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Microstructure and Mechanical Properties of a Multi-modal Al Alloy with
High Strength
Xiaoning Hao1,a, Ruixiao Zheng1,b, Lirong Hao2,c, Han Yang1,d and Chaoli Ma1,e 1Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education,
School of Materials Science and Engineering, BeiHang University, Xue Yuan Road No.37,HaiDian District, Beijing 100191, China
3Hebei Sitong New Metal Material co., Ltd, Baoding Hebei 071100, China
[email protected] ,
[email protected] ,
[email protected] ,
[email protected] ,
[email protected]
Keywords: Al alloy, High energy ball milling, Multi-modal, Nanocrystalline.
Abstract. Nanocrystalline (NC) Al alloy powder was fabricated by milling 2024 Al alloy powder and
Fe-based metallic glass (FMG) particles. The NC Al alloy powder was consolidated into bulk sample
by adding a part of atomized coarse-grained (CG) 2024 alloy powder. The microstructure and
mechanical properties of powder and consolidated bulk materials were examined by X-Ray
Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy
(TEM) and mechanical test. It revealed that the FMG particles were uniformly distributed in the NC
aluminum alloy powder. In the consolidation process, the grain size increased, and Al2CuMg phase
precipitated. The multi-modal Al alloy by consolidation of FMG particles, NC and CG powder,
exhibited higher yield strength up to 517 MPa and better plasticity in comparison to the samples
without CG powder.
Introduction
Aluminum alloy is one of the most widely used materials in metal matrix composites as matrix
from research and industrial viewpoints [1]. It results from their outstanding properties, for instance,
light weight, high strength, high specific modulus, low thermal expansion coefficient and good wear
resistance [2]. But the development of nanostructure metals for structural applications must take into
consideration both fabrication of bulk samples and overall material performance. Bulk NC Al alloy
has shown significant improvement in hardness and strength when compared to their microcrystalline
equivalents. From the well-known Hall-Petch equation, we know that these enhanced properties are
caused by the reduced grain size. Besides grain size strengthening, improvement in mechanical
strength in such materials can also be achieved by the incorporation of reinforcement particles into the
material matrix [3]. Another way to improve these properties is precipitation into the metal matrix.
These particles strengthen the metal by blocking the movement of dislocations through the crystal
structure of the metal matrix.
The idea behind strength enhancement in dispersion strengthened materials lies in the introduction
of a high strength phase dispersed into the matrix. Nonetheless, to obtain a desirable microstructure
and to improve mechanical properties, it is essential for all the particles to be homogeneously
distributed in the mixture [1,4]. To improve the homogeneity of particle distribution, one can use the
mechanical process of ball-milling (BM) [4,5]. Compared with Al alloy, FMG is the strengthening
phase. What’s more, the diffusivity of Fe in Al is smaller than that of Al, so the microstructure is
expected to be thermally stable. In this paper, BM was used to produce a nanocomposite by dispersing
FMG particles into commercial 2024 Al alloy. The commercial 2024 Al alloy exhibits high strength
and can be heat treated. It is used for high loaded parts and components such as the skeleton parts on
the aircraft.
Materials Science Forum Vols. 745-746 (2013) pp 286-292Online available since 2013/Feb/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.745-746.286
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The focus on this work was to evaluate the strength and plasticity of the commercial 2024 Al alloy.
However, the increase in strength is always accompanied with some sacrifice in plasticity for NC
metals in comparison to their microcrystalline counterparts [6]. Ductility enhancement has been
achieved in nanostructure metals through the incorporation of larger grains in a fine-grained matrix.
David Witkin et al. observed this in tensile tests of Al-7.5Mg alloy [7]. A bulk composite with 10wt.
% B4C, 50wt. % coarse-grained 5083 Al and the balance NC 5083 Al was fabricated by Jichun Ye et
al. [8]. This multi-modal composite exhibited an extremely high strength (1065 MPa) in compression
at room temperature and the plasticity was improved to 2.5% after annealing.
