,..,..,~i ~- -AI 2024 + 2.5 LI

15 ~ ""(3 \ \ ~ - A I 2024

,< AI 2024 + 1.3 LI 7 \

~E a,.. - - .-A-~ / \ - ~,. "--<~. % ~: ~ ',,q. -...

Ai 2024 + 2 . s ,i-- '~'......p_.'._'-~_...--.-.'= I l l l

300 400 500 600 AGING TEMPERATURE (K)

Fig. 18-Tensile properties as a function of aging temperature of solution treated powder source A 1 2024 extrusions containing 0, 1.3, and 2.5 pct Li.

z

o z

I

g . J

400

300

200

20

10

0

10

• � 9 CAST PRODUCTS / ~ EXTRUDED 10:1

/ ~ \ UTS YS / - \ 0%u �9 o

/ ..... \ 1.6% Li �9 [] ../." A , 4 2.5%U �9 A

. . / / %-

......'"

......... "i / ~ "-......

li~llm i �9 ............. / / / "...

, / / - . \ ~ - 'A f - \ .

i , I i

"A

A Z~ ....................................... ZX. ..................... .Z~.. ~ .................... eX

><

I I I 300 400 500

AGING TEMPERATURE (K) Fig. 19-Tensile properties as a function of aging temperature of solution treated ingot source A1 2024 containing 0, 1.6, and 2.5 pct Li.

M E T A L L U R G I C A L T R A N S A C T I O N S A

AI 2020 T651. INGOT SOURCE (REF. 7 ) 7

/ ,'///c"., / / /o - t 1~ o. - - 0

0,01

4 l0 20 ~tK (MPA ~4)

Fig. 20-Fatigue crack growth rate in laboratory air of heat treated powder source extrusions of A1 2024 and A1 2024 + 2.5 pct Li. Other aluminum alloys are shown for comparison.

Table IV. Effect of Orientation on Mechanical Properties of AI 2024 Containing Lithium Solution Treated and Aged, 460 K, 16 h

Tensile Strength

MN/m2 Charpy

Alloy Orientation UTS 0.2 YS E 1, Pct Value, J

AI 2024 + 2.5 Li T 352 352 0,2 0.8 Powder L 420 400 3 2.2 A1 2024 + 1.6 Li T 325 193 11 4.1 Cast L 345 255 11 3.4 A1 2024 + 2.5 Li T 345 262 3 1.0 Cast L 360 262 7 1.4

alloy is aged to i ts max imum s t reng th condit ion (16 h 464 K) there is no d i sce rn ib l e change in the f r ac tu re appearance (Fig. 12).

Cast Al loys . The cas t a l loys af ter ex t rus ion and heat t r e a t m e n t have an elongated g ra in s t r u c t u r e e s - t imated to be about 30 pm wide and s e v e r a l t imes as long (Fig. 13). Both l i th ium free and l i th ium con ta in - ing a l loys have large i n t e r m e t a l l i c compounds up to 10 ~m in d iam which c rack t r a n s v e r s e l y under an ap- plied tens i le s t r e s s . The n u m b e r of la rge pa r t i c l e s is g r e a t e r in the 2.5 pet Li al loy (Fig. 13(b)) than the l i th ium free alloy (Fig. 13(a)). The re is a marked change in the f r ac tu re behavior when l i th ium is added and the f r ac tu re path in the T6 condit ion changes f rom t r a n s g r a n u l a r (Fig. 13(a)) to main ly i n t e r g r a n u l a r (Fig. 13(b)).

Scanning elect ion mic rographs of the t r a n s g r a n u l a r f r ac tu re of cast A1 2024 and cas t A1 2024 + 2.5 Li, both heat t r ea ted to the ma x i mum s t rength condit ion, (464 K 16 h) a re shown in F i g s . 14 and 15 r e spec t ive ly . Both a l - loys show a b imodal dimpled f r ac tu re with the la rge in - t e r m e t a l l i c pa r t i c l e s in i t ia t ing the l a r g e r d imples . The main di f ference between the f r ac tu re appearance of the two al loys is the p reva lence of g ra in boundary f r ac tu re and the c lose r spacing of the large i n t e r m e t a l l i c p a r - t i c les in the l i th ium containing al loy.

