On the transformation characteristics of LA141A (Mg-LiAl) alloy

Journal of

ELSEVIER

Journal of Materials Processing Technology 56 (1996) 108-118

Materials Processing Technology

ON THE TRANSFORMATION CHARACTERISTICS OF LA141A (Mg-Li-AI) ALLOY

P. Crawford, R. Barrosa, J. Mendez, J. Foyos and O.S. Es-Said Mechanical Engineering Dept., Loyola Marymount University, LA, U.S.A.

ABSTRACT The use of Magnesium-Lithium LA141A-T7 alloy for nonstructural or secondary structural aerospace applications was abandoned mainly because of the loss in strength above room temperature. At temperatures of 90 - 150°C the ultimate strength decreased by about 50% from the room temperature strength. Two processing methods were used in this study in an effort to retain the strength at the intermediate temperatures. The first method consisted of a series of cold rolling and annealing treatments. The cold rolling was employed in the longitudinal, transverse and 45 ° directions. The second method was similar to the first but included a solution treatment at 315°C. Both methods resulted in a retention of strength at the elevated temperatures by stabilizing the structure and inhibiting recrystallization.

1.

INTRODUCTION

Magnesium-Lithium alloys are attractive for nonstructural or secondary structural applications for aerospace, missile and armor applications, [1]. The addition of lithium to magnesium with greater than 11 wt% (30 at %), causes the crystal structure to transform from HCP to B.C.C., which allows the magnesium to be cold worked. In addition, Mg-Li alloys are twenty-five percent lighter than conventional magnesium alloys, [1]. The alloy used in this study is LA141A. It has a nominal composition of Mg14Li-1A1, and an elastic modulus equivalent to those of commercial magnesium alloys. It is as light as most plastics but has the stiffness of a metal [1]. The mechanical properties of LA141A tested at temperatures between -240°C to 150°C indicate that it does not exhibit brittle characteristics at cryogenic temperatures, however it does lose strength at moderately elevated temperatures, above 66°C, [1]. The major hardening phase is MgLi2A1, 0 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10924-0136(95) 01826-Z

P. Crawford et al. / Journal of Materials Processing Technology 56 (1996) 108-118

109

phase, which transforms on overaging at intermediate temperatures to an equilibrium phase A1-Li [2-4]. The precipitation hardening mechanism in this alloy is similar to that observed in A1-Cu alloys [2,5]. The present study reports a thermomechanical process aimed to improve the strength of LA141A-T7 aged at temperatures between 66 and 121°C.

2.

EXPERIMENTAL WORK

2.1

Material

The LA141A-T7 magnesium-lithium bar was provided by Hughes Aircraft. It was naturally aged since August 1977, 16 years. The as-received bar was 565mm x 82.5mm x 57.2mm. The composition of the alloy is shown in Table 1.

Table 1, Chemical Analysis of LA141A-T7 alloy, in weight percent.* specification AMS-*'4386

Li

A1

13.0-15.0 1.00-1.50

Mn

Si

Cu

Ni

0.15

0.10

0.04

0.005

Fe

Na

M~

0.005

0.005

Balance

*As provided from Continental Metals, Inc., Inglewood, CA. **T7 stabilized condition, the treatment consists of heating for 3-6 hours at 177°C. 2.2

Thermomechanical Processing.

Two processing methods were used, Figure 1. In the first method 25.4mm thick samples were reduced 30% in thickness by rolling in the transverse direction (long transverse, TD), then annealed at 93°C and air cooled. Another reduction of 30% in the 45 ° direction, followed by the same annealing procedure was performed. The samples were then reduced approximately 85% in the longitudinal direction to a final thickness of 1.3 - 2.3ram and aged. The aging treatments were performed at 65°C, 93°C and 121°C for the time blocks of 0.5 hour, 1 hour and 5 hours. The second processing method was similar to the first except that an initial solution treatment was performed in an atmosphere of SO2 (6%) at 315°C for 30 minutes, to produce a complete solid solution [2], Figure 1. The first and second processing methods with and without solution treatment will be referred to as conditions I and II respectively. Jackson and Frost [1] did not indicate the time intervals at which LA141A loses strength above 66°C, accordingly time blocks of 0.5 hour, 1 hour and 5 hours were chosen as an aging time of exposure at 66°C, and above.

