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


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


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


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


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


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]


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


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.

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