A. KRA^UN et al.: MICROSCOPIC CHARACTERIZATION AND PARTICLE DISTRIBUTION ...
451–454
MICROSCOPIC CHARACTERIZATION AND PARTICLE
DISTRIBUTION IN A CAST STEEL MATRIX COMPOSITE
MIKROSKOPSKA KARAKTERIZACIJA IN RAZPOREDITEV
DELCEV V KOMPOZITU Z MATRICO LITEGA JEKLA
Ana Kra~un1,2, Matja` Torkar1, Jaka Burja1, Bojan Podgornik1
1Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
2International postgraduate school Jo`ef Stefan, 1000 Ljubljana, Slovenia
ana.kracun@imt.si
Prejem rokopisa – received: 2015-10-01; sprejem za objavo – accepted for publication: 2015-12-24
doi:10.17222/mit.2015.310
The purpose of this investigation was to identify the distribution of ultrafine particles in a steel matrix introduced through a
conventional melting and casting method, and above all to determine the methodology and analysing techniques suitable for the
analysis and identification of ultrafine particles incorporated into the steel matrix. In the frame of this work, steels dispersed
with Al2O3 ultrafine particles were produced by a conventional casting method and their microstructure investigated with light
microscopy (LM), scanning electron microscopy (SEM) and auger electron spectroscopy (AES). Microstructural analyses show
that the distribution of the Al2O3 ultrafine particles is non-uniform and has a high degree of agglomeration. Furthermore, for a
detailed analysis of the nanoparticles a specific preparation and characterization using advanced microscopic techniques is
required.
Keywords: particle distribution, microscopic characterization, steel matrix
Namen raziskave je bil ugotoviti porazdelitev ultrafinih delcev v jekleni matrici, ki je bila proizvedena s konvencionalnim
postopkom litja, predvsem pa dolo~iti metodologijo in analizne tehnike, primerne za analizo in identifikacijo ultrafinih delcev,
ki so bili vklju~eni v jekleno matrico. Delci Al2O3 so bili dodani med procesom konvencionalnega litja in so bili analizirani s
pomo~jo razli~nih analiznih tehnik, in sicer: z uporabo opti~nega mikroskopa (LM), vrsti~nega elektronskega mikroskopa
(SEM) in spekroskopije Augerjevih elektronov (AES). Analiza mikrostrukture je pokazala neenakomerno porazdelitev in
aglomeracijo Al2O3 delcev. Za podrobno analizo je potrebna karakterizacija mikrostrukture s pomo~jo naprednih mikroskopskih
tehnik.
Klju~ne besede: porazdelitev delcev, mikroskopska karakterizacija, jeklena matrica
1 INTRODUCTION
The insertion of ceramic reinforcements into metal
matrices to produce composite materials with improved
properties has been a subject of intensive research during
the past three decades.1–3 Ceramic particulates such as
borides, carbides, oxides and nitrides are added to metal
matrix composites (MMCs) to improve their elastic mo-
dulus, wear resistance, creep and strength.4–5
The ductility of MMCs, however, deteriorates at high
ceramic particle concentrations5. The metal matrix, the
so-called metal-matrix nano-composite (MMnCs) is
strengthened by nano-sized ceramic particles.6 These
nanoparticle reinforcements can significantly increase
the mechanical strength of the metal matrix, as they pro-
mote particle hardening more effectively than micro
particles. Moreover, MMnCs improve the performance
significantly at elevated temperatures, because the cera-
mic nanoparticles can maintain their properties at high
temperatures.6
Steel matrix composites commonly have a combi-
nation of hard ceramic (e.g., TiC, TiB2, WC and Al2O3)
reinforcements and a ductile metallic matrix, which
makes them promising candidates for high-strength and
wear-resistance applications. There are several methods
for fabricating particulate-reinforced steel matrix compo-
sites, such as powder metallurgy, conventional melting
and casting, reactive sintering and self-propagating
high-temperature synthesis (SHS). The casting process is
simple and more economical than the other available
routes for integrating nanoparticles into the microstruc-
ture of steel. However, it is extremely difficult to obtain a
uniform dispersion of ceramic nanoparticles in liquid
metals due to the poor wettability and the difference in
the specific gravity between the ceramic particles and the
metal matrix.7
The microstructure of metals is generally charac-
terized by advanced microscopic techniques (e.g., LM,
SEM and TEM) which probe and map the surface and
sub-surface structure of a material. These techniques can
use photons, electrons, ions or physical cantilever probes
to gather data about a sample’s structure on a wide range
of length scales.8 Auger electron spectroscopy (AES)
also provides quantitative elemental information from
the surfaces of solid materials.9
The current work aims at contributing to the knowl-
edge and understanding of the conventional casting route
for ultrafine particle inoculation in a steel matrix. This
Materiali in tehnologije / Materials and technology 50 (2016) 3, 451–454 451
UDK 621.74:620.18:669.13 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(3)451(2016)
production route seems to show potential and offers
more cost efficiency in achieving the dispersion of
second-phase ultrafine particles compared to the powder
and metallurgical techniques used until now. The aim of
the work was therefore to study the influence of Al2O3
ultrafine particles on the microstructure of a steel matrix
using a conventional casting method. The additional aim
is to determine the methodology and analysing tech-
niques suitable for analysing and necessary to identify
the ultrafine particles incorporated in the steel matrix.