P. Crawford et al. /Journal of Materials Processing Technology 56 (1996) 108-118

110

2.3

Testing

The tensile data were d e t e r m i n e d at r o o m t e m p e r a t u r e at a c o n s t a n t crosshead s p e e d (1.27mm per minute) by using a screw d r i v e n I n s t r o n Machine (Model No. 4505) in a uniaxial tension mode. Each data p o i n t represents the mean of 3 samples. Rockwell superficial hardness m e a s u r e m e n t s (15T), light optical microscopy a n d electrical resistivity measurements were used to evaluate the aging characteristics. 25.4mm thick samples of LA141A

4

4

Condition II

4 Condition I

$ Homogenization at 315°C in an atmosphere of 6% SO2 for 30 minutes

$ Cold work to 30% Reduction in thickness in the Transverse direction

$

I

Anneal at 93°C/2 hours and air cool

[

$ Cold work to 30% Reduction in thickness in the 45 ° direction

4 Anneal at 93°C/2 hours and air cool

$ Cold work to ~ 85% Reduction in thickness till 1.3-2.3mm in the Longitudinal direction

$ Aging at 65°C, 93°C, and 121°C for 0.5 hour, 1 hour and 5 hours and air cool

4 Evaluation of Mechanical Properties Figure 1. Thermomechanical Processing of LA141A-T7 Mg-Li alloy

I

[

P. Craw/brd et al. / Journal of Materials Processing Technology 56 (1996) 108-118

. 3.1

111

RESULTS AND DISCUSSION Material Characterization

The microstructure of the as-received bar is shown in Figure 2. The grain structure is bimodal, Figure 2(a), the majority of the grains are equiaxed with a grain size of 25-35p,m, Figure 2(b). Coarse isolated grains of 65-85~m were observed in the longitudinal view, Figure 2(c), some extending to 20040011m in the transverse view, Figure 2 (d)

(a)

Co)

t

r

a

m

(c) (d) Figure 2: The grain structure of LA141A-T7 naturally aged from 1977 until 1993 with a bimodal grain structure, (a), a majority of fine grains, (b), and some isolated coarse grains,(c & d), (a), (b), and (c) are longitudinal views and (d) is a transverse view.

112

P. Crawford et al. /Journal of Materials Processing Technology 56 (1996) 108-118

Jackson and Frost [1], indicated that the material in sheet

form,.~with

a !higher e l a s t i c

is a n i s o t r o p i c m o d u l u s transverse to the

rolling direction. The same p h e n o m e n a was studied by McDonald [7] as a function of lithium content. Accordingly, to determine the effect of strai~:ht rolling in different m o d e s on the mechanical properties, 25.4mm thick samples were reduced 91% in thickness to 2.3ram along the length of the asreceived bar (longitudinally), across the width (transversely) and at a 45 ° direction between both. The results are summarized in Table 2. The ultimate and yield strengths were similar, however the difference in the ductility levels in terms of % elongation was significant. The h i g h e s t percent elongation is in the longitudinal mode (51%), while those in the transverse and 45 ° modes are comparable (-30%).

Table 2: Effect of Rolling Mode Property Tensile Strength KSI(MPa) Yield Strength KSI(MPa) Elongation % Electrical Resistivity ~cm

As received with a 57.2mm thickness

ASrolled from 25.4mmto 1.3-2.3mm Longitudinal Transverse

45°

22(151.6)* (minimum)

19.9 (137)

19.7 (135.7)

18.9 (130.2)

15.9(109.6)* (minimum)

18.7(128.8)

18.5(127.5)

18.4(126.8)

15 (minimum)*

51

29

32

21.3

17.9

18.1

18.0

*As provided by the specification sheets from Continental Metals Inc, Inglewood, CA, to Hughes Aircraft Co., Culver City, CA.

The deformation of the samples rolled in all three m o d e s s h o w e d similar patterns. The deformation pattern, for example, in the 45 ° direction is shown in Figure 3. The as-received grain structure was smeared off. The intermetallic particles which were spherodized and uniformly dispersed in the as-received bar, Figure 2, were fragmented and some were aligned as stringers parallel to the rolling direction, Figure 3(a). No evidence of partial recrystallization was observed. The electrical resistivity of the cold worked samples dropped by - 15% indicating a loss of supersaturation content and probably the precipitation of the A1-Li phase.

