MACIEJ DYZIA, JÓZEF ĝLEZIONA, MARIOLA SATERNUS
Mechanism of nitride phases’ formation in the reaction
of liquid aluminium with nitrogen
Mechanizm powstawania faz azotkowych w reakcji ciekáego
aluminium z azotem.
ABSTRACT
STRESZCZENIE
The paper presents the main factors influencing the process of
nitriding aluminium. The ability of controlling the kinetics of the
reaction helps obtain an appropriate phase composition and
morphology of reinforcement. During the liquid-phase process, the
following factors determine the possibility of controlling the process
of the dispersion reinforcement phases’ formation: the matrix alloy
composition, the temperature, the reactive gas pressure and the time
of synthesis.
The presented research results refer to the potential for nitride
dispersion phases’ formation based on the reactions of liquid
aluminium containing Mg and Ti additions plus nitrogen, while under
increased gas pressure.
W pracy przedstawiono gáówne czynniki wpáywające na proces
azotowania aluminium. MoĪliwoĞü kontrolowania kinetyki reakcji
umoĪliwia uzyskanie odpowiedniego skáadu fazowego i morfologii
zbrojenia. W warunkach procesu ciekáofazowego czynnikami
decydującymi o moĪliwoĞci sterowania procesem powstawania
dyspersyjnych faz zbrojenia mogą byü: skáad stopu osnowy,
temperatura, ciĞnienie gazu reaktywnego i czas syntezy.
Prezentowane wyniki badaĔ dotyczą moĪliwoĞci wytworzenia
dyspersyjnych faz azotkowych w oparciu o reakcje ciekáego
aluminium zawierającego dodatki Mg i Ti z azotem w warunkach
podwyĪszonego ciĞnienia gazu.
INTRODUCTION
Nitriding reactions in those two cases can be written in the
following way:
2Al + N2 = 2AlN
(1)
2Al + 2NH3 = 2AlN + 3H2
(2)
0
At 700 C , the process enthalpy for reaction (1) amounts to 658.99 kJ/mole, while free enthalpy amounts to -429.87 kJ/mole; for
reaction (2), the enthalpy value (-549.23 kJ/mole) is higher, while free
enthalpy is lower (-547.09 kJ/mole). For reaction (2), the enthalpy and
free enthalpy values are similar. Ammonia is more active than
nitrogen, which results from its dissociation in high temperatures [1].
The nitriding process is influenced by factors such as:
a) nitrogen’s solubility in liquid aluminium and its alloys,
b) passivation effect,
c) reactivity of gases or gas mixtures used for the process (N2,
NH3),
d) alloying additions,
e) reaction atmosphere,
f) process temperature and pressure [1, 2, 3].
According to thermodynamically point of view, aluminium
nitriding is an exothermic process and is quite energetically
favourable throughout a wide range of temperatures. Fig. 1 presents
the temperature-dependence of enthalpy and free enthalpy (Gibbs free
energy) for nitriding reactions in the cases when nitrogen and
ammonia were the gases.
Fig. 1. Dependence of enthalpy and free enthalpy on temperature
for the nitriding process (forming of A1N) with the use of
nitrogen: 2Al + N2 = 2AlN (1) and ammonia: 2Al + 2NH3 =
2AlN + 3H2 (2) as reaction gases [1]
Rys. 1. ZaleĪnoĞü entalpii i entalpii swobodnej od temperatury dla
procesu azotowania (tworzenia AlN) z wykorzystaniem azotu:
2Al + N2 = 2AlN (1) i amoniaku: 2Al + 2NH3 = 2AlN + 3H2
(2) jako gazów reakcyjnych [1]
__________________________
dr inĪ. Maciej Dyzia , prof. dr hab. inĪ. Józef ĝleziona,
dr inĪ. Mariola Saternus
Politechnika ĝląska w Katowicach
Wydziaá InĪynierii Materiaáowej i Metalurgii
Nitrogen is practically insoluble in liquid and solid aluminium. At
the melting point, the solubility of nitrogen in aluminium amounts to
0.66 cm3/100g Al (8,25 ppm or 8.25.10-4). For a comparison,
hydrogen’s solubility in aluminium at the melting point amounts to
0.88 cm3/100 g Al (0.785 ppm) [4]. Nitrogen’s solubility does not
change significantly with the change of temperature [1]. Nitrogen’s
solubility in a liquid metal may increase under the influence of
appropriate alloying additions. From this perspective, manganese
turned out to be a favourable chemical element. Ca. 0.1 – 0.13 % of
nitrogen can be dissolved in pure manganese at a temperature of 1300
– 1400 0C. Under the same conditions, nitrogen’s solubility is much
lower (about 8 ppm). In industrial aluminium alloys containing up to
1.25% of manganese, nitrogen’s solubility increases seven times at
1100 0C. The studies conducted [1] showed, however, that a Mn
addition does not substantially improve the nitriding process.
