Oxidation of Aluminium Alloy Melts and Inoculation … · Oxidation of Aluminium Alloy Melts and...
Transcript of Oxidation of Aluminium Alloy Melts and Inoculation … · Oxidation of Aluminium Alloy Melts and...
TECHNICAL PAPER TP 2606
Oxidation of Aluminium Alloy Melts and Inoculation by OxideParticles
Yun Wang • Hu-Tian Li • Zhongyun Fan
Received: 31 July 2012 / Accepted: 11 September 2012
� Indian Institute of Metals 2012
Abstract One of the main concerns in recycling alu-
minium alloy scrap is the removal of oxide inclusions.
Understanding the nature and behaviour of oxide films in
the alloy melts is an important step for developing efficient
recycling technologies. In this work, characterisation of
oxides formed in pure Al and Al–Mg alloy melts was
carried out. In commercially pure Al melt, c-Al2O3 plate-
lets and a-Al2O3 particles were found to form at 750 and
920 �C, respectively. The oxides were in the form of
liquid-like films consisting of numerous individual parti-
cles. The addition of 0.49 and 0.70 wt% Mg resulted in the
formation of MgAl2O4, and the MgAl2O4 particles were
{1 1 1} faceted and had a cube-on-cube orientation relation-
ship with a-Al. The MgAl2O4 films were also liquid-like in
which large numbers of the particles were held by the melt.
Grain refinement was achieved by intensive shearing of the
melts prior to solidification. It is believed that intensive melt
shearing broke up the oxide films and dispersed the potent
oxide particles which in turn enhanced the heterogeneous
nucleation, resulting in grain refinement. The potency of the
oxide particles and the mechanism of the inoculation by the
oxides were discussed on the basis of the TEM results and
theoretical analysis of the lattice misfits at the interfaces
along specific orientation relationships.
Keywords Oxidation � Aluminium alloy � Solidification �Grain refinement
1 Introduction
Aluminium oxides are usually treated as one of the fun-
damental defects detrimental to the performances of alu-
minium alloys. A major task during the melt cleaning
treatment in aluminium industry is to reduce or eliminate
the oxides prior to solidification. The detrimental effects of
the oxides on the microstructure and properties have been
extensively investigated in both cast and wrought alumin-
ium alloys. Previous studies have shown that oxide films,
particularly formed at liquid state at high temperatures, are
frequently associated with the casting porosity, hot tearing
and cracks, resulting in decreased strength, ductility and
corrosion resistance of the castings [1–3]. The structure,
composition, size and growth morphology of the oxides
themselves are greatly influenced by various alloying ele-
ments in the alloys and diverse processing conditions, so
that the mechanisms of their formation and growth
behaviour are not well understood yet. On the other hand,
as the solid phases in the melt prior to solidification, the
oxides could possibly act as inoculants to contribute to the
heterogeneous nucleation during the solidification process,
resulting in grain refinement.
Due to the high affinity between oxygen and Al, the
oxidation on the surface of Al alloy melts at high tem-
perature is inevitable when they are exposed to oxygen-
containing atmospheres. The oxides formed at the surface
of the melts are readily entrained into the castings by the
turbulence of melt handling such as stirring and pouring [1,
4–7]. In addition, alloying elements in Al alloys, Mg in
particular, will affect the mechanism of the oxidation and
thus the structure and morphology of the resultant oxides
will be altered. It is the entrained oxides inside the melts, in
the form of either films or separated particles, that play an
important role in affecting the subsequent solidification
Y. Wang (&) � H.-T. Li � Z. Fan
The EPSRC Centre for Innovative Manufacturing in Liquid
Metal Engineering, BCAST (Brunel Centre for Advanced
Solidification Technology), Brunel University, Uxbridge,
Middlesex UB8 3PH, UK
e-mail: [email protected]
123
Trans Indian Inst Met
DOI 10.1007/s12666-012-0194-x
process and thus determining the microstructure and
properties of the castings. To characterise the nature of the
oxides in the melt is therefore essential in order to mini-
mise their detrimental effects. However, the number den-
sity of the oxide is usually too low to find them with
electron microscopy. In this work, a pressurised melt fil-
tration was used to collect the oxides present in alloy melt
and the local number density of the oxides was therefore
considerably increased. This facilitated the observations of
the oxides by the available analytical technologies, such as
electron microscopy and X-ray diffractometry. The com-
prehensive characterisation of the nature of the oxides
formed in commercially pure Al melt and two dilute
Al–Mg alloys melts was present in this paper. Assessment on
grain size of the solidified materials has also carried out by
deliberately dispersing the oxide particles using intensive
melt shearing. The experimental results were discussed
based on the mechanism of heterogeneous nucleation and
in terms of the lattice matching at the interface between Al
and the oxides along specific crystallographic orientation
relationships.