In this paper, we describe the fabrication of multi-modal composite engineered for high strength
with enhanced plasticity. Unlike Jichun Ye’s work, our NC Al alloy powder was fabricated by milling
mixed gas atomized of commercial 2024 Al alloy powder and with FMG particles, which was
followed by blending with an equal quality of un-milled CG 2024 Al alloy powder and then
hot-extrusion. The microstructure was characterized by X-Ray Diffraction (XRD), Scanning Electron
Microscopy (SEM) and Transmission Electron Microscopy (TEM) while the mechanical properties
were determined by compression test. The results obtained are discussed from both the
microstructural and mechanical points of view.
Experimental procedure
The raw materials used in this study were 2024 commercial Al alloy and Fe-based alloy, which
were both produced by argon gas atomization process. The chemical composition of 2024 Al alloy is
4.0wt. % Cu, 1.5wt. % Mg and balance Al with 10-20µm in diameter. Fe-based alloy powder is a kind
of amorphous metallic with 5-20µm in size. A generalized manufacturing process for multi-modal Al
alloy is illustrated in Fig. 1. The milling powder, which contained 40wt. % FMG powder and 60wt. %
2024 Al alloy, was produced by using BM in a planetary high energy ball mill (XQM-2L) with a
stainless steel vessel and balls. The ball-to-powder weight ratio of 10:1 was used in all runs, with 8wt.
% alcohol as process controlling agent (PCA). The planetary high energy ball mill was operated at
280 rpm at room temperature. The milling time was 48h. The as-milled powder was transferred into a
vacuum glove box, in order to prevent from oxidation. The as-milled composite powder was
homogeneously blended with an equal quality of as-atomized commercial 2024 Al for 1h in the same
condition. The blended powder was extruded in a vacuum hot-pressing furnace at 500℃ for
compression test and microstructure analysis. An extrusion ratio of 10:1 was used for all composites.
Fig. 1 Typical manufacturing processing of multi-model Al alloy
The rod compression test samples are 3 mm diameter and a height in 6 mm. The rod samples were
compression tested uniaxially along the extrusion direction at room temperature. The samples, which
consisted of 20wt. % FMG and 80 wt. % 2024 Al alloy powder, were prepared in the same condition
for comparing.
Coarse-grained Al FMG powder
Milling for 48h
Blending with CG Al
Hot-extrusion at 500℃
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The microstructure characterization was carried out by a CamScan-3400 operated at 15 kV and
TEM using an FEI Tecnai G2 F20 instrument operated at 200 kV. Phase identification was performed
by a RIGAKU RINT-2000 X-ray diffractometer with Cu Kα radiation and an image plate detector
over the 2 range of 20°～90°at a 0.02º step size. The compression test was carried out by INSTRON
3367 machine.
Results and discuss
Fig. 2a. and Fig. 2b are the SEM images of the commercial 2024 Al alloy and FMG alloy powder
respectively. The Al alloy particles are sphere, but some of the FMG particles are irregular sphere, and
others are rod-like. The morphology of the as-milled powder is irregular shape. However, the
multi-modal composites powder has some spherical particles compared with the as-milled powder
without CG 2024 Al alloy, as shown in Fig. 2c and d. The spherical particles are CG Al particles and
they can improve the plasticity of the material. There are also some irregular particles, in which some
of them increased while others decreased. This is caused by the spherical Al particles repeated
welding and fracturing during the milling process. But FMG alloy belongs to fragility phase, its size
decreased with the increase of the milling time.
Overall, the grain size decreased after milled for 48h. This can also get from the XRD profile in
Fig. 3. The diffraction patterns show a small broadening and lowing of Al peaks after milling, which
may be a result of the deformation induced by the processing and the grain refinement and straining,
as can be got from the Scherrer formula:
Fig. 2 SEM for the powder of (a) the commercial 2024 Al alloy, (b) FMG alloy, (c) the commercial
2024 Al alloy with FMG alloy milled for 48h and (d) the multi-modal composites
Bcosθ
0.9λd = . (1)
Where d is the crystallite size, λ is the wavelength of the X-radiation used, B is the peak width at half
the maximum intensity, and θ is the Bragg angle [9].