Mechanica l P r o p e r t i e s

Hardness Va lues . The ha rdnes s va lues for the so lu - t ion t rea ted ex t rus ions f rom cast ingots as a funct ion of aging t e m p e r a t u r e a re shown in F ig . 16. Peak ha rdness

VOLUME 10A, DECEMBER 1 9 7 9 - 1 9 1 9

a f t e r 16 h aging o c c u r s at about 460 K r e g a r d l e s s of the l i th ium content , al though the peak h a r d n e s s is r e - duced s l igh t ly in the l i th ium conta ining m a t e r i a l s . The aging r e s p o n s e of the powder m e t a l l u r g y e x t r u s i o n s i s shown in F ig . 17. The peak aging t e m p e r a t u r e of the A1 2024 is r educed f rom the 460 K o b s e r v e d in the cas t m a t e r i a l to 340 K, p r e s u m a b l y as a r e s u l t of the en - hanced dif fus ion r a t e p o s s i b l e in a v e r y fine g r a ine d m a t e r i a l . The A1 2024 + 2.5 L i a l loy shows a h a r d n e s s peak at 420 K only s l igh t ly lower than that o b s e r v e d for the iugot s o u r c e equiva lent , while the A1 2024 + 1.3 Li shows two h a r d n e s s peaks which may be due to the copper and l i th ium harden ing p r e c i p i t a t e s , r e s p e c t i v e l y

P h y s i c a l P r o p e r t i e s . The dens i t y and modulus v a l - ues obta ined on e x t r u s i o n s f o r m e d f r o m the r ap id ly cooled powde r s a r e given in Tab le II . The e x t r u s i o n s a r e so lu t ion t r e a t e d at 770 K, and aged at the t e m p e r - a t u r e s ind ica ted below.

The r e d u c e d dens i ty and i n c r e a s e d modulus of the powder a l loys is s i m i l a r to that obta ined by p r e v i o u s w o r k e r s on cas t a l u m i n u m - l i t h i u m a l l oys 1,~ and ind i - c a t e s that the l i th ium in the powder a l loys i s p r e s e n t in a usefu l f o rm as an i n t e r m e t a l l i c compound r a t h e r than as an oxide .

T e n s i l e and_Impact P r o p e r t i e s . The impac t p r o p e r - t i e s and h a r d n e s s of the t h r e e powder s o u r c e a l l o y s in the a s - h o t p r e s s e d condi t ion a r e given in Tab le III.

It can be seen that the addi t ion of l i th ium e m b r i t t l e s the A1 2024 in the hot p r e s s e d condi t ion . Th is e m b r i t - t l emen t is not r e m o v e d by anneal ing but is a l l e v i a t e d by so lu t ion t r e a t m e n t and n a t u r a l aging.

The t en s i l e and impac t p r o p e r t i e s of the solut ion t r e a t e d powder m e t a l l u r g y ex t ru s ions as a function of aging t e m p e r a t u r e a r e shown in F ig . 18. T h e r e is v e r y l i t t le d i f f e rence in s t r e n g t h be tween the t h r e e a l loys when each i s c o m p a r e d at i t s own peak aging t e m p e r - a t u r e . T h e r e i s , however , a m a r k e d deg rada t i on of the t ens i l e e longat ion and impac t toughness in the l i th ium conta in ing a l l o y s at a l l aging t e m p e r a t u r e s .

The t ens i l e and impac t p r o p e r t i e s of the ca s t A1 2024 a l loys conta ining up to 2.5 pct L i a r e shown in F ig . 19. As in the c a s e of the powder p roduc t s , the addi t ion of l i t h ium to A1 2024 has r educed the toughness and duc - t i l i t y . T e n s i l e s t r e n g t h was a l so r educed p a r t i c u l a r l y in the underaged condi t ions .