2 Materials and Methods
Commercially pure Al, Al–0.7 wt%Mg and a modified
A380 cast aluminium alloy containing 9.38Si, 2.31Cu,
1.02Zn, 0.49 Mg, 0.26Mn and 0.80Fe (all in wt%) were
used in this work. The pure Al ingot (2.0 kg for each
experiment) was melted at 750 and 920 �C respectively,
and then oxygen was introduced by a ceramic tube into the
melts for 30 min in order to enhance the oxidation. Also,
2 kg Al–0.7Mg and 2 kg Al–9.38Si–2.31Cu–1.02Zn–0.49Mg
alloys were melted at 750 �C and then isothermally holding
at 700 �C for 2–4 h during the period both the melts being
subjected to slightly stirring. The melts of Al and the Al
alloys were then ready for the pressurised melt filtration. In
the melt filtration process, the melt (1.5 kg for each run)
was transferred to the pre-heated crucible of the filtration
unit. Argon was then introduced to the pressure chamber to
force the melt to flow through the porous ceramic filter
attached at the bottom of the crucible. Oxide films and
other solid particles were then collected above the filter in
the crucible together with the remaining melt. Then the
solidified alloy material above the filter containing large
number of the collected oxides was sectioned, mounted and
polished for metallographic and electron microscopic
examinations.
The scanning electron microscopy (SEM) examination
was carried out using a Zeiss Supera 35 FEG microscope,
equipped with an energy dispersive spectroscopy (EDS)
facility, operated at an accelerating voltage of 5–20 kV. To
prepare thin foils for conventional transmission electron
microscopy (TEM) and high-resolution TEM examina-
tions, slices from the filtered residue material were
mechanically ground and cut into 3 mm diameter discs.
The discs were then ground to a thickness of less than
80 lm and finally ion-beam-thinned using a Gatan preci-
sion ion polishing system (PIPS) under conditions of
5.0 kV and an incident angle of 4–6�. Conventional TEM
and high resolution TEM analyses were conducted on a
JEOL 2000FX and a Tecnai FEG F30 TEM microscope
operated at an accelerating voltage of 200 and 300 kV,
respectively. The phase crystallography and orientation
relationships were determined using selected area diffrac-
tion, high resolution TEM combined with EDS. The oxides
were also identified by X-ray diffractometry (XRD), which
was performed using a Bruker D8 Advance X-ray dif-
fractometer with Cu radiation at a voltage of 40 kV and a
current of 40 mA.
3 Results
3.1 Oxides Collected from Commercial Purity Al Melt
The SEM micrographs in Fig. 1 show the typical mor-
phology of the oxide collected from the commercially pure
Al melt which had been isothermally held at 750 �C for a
period of 4 h. The oxide formed in the pure Al melt at this
temperature appeared as the typical bifilms morphology at
low magnifications [1, 2], as shown in Fig. 1a. Detailed
observation revealed that these films were not necessarily
continuous solid films, but contained many extremely small
particles. As shown in Fig. 1b, it was also found that small
oxide particles were separated from each other and dis-
persed, as shown in Fig. 1b. TEM revealed that the oxide
particles, which were identified as c-Al2O3 by X-ray dif-
fractometry [8], exhibited the morphology of platelets
about 200–300 nm long and 40–60 nm wide, as shown by
the TEM bright field image in Fig. 2. Further detailed
observation by high resolution TEM indicated that these
c-Al2O3 platelets were morphologically faceted and also
twinned with both the surface faceting plane and the
twining plane being its {1 1 1}, as shown in Fig. 3.
