ELSEVIER Journal of Materials Processing Technology 67 (1997) 62-66

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Materials Processing Technology

Thermal cycling processes in metal-matrix composites

M.J. Tan a,,, M.C. Chew a, N.P. Hung a, T. Sano b a School of Mechanical and Production Engineering, Nanyang T:'v/:nologica! Unirersity, Singapore~ Singapore

b Mechanical Engineering Laboratory MIST, MITI). lharaki. Japan

Abstract

Aluminium-matrix composites have attracted considerable academic and industrial attention in recent years, where modulus increases of 100% and strength increases of 60'¼, over conventional metal alloys have been reported. Further, the emphasis on the use of recyclable and environmentally-friendly materials has lent support to the aluminium industries vis-',i-vis other materials. The high specific strength and modulus makes this class of materials highly desirable for the aerospace and transport industries. Aluminium-matrix composites are, however, inherently brittle, thus making superplastic forming an alternate and attractive option.

In this work, a study of the behaviour of AA6061 reinforced with alumina particles produced by casting and extrusion, under the action of strain control and thermal cycling conditions, is presented. Comparisons are made with unreinforced AA6061 matrix, and also with the same composite re-extruded to obtain finer grain sizes. Elongations obtained via thermal cycling are compared with those from room temperature and i~othermal testing. The work also looks at the differences in cycling at different frequencies, range and rate, for the AA6061 matrix composites. © 1997 Published by Elsevier Science S.A.

Keywords: Temperature cycling process: Metal-matrix composite: Aluminum

I. Introduction

Internal stress superplasticity is the extensive elonga- tion that occurs in some materials under conditions where internal stress exists concurrently with deforma- tion, which enhances plastic flow in the direction of the applied stress and can result in high ductility during deformation. Generally there are two methods by means of which to induce internal stresses: (i) thermal cycling of a material through a phase change with concurrent application of a small external stress; and (ii) thermal cycling of a material with anisotropic coeffi- cients of thermal expansion whilst simultaneously ap- plying a load, on which this study using a metal-matrix composite is based. In contrast to fine structure super- plasticity, work dene on internal stress superplasticity is sparse [1-4].

In MMCs, the coefficient of thermal expansion (CTE) values of metal matrices are approximately 25 x 10-6°C - ~ and those for ceramic particles approxi- mately 5 x 10-6°C-~, i.e. a difference of a factor of about 5. The residual strain, or strain accumulation,

* Corresponding author. E-mail: mmjtan@ntuvax.ntu.ac.sg.

0924-0136/97/$17.00 © 1997 Published by Elsevier Science S.A. All rights Pll S0924,013 6( 96)0 2819-1

thus produced as a result of the thermal mismatch due to such large differences in CTEs between matrices and reinforcements can be calculated from the following equation:

e = A~ AT (1)

where AT is the temperature change during the solidifi- cation of the MMCs and A~ is the difference between the CTEs of the reinforcement and the matrix. Because of the thermal mismatch strains, the matrix in the region adjacent to the ceramic reinforcements experi- ences a residual stress concentration, where if the ther- mal strain is sufficiently high, the matrix yields, and the ensuing plastic flow as a result of partial relaxation of the residual stress leads to the formation of a disloca- tion network in the matrix adjacent to the interface.

In a typical 6061 AI/AI203 composite, a temperature difference in processing of about 600°C achieved during cooling can generate strains of approximately 1%. Un- der this strain, the estimated stress close to the inter- faces in the 6061 Al matrix is beyond the yield strength of a T6 treated AA6061 alloy, thereby potentially en- hancing the plasticity of composite materials. Further- more, most of the residual stress is located at the grain

reserved.

M.J. Tan et al. Journal o[ Materials Processhtg Technoh~gy 67 (1997) 62-66 63

Fig. !. Extruded AI_,O~ in AA6061 matrix.

boundaries due to the anisotropy of the grain structure, thus facilitating movement by sliding.

Temperature cycling of materials with anisotropic coefficients of expansion has also been found to lead to grain refinement [5], although grain growth may also occur when the material is exposed to high tempera- tures for prolonged periods of time, especially if the grain structure is not stabilised.