2 EXPERIMENTAL WORK
2.1 Material
Austenitic stainless steel was used for the work,
mainly due to the distinctive two-phase microstructure of
austenite and ferrite. The chemical composition of this
alloy is given in Table 1. These are the most used group
of stainless steels. They are paramagnetic, have a face-
centred cubic lattice and excel with a good combination
of hot and cold workability, mechanical properties and
corrosion resistance.
Table 1: Chemical composition of austenitic stainless steel in mass
fractions, (w/%)
Tabela 1: Kemijska sestava avstenitnega nerjavnega jekla v masnih
dele`ih, (w/%)
Elements w/%
C 0.02
Si 0.33
Mn 1.24
Cr 17.4
Ni 10.1
Cu 0.36
Mo 1.29
V 0.08
As the reinforcement particles, commercial ultrafine
Al2O3 powder with a mean particle size of 500 nm was
used, as shown in Figure 1. The Al2O3 ultrafine particles
were selected due to their high chemical stability to Fe
and high specific gravity. In particular, it was reported
that the wetting angle  between Al2O3 and molten iron
alloy is less than 50°, even at high temperatures and in
many different types of atmospheres.10
2.2 Specimens preparation
A weighed quantity (10 kg) of the austenitic stainless
steel was melted in an induction furnace. In the first
experiment 20 g of the ultrafine Al2O3 particles were
wrapped in an Al foil and put into the ingot and the
molten metal was poured over it into the same ingot. In
the second experiment a mixture of 24 g of Al2O3 and
2.4 g of dry glue was prepared. The mixture was then
filled in the steel tube and flooded with paraffin. The
tube was inserted into the molten metal and when
melted, the molten metal was poured into the ingot.
2.3 Characterization
The microstructural changes and the dispersion of the
ceramic particles in the steel matrix were observed and
analysed using light microscopy (LM), scanning electron
microscopy (SEM) and auger electron spectroscopy
(AES). Samples for the microstructure analysis were
taken from the bottom, middle and top portions of the
cast piece. Metallographic samples were prepared by
grinding, polishing, followed by chemical etching and
analysed to reveal the particle distribution. Samples for
Auger electron spectroscopy were prepared by grinding
and polishing the surface. These samples were attached
to the bracket, placed in an experimental container-air-
lock, pumped to UHV and transferred into an analytical
container. The surface of the sample was ion etched and
analysed to determine the elemental composition in the
surface region of the sample.
3 RESULTS AND DISCUSSION
Figure 2 shows a LM micrograph of the microstruc-
ture of pure austenitic stainless steel with a distinctive
A. KRA^UN et al.: MICROSCOPIC CHARACTERIZATION AND PARTICLE DISTRIBUTION ...
452 Materiali in tehnologije / Materials and technology 50 (2016) 3, 451–454
Figure 1: SEM image of Al2O3 ultrafine particles at various magni-
fications
Slika 1: SEM-posnetek ultrafinih delcev Al2O3 pri razli~nih pove-
~avah
two-phase microstructure of austenite and -ferrites. A
LM micrograph of the microstructure and ultrafine
particles’ distribution of the sample produced by the
casting process of the austenitic stainless steel poured
over the Al2O3 ultrafine particles is shown in Figure 3.