Figure 4a shows the typical morphology of the oxide
films collected from the commercially pure Al melt at a
higher temperature 920 �C. It is seen that the oxide film
consists of many separated individual oxide particles rather
than continuous solid film, and that these particles are
about 0.5–1.5 lm in size, much larger than the c-Al2O3
particles formed in the pure Al melt at a lower temperature
750 �C (see Figs. 1, 2). In addition, it was frequently found
that a lot of such collected oxide particles were apparently
separated and dispersed, as shown in Fig. 4b, although the
melt was subjected only slightly stirring during the
Trans Indian Inst Met
123
isothermal holding at the temperature. XRD and electron
diffraction plus EDS in TEM revealed that the oxide par-
ticles formed at this temperature were a-Al2O3 and
exhibited a faceted morphology, as shown by the TEM
micrograph in Fig. 5.
3.2 Oxide Collected from Al–0.7 wt% Mg Alloy Melt
Oxides formed in Al alloy melts may be different from those
in pure Al melt due to the effects of the alloying elements on
the oxidation process and the formation mechanism of the
oxidation products. Figure 6 shows the morphology of the
oxide collected from Al–0.7 Mg alloy melt at 700 �C. It is
seen that a lot of individual discrete oxide particles were
associated with the films, which appeared as the morphology
of bifilms [1], Fig. 6a, whilst some of the oxide particles were
found to have no features of a film but were already dispersed
in the Al matrix, Fig. 6b, indicating that the particles in the
films were easily separated and dispersed by slightly stirring.
The oxide particles, which were about 300–800 nm in size,
were identified as spinel MgAl2O4 by electron diffraction
and EDS in TEM, with the evidence being given in Fig. 7. It
is seen that the fcc spinel MgAl2O4 apparently grew in the
melt in a faceted manner. The faceting of the MgAl2O4
particles was clearly observed by TEM as shown in Fig. 7a
where such a MgAl2O4 particle embedded in a-Al matrix is
faceted with its {1 1 1} crystal planes. Selected area electron
diffraction (SAED) patterns taken from various zone axes by
tilting this crystal confirmed that these collected oxide par-
ticles were MgAl2O4 which has an fcc structure with the
lattice parameter a = 0.80831 [9]. Two examples of such
SAED patterns along [3 1 0] and [1 0 0] zone axes are given in
Fig. 7b, c.
Fig. 1 SEM micrographs showing the typical morphology of the oxides collected from a commercially pure Al at 750 �C by melt filtration. The
oxide is identified as c-Al2O3 which exhibits either a discontinuous films or b separated particles (platelets)
Fig. 2 TEM micrograph showing the typical morphology of the
faceted c-Al2O3 platelets about 50 nm wide and 200–300 nm longFig. 3 High resolution TEM micrograph showing a c-Al2O3 platelet
which is faceted and twinned along its {1 1 1} crystal planes
Trans Indian Inst Met
123
TEM examination was also conducted to investigate the
interface between spinel MgAl2O4 and Al matrix. Figure 8a
shows the MgAl2O4 particles and Fig. 8b is the SAED pat-
tern taken from both the MgAl2O4 and the adjacent a-Al
matrix along their [0 0 1] zone axes across the MgAl2O4/a-Al
interface. Figure 8c gives the schematic indexing of the
SAED pattern, indicating that the same crystal planes and
the same crystal directions are parallel to each other for the
MgAl2O4 and a-Al crystals. From the evidence of the SAED,
it is clear that there is a cube-on-cube orientation relationship
(OR) between the MgAl2O4 and a-Al matrix, which is:
ð0 1 0Þ½0 0 1�MgAl2O4==ð0 1 0Þ½0 0 1�a-Al
This OR is much expected because MgAl2O4 and a-Al
have the same fcc crystal structure and the lattice
parameters for MgAl2O4 is 0.80831 nm [9], about double
of that (0.40494 nm) for Al.
Fig. 4 SEM micrographs showing the typical morphology of the oxides collected from a commercially pure Al at 920 �C by melt filtration. The
oxide is identified as a-Al2O3 which exhibits either a discontinuous films or b separated particles about 0.5–1.5 lm in size
Fig. 5 TEM micrograph showing the typical morphology of the
faceted a-Al2O3 particles collected from pure Al melt at 950 �C
Fig. 6 SEM micrographs showing the typical morphology of the oxides collected from Al–0.7 wt% Mg alloy melt at 700 �C by melt filtration.