Therefore, it can be confirmed that nitrogen’s solubility in liquid
aluminium alloys does not influence the nitriding process, especially
directly. Therefore,, dissolved and gaseous nitrogen’s low reactivity
with liquid aluminium is due to kinetic limitations.
The passivation effect connected with nitriding can be determined
by the Pikling-Bedworth ratio (PBR) – the ratio of products’ molar
volume to substrates’ molar volume (VAlN/VAl). If PBR is higher than
unity, the passivation effect is observed [5]. In liquid aluminium’s
15BBBBBBBBBBBBB,1ĩ<1,(5,$0$7(5,$à2:$BBBBBBBBBBBBBBBBBBB531
nitriding process, the PBR is higher than unity (e.g. ca. 1.14 at
aluminium’s melting point). As a result,, a protective surface layer of
nitrides (products) is formed, guarding aluminium against further
reactions. The formation of such a protective “shell” around the lance
introducing the reactive gas prevents its access to the liquid
aluminium, which inhibits the nitriding process. However, the
passivation layer’s strength can be reduced by adding the following
alloying elements to liquid aluminium: magnesium or silicon.
Ammonia decomposes in high temperatures (fig. 1), making it a good
source of reactive nitrogen. Kinetically, nitriding with ammonia is
more favourable than directly with nitrogen, due to a lower activation
energy for nitriding. However, autors [1] indicate that there is no
significant difference in the nitriding results obtained when different
reactive gases were used (nitrogen, ammonia). Other studies were also
conducted, during which liquid alloy AlMnMgSi underwent nitriding
by introducing the mixture N2 – NH3 (3 vol.-%). The result obtained
was similar to that obtained by nitriding with the incorporation of
nitrogen or ammonia.
The role of alloying additions such as: Mg and Si was studied and
elaborated on in works [6,7]. It was found that magnesium is a
necessary admixture, whose absence prevents the composite’s
formation [8].
It was found that magnesium and aluminium likewise can take the
form of Mg3N2. Obtained: magnesium nitride and magnesium oxide
occur in a porous form (PBR amounts to circa 0.77 at 900 0C) and not
compressed, as is the case with aluminium oxide or aluminium
nitride. By adding magnesium, the passivation effect still occurs
locally, in a porous form. Scholz and Greil [5] concluded that
magnesium has a catalytic influence on nitrides’ formation, enabling
the transferring of nitriding reaction from the alloy’s upper layer to
the whole alloy’s volume. LeHuy and Dallaire [7] found that both
magnesium and silicon should be present in the alloy.
The results of the studies they have conducted show that Al-Mg
and Al-Si do not result in a constant weigh growth, even at the
temperature of 1450 0C, while the Al-Mg-Si alloy indicates a weight
growth acceleration already after exceeding 1200 0C; it achieves the
nitriding effect yet before attaining the temperature of 1400 0C. It was
also found [7] that during the nitriding process of the Al-Mg-Si alloy,
magnesium loss from the alloy’s surface is observed, which is
signified by its transition into a volatile state, while the amount of
silicon in the alloy remains constant. Kagawa et al. [6] concluded that
due to magnesium loss and conversion of Al into AlN, the content of
silicon in the alloy increases. This can lead to a reduction in the
aluminium’s diffusion stream towards the reaction front and as a
result, to a decrease in the AlN’s growth rate. Therefore, it can be
claimed that Mg serves as an initiator, accelerating the nitrides’
formation effect in the alloy, while Si inhibits the process.