As shown in Figure 3, the microstructure of the
austenitic stainless steel is modified after the addition of
Al2O3 ultrafine particles, being incorporated into the me-
tal matrix. However, the distribution of Al2O3 particles is
non-homogeneous and concentrated in a certain area.
In Figure 4 the particle distribution of the sample
taken from the second experiment, where the steel tube
filled with Al2O3 particles was inserted into the melt is
shown. As in the case of the first experiment, with the
molten steel being poured over the Al2O3 ultrafine parti-
cles, the distribution of the particles is non-uniform and
has a high degree of agglomeration (Figure 4). However,
the degree of particles is lower when inserting the parti-
cles-filled steel tube into the molten metal.
From the SEM elemental analysis, shown in Figure
5, it was confirmed that the bright, small, spot-like feat-
ures represent the Al2O3 ultrafine particles that are non-
uniformly distributed in the steel matrix.
In Figure 6 the AES spectrum of the Al2O3 ultrafine
particles in the cast microstructure of austenitic stainless
steel is shown. The spectra of particles (P1 and P2)
showing only O and Al peaks confirm the successful
introduction of Al2O3 ultrafine particles into the steel
matrix (P3) without any intermetallic reaction taking
place.
4 CONCLUSIONS
Steel matrix composites with non-uniformly dis-
persed Al2O3 ultrafine particles were produced by a con-
ventional melting and casting method. The purpose of
this investigation was to determine the methodology and
analysing techniques suitable for the analysis and identi-
A. KRA^UN et al.: MICROSCOPIC CHARACTERIZATION AND PARTICLE DISTRIBUTION ...
Materiali in tehnologije / Materials and technology 50 (2016) 3, 451–454 453
Figure 5: SEM elemental analysis of Al2O3 ultrafine particles in the
cast microstructure of austenitic stainless steel
Slika 5: SEM-posnetek elementne analize Al2O3 ultrafinih delcev v
liti mikrostrukturi avstenitnega nerjavnega jekla
Figure 3: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite and Al2O3 ultrafine particles
Slika 3: Lita mikrostruktura avstenitnega nerjavnega jekla s 6 % -fe-
rita in Al2O3 ultrafinimi delci
Figure 2: Cast microstructure of austenitic stainless steel with 6 % of
-ferrite
Slika 2: Lita mikrostruktura avstenitnega nerjavnega jekla s 6 % -fe-
rita
Figure 4: Cast microstructure of austenitic stainless steel with Al2O3
ultrafine particles inserted into the melt (experiment 2)
Slika 4: Lita mikrostruktura avstenitnega nerjavnega jekla z Al2O3
ultrafinimi delci, vstavljenimi v talino (preizkus 2)
fication of ultrafine particles incorporated in the steel
matrix.
The microstructural changes and the dispersion of the
Al2O3 ultrafine particles in the steel matrix were ob-
served and analysed by light microscopy (LM), scanning
electron microscopy (SEM) and auger electron spectro-
scopy (AES). This work clearly shows that for a proper
analysis and identification of the successful nano-parti-
cles’ incorporation, different analysing techniques need
to be used and combined.
Based on the experimental results the dispersion of
the Al2O3 ultrafine particles in the steel matrix is non-
homogeneous and concentrated in certain areas.
In order to be able to obtain a homogeneous distri-
bution of reinforcements in the metal matrices the
following factors need to be understood and taken into
consideration for future work:
• particle density, size, shape and volume fraction will
influence the reinforcement settling rate,
• surface properties of the particles will affect the
wetting with molten metal,
• rheological behaviour is influenced by the reaction of
the particles with the melt and each other,
• in general, the reinforcement particles occupy inter-
dendritic or between secondary dendrite arm spa-
cings, while the particle distribution is also metal-
matrix dependent.
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454 Materiali in tehnologije / Materials and technology 50 (2016) 3, 451–454
Figure 6: AES spectrum of the Al2O3 ultrafine particles in the cast
microstructure of austenitic stainless steel
Slika 6: AES-spekter analize Al2O3 ultrafinih delcev v liti mikrostruk-
turi avstenitnega nerjavnega jekla