HIGH TEMPERATURE DEFORMATION OF AN ECAP Al-Mn-BASED...

23. - 25. 5. 2012, Brno, Czech Republic, EU

HIGH TEMPERATURE DEFORMATION OF AN ECAP Al-Mn-BASED ALLOY

Přemysl MÁLEKa, Miroslav CIESLARa, Peter SLÁMAb

a Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 5, 12116 Prague 2, Czech

Republic, [email protected]

b COMTES FHT a. s., Průmyslová 995, 33441 Dobřany, Czech Republic,

ABSTRACT

The equal-channel angular pressing (ECAP) is one of the most efficient methods of grain refinement. The

fine grain size can enhance the plasticity of materials at high temperatures and many ECAP alloys exhibit

even superplastic behaviour in the case that the fine-grained structure remains stable at high temperatures.

An Al-Mn-based alloy modified by the addition of Sc and Zr was produced using ECAP. The microstructure

investigation using TEM and EBSD methods revealed a mixture of well defined grains with the

submicrometer size, and severely deformed regions in the as-prepared state. Annealing at elevated

temperatures resulted in a recovery of the severely deformed microstructure. The submicrocrystalline

microstructure is retained after annealing up to 400 °C, a slight grain growth is observed at higher

temperatures. A relatively large fraction of low-angle boundaries is typical for the investigated material even

after annealing at the highest temperatures.

Special tensile tests performed at temperatures above 400 °C revealed the maximum values of the strain

rate sensitivity parameter m > 0.3 at strain rates of the order of 10-2

s-1

. The maximum ductility of 350 % was

found at 500 °C. This behaviour can be interpreted as the high strain rate superplasticity.

Keywords: Al-Mn-based alloy, ECAP, microstructure, strength, superplasticity

1. INTRODUCTION

The foils from Al-Mn alloys are used in automotive heat exchangers. These alloys are exposed to

temperatures very close to the melting point where recrystallization and significant grain coarsening can

result in deterioration of strength. In order to suppress these processes, the composition is modified by the

addition of Zr or Sc. Both elements form very fine particles of the Al3Zr and Al3Sc phases. The main

drawbacks of the Al3Zr phase are its inhomogeneous distribution and the transition from the metastable L12

modification to the stable DO23 modification at temperatures close to 400 °C resulting in a drastic decrease in

the stabilizing effect [1]. The Al3Sc phase forms directly in the stable L12 modification with excellent

stabilizing effect [2], however, Sc is extremely expensive and, therefore, the combination of Zr and Sc is

considered as a compromise. Homogeneously distributed particles of the Al3(ScxZr1-x) phase with a core-

shell structure are formed (Sc in the core, Zr in the shell) [3].

The fine-grained structure frequently improves plasticity of metallic materials and especially Al-based alloys

exhibit even superplastic behaviour if strained at elevated temperatures, e.g. [4, 5]. The very fine grain size

of about 250 nm was observed in the Al-Mn alloy processed by accumulative roll bonding [6]. The method of

equal-channel angular pressing represents an alternative route to the processing of very fine-grained

materials. The present paper shows the structure development and superplastic like deformation behaviour

of the ECAP Al-Mn alloy stabilized by Sc+Zr at high temperatures. The EBSD and atom force microscopy

investigations are used for the determination of the deformation mechanism operating under these

conditions.


2. EXPERIMENTAL MATERIAL AND PROCEDURE

The chemical composition of the studied alloy is given in table 1. The casting was annealed at 300 °C for 2

hours in order to support the precipitation of the Al3(ScxZr1-x) particles and then pressed at room temperature

through a die consisting of 2 channels (cross section 8 x 8 mm) intersecting at an angle of 90o. Samples with

1 to 8 passes were produced using the Bc rotation between subsequent passes.

The microstructure was studied using transmission electron microscopy both in the as-pressed state (after

ECAP) and after annealing at elevated temperatures. Additionally, the EBSD experiments were performed in

order to obtain data on the crystallographic orientation of individual grains and on the type of interfaces. The

deformation behaviour was investigated using tensile tests at temperatures above 400 °C. The tensile

samples with the cross section of 1 x 5 mm and gauge length of 17 mm were cut parallel to the ECAP

direction. To verify the potential for superplastic behaviour at elevated temperatures, special tensile tests

were performed – the samples were pre-strained to 10 % of elongation at the strain rate of 10-3

s-1

,

afterwards the strain rate was reduced to 10-5

s-1

and gradually increased in small steps up to 10-2

s-1

. The

strain rate sensitivity parameter m (defined as ∂log / ∂log ∂.

where represents the true stress and .

the

true strain rate), was evaluated from the individual strain rate jumps. The ductility was determined from a

classical tensile test performed at a constant crosshead velocity. The surface relief of the sample strained to

the elongation of 20 % at straining conditions corresponding to the highest values of the parameter m and

ductility was studied using atom force microscopy.

Table 1 Chemical composition of the investigated material in wt. %

Element Mn Sc Zr Fe Si Al

Composition in wt. % 1.35 0.27 0.23 0.072 0.034 balance

3. EXPERIMENTAL RESULTS AND DISCUSSION

The ECAP processing introduces a large deformation into the material. The fragmentation of original grains

starts already during the 1st pass. Figure 1documents that the microstructure after 8 passes contains grains

with the sub-micrometer size frequently arranged into bands. Simultaneously, strongly deformed regions

without any visible grain boundaries are still present. The EBSD experiments prove that, due to a strong

deformation, the crystallographic orientation cannot be determined at some places (white places in fig. 2).

