# TEM Nanostructural Study of Al-6Si-3Cu-xMg Melt-Spun Ribbons

Authors: Ismeli Alfonso López; Cuauhtémoc Maldonado Zepeda; Gonzalo González Reyes; Ariosto Medina Flores; Juan Serrato Rodríguez
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

Three quaternary Al-6Si-3Cu-xMg (x = 0.59, 3.80, and 6.78 wt.%) alloys were produced by melt-spun and characterized using X-ray diffractometry (XRD), transmission electron microscopy (TEM), and microhardness techniques. Obtained second phases were Al2Cu(θ) for the alloy with 0.59% Mg and Al5Cu2Mg8Si6 (Q) for the alloys with 3.80 and 6.78% Mg. These phases are present as 30–50 nm or as 5–10 nm nanoparticles. Alloying elements content in solid solution increased, mainly for Si and Mg. The high alloying elements content in solid solution and the small α-Al cell size for melt-spun alloys leads to microhardness values about 2 times higher than those of ingot counterparts. The microhardness increase for melt-spun alloys with 3.80 and 6.78% Mg depends on Mg content in solid solution.


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## Body

## 1. Introduction

Among the aluminum alloys, the 319 is one
of the most widely used Al-Si-Cu-Mg alloys
in the automotive industry. This alloy presents Si contents from 5.5 to 6.5 and
Cu in the range from 3.0 to 4.0 wt% [1]. The cast structure for Al-Si-Cu-Mg alloys includes
α-Al, Si eutectic particles, Mg2Si, Al2Cu, Al5Cu2Mg8Si6, and other complex intermetallics [1, 2]. Applying a rapid
solidification process (RSP) as melt-spinning, it is possible to avoid or
minimize second phase formation and obtain a supersaturated solid solution. Melt-spun
produces metastable crystalline, nanocrystalline, and amorphous phases, with improved
mechanical properties [3].There was a previous work about the
characterization of Al-Si-Cu alloys [4] obtained by melt spun, remarking the
solid solubility extension of Si in α-Al. However, the effects of RSP on structure and properties when Mg is
added have not been reported in the literature. Mechanical properties
improvement for these alloys has been related to nanocrystalline and amorphous
phase formation. In this work, nanostructure and mechanical properties for the
experimental 319 alloy are dealt with by examining melt-spun ribbons with three
different Mg concentrations, analyzing the effect of Mg on
nanostructure and on the quantity of alloying elements in solid solution, and their relationship with the mechanical properties.


## 2. Experimental

The master alloy was an A
356 alloy ingot with Al-8.5Si-0.3 Mg (wt%), adding pure Cu and Mg. Castings
were carried out in graphite crucibles using an induction furnace under a controlled
Ar atmosphere. Ingots
were then remolten in a quartz tube and ejected through a 0.5-mm diameter
aperture on to the surface of a 200-mm in diameter polished
copper wheel rotating at
30 meters per second−1. The ribbons were 20–60 μm in thickness. Table 1 shows the alloys
chemical composition.Table 1
Average chemical
composition (in wt%) of the experimental alloys.
Alloy codeSiCuMgFeMnZnTiAlAM016.403.020.590.340.090.040.14BalanceAM036.313.033.800.320.080.030.13BalanceAM065.842.956.780.310.070.030.12BalanceExperimental
alloys were characterized using XRD, TEM/STEM, and microhardness techniques. XRD
measurements were carried out in a Siemens 400 X-ray diffractometer using CuKα radiation at 30 kV and 25 mA. TEM/STEM and HRTEM investigations were conducted using a FEG-Philips Tecnai F20 scanning/transmission
electron microscope operated at 200 kV, equipped with an energy dispersive spectrometry (EDS) with internal software
for qualitative, semiquantitative and quantitative chemical analysis. In order to obtain repetitive and
accurate results for each specimen 10 spectra were collected and averaged. Each
spectrum was collected in only 10 seconds to avoid the electron beam to
drift over the sample resulting in reduced statistics. Specimens for TEM were prepared by
dimpling using a Gatan 656 Dimple Grinder followed by Argon ions milling using
a Gatan 691 Precision Ion Polishing System (PIPS). HRTEM micrographs were
processed using Digital Micrograph(TM) 3.7.0 (Gatan). Microhardness
measurements were made with a Vickers diamond indenter in a Leitz Wetzlar
microhardness tester employing a load of 25 g for 15 seconds. Statistical treatment
criteria were used to get repetitive and accurate results.


