Download Environmental Embrittlement Characteristics of the AlFe and AlCuFe

Journal of New Materials for Electrochemical Systems 8, 97-100 (2005)
c J. New. Mat. Electrochem. Systems
Environmental Embrittlement Characteristics of the AlFe and AlCuFe Intermetallic Systems
M. Salazara , R. Pereza * and G. Rosasb
a Programa
de Investigación y Desarrollo de Ductos, Instituto Mexicano del Petróleo, Eje Central Lazaro Cárdenas 152, Col. San Bartolo
Atepehuacan, 07730 México D. F
b Instituto de Investigaciones Metalúrgicas, UMSNH
P. O.Box 52-B, 58000, Morelia, Mich., México.
( Received September 24, 2004 ; received in revised form December 20, 2004 )
Abstract: In this work, environmental factors as a function of the alloy compositions were evaluated in order to characterize the embrittlement
process in AlFe and AlFeCu intermetallic alloys. This research was carried out subjecting different alloys to different atmospheric conditions
such as: water vapor, hydrogen and oxygen atmospheres. Through these experiments it has been possible to evaluate the characteristics of the
embrittlement process in the AlFe and AlFeCu systems. The experimental results show that the embrittlement phenomenon tends to decrease with
the increment of the crystallographic phases. Also, the environmental embrittlement time (EET) increased as the aluminum solubility in different
phases is increased. The experimental results show that the water vapor has a strong influence on the intermetallic deterioration and decreasing
the embrittlement time. All the experiments demonstrate that the time of embrittlement under water vapor + O2 were shorter than under the other
atmospheric conditions.
Key words : Alloys, Intermetallic Compounds, Metal, Quasicrystals.
1.
INTRODUCTION
AlFe structure has been found to be the driving force in the
formation of the AlFeCu quasycristalline phase[10] . Thus, both
structures share similar properties, for example, they are brittle in nature. It is, therefore, expected that the AlFeCu suffers
similar embrittlement mechanism than the AlFe intermetallic
system.
The environmental embrittlement mechanism in intermetallic
compounds has been studied in the past[1−10] . This mechanism
indicates that these materials are brittle due to a chemical reaction occurring on the intermetallic compound surface. This
phenomenon is mainly related with a reaction between the humidity and aluminum. Such reaction produces an Al2 O3 layer
and monoatomic hydrogen. The oxide layer is a porous material
and allows the diffusion of hydrogen toward the intermetallic
compound. The fugacity of the hydrogen produced by the reaction is greater than the molecular hydrogen. This monoatomic
hydrogen mechanism is the main reason for the environmental
embrittlement.
In this work, we used an experimental arrangement to accelerate the environmental embrittlement process in the AlFe and
AlFeCu alloys under different atmospheres: H2 O (vapor), O2
and H2 . Also, it has been possible to evaluate the embrittlement
process as a function of the alloy composition, the lattice parameters and the structure type in these systems. Furthermore,
an embrittlement evaluation of the addition to the AlFe alloy of
different elements such as: B, Ce, Co, Ni, Li has been carried
out. These minor additions are commonly used to enhance the
ductility properties of these intermetallic compounds.
The AlFe intermetallic compounds are closely related to the
AlFeCu quasicrystalline phase[11−13] . For example, the B2∗ To
whom correspondence should be addressed: email: [email protected]
97
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2.
M. Salazar et al./ J. New Mat. Electrochem. Systems 8, 97-100 (2005)
Experimental Procedure
Different AlFe and AlFeCu alloy compositions were obtained
using melt spinning technique. In the case of AlFeCu, the aluminum content was varied from 50 to 75 at.%, with a constant
Cu/Fe relationship of 1.33. In the AlFe case, several elements
were added in amounts of 1, 2 and 4 at%. These elements were
Ni, Si, Ti, Cr, Co, Ce, B and Li. These binary alloys with minor elemental additions were based on Al60 Fe40 (at %). Table 1
summarizes all the obtained ternary alloy compositions. Such
alloys were subsequently subjected to the exposition of different gas mixtures (water vapor, O2 , H2 ) inside a small reactor
(Figure 1). Initially, the produced alloys were placed in a steel
mesh -labeled (1) in Figure 1-, with 1 mm2 holes on the surface. Then, the mesh was placed in the reactor –labeled (2)-,
with three different ports. One of the ports was used for water
vapor injection (PH2 O = 1.8 atm.), the other was used for either
H2 or O2 injection (PH2 or O2 = 0.5 atm.) and the third port for
gas expulsion. The total pressure of the gases was maintained
at a constant pressure of 2.3 atm. The reported environmental
embrittlement time (EET) has been defined as the period of time
where the material was completely degraded, i.e. from ribbon
form to powder. The powders obtained in the degradation experiments were characterized by X-ray diffraction (XRD) and
differential thermal analysis (DTA). The phases and the lattice
parameters obtained from each alloy were also characterized.
Table 1: Chemical composition, phases obtained by XRD and
degradation time for the different ternary compositions studied.
Phases in
the Alloy
(%)
Al50 Cu29 Fe21
100 β
Al55 Cu26 Fe19
100 β
Al60 Cu23 Fe17
100 β
Al60 Fe40
100 β
Al65 Cu20 Fe15 40 β + 45 Ψ + 15 λ
Al68 Cu18 Fe14 35 β + 50 Ψ + 15 λ
Al75 Cu14 Fe11 40 β + 45 Ψ + 15 κ
*: (without degradation)
Alloy
Degration
Time
(min.)