2. Experimental details

The materials used are cast l0 volume percent A I 2 0 3

reinforced AA6061 ( A I - M g - C u - Z n - M n ) as provided by Duralcan USA, which has been extruded at 35:1 (Fig. 1); and AA6061 extruded rods. The AI,O~ parti- cles in the former were originally 10 lain in size, but can be seen to be 5 pm or smaller after extrusion due to fracture during processing. A 500 kN Fogg and Young extrusion press was used for extruding compacts (di- ameter 32 mm and lengths up to 80 ram) at tempera- tures of 500°C and at a further 20:1. Immediate water quench was applied on the exit of the extrudates, after which the structures were peak aged (for room-temper- ature testing) and tested. Room temperature, isother- mal and thermal cycling tests were done on an Instron 8502 digital servohydraulic testing machine, with an output at full load of 10 kW, a DC current at full load of 52 A, an operating frequency range of 50-200 kHz

SpecJ~eu and Neaf i~ ¢7~il Dimension

[ - -

57.0

- - - - i 5.(I

-------___~,

Test Specimen Heating Coil

40.0

Fig. 2. Set-up for the test specimen and the induction heating coil (dimensions: mm).

and heating and cooling rates of 200°C min I. The test-piece and the induction coil arrangement are given m Fig. 2.

3. Temperature cycling profiles

The work here looks at the influence of the tempera- ture cycliog profiles on the l0 vol.% A1203 reinforced AA6061 matrix. Each of the parameters were changed for each test and compared with the standard, i.e.: a temperature range of 150°C (300 to 450°C), mean 375°C; a heating-up time of 90 s; a cooling-down time of 60 s; and a strain rate of 0.4 × i0 3 s - ~ (see Table 1).

For this composite, when the range was changed and extended to lower temperatures, i.e. the lower-bound temperature from 300 to 200°C, greater extensions, as anticipated, were not obtained. Pickard and Derby [1] found that for low thermal cycle amplitudes (e.g. a 80-200°C cycle), the deformation behaviour was simi- lar to that seen isothermally. Hevce, accordingly, for efficacy, the range should be greater from 240 to 300°C. It is noted, however, that the work mentioned [l] was based on much lower strain rates, and the test exposure time in that case was increased many fold; also the reinforcement particle size was 3 pm, which led to a more homogeneous microstructure.

Table 1 temperature cycling profiles of 10 vol.% AI_,O~ reinforced AA6~ol matrix

Test Type Range (°C) Mean (°C) Cycle (s) Strain Rate Elongation

Standard 150 (300-450) 375 90/60 0.4 x 10 --~ s -~ 28% Range 250 (200-450) 325 90/60 0.4 x 10 -3 s ~ 22% Mean 150 (200-350) 275 90/60 0.4 x 10- -~ s - ~ 36% Cycle 150 (300-450) 375 60140 0.4 x 10- 3 s-~ 28% Strain rate 150 (300- 450) 375 90/60 40 x 10 -3 s - ~ 64%

4

¢= _o

I = o l u

50%

45%

40%

35%

30%

25%

20%

15%

10%

5%

0%

M.J. Tan et al. / Journal of Materials Process#zg Technology 67 (1997) 62-66

28%

Room Isothermal TemPerature

50%

% 28%

E! 6061

16061 composite

13 Re-extruded 6061 composite

Thermal Cyclic

Fig. 3. % Elongation for various tests conditions and material systems.

When the mean of the temperature-cycled was re- duced to 275°C, the elongation increased by as much as a quarter, up to 36%, indicating that the effects of temperature cycling are more significant for a iower mean than for a higher mean (assuming that the range is kept constant). There was not much change recorded here in the elongation when changing the cycling rate to faster times.

By changing the strain rate, the effective stresses at the interface between the matrix and reinforcement is being changed and thus it would be expected that different grain deformation mechanisms would be ex- hibited! By increasing the strain rate in this case from 0,4 × IO-3 to40 × 10-3 s - =, the specimen increased its overall elongation to 64%, the largest amount obtained in this work. This was concluded to be the most effective way of obtaining an optimum combination of deformation mechanisms under thermal cycling and strain control conditions.

4. MMCs behaviour in different tests

4.1. Comparisons o f room temperature, ~othermal and thermal cyclic tests

From Fig. 3, it can be seen that improvements in ductility can be made at higher temperatures, either via isothermal testing at 375°C, or thermal cycling at around this temperature mean. (N.B. All tests here were performed at strain rates of 0.4 x 10-3 s - !.) Higher strengths and ductility were obtained for the specimens which were re-extruded, as finer grain sizes were ob- tained (Fig. 4). Specimens that were unreinforced showed the highest elongations, despite having the largest grain sizes.