## 3. Results and Discussion

In
ternary Al systems, previous works report ample solid solubility of alloying
elements inα-Al matrix and minimum second phase formation as result of rapid solidification [3, 4]. These effects are also expected for quaternary Al-Si-Cu-Mg alloys. XRD evidence in Figure 1(a) suggests limited solid
solution for the melt-spun ribbons. Besides diffraction peaks corresponding to α-Al, diffraction peaks associated to θ (Al2Cu) were visible for ribbons with 0.59% Mg, while peaks
associated to Q (Al5Cu2Mg8Si6) were
observed for ribbons with 3.80 and 6.78%. The intensity of Q peaks increased for the alloy with higher Mg content.
This suggests the content of free Q phase increases, an expected result because
of the high quantity of Mg in the alloy with 6.78% Mg. Despite the limited solubility, XRD analysis shows that melt-spun reduced formation of second phases. No evidence of diffraction
peaks belonging to Si, Al-Si eutectic, or other complex intermetallic phases
(present in conventionally cast alloys for this quaternary system [5]) were
observed from melt-spun XRD spectra. TEM techniques were applied to correlate
XRD diffraction peaks with microstructure and to further study second phases. Bright
field transmission electron micrograph of the ribbons shown in Figures 1(b)–1(d) revealed the presence of 30–50 nm second
phase irregular particles (arrowed). One can see the contrast
in the images, revealing the existence of other phases besides α-Al
matrix. Diffraction peaks belonging to θ and Q (see Figure 1(a)) are correlated to the
presence of these nanoparticles. The
presence of these second phases also corroborates the limited solid solubility revealed
using XRD. The quaternary Q phase is present as an equilibrium phase at most of
the compositions in these quaternary systems. In our study, the distinction
between the Q stable phase and its precursors (Q′, QC, and QP, all metastable
phases) was made taken into account
XRD results. These precursors are coherent phases [6, 7]. Nevertheless, a large
overlapping between different Qs phases exists and then the presence of
metastable or precursor phases can always be possible.XRD profiles of the ribbons for the alloys with
different Mg contents (a) and bright field transmission electron
micrographs of the as-melt-spun ribbons for the alloys with: (b) 0.59% Mg, (c)
3.80% Mg, and (d) 6.78% Mg. Diffraction peaks belonging toθ and Q are correlated to the presence of arrowed
nanoparticles.
(a)(b)Relationship between diffraction peaks
and particles commented above is based on EDS studies of regions with a darker
contrast observed in Figures1(b)–1(d). EDS analysis carried
out in particles of alloy with 0.59% Mg, as depicted in Figure 2(a), shows Al
and Cu signals. While analysis for particles in the alloy with 3.80% Mg, as seen
in Figure 2(b), shows Al, Si, Cu, and Mg signals. For the alloys with 0.59% Mg an
Al:Cu ratio close to 2:1 confirms that regions with a darker contrast observed
in Figure 1(b) correspond to θ. For the alloys with 3.80 and 6.78% Mg Al:Cu:Mg:Si ratios indicate that regions with a
darker contrast observed in
Figures 1(c) and 1(d) correspond to Q. These
compositions are consistent with XRD results.EDS analysis obtained
from the second phases:  (a) for ribbons with 0.59% Mg and
(b) for ribbons with 3.80% Mg.
(a)(b)Despite limited solid solubility, RSP led to
high concentration of alloying elements in the supersaturated solid solution (α-Al), as seen in Table 2. EDS analysis was used to
get comparable results. Alloying elements concentration in solid solution was increased
for melt-spun ribbons, mainly for Si and Mg. These high alloying elements in
solid solution are not possible using conventional casting even after
quenching.Table 2
Average atomic percent of alloying elements
in the supersaturated solid solution for the alloys obtained using rapid
solidification and conventional casting.
Alloy codeRibbonsIngotsSiMgCuSiMgCuAM019.00.21.31.40.10.3AM036.62.51.00.70.10.7AM064.46.70.90.30.11.5An exhaustive
analysis of the ribbons showed the presence of well spread approximately 5 nm nanoparticles
for all the melt-spun alloys. Dark field technique was advantageously used to