Some t r a n s v e r s e t en s i l e and impac t p r o p e r t i e s were d e t e r m i n e d on one powder ex t ru s ion and two ingot sou rce e x t r u s i o n s . The r e s u l t s a r e given in Table IV. As would be expected , the t ex tur ing of the powder sou rce s amp le , due to i t s tendency to f r a c t u r e along the e longated b o u n d a r i e s of the in i t i a l powder f l akes r e s u l t s in a g r e a t e r o r i en ta t ion dependence of s t r eng th , toughness , and duc t i l i ty than the cas t m a t e r i a l s .

Fa t igue C r a c k Growth R a t e . The fat igue c r a c k g rowth r a t e s of the powder source A1 2024 with and without l i th ium a r e given in F ig . 20. Both a l loys show a r educed c r a c k g rowth r a t e c o m p a r e d to t y p i c a l v a l - ues for cas t A1 2024.6 T h e r e is no appa ren t bene f i c i a l effect of the l i th ium on the c r a c k g rowth r a t e of the powder a l loy, al though Sande r s and Ba lmuth 7 point out that A1 2020 which is an a l loy p roduced f r o m cas t ingot containing a p p r o x i m a t e l y 1 pct l i th ium has a c r a c k growth r a t e about four t i m e s s lower than A1 7075. T h e i r data , toge the r with da ta fo r ano ther powder s o u r c e a l loy , MA87, a r e a l so given in F ig . 20. It should be noted that no s t r e s s r e l i ev ing t r e a t m e n t was

conducted on the A1 2024 and A1 2024 + 2.5 L i powder m e t a l l u r g y a l loys be fore fat igue t e s t ing .

DISCUSSION

The above r e s u l t s indica te that a l u m i n u m - l i t h i u m powder with up to 2.5 pc t Li can be r e a d i l y manufac - t u r e d if s imp le p r e c a u t i o n s a r e taken t0 avoid e x c e s - s ive l i th ium oxida t ion dur ing the c r i t i c a l powder f o r m a - t ion s t age . The use of a powder p r o c e s s n a t u r a l l y in- cu r s some i n c r e a s e in cos t but does a l l ev i a t e the p r o b l e m of ingot c r a c k i n g . The o ther m a j o r p r o b l e m of d i lu te ca s t a l u m i n u m - l i t h i u m a l loys , i . e . , the r e - duced toughness and duc t i l i ty , does not s e e m to be g r e a t l y a f fec ted by use of the r a p id ly cooled powder approach , a t l e a s t when the A1 2024 base compos i t i on is used . Th is finding s u g g e s t s that the e m b r i t t l e m e n t of a l u m i n u m - l i t h i u m a l l o y s is a fine s c a l e phenomenon and i s not m e r e l y a r e s u l t of the l a r g e (>10 ~m) i n t e r - me ta l l i c compounds that f o r m dur ing ingot cas t ing . The exce l l en t toughness and duc t i l i t y of the powder sou rce A1 2024 ind ica t e s that in the a bse nc e of l i th ium, the conso l ida t ion and t h e r m o - m e c h a n i c a l p r o c e s s i n g used in th is work a r e suff ic ient to deve lop the po ten t ia l of the a l l o y s . It has been sugges ted 8 that sodium, which i s a m a j o r i m p u r i t y in the l i th ium a l loy ing addi t ions , is r e s p o n s i b l e for the e m b r i t t l i n g ef fec t of l i th ium on a luminum if the concen t ra t ion is above 50 ppm. It would a p p e a r f r o m the a n a l y s e s in Tab le I that the l eve l s of ne i t he r sod ium nor p o t a s s i u m c o r r e l a t e with the o b s e r v e d toughness or duc t i l i ty r e g a r d l e s s of whether the a l loy a l so conta ins l i th ium. F o r example the impac t ene rgy of the powder sou rce A1 2024 + 1.3 L i i s only m a r g i n a l l y h igher than the powder s o u r c e A1 2024 + 2.5 L i which conta ins the s a m e amount of p o t a s - s ium and 27 t i m e s the sod ium leve l . In addi t ion t h e r e is more sod ium in the ingot sou rce A1 2024 which does not show e m b r i t t l e m e n t than in both the powder s o u r c e A1 2024 + 1.3 L i and the ingot s o u r c e A1 2024 + 2.5 L i which do show e m b r i t t l e m e n t . The ev idence t h e r e f o r e i nd i ca t e s that the b r i t t l e n e s s of A 1 - C u - M g - L i a l loys is due to l i th ium i t s e l f r a t h e r than to sod ium or p o t a s s i u m i m p u r i t i e s that a r e in t roduced with the l i th ium.