The oxide is identified as spinel MgAl2O4 which exhibits either a discontinuous films or b separated faceted particles about 0.5–2.0 lm in size
Trans Indian Inst Met
123
3.3 Oxides Collected From Al–9.4Si–2.3Cu–1.0Zn–
0.49Mg Alloy Melt
For the Al–9.4Si–2.3Cu–1.0Zn–0.49Mg alloy, oxide films
containing large number of discrete particles were also
observed. Under optical microscope at low magnification,
the oxides appeared as curved dark films as shown in
Fig. 9a. However, as shown in Fig. 9b, SEM examination
revealed that the oxide films actually consisted of numer-
ous discrete particles held together by the liquid Al matrix.
Similar to the oxide in Al–0.7Mg alloy, the oxide particles
in the A380 alloy were identified as spinel MgAl2O4 by
XRD and electron diffraction plus EDS in TEM. Figure 10
gives an example of the TEM observation where the TEM
Fig. 7 a TEM micrographs showing a spinel MgAl2O4 particle embedded in a-Al matrix which is faceted with its {1 1 1} crystal planes; b and cSelected area electron diffraction (SAED) patterns taken from the spinel particle along its [3 1 0] and [0 0 1] zone axes, respectively
Fig. 8 a TEM micrograph showing a spinel MgAl2O4 particle embedded in an a-Al grain; b selected area electron diffraction (SAED) pattern
taken from both the spinel particle and the adjacent Al; and c the schematic of the pattern indexed along the [0 0 1] axis for both the spinel and Al
crystals, showing the cube-on-cube crystallographic orientation relationship between the two phases
Trans Indian Inst Met
123
bright field image (Fig. 10a) shows the faceted morphology
of several such MgAl2O4 particles and the two SAED
patterns in Fig. 10b, c were taken from [0 1 1] and [3 1 0]
zone axes of one of the MgAl2O4 crystals, respectively.
High resolution TEM examination was carried out on the
MgAl2O4/a-Al interface and the cube-on-cube OR was
once again observed. However, deviation from this OR was
also observed. For instance, the high resolution TEM
micrograph in Fig. 11 shows a MgAl2O4/a-Al interface
with both the MgAl2O4 and a-Al being viewed in their [0 1 1]
zone axes. The MgAl2O4 particles are clearly {1 1 1}
faceted, but the (1 -1 1) planes for MgAl2O4 and a-Al
deviated from each other, about 18� away from the cube-
on-cube orientation relationship. This deviation in orien-
tation may be attributed to segregation of the alloying
elements or impurities at the MgAl2O4/a-Al interface.
3.4 Grain Refinement by Inoculation of Oxide in Al–Mg
Alloys
As observed above, the oxide films in Al and Al–Mg alloys
were not necessarily continuous solid films and they
actually contained a large number of sub-micron sized
alumina and spinel MgAl2O4 particles, respectively.
Therefore the films can be broken up and the oxide parti-
cles dispersed. The dispersed crystalline oxide particles
Fig. 9 a Optical and b SEM micrographs showing the general view and detailed morphology of the oxide films collected from Al–9.4Si–2.3Cu–
1.0Zn–0.49Mg alloy melt by melt filtration. It is seen from (b) that the films actually consist of large number of spinel MgAl2O4 particles
Fig. 10 a TEM micrograph showing the morphology of several faceted MgAl2O4 spinel particles collected from the Al–9.4Si–2.3Cu–1.0Zn–
0.49Mg alloy melt; b and c selected area electron diffraction (SAED) patterns taken from [0 1 1] and [3 1 0] direction of the spinel particle,
respectively
Trans Indian Inst Met
123
could contribute to the heterogeneous nucleation during the
solidification process if these particles were potent for Al
and also had a sufficient number density. Experiments were
therefore carried out for the Al–0.7Mg alloy with intensive
melt shearing aiming to dispersing the oxide particles prior
to solidification. The sheared melts were then cast into TP-1
samples to assess the grain size. Figure 12 shows the rep-
resentative grain structures along the longitudinal sections
of the TP1 cast Al–0.7Mg samples with and without melt
shearing [10]. It is clear that the intensive melt shearing
resulted in significant grain refinement for the binary alloy.