Due to aluminium content in the alloy, three areas of the nitriding
reaction mechanism have been distinguished [5]:
x forming a compact, nitride passivation layer on an alloy’s
surface,
x volume reaction, in which AlN is distributed throughout the
whole matrix,
x complete conversion of Al into AlN
A necessary, but minor condition in nitrides’ formation are
oxygen and humidity [6, 8]. When nitriding aluminium, oxygen’s
partial pressure should be reduced to a boundary value. When
nitrogen’s partial pressure is pN2 = 1 atm and the temperature T =
1000 0C, then oxygen’s critical, partial-pressure necessary for nitride
formation is below 10-20 Pa. However, nitride formation is observed
long before such partial-pressure is obtained. Based on relevant
Pourbaix diagrams, it is known that magnesium oxidation requires a
very low oxygen’s partial pressure (pO2 = 10-30 Pa). This allows
magnesium to serve as a local absorbent of gases, providing a liquid
metal to “see a reductive atmosphere”, which favours nitrides
formation [5].
The process temperature, plus atmosphere and alloy additions, is
an important variable in the nitriding process. The reaction between
nitrogen and aluminium proceeds at a temperature of above 700 0C.
While pure aluminium forms a stable passivation layer, aluminium
alloys such as Al-Mg allow constant nitride formations. After was
noticed that an increase in temperature from 800 to 1050 0C causes
the composite’s microstructure to become significantly finer, and
ceramics then dominates. This enables production of both composites:
with ceramic and metallic matrices. At variable temperatures,
Schweighofer and Kudela [9] determined a critical temperature at
which the reaction rate change corresponds to the change in reaction
mechanism. In Fig.2, T2 refers to the temperature in which the
reaction’s mechanism changes form surface into volume-type.
Accelerating the reaction in the area of volume mechanism takes
place at a temperature higher than T1, which is mainly caused by the
fact that the reaction is no longer thermodynamically stable. What
follows is a fast transition of Al into AlN, resulting in a sudden drop
of nitrogen’s pressure.
Autors [9] found that with an increase in nitrogen’s pressure there
is a decrease in temperature, causing a change in the reaction
mechanism from a surface-type into volume-type – Fig. 2. It was also
discovered that magnesium content influences the process
temperature. At a pressure of 1 MPa, nitrides for the Al-5%Mg alloy
will be formed at 1060 0C, while the Al-2%Mg alloy forms at
1260 0C. A 3% increase magnesium content in the alloy’s allows
decreasing the temperature by 200 0C. When increasing the nitrogen’s
pressure, the difference in temperatures is insignificant and
diminishes to 100 0C.
Fig. 2. Temperature and pressure curves as time functions
presenting the areas of proceeding reaction mechanism [9]
Rys. 2. Krzywe temperatury i ciĞnienia jako funkcje czasu
przedstawiające obszary zachodzącego mechanizmu reakcji
[9]
MATER IALS FOR EXPERIMENT
Aluminium alloy with a 5% addition of titanium was used as
matrix material by IMN OML in Skawina. The alloy’s composition
was modified by adding 3% of Mg. The 5.0 nitrogen of 99.999%
purity and 5.0 argon of 99.999% purity provided by Messer Polska
were used in the reaction.