detect low-contrast nanoparticles. A TEM dark field image of ribbons with 0.59% Mg is observed in Figure3. Nanoparticles can be observed. This image was
obtained from the most intense halo ring of the diffraction pattern.Figure 3
TEM-dark
field micrograph showing the presence of nanoparticles for ribbons with 0.59%
Mg.We further
tried to clarify the details of nanoparticles by HRTEM. Figure4(a) shows HRTEM
micrograph of ribbons with 0.59% Mg. As noted, nanocrystallites
are present in the α-Al matrix. The fast Fourier transform (FFT) depicted in
Figure 4(b) shows the frequencies attributed to
crystallites of θ and the α-Al phase. The composition of the
nanosized particles was determined using nanobeam EDS (3 nm in
diameter spot size), indicating that it is close to Al2Cu (θ).
To study the morphology
of the nanosized particles inverse-FFT (IFFT) using θ frequencies were obtained (see
Figures 4(c), 4(d)). As observed, nanoparticles have irregular shapes. The
filtered image for the (200) plane showed the interplanar distance matching to
Al2Cu (3.04 Å).(a) HRTEM micrograph of
the alloy with 0.59% Mg showingθ nanocrystallites; (b) FFT of the image; (c) frequencies
corresponding to θ to produce the IFFT image and (d) IFFT image
isolating the θ nanoparticles.
(a)(b)(c)(d)The increase in Mg content (alloy with
3.80% Mg) also leads to formation of nanoparticles distributed in a similar
fashion as those of 0.59% Mg alloy. The nanobeam EDS
analysis of these nanoparticles revealed the presence of elements with ratios
near to the stechiometric ratios for Q phase (Al5Cu2Mg8Si6). Q phase
was originated by the
increase in Mg content and is also observed for the alloy with 6.78% Mg. Figure 5(a) shows an HRTEM micrograph for ribbons with 3.80% Mg. Q phase nanocrystallites
can be observed. The
FFT depicted in Figures 5(b), 5(c) shows the frequencies attributed to crystallites of Q
phase. IFFT image obtained for
Q frequencies revealed the irregular shape of nanoparticles (see Figure 5(d)). The interplanar distance (3.16 Å)
corresponding to Q(200) plane corroborates EDS results.(a)
HRTEM image of the melt-spun ribbons with
3.80% Mg showing
Q nanocrystallites; (b)
FFT of the image; (c) frequencies corresponding to Q to produce the IFFT image and (d) IFFT
image isolating the Q nanoparticles.
(a)(b)(c)(d)Table3 shows microhardness values for
ribbons and ingots. Ribbons hardness values are about 2 times
higher than those of ingots with the same composition. Hardening mechanisms of Al alloys can
be described by several
contributing factors: Hall-Petch hardening from the α-Al cell size, solid solution
hardening of α-Al matrix, and particle size
(according to Orowan hardening). Wang et al. [8] found that for melt-spun
alloys, microhardness is mainly governed by a combination of Hall-Petch
hardening from the small α-Al cell size and solid solution
hardening of the α-Al matrix, whereas the Orowan
hardening from the nanoparticles is negligible. The increase in microhardness for
the ribbons with 3.80 and 6.78% Mg could be related to the presence of a higher
Mg content in solid
solution, besides the increase of the volume fraction of Q phase.Table 3
Microhardness values for ribbons and ingots.
Mg content (wt%)Microhardness (Vickers hardness no.)RibbonsIngots0.59164.3±4.286.4±3.43.80183.4±4.188.2±2.66.78190.6±5.287.7±2.1


## 4. Conclusions

From XRD and TEM analysis, it can be concluded that using
melt-spun second phase formation was minimized and alloying elements
concentration in solid solution was increased. Second phases composition
depends on Mg content: Al2Cu for the alloy with 0.59% Mg and Al5Cu2Mg8Si6 for the alloys with 3.80 and 6.78% Mg. These phases are present as 30–50 nm nanoparticles. HRTEM studies also concluded the presence of these phases as 5–10 nm
nanoparticles. Microhardness values of melt-spun ribbons are between 160 and 190 VHN, about
2 times higher than those of conventionally cast alloys with
the same composition. Hardness improvement is
originated from the high alloying elements concentration in the supersaturated
solid solution and from nanostructure originated for the small α-Al
cell size, besides the presence of nanoparticles.



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*Source: 737546-2008-05-26.xml*