The average grain size reduced from about 600 lm down
to about 350 lm [10]. In fact, without melt shearing, the
solidification structure of the TP1 sample exhibited
columnar grains, while with melt shearing the alloy had a
fully equiaxed grain structure with much finer grain size. It
was significant for the columnar to equiaxed transition
(CET) to be achieved by melt shearing prior to solidifica-
tion. The grain refinement resulted from melt shearing was
also observed in the commercially pure Al [8]. The
mechanism of the grain refinement is to be discussed in the
sections below.
4 Discussion
4.1 Oxidation of Al and Its Alloy Melt
Liquid Al oxidises readily at the surface when it is exposed
to an atmosphere containing oxygen and/or water moisture.
Oxidation of pure Al melt has been investigated at different
temperatures. It was found that a crystalline alumina,
usually c-Al2O3, nucleates and grows at melt/oxide surface
after a thin amorphous alumina layer initiates at the very
beginning. After the formation of the crystalline c-Al2O3,
the alumina will be subjected to a further transformation to
a-Al2O3 with increasing temperature and time [11, 12]. The
complete transformation from c-Al2O3 to a-Al2O3 was
reportedly achieved at 750 �C after an incubation period of
5 h in commercially pure Al [11], although the reported
temperature and time for this phase transformation to occur
and complete were not necessarily in agreement with each
other in the literature. This is probably due to the diverse
Fig. 11 High resolution TEM micrograph showing the interface
between a faceted spinel MgAl2O4 particle (right part) and Al matrix
(left part) with both the two crystals being view along [0 1 1] zone
axis. The (1 -1 1) planes for the two phases deviated by about 18�from each other
Fig. 12 Optical micrographs showing the longitudinal sections of the TP1 cast samples indicating a grain refinement achieved by intensive melt
shearing of the Al–0.7Mg alloy: a with shearing and b without shearing [10]
Trans Indian Inst Met
123
alloy systems and wide range of the oxidation conditions.
This study confirms the reported temperature range over
which both the c-Al2O3 and a-Al2O3 alumina form. More
significantly, the extensive SEM and TEM work in this study
revealed for the first time that the c-Al2O3 and a-Al2O3
films were not necessarily continuous solid films but con-
sisted of discrete individual oxide particles. This finding
can explain why the oxide films were found to be broken
up and the oxide particles dispersed by melt shearing in our
previous studies.
As an important alloying element, Mg is required in
majority of both cast and wrought Al alloys to achieve a
high strength. The presence of Mg in Al alloys increases
the oxidation tendency, particularly with increasing Mg
concentration. Depending on the Mg content, the oxidation
reaction usually starts with the formation of amorphous
MgO, MgAl2O4, or Al2O3, which then transforms to
crystalline MgO, MgAl2O4 or c-Al2O3 films, respectively.
In their investigation of the oxidation behaviour of
Al-2–12 wt% Mg alloy melts in air, Haginoya and Fukusako
[13] found that MgO was produced at early stage of oxi-
dation, and its amount increased temporarily and then
decreased gradually, while the amount of MgAl2O4
increased with increasing time. In fact, Mg can be effec-
tively segregated on the surface of Al alloy melt and such a
high Mg concentration allows MgO to form at the initial
stage of oxidation. Since MgAl2O4 is thermodynamically
more stable than MgO, the initially formed MgO will give
way to MgAl2O4 with increasing oxidation time. In the two
Mg-containing Al alloys in this work, MgAl2O4 was the
major oxide observed, in agreement with their result [13].
However, the significant findings through the extensive
SEM and TEM examinations are that there are large
numbers of MgAl2O4 particles in the oxide films and these
particles could be dispersed and thus act as heterogeneous
nucleation substrates provided that they are potent and
have an efficiently high number density in Al alloy melts.
4.2 Oxide Particles as Potent Heterogeneous
Nucleation Substrates
Experiments have demonstrated that intensive melt shear-
ing prior to solidification resulted in significant grain
refinement in both the commercially pure Al and dilute
Al–Mg alloys without addition of any grain refiner. In our
previous studies, it has been experimentally confirmed that
the intensive melt shearing can effectively disperse the sub-
micron oxide particles in both Al- and Mg alloy melts
[8, 10, 14–16]. If such dispersed oxide particles are potent
to act as heterogeneous nucleation sites, the reason for melt
shearing lead to grain refinement must be attributed to the
enhanced nucleation process by the dispersed oxide parti-
cles. A theoretical approach to assess the potency is to
calculate the lattice misfit at the interface between the
oxides and Al matrix along some specific orientation
relationships. The theoretically calculated lattice misfit,
which is defined as f = (dAl - dS)/dAl 9 100 %, where dAl
and dS are the atomic spacing along a specific direction on
the matching planes of the Al matrix and the oxide sub-
strate respectively, is given in Table 1. It is seen that the
lattice misfit with Al at 660 �C is 3.38 % for c-Al2O3,
-0.48 % for a-Al2O3, and 1.41 % for spinel MgAl2O4,
comparable with -4.22 % for TiB2 and 0.09 % for Al3Ti.