One of the m a j o r benef i t s of l i th ium addi t ions to a l u - minum is of c o u r s e the modulus enhancement that it p r o v i d e s . Al though no inves t iga t ion of the m e c h a n i s m of th is enhancement has been conducted in th is study, t h e r e is ev idence that the m a x i m u m s t i f fnes s of s o l u - t ion t r e a t e d a luminum l i th ium a l loys occur s only a f t e r aging, ind ica t ing that a p r e c i p i t a t e r a t h e r than a so l id so lu t ion is r e s p o n s i b l e for the i n c r e a s e d modulus . F o r e xa mp le so lu t ion t r e a t e d A1 2020 a l l o y s (which conta in about 1 pct Li) show a m a x i m u m modulus a f t e r aging 500 h in the r ange 422 to 450 K. ~

CONCLUSIONS

I. Aluminum alloy powders containing up to 2.5 wt pet lithium were satisfactorily produced by meR-spin- ning onto a rotating copper disk using an inert gas flow to prevent lithium oxidation.

2. Extrusions from rapidly cooled powder compacts of 2024 and 2024 + lithium afloys had mechanical and physical properties similar to those produced from cast ingots of the same compositions. The more rapid

1920-VOLUME 10A, DECEMBER 1979 METALLURGICAL TRANSACTIONS A

cooling ol the powder did not reduce the embrittling effect of lithium in these alIoys.

3. Fatigue crack growth rates of extrusions from the rapidly cooled powder alloys of 2024 and 2024 + 2.5 pct lithium were 5 to 10 times lower than typical values for ingot source 2024.

4. Use of rapidly cooled powder of aluminum-lithium alloys for producing wrought products may alleviate the problems associated with production of sound ingots of these alloys.

ACKNOWLEDGMENT S

The author would like to thank Donald D. Crooks for valuable assistance with the experimental work, Harold

A. Kawayoshi for the t ransmission electron micros- copy and A. S. Gleason for the scanning election mi- croscopy.

RE FERENCES

1_ J. W. Evancho: Final Report on NADC Contract No. N62269-73-C-O219, 1974.

2. W. R. D. Jones and P. P. Das: J. Inst. Metals, 1959-60, vol. 88, pp. 435-43. 3. R. E. Maringer and C. E. Mobley: Proceedings o f the International Conference

on Rapid Solidification Processing, pp. 208-21, Reston, VA, Claitor's Pub- lRhing Division, 1977.

4. J. M. Silcox: J. Inst. Metals, 1959-60, vol. 88, p. 357. 5. B. Noble and G. E. Thompson: Met. Sei. J., 1971, vol. 5, pp. t 14-20. 6. W. G. Truckner, J. J. Staley, R. J. Bucci, and A. B. Thakker: Final Report on

AFML Contract F33615-74-C-5079, 1976. 7. T. H. Sanders and E. S. Balmuth: Met. Progr., 1978, vol. 114, pp. 32-37. 8. R. J. M. Payne and J, D. L. Eynon: British Patent 787665, 1957. 9. J. G. Sessler and V. Weiss: Aerospace Structural Metals Handbook

AFML-TR-68-115, vol. 11, January 1968.

METALLURGICAL TRANSACTIONS A VOLUME 10A, DECEMBER 1979-1921