Proceedings of the 12th International Conference on
Aluminium Alloys, September 5-9, 2010, Yokohama, Japan
©2010
2010 The Japan Institute of Light Metals
pp. 1567-1572

1567

Creep Deformation Mechanism of Al-Mn Alloy Sheetswith a Wide Range of Grain Size
Shoichi Hirosawa 1, Shoji Yokawa1, Akinori Sakai1,
Makoto Ando2, Akio Niikura2 and Yoshikazu Suzuki2
1

Department of Mechanical Engineering and Materials Science, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPAN2

Furukawa-Sky Aluminum Corp., Uwanodai, Fukaya-shi, Saitama 366-8511, JAPAN

Creep performance of Al-1.05mass%Mn alloy sheets with various grain size has been investigatedand correlated to the microstructural parameters such as average grain size, residual soluteconcentration in the matrix and volume fraction of Mn-containing compounds. The obtained grain
size dependence of creep characteristics (e.g. rapture time and minimum creep rate) suggests that
50-70µm is an optimal grain size that shows the most excellent creep performance at 523K under45MPa. The appearance of such an optimal size could be explained by inversely changing sizedependence of two creep deformation mechanisms; i.e. the strengthening by coarse grains due to theretained solid-solution strengthening by supersaturated Mn and the softening by coarse grains due tothe inhomogeneous deformation of the 2mm thick sheets with few grains in the thickness direction.
Keywords: Al-Mn alloy, creep characteristics, strengthening mechanism, microstructure, grain size

1. Introduction
Al-Mn alloys have been utilized as a fin material for heat exchangers due to their high formability,
moderate mechanical strength and excellent corrosion resistance. The creep performance of the alloys
is also important because heat exchangers are expected to be in service at more elevated temperatures.
The authors [1] reported that prolonged heat treatment at 873K before blazing (e.g. 1000hr) results inlonger rapture time and smaller minimum creep rate during creep of an Al-1.05mass%Mn alloy at473-523K. The predominant strengthening factor was found to be the strengthening by coarse grains,rather than by solid-solution or dispersion strengthening, in marked contrast with that of 0.2% proof
stress at room temperature. However, such a prolonged heat treatment at high temperatures isindustrially impractical so that different routes to fabricate coarse grained sheets; e.g. strain-anneal
grain growth treatment and low-reduction rolling treatment, are worthy of being developed.
In this work, creep characteristics of Al-1.05mass%Mn alloy sheets with various grain size havebeen investigated, and correlated to the microstructural parameters (i.e. average grain size, residual
solute concentration in the matrix and volume fraction of Mn-containing compounds) in order tomake clear whether similar improvement in creep characteristics could be achieved by the coarse
grained sheets fabricated through strain-anneal grain growth treatment and low-reduction rolling
treatment. The grain size dependence of creep performance was also clarified in conjunction with thecreep deformation mechanisms for which microstructural evolution during creep and geometric
relationship between average grain size and thickness of the creep test specimens were taken into
account.
2. Experimental
The chemical composition of the alloy utilized in this work is listed in Table 1. Three different routes
after hot-rolling were adopted to fabricate 2mm thick sheets with various grain size; i.e. (A)cold-rolled by 70-80%, annealed at 643K for 2hr and heat-treated at 873K for 1 to 1000hr (same as the1568

specimens in [1]), (B) cold-rolled by 70-80%, annealed at 643K for 2hr, stretched by 2.5-10% andheat-treated at 873K for 10hr (strain-anneal grain growth treatment) and (C) cold-rolled by 10 or50%, annealed at 643K for 3hr and heat-treated at 873K for 10hr (low-reduction rolling treatment).
Hereafter, the name of each specimen is regarded as A-1000hr, B-2.5%stretch+10hr, C-50%rolling+10hr and so on. The specimens for optical microscopy were prepared either by chemical etching
using 0.5% hydrofluoric acid for observation of Mn-containing compounds or by anodic oxidation
using a Baker’s regent (2.7% hydrofluoroboric acid) for observation of crystal grains. Optical
microstructures were observed using a microscope and quantitatively estimated through image
analysis software. Residual solute concentration of i in the matrix, Ci, was estimated from electricalconductivity (in %IACS) measured by an eddy current tester through the Matthiessen’s law [1]. The
specimens for creep test were machined from the sheets with a gage length of 30mm and a rectangularcross section of 5mmX2mm. Creep rapture test was performed in air by lever arm creep machines at
T=523K under σ=45MPa. The creep strain was measured every 10s using a linear gage to an accuracyof 1/1000mm.
Table 1 Chemical composition of the alloy utilized in this work (mass%).
MnFeSiCuTiZn

MgAl

1.05

0.61

0.24

0.15

0.02

0.02

0.01

Bal.