It appears from the results that the larger reinforce- ments (10-15 ~tm) do not assist recrystallisation nor pin grain boundaries or limit grain growth at the high temperatures, as most of the sub-grain sizes are of

-~ 12 E =. 10

• 8 N ca 6

4 l , l l ,

E 2 O O'

6061 Re-exlruded 6061 Composite

Fig. 4. Grain size comparisons.

M.J. Tan el al. / Journal ~I" Materials Processing Tectmoh,gy 67 (1997) 62-66 65

80

70

6o ct 0 ,~ 50

= 40 0

w 30

20

10

0 (4 x 10)-5 (4 × 10~-4 14 × 10)-3 (4 × 10)-2

Strain Rate (/see)

Fig. 5. Elongation vs. strain rate for the thermal cyclic test [re-extruded composite).

comparative or smaller size ( < 10 lam) than the rein- forcements. Tht~ re-extruding these specimens has only a limited effect, except in securing smaller grains and sub-grains. The role of finer particle reinforce- ments is thought to be to produce a stable and fine grain structure, which latter will also enable more widespread plasticity throughout the material at the interfaces.

4.2. Strain rate sensitivity e.x'ponent

Fig. 5 shows that the strain rate control of the test is very important in obtaining optimum elongation: for each set of material and test conditions, this has to be determined. High temperature deformation is by a mixture of diffusion creep, grain boundary sliding, dislocation glide and dislocation creep. Figs. 6 and 7 present plots of strain rate versus stress. For the ther- mally cycled material (Fig. 7) at lower strain rates, the strain-rate sensitivity exponent m is low (from

a = K?,'"), and mirrors that of isothermal creep and the isothermal tests at lower strain rates (Fig. 6); but is larger at higher strain rates: m = 0.3 for the ther- mally cycled material; m = 0.17 for the :,.sothermally tested material. A high value of m is always associ- ated wzth a greater resistance to neck formation and failure is thus delayed; and superplastic materials have m values from 0.3 up to 1, the latter value being for a Newtonian viscous material.

4.3. Microstructure at fi'acture tip

Figs. 8 and 9 show the fracture tip of re-extruded composites tested at 10 -4 and 10 -2 s -~ There is evidence of equi-axed grains present, indicating that recrystallisation has taken place during high tempera- ture deformation. For the specimen that elongated 28% (10 -4 S - l ) , widespread cavitation can be seen, joining interfaces between the particles and matrices that have failed, rather than inter-granularly. For the

1.00E~O

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!

j .., ! ° i

-J 1.00E-03 .

= 1 i

1.00~04

t .00E-05 0.1

!

I i

Log S t ress

Fig. 6. Isothermal test.

1.OOE,~

1.00E-O1

i 1 O 0 ~

i

1.00E-O4

O.1

I i t i : ; i l i ~ : '- i : i ! ~ } i

t '1-- I J J i l l J _ - L . I - I

1 I i : i ~ ' ; ~ , ' t I t

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Log St ress

Fig. "7. Thermal cycfi¢ test.

! 10

i ¸ ~ /! ~ ! ~ % j

66: ~ i J . T ~ Materials Processhtg Technology 67 (1997) 62"66

Fig. 8. Strain rate 0.4 × 10 - ~ s - t; 28% elongation.

Fig. 9. Strain rate 40 x 10- 3 s - ~; 64% elongation.

10 - 2 S - l strained specimen, there was a lot less of the decohesion between the interfaces that caused cav- itation failure.

grain sizes, and more homogeneous plasticity through- out the material, should be used.

5. Conclusions

From this work, it can be shown that metal-matrix composites behave differently under the action of thermal cyclic stresses, and they can enhance the plas- ticity at the interface between the reinforcement and the matrix. There exists an optimum strain-rate condi- tion where this effect is maximum. For greater effi- cacy, finer reintbrcements that enable enhanced recrystallisation during processing, finer and stable

References

[1] S.M. Pickard and B. Derby. Mater. Sci. Eng., 135,4 (1991) 213-216.

[2] M.Y. Wu. J. Wadsworth and O.D. Sherby, Metail. Trans., IgA (1987) 451.

[3] G. Gonzalez-Doncel, S.D. Karmarkar, A.P. Divacha and O.D. Sherby, Comp. Sci. Technol., 35 (1988) 105.

[4] S.M. Pickard and B. Derby, Acta Metall. Mater., 38 (1990) 2537.

[5] R.C. Lobb, E.C. Sykes and R.H. Johnson, J. Met. Sci., 6 (1972) 33.