This indicates that, in terms of the lattice misfit at the
interface, the oxides observed in this work are as potent as
the commonly used grain refiner TiB2, although the
potency is affected by many other factors [6, 7]. In prac-
tice, in order to enhance heterogeneous nucleation and
achieve grain refinement, the nucleation substrates not only
need to be potent, but also need to have an adequate
Table 1 Calculated lattice misfit f between Al and some substrates at 660 �C
Interface
of Al/S
Crystal structure & lattice
parameters, nm
OR: (hkl)[uvw]Al//(h0k0l0)[u0v0w0]S d[uvw]Al,
nm
d [u0v0w0]S,
nm
f (%)
Al/MgAl2O4 Al: fcc, a = 0.41212, (111)[110]//(111)[110] 2 9 0.29141 0.57462 1.41
S: fcc, a = 0.81263
Al/a-Al2O3 Al: fcc, a = 0.41212, (100)[001]//(0001)[10-10] 0.41212 0.82818 -0.48
S: Rhomb. a = 0.47823; c = 1.30575
Al/c-Al2O3 Al:fcc, a = 0.41212, (111)[110]//(111)[110] 2 9 0.29141 0.56310 3.38
S: fcc, a = 0.79634
Al/Al3Ti Al: fcc a = 0.42112 (111)[110]//(112)[20-1] 0.29141 0.29116 0.09
S: Tetragonal, a = 0.3883, c = 0.8679
Al/TiB2 Al: fcc, a = 0.42112 (111)[110]//(0001)[11-20] 0.29141 0.30372 -4.22
S: hcp, a = 0.30372, c = 0.32368
S substrate. The lattice parameters are modified with the relevant thermal expansion coefficient. f = (d[u v w]Al - d[u0v0w0]S)/d[u v w]Al
Trans Indian Inst Met
123
number density, a proper particle size and a narrow size
distribution [17, 18]. With this understanding, the grain
refinement achieved by the melt shearing in Al and Al-
alloys can be explained in terms of the inoculation of the
oxide particles. It is the melt shearing process that dis-
perses the enormous oxide particles in the oxide films, and
thus provides enough number of the potent oxide particles
as nucleation substrates which enhance the heterogeneous
nucleation throughout the whole melt volume, resulting in
grain refinement.
The high resolution TEM results present in above sec-
tions have already shown the evidence of the potency of the
c-Al2O3, and MgAl2O4 as heterogeneous nucleation sub-
strates for Al. For instance, both c-Al2O3 and MgAl2O4 are
faceted with their closest packed {1 1 1} crystal planes
(Figs. 2, 3, 7, 8, 10), providing the required substrate sur-
faces on which for Al grain to nucleate. Furthermore, the
well defined cube-on-cube OR observed between MgAl2O4
and a-Al in the Al–0.7Mg alloy (Fig. 8) indicates that the
very spinel particle did nucleate the adjacent a-Al grain.
Along this OR, the lattice misfit at the interface between
MgAl2O4 and a-Al is very small, i.e., 1.41 % (Table 1).
Although specific ORs between c-Al2O3 and Al were not
directly observed yet in this work, the high resolution TEM
has shown that c-Al2O3 formed in the pure Al melt is {1 1 1}
faceted, indicating that the c-Al2O3 particles have the {1 1 1}
surface planes matching the same planes of Al grains so that
an interface with as low lattice misfit as 3.38 % can be
established, as indicated by the theoretical calculation in
Table 1. Well defined ORs at the interfaces between MgO
and a-Mg have also been observed in Mg alloys in the pre-
vious studies [16, 19], confirming further that some oxides
can be potent heterogeneous nucleation substrates. Indeed,
significant grain refinement has been also achieved by melt
shearing in Mg alloys [16, 20, 21].