3. Results
3.1 Optical microstructures
Fig.1 shows optical microstructures of A-1000hr, B-2.5%stretch+10hr, C-50%rolling+10hr andC-10%rolling+10hr specimens. The distribution of crystal grains is well illustrated by the differencein polarization contrast, and thus the average grain size D can be clearly quantified. From the fact thatall D values of B and C specimens are much larger than D of A-1000hr specimen, it was confirmedthat both of strain-anneal grain growth treatment and low-reduction rolling treatment are easier
methods to fabricate coarse grained sheets.
Fig.2 illustrates optical microstructure of each specimen showing distribution of Mn-containing
compounds. Equiaxed compounds are observed with diameters of <10µm in A-1000hr and B-2.5%
stretch+10hr specimens, whereas elongated compounds with several tens of µm in longer axis are
inhomogeneously distributed in C-50%rolling+10hr and C-10%rolling+10hr specimens. The Mncontaining compounds were identified by X-ray diffraction (XRD) analysis to be a mixture ofα-Al(Mn,Fe)Si and Al6Mn phases, as expected from the phase diagram of Al-Mn-Si system at 873K
[2]. As for residual Mn concentration in solid solution CMn, furthermore, because the final heat
treatment at 873K was identically applied to all the specimens, CMn was limited to a narrow range
from ~0.3mass%Mn (A-1000hr) to <0.5mass%Mn (C-50%rolling+10hr and C-10%rolling+10hr).
3.2 Creep characteristics
The creep curves at 523K under 45MPa are shown in Fig.3 for A-1000hr, B-2.5%stretch+10hr,C-50%rolling+10hr and C-10%rolling+10hr specimens. It is obvious that creep characteristics of B
and C specimens are inferior to those of A specimen, as indicated by shorter rapture time tr and larger．
minimum creep rate εmin (i.e. minimum slope of tangent in secondary creep region). This suggeststhat strain-anneal grain growth treatment and low-reduction rolling treatment are not as effective in
improving creep performance as prolonged heat treatment at 873K (i.e. A-1000hr). Fig.4 illustrates．
the grain size dependence of tr and εmin for all the investigated specimens crept at 523K under 45MPa.
It is clarified from the positions of apexes on the convex curves that 50-70µm is an optimal grain sizethat shows the most excellent creep performance for the 2mm thick sheets.

1569

Fig.1 Optical micrograph of each specimen showing distribution of crystal grains.

Fig.2 Optical micrograph of each specimen showing distribution of Mn-containing compounds.

B-2.5%
stretch+10hr

C-10%rolling+10hr
C-50%
rolling+10hr

A-1000hr

A-1000hr

Fig.3 Creep curve of each specimen at 523K under 45MPa.