EXPERIMENTAL STUDIES
A 300 g matrix alloy ingot was placed in a corundum crucible and
closed in a reaction retort, inside a resistance furnace. Afterwards, the
air was pumped out of the retort using a vacuous pump, while forcing
in argon. The process was repeated twice in order to ensure protective
atmosphere in the chamber. After the matrix had been melted, argon
was pumped out at a temperature of 780°C and nitrogen was forced
into the chamber, obtaining the pressure of 300 kPa. The sample was
heated to 1100°C and held at that temperature for 6 hours. Then, the
material was cooled in the furnace until it reached ambient
temperature. Upon opening the reaction chamber, a yellowish deposit
produced on the surface of the crucible was noticed. The authors [3]
observed a similar phenomen upon identifying the product as
magnesium nitride Mg3N2, which can confirm an indirect reaction
between the matrix’s liquid metal and nitride. Samples of the ingot
were taken for X-ray examinations, and metallographic microsections
were made to evaluate the structure of the materials obtained.
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Fig. 3. X-ray analysis of the Al5%Ti 3% Mg alloys after reaction with N2 at a temperature of 1100°C, 300 kPa pressure.
Rys. 3. Analiza rentgenowska stopów Al5%Ti z Mg po reakcji z N2 w temperaturze 1 100°C, ciĞnienie 300 kPa
Phase analysis was performed via X-ray diffraction on policrystals by
using the JDX-7S diffractometer from JEOL. The source of radiation
was a lamp with a copper anode, powered by a constant voltage of 40
kV and current of 20 mA.
Monochromatisation of the beam was performed on a graphite
monochromator. Phasal identification was supported by PCSIWIN
computer programme, based upon the JCPDS-International Centre for
Diffraction Data 2000 file, whose studies confirmed the presence of
AlN and Al3Ti phases.
Point 1
Point 2
(a)
1
3
Point 3
2
(b)
Fig. 5. a) Composite material’s microstructure,
b) EDS analyse area
Rys. 5. a) Mikrostruktura materiaáu kompozytowego,
b) obszar analizy EDS
Fig. 6. EDS phase analyse from fig 5 b area
Rys. 4. Analiza EDS z obszaru przedstawionego na rys. 5 b
Metallographic studies were done in Skawina’s IMN OML, using the
PHILIPS XL30 TMP electron microscope, empowered by EDAX’s
EDS system.. Fig. 4 presents the microstructure of the sample taken
15BBBBBBBBBBBBB,1ĩ<1,(5,$0$7(5,$à2:$BBBBBBBBBBBBBBBBBBB533
form the upper part of the ingot. With the use of EDS system (fig. 5b),
phases including magnesium, nitrogen and oxygen were identified in
the porous area; the presence of oxygen can be probably accounted
for by magnesium oxidation during the matrix’s alloy modification.
In point 2 the nitrogen’s atomic fraction was determined as
20.84%, oxygen’s: 11.21% (Al – 67.95 at.-%), while in point 3:
nitrogen’s 13.26 at.-%., oxygen’s 35.62 at.-%, Mg 2.72 at.-%. and Al
48.40 at.-%, and in point 4, only oxygen’s 56.12 at.-%, Mg 5.06 at.-%
and Al 38.79 at.-% These results, as well as the presence of the
Mg3N2 phase, can corroborate the presence of a mechanism resulting
in magnesium being the “carrier” of nitrogen reacting with liquid
aluminium. When the gas’s pressure affects the surface of liquid
metal, this reaction occurs on the surface only and can be inhibited by
passivation.
CONCLUSIONS
Both the X-ray analysis and chemical composition analysis in
microareas (EDS) confirmed the presence of the Al-N system’s
phases in the samples, in which the matrix’s alloy was enriched with
3% magnesium. The results confirm the hypothesis that the presence
of magnesium accelerates the intensity of the reaction between liquid
aluminium and nitrogen. Further research will concentrate on creating
better conditions for a fuller conversion of components. By extending
the reaction time at a temperature above 1000°C and by mixing liquid
metal, the process of forming AlN phases can be intensified in an in
situ reaction. Mechanical mixing is supposed to favour gas’s
penetration into the liquid alloy by fragmentation of the reaction’s
products layer, which is formed of the surface.
The research financed from funds allocated for science in the
years 2005-2007 under Research Project No. 1134/T08/2005/29.
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