5 Summary
Characterisation of oxides formed in commercially pure
aluminium and two dilute Al–Mg alloys (0.49 and
0.70 wt% Mg) has been carried out extensively by
advanced analytical electron microscopy. The oxides were
collected by pressurised melt filtration from the molten
pure Al and Al alloys so that direct examinations of the
oxides and their interfaces with Al matrix were facilitated.
The experimental results showed that the oxide films
formed in the pure Al and Al alloy melts were not neces-
sarily continuous solid films. In fact, the oxides were in the
form of liquid-like films consisting of densely populated
oxide particles embedded in the liquid Al. The oxide par-
ticles collected from Al melt were identified as c-Al2O3
platelets about 200 nm in length and a-Al2O3 particles
about 0.5–1 lm in size at holding temperatures 750 and
920 �C, respectively. The presence of 0.70 and 0.49 wt%
magnesium in Al and Al–9.4Si–2.3Cu–1.0Zn alloy resulted
in a change of the oxide from alumina to spinel MgAl2O4.
The MgAl2O4 particles in the liquid-like films in the Al-
0.7 Mg alloy were about 0.5–1.5 lm in size. High resolu-
tion TEM revealed that both c-Al2O3 and MgAl2O4
particles were morphologically faceted with {1 1 1} fac-
eting planes. Theoretical analysis and the extensive TEM
work confirmed that c-Al2O3, a-Al2O3 and MgAl2O4 were
potent substrates for heterogeneous nucleation of Al grains.
Assessment on grain size demonstrated that grain refine-
ment can be achieved by intensive shearing of the melts
prior to solidification. It was intensive melt shearing that
dispersed oxide particles in the films, and provided enough
number of the potent oxide particles which in turn
enhanced the heterogeneous nucleation, resulting in grain
refinement.
References
1. Campbell J, Castings, Second ed., Butterworth Heinemann, 2003.
2. Campbell J, Mater Sci Technol 22 (2006) 127.
3. Nyahumna C, Green N R, and Campbell J, AFS Trans 106 (1998)
215.
4. Dai X, Yang X, Campbell J, and Wood J, Mater Sci Eng A354(2003) 315.
5. Campbell J, Metall Mater Trans B, 37B (2006) 857.
6. Cao X, and Campbell J, Metall and Mater Trans A, 34A (2003)
1409.
7. Cao X, and Campbell J, Metall Mater Trans A 35A (2004) 1425.
8. Li H T, PhD Thesis, Brunel University, UK, 2011.
9. JCPDS. International centre for diffraction data (#21-1152);
2002.
10. Li H T, Wang Y, and Fan Z, Acta Mater 60 (2012) 1528.
11. Impey S A, Stephenson D J, and Nicholls J R, Mater Sci Technol,4 (1988) 1126.
12. Narayanan L A, Samuel F H, and Gruzleski J E, Metall MaterTrans A, 25A (1994) 1761.
13. Haginoya I, and Fukusako T, Trans Jpn Inst Metals, 24 (1983)
613.
14. Li H T, Wang Y, Xia M, Zuo Y, and Fan Z, in SolidificationScience and Technology, Proc. John Hunt International Sympo-sium, (eds.) Fan Z and Stone, Brunel University, Dec. (2011)
p 93.
15. Li H T, Wang Y, Fan Z, The 3rd International Conference on
Advances in Solidification Processes, Mater Sci Eng 27 (2011)
012047.
16. Fan Z, Wang Y, Xia M, and Arumuganathar S, Acta Mater 57(2009) 4891.
17. Greer A L, Bunn A M, Tronche A, Evans P V, and Bristow D J,
Acta Mater 48 (2000) 2823.
18. Quested T E, and Greer A L, Acta Mater 52 (2004) 3859.
19. Wang Y, Fan Z, Zhou X, and Thompson G E, Philos Mag Lett 91(2011) 516.
20. Fan Z, Wang Y, Zhang Z F, Xia M, Li H T, Xu J, Granasy L, and
Scamans G M, Inter J Cast Metals Res 22 (2009) 318.
21. Zuo Y, Xia M, Liang S M, Wang Y, Scamans GM, and Fan Z,
Mater Sci Technol 27 (2011) 101.
Trans Indian Inst Met
123