Figure 3 shows no boundaries in these regions so that the misorientation of neighbouring “grains” is lower

than 2° adopted as a threshold value for the identification of low-angle boundaries. The microstructure also

contains numerous low-angle boundaries with misorientation between 2 and 15° (black in fig. 3). This result

is contradictory to the microstructure development in the AA7075 alloy where a high fraction of high-angle

boundaries was observed already after 6 ECAP passes [5].

Figure 4 documents well developed grain boundaries after annealing at 400 °C, however, some grains are

still strongly deformed. The majority of grains retain the grain size below 1 m. The EBSD experiments

performed on the sample annealed at 500 °C show a slight grain growth (fig. 5). The fraction of low-angle

boundaries remains high (fig. 6).


Fig. 1 The microstructure of the material after

8 passes of ECAP

Fig. 2 The EBSD orientation map of the material

after 8 passes

Fig. 3 The distribution of grain boundaries in the

material after 8 passes, low-angle boundaries (2 –

15°) in black, high-angle boundaries (> 15°) in red

Fig. 4 The microstructure of the material after

8 passes of ECAP and following annealing

400°C/ 30 min

The surviving fine-grained microstructure should be a good prerequisite for superplastic behaviour at high

straining temperatures. The measurement of the parameter m as a function of the number of ECAP passes

and straining temperature revealed the values slightly exceeding the value of 0.3 (which is frequently

considered as a bottom limit of superplastic behaviour [7]) in samples with at least 4 passes of ECAP if

strained at temperatures above 450 oC. Fig. 7 shows the strain rate dependences of the parameter m for

selected straining temperatures. All curves exhibit the maxima at the strain rate of the order of 10-2

s-1

. The

maximum values of the parameter m exceeding slightly the bottom limit of superplasticity suggest the

potential of the Al-Mn-Sc-Zr alloy to exhibit high strain rate superplasticity. To verify it, one sample after 8

ECAP passes was strained up to the rupture and the obtained ductility exceeds 300 % (fig. 8).



after 8 passes and following annealing 500 °C/ 1

hour

Fig. 6 The distribution of grain boundaries in the

material after 8 passes and following annealing

500 °C/ 1 hour

Fig. 7 The influence of straining temperature on the

strain rate sensitivity parameter m (4 to 8 passes) Fig. 8 True stress vs. elongation curve (8 passes)

The values of the parameter m above 0.3 and ductility values above 300 % can result either from the

operation of grain boundary sliding [7] or from the mechanism of viscous glide of dislocations [8]. In order to

distinguish between these two completely different mechanisms the material from the sample strained to

more than 300 % was tested using EBSD. Fig. 9 shows very small elongation of individual grains along the

tensile axis. A reasonably homogeneous grain growth occurred during straining and the fraction of low-angle

boundaries in the strained material was much lower than in the grip region (compare figs. 10 and 6). The

texture measurements revealed that straining resulted into a weaker texture as compared to the only

annealed grip region. All these observations prove that grain boundary sliding accompanied by grain

rotations play an important role during high temperature deformation of the studied alloy. A direct evidence

for the operation of grain boundary sliding was obtained from the AFM measurements. Figure 11 shows the

surface of the initially polished sample after straining to 20 %. The image was obtained in the mode where

different colours are attributed to places with different slopes of the surface. The places with the highest


slope are represented by the extreme black or white colours. In order to quantify the surface relief, a

horizontal line was selected within the scanned area and the corresponding height profile along this line is

visualized in fig. 12. The profile shows the mutual displacements of neighbouring grains up to 100 nm. The

operation of grain boundary sliding results into the conclusion that the deformation of the ECAP Al-Mn-based

alloy can be interpreted as the high strain rate superplasticity.


after 8 passes and following straining to about 300

% of elongation, tensile axis horizontal

Fig. 10 The distribution of grain boundaries in

the material after 8 passes and following

straining to about 300 % of elongation, tensile

axis horizontal

Fig. 11 The surface of the sample strained to 20%

at 500 °C, .

= 2x10-2

s-1

Fig. 12 The surface profile of the sample

strained to 20 % at 500 °C, .

= 2x10-2

s-1


CONCLUSIONS

An ultrafine-grained structure can be prepared in the Al-Mn-Sc-Zr alloy using ECAP. The

microstructure after 8 passes contains both individual grains with the submicrometer size and

severely deformed regions where grain boundaries are not well developed.

The microstructure is very stable up to the temperature of 500 °C. A high fraction of low-angle

boundaries was observed even in annealed samples.

The ECAP Al-Mn-Sc-Zr alloy exhibits deformation behaviour at the bottom limit of superplasticity

(parameter m > 0.3, ductility > 300%) at 500 oC and strain rates of the order of 10

-2 s

-1.

The EBSD and AFM measurements proved the operation of grain boundary sliding which is known

as the principal deformation mechanism during superplastic deformation. The deformation

behaviour of the Al-Mn-Sc-Zr alloy can be interpreted as the high strain rate superplasticity.

ACKNOWLEDGEMENT

The work was supported by the grant of the GACR N. 107-12-0921.

REFERENCES

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[7] EDINGTON, J. W., MELTON, K. N., CUTLER, C. P. Superplasticity. Prog. Mater. Sci., 1976, V. 21, p. 61-158.

[8] McNELLEZ, T. R., MICHEL, D. J., SALAMA, A. The Mg-concentration dependence of the strength of Al-Mg

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