P. Crawford et al. / Journal of Materials Processing Technology 56 (1996) 108-118

75/zm (a) Figure 3:

(b)

The grain structure of LA 141A cold rolled to 2.3 mm in the 45 ° direction, (a) longitudinal view and (b) transverse view.

McDonald [2,7], in the early work, recommended a rolling mill practice, involving intermittent annealing, to create a stable condition before a considerable final cold rolling and subsequent aging at 25°C and 50°C. Cold work in this case actually accelerated softening at both temperatures. In this study inducing a stable condition was performed by cold rolling in the transverse and 45°directions (both with ~ 30% elongation) in equal increments and then performing the considerable cold rolling in the longitudinal direction. The intermittent annealing at 93°C produced recovery without recrystallization, Figure 1. 3.2

Conditions I and II

The objective of condition I (without solution treatment) was to measure the stability of the cold worked samples in resisting recrystallization (softening) at the intermediate temperatures of 66°C, 93°C and 121°C. The results are shown in Table 3. The ultimate and yield strengths of the aged samples were similar to the cold worked strength. This was also confirmed by the hardness measurements. The electrical resistivity data indicated ~ 15% drop in solute supersaturation content during processing, decreasing from 21.3gf2cm in the as-received state to 18.3gf2cm in the cold rolled state of condition I. However, the similar values of the electrical resistivity after aging indicated no further precipitation in all the aged samples. The only variation in the results shown in Table 3 is in the range of % elongation which is mostly between 30-35.

113

114

P. Crawford et al. / Journal of Materials Processing Technology 56 (1996) 108-118

Table 3: Mechanical Properties of C o n d i t i o n I (1.3-2.3mm thick sheet samples) W i t h o u t a Prior Solution T r e a t m e n t Thermal Yield Strength Treatment KSI(MPa)

Tensile Strength KSI(MPa)

Elongation %

Hardness (15 T)

Resistivity

Electrical

Cold Rolled Annealed 66°C/0.5h

19.9(137.1)

20.8(143.3)

30

64

g.Qcm 18.3

20(137.8)

20.9(144.0)

35.6

64

17.4

66°C/lh

19.4(133.7)

20.7(142.6)

38.0

63

17.6

66°C/5h

20.6(141.9)

20.9(144.0)

35

63

18.2

93°C/0.5h

20.0(137.8)

21.0(144.7)

35.5

61

18.2

93°C/lh

20.8(143.3)

20.9(144.0)

32.0

63

18.2

93°C/5h

20(137.8)

20.6(141.9)

40.0

63

17.6

121°C/0.51 20.4(140.6)

20.6(141.9)

30.0

64

18.9

121°C/lh

19.9(137.1)

20.6(141.9)

28.0

62

19.0

121°C/5h

19.2(132.3)

19.8(136.4)

30.5

61

19.0

(a) Figure 4:

(b)

The grain structure o f Condition I aged at 9 3 o c for 5 hours, (a) longitudinal view and (b) transverse view.

P. CrawJbrd et al. /Journal ~'Materials Processing Technology 56 (1996) 108-118

115

The inhibition of softening might be explained by Humphrey's theory [8]; during deformation, hard particles may deform if they are small or thin. A refinement of the particle dispersion will occur where the volume fraction increases and the interpraticle distance decreases causing retardation of the nucleation of recrystallized grains. The microstructure of all the samples aged at 66°C, 93°C and 121°C revealed a deformed microstructure with fragmented dispersoids, Figure 4. The objective of condition II (with a solution treatment) was to evaluate precipitation hardening after inhibiting recrystallization (softening) effects. The drop in solute supersaturation content was ~ 20% during processing, the electrical resistivity value was 21.3~f2cm in the as-received state and 17.1 in the cold rolled state of condition II. The mechanical properties of the samples aged at 66°C and 121°C are shown in Table 4. At both temperatures a hardening effect and a decrease in % elongation was observed which was confirmed by the variations in hardness and electrical resistivity changes, Figure 5. A drop in resistivity values was accompanied by a hardening effect. Similar to condition I, the microstructure of the samples aged at 66°C and 121°C revealed a deformed structure without partial recrystallization, Figure 6. 75

75

70 C, 65

55 22

12

,C- 65

: ..................