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Figure 4 shows the backscattered electron SEM images of the multi-modal composite in the
transverse directions. The white regions correspond to the FMG particles, with a spherical shape and
needle-like, while the small gray dots are precipitated phases (Al2CuMg). In the Al-Cu-Mg series, the
main strengthening phase changes from plate-like θ’ (metastable form of Al2Cu) to rod-shaped S’
(metastable form of Al2CuMg). In the consolidation process, the grain size increased, and Al2Cu and
Al2 (Cu, Mg, Si, Fe, Mn) intermetallic-phases precipitated [10]. Al2Cu was also found during
sintering-extrusion sequence by C. Carreno-Gallardo et al. [11]. The gray regions are Al matrix.
Al2CuMg and FMG particles were uniformly distributed in the Al matrix. Presence of Al2CuMg was
also corroborated by XRD analyses, which is shown in Fig. 5.
The peak value of Al2CuMg is much smaller than that of Al. In the present case, Cu and Mg can be
soluble in Al [12]. As a precipitated phase, Al2CuMg can improve the strength of the material greatly:
inhibit grain growth and via Orowan mechanism hinder dislocation motion due to their small size and
homogeneous distribution. We can also find Al2CuMg in the CG regions of the bright and dark field
TEM images (Fig. 6). Because the presence of potent heterogeneous nucleation sites, such as grain
boundaries can prevent the S’ Al2CuMg phase from precipitating, and Al2CuMg precipitated within
the CG regions but not in the NC grains [12]. As a result of repeated cold welding and fracturing
during milling, the FMG particles were eventually uniformly distributed in the Al matrix. We also
have got this in the SEM image (Fig. 4). We saw the presence of trace FMG in the energy spectrum
but not in the TEM image. Maybe because the size of the FMG particles is too small. Sharp peaks are
observed in sample identified as “extruded”, indicating a grain growth in the composites after hot
extrusion processes.
Fig. 3 XRD profiles of the commercial 2024 Al
alloy powder, the multi-modal composites powder
BM for 48h and FMG alloy powder
Fig. 4 Backscattered electron SEM for the
multi-modal composites in the transverse
direction.
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Fig. 5 XRD profiles of the multi-modal composites (as-extruded and as-milled condition).
Fig. 6 Bright (a) and dark (b) field TEM images of the multi-modal composites in the transverse
direction.
The mechanical properties of the multi-modal composite were evaluated with the compression
test. The compression stress-strain plots are shown in Fig. 7. The sample without CG particles
fractured before yielding, but the strength is the highest among the three kinds of samples. It is notable
that the introduction of 50wt. % of CG particles can greatly improve the plasticity of the material.
Compared with the as-atomized 2024 Al, the yield strength increased about 23.4%, with only a slight
reduction in the plasticity. And the plasticity of multi-modal composites represents more than a
four-fold increase over the sample without CG particles. The results show a correlation between
plasticity and the proportion of CG powder. This had been demonstrated by David Witkin et al. [7]
and Jichun Ye [8]. The high yield strength of this multi-modal composite is attributed to various
microstructural features: (1) particulate strengthening from the FMG particles; (2) grain size
strengthening from the NC Al; and (3) Orowan strengthening from the dispersoids (Al2CuMg)
formed during extrusion.
Conclusions
A bulk composite consisting of FMG, NC Al alloy and CG Al alloy was produced by BM and
subsequently hot-extrusion with the FMG particles distributed uniformly in the NC Al matrix.. The
microstructure played an important role in mechanical properties. The present study provided an
290 Advances in Functional and Electronic Materials
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approach to engineer nanostructure materials with enhanced strength and plasticity derived from a
multi-scale microstructure. This multi-modal composite exhibited a high strength (517 MPa) and a
good plasticity with the failure strain of 21% in compression at room temperature.
Fig. 7 Compression curves of extruded the samples
Acknowledgement
The financial support was provided by Beihang University (BUAA). The authors would like to
thank Hebei Sitong New Metal Material Co., Ltd (STNM) for providing BM experimental facilities to
carry out this work.
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