1570(a)A-1000hr
A-100hr
A-10hr
A-1hr
B-2.5%stretch+10hr
C-50%rolling+10hr

C-10%rolling+10hr
B-10%stretch+10hr
B-5%stretch+10hr(b)
B-5%stretch+10hr

C-50%rolling+10hr
A-1hr
A-10hr

B-2.5%stretch+10hr

B-10%stretch+10hr
C-10%rolling+10hr

A-100hr
A-1000hr．
Fig.4 Grain size dependence of (a) rupture time tr and (b) minimum creep rate εmin for all theinvestigated specimens crept at 523K under 45MPa.
4. Discussion
Creep performance of Al-1.05mass%Mn alloy sheets was found to strongly depend on the averagegrain size within the investigated range of 30 µm to a few mm (Fig.4). Unfortunately, strain- anneal
grain growth treatment and low-reduction rolling treatment were not effective in improving creep
performance (Fig.3) although much larger grain size than that of A-1000hr specimen wassuccessfully produced (Fig.1). However, 50-70µm was found to be an optimal grain size that shows
the most excellent creep performance, and thus becomes a target for any routes to fabricate 2mm
thick sheets. The appearance of such an optimal size could be explained by inversely changing size
dependence of two creep deformation mechanisms, as schematically illustrated in Fig.5.
In general, the strengthening by coarse grains (i.e. solid arrow in Fig.5) is believed to occur under
the creep test conditions within diffusional creep or plasticity region of deformation mechanism maps
[3, 4], whereas the softening by coarse grains (i.e. broken arrow in Fig.5) works mainly in dislocation
glide region; e.g. Hall-Petch equation. The experimental conditions of this work; i.e. T/Tm=0.56Tm
and σ/G=~2.1X10-3 (Tm: melting temperature, G: shear modulus), are likely to locate within
dislocation glide region, or at least dislocation creep region, so that a new mechanism is needed toexplain such a positive grain size dependence of creep performance (i.e. solid arrow in Fig.5)
applicable even in the dislocation glide or dislocation creep region. Transmission electron
microscopy (TEM) observation of microstructural evolution during creep suggested that large
precipitates of Mn-containing compounds are formed along grain boundaries, leading to the1571

decreased contribution of solid-solution strengthening by Mn atoms in the matrix. This type of creep
deformation mechanism will be feasible in the alloys with supersaturated solid solution at creep test
temperatures, and therefore the coarse grained specimens are thought to possess few chances to form
such grain boundary precipitates, resulting in the retained solid-solution strengthening by 0.30.5mass%Mn (Fig.2). Remember that all the specimens crept in this work possess supersaturated
solid solutions because of rapid water-quenching after the final heat treatment at 873K.
On the other hand, the softening by coarse grains (i.e. broken arrow in Fig.5) is a result of thedecreased number of grains existing along the thickness of creep test specimens. The horizontal axis
in Fig.4 shows not only the average grain size D in the range of 30 µm to a few mm but also thenumber of grains N in the range of 66 grains to less than one grain for the 2mm thick sheets. Thelarger D values are, the more inhomogeneously the specimens deform because of the fewer grains in
the thickness direction, resulting in the lower creep strength. By the combination of such two creep
deformation mechanisms, therefore, the investigated Al-Mn alloy sheets are supposed to show themost excellent creep performance at a grain size of 50-70µm.

Fig.5 Schematic illustration of grain size dependence of (a) rupture time tr and (b) minimum creep．
rate εmin during creep of Al-Mn alloy sheets. Inversely changing size dependence of two creep
deformation mechanisms is also shown by solid arrow for the strengthening by coarse grains or bybroken arrow for the softening by coarse grains.
5. Conclusions
Grain size dependence of creep characteristics of an Al-1.05mass%Mn alloy has been investigatedusing the sheet specimens fabricated through three different routes; i.e. prolonged heat treatment athigh temperatures, strain-anneal grain growth treatment and low-reduction rolling treatment. The
optimal grain size within the investigated range of 30µm to a few mm was found to be 50-70µm at
which the most excellent creep performance was obtained at 523K for the 2mm thick sheets. The
appearance of such an optimal size could be explained by inversely changing size dependence of two
creep deformation mechanisms; i.e. the strengthening by coarse grains which occurs in coarse
grained specimens with supersaturated Mn and the softening by coarse grains at which the specimens
are inhomogeneously deformed because of few grains in the thickness direction.
Acknowledgements
The authors gratefully acknowledge Mr. Y.Ogaki (present: IHI Metaltech Co.,Ltd) and Mr. S.
Matsuda (present: Yokohama Fire Prevention Bureau) at Yokohama National University for their1572

experimental assistance. The generous financial support by The Light Metal Educational Foundation
Inc. is also acknowledged.
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Skrotzki and G.Gottstein (Wiley-VCH, Weinheim, 2008), pp.1532-1538.
[2] G.Effenberg: COST507 Group B, Definition of thermomechanical and thermophysical properties
to provide a database for the development of new light alloys, Critical evaluation of ternary systems,
3 (1998), pp.201.
[3] H.J.Frost and M.F.Ashby: Deformation mechansim maps, (Pergamon press, Oxford, 1982), pp.
26-28.
[4] K.Kitazono: J. Japan Inst. of Light Metals, 59(2009), 458-463.