" ................. 20 40 60 80

100

120

55 22

"''

12

' ' '

140

20

i

40

T E M P E R A T U R E (°(2)

60

i

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80

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i i i

100

i

120

i

i

140

(b)

75 I

j

4t~.~

"I

m60 55 22

i

i

T E M P E R A T U R E (°C)

(a) 70

i

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Or"

i

20

I I I

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40 60 80 100 120 T E M P E R A T U R E (°C)

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(c) Figure 5: The isochronous hardness and electrical resistivity changes at 0.5 hour (a), 1 hour (b) and 5 hours (c).

P. CrawJbrd et al./ Journal ¢~'Materials Processing Technology 56 (1996) 108-118

116

Table 4: Mechanical Properties of Condition II (1.3-2.3mm thick sheet samples) W i t h Solution Treatment.

Yield Strength KSI (MPa)

Tensile Strength KSI (MPa)

Elongation %

Cold Rolled Annealed 66°C/0.5h

23.0 (158.5)

24.0 (165.4)

28.0

22.0 (151.6)

23.2 (159.8)

21.1

66°C/lh

23.7 (163.3)

24.3 (167.4)

66°C/5h

24.9 (171.6)

25.0 (172.3)

20.4

121°C/0.5h

27.7 (190.9)

28.4 (195.7)

16.3

121°C/lh

22.0 (151.6)

22.6 (155.7)

23.9

121°C/5h

24.6 (169.5)

24.7 (170.2)

1.6

Thermal Treatment

i

27.9

75ttm (a) Figure 6:

qo)

The grain structure o f Condition II aged at 121 o c for 5 hours, (a) longitudinal view and (b) transverse view.

117

P. CrawJbrd et al. / Journal of Materials Processing Technology 56 (1996) 108-118

Figure 7, summarizes the results of conditions I and II.

The results

reported by Jackson and Frost [1] are included for comparison.

"6"

~ 220

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t--, 170 t..9 Z 120

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40 60 80 100 120 140 TEMPERATURE (°C)

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40 60 80 100 120 140 TEMPERATURE (°C)

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REF (1)

®

REF (1)

(~

REF (l)

•

COND. I@ 0.5hr

•

COND. I@ lhr

#

COND. I@ 5hr

x

COND. 1I @ 0.5hr

X

COND. II@ lhr

×

COND. 1I@ 5hr

Figure 7:

(b)

(c)

The isochronous variations in mechanical properties at 0.5 hour

(a), 1 hour (b) and 5 hours (c).

1 1

40 60 80 100 120 140 TEMPERATURE (°C)

20

®

(a)

(1

P. CrawJbrd et al. / Jountal t~ Materials Processing Technology 56 (1996) 108-118

118

4.

CONCLUSIONS

In the time range of aging, 0.5 hour, 1 hour and 5 hours, the mechanical strength of LA141A is retained at 66°C and above. 1 -

2 - The inhibition of softening in condition I is related to the interaction of dislocations with second phase particles.

Acknowledgment D.F. Fost, R. Bojorques, F. Flores, P. Gealogo and F. Quintanilla are gratefully acknowledged for their help in the experimental work.

REFERENCES

[1] [2] [3]

[4]

R. J. Jackson and P.D. Frost, Properties and Current Applications of Magnesium - Lithium Alloys, Washington, D.C., NASA SP-5068, 1967. J. C. McDonald, Precipitation Phenomena in the ~ Phase of Mg-Li-AI Alloys of Low Aluminum Content, J. of the Institute of Metals, 97 (1969) 353. AA. Kazakov, M.A. Timonova and L.G. Borisova, Properties of Magnesium - Lithium Alloys, Translated by Plenum Publishing Corporation, Metallovendenie i Termicheskaya Obrabotka Metallov, No. 9, September, (1983), 31. R. M. Brodskaya, A.M. Glagoleva and B.D. Chukhin, Electron

Microscopic Study

[5] [6] [7]

of the Structure of Magnesium

Lithium ~ alloys,

Translated by Plenum Publishing Corporation, Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, May (1975) 78. A. Alamo and A.D. Banchik, Precipitation Phenomena in the Mg-31 at% Li-1 at % AI Alloy, J. of Materials Science, 15 (1980) 222. J. C. McDonald, Strength and Hardness of Some Beta Phase Mg-Li-AI Alloys of Low Aluminum Content, J. of the Institute of Metals, 99 (1971) 24. F. J. Humphreys, Recyrstallization Mechanism in Two-Phase Alloys, Met. Sci, 13 (1979) 136.