4470*
275
140
40
300
365
605
range of 50 to 60 at.% present a single solid state phase, the
β-Al(Cu,Fe) cubic phase (Figure 2b,c). However, in the range
of 61 to 68 at. % of aluminum content, regions with different
phases (β-Al(Cu,Fe) + ψ-Al6 Cu2 Fe) are obtained. This is illustrated in Figure 2(d) where the X-ray diffraction pattern for an
alloy with 65 at % of Al is displayed. Finally, in alloys with
approximately 70 and higher atomic percent of Al, other crystalline phases have also been obtained: the κ-Al cubic, the θAl2 Cu tetragonal and the λ-Al13 Fe4
monoclinic
phases, (Figure 2e).
Figure 1: Experimental set-up used to evaluate the degradation
time in the samples.
3.
RESULTS AND DISCUSSION
Figure 2 shows the X-ray diffraction patterns for the different
ternary alloys as a function of the aluminum content. This figure shows that the number of crystallographic phases increased
with the increment of aluminum. The cubic phase (β), which is
based on the intermetallic compound B2-AlFe (see Figure 2a),
presents a wide solubility of Al, Cu and Fe in its structure. The
aluminum contents of these alloys are in the range of 50% up
to 75 at.%. The alloys which have aluminum contents in the
Figure 2: X-ray diffraction pattern obtained for the different
alloys.
Figure 3, illustrates the degradation time against the different
atmospheres used for the Al55 Cu26 Fe19 alloy. In the case of
H2 and O2 atmospheres no degradation phenomena was observed up to a maximum experimental degradation time of 2400
min. When the water vapor was used as the experimental atmosphere, lower degradation times were observed. However, in
Environmental Embrittlement Characteristics of the AlFe and AlCuFe Intermetallic Systems . / J. New Mat. Electrochem. Systems 8, 97-100 (2005)
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the cases of water vapor plus O2 or H2 the observed degradation times decreased even more. Past investigations[5] have proposed the reaction: 2Al + 3H2 O = Al2 O3 + 6H+ as the main responsible mechanism of the degradation phenomena, where the
monoatomic hydrogen is strongly involved in this phenomenon.
The experimental results illustrated in Figure 3, seem to be in
agreement with previous studies[1−8] . However, in the cases of
water vapor plus O2 or H2 the degradation times were even
lower. Also, the experimental results indicate that the lowest
degradation times were obtained for atmospheres of water vapor + oxygen.
Figure 4: Embrittlement degradation time (EET) variation with
the atomic percent of Al content under a water vapor atmosphere.
Figure 3: Degradation time variation with the atmosphere for an
Al55 Cu26 Fe19 alloy composition.
Figure 4 shows the results obtained from the different AlCuFe
alloy compositions under water vapor atmosphere. This figure
illustrates the variation of the environmental embrittlement time
(EET) with the increment of Al content in the alloy system. It
is observed that the EET increases near the composition values
at the left and right hand sides of the curve. These regions correspond to the intermetallic relationships 1:1 and 3:1 Al:(Cu or
Fe). It is also interesting to realize that the presence of different
phases in the system has strong influence on the obtained values
of the EET.
Figure 4 shows the alloys with aluminum compositions in the
range 50 to 75 at.%. In the range 50 to 60 at%, all the obtained
alloys give rise to phases with cubic structure, which form solid
solutions. Also, in this range, the EET dramatically diminished
with the increment of aluminum or the decrement of Cu and Fe.
The EET for the Al55 Cu26 Fe19 and Al60 Cu23 Fe17 alloy compositions were 300 and 350 min. respectively. The aluminum
increment in the compositional values of the ternary alloys increases the Al-Al bonds in the structure. The decrement of the
degradation time observed in Figure 4 suggests that the hydro-
gen could have a stronger effect in the Al-Al breaking bonds,
than in the Al-Cu and Al-Fe bond cases. Therefore, the degradation of the material cannot be directly attributed to the copper
presence in the lattice. Instead, it could be related to the aluminum increment in the cubic phase. In the β phase, the lattice
parameter (a0 ) was calculated through refinement of the Cohen
method as a function of the copper increment in its structure.
The results are plotted in Figure 5, which includes compositional values of 50 to 60 at. % aluminum and 0 to 29 at.% of
copper. It is clearly observed an increment of the lattice parameter with the copper content increment in the alloy. The increase
of a0 in the β-cubic phase is related with the increment of the
atomic radius of the Cu. However, with the increments of copper content in the alloy the EET also increased.
Figure 5: Lattice parameter variation with the copper content in
the AlFeCu alloy compositions
The alloys with 65 and 68 at.% Al (For instance, Al65 Cu20 Fe15
and Al68 Cu18 Fe14 ) have the following characteristics (Figure 4).
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M. Salazar et al./ J. New Mat. Electrochem. Systems 8, 97-100 (2005)
Firstly, more crystallographic phases are commonly obtained
(see Figure 2d). Furthermore, it can be observed in Figure 4 that
the degradation time increase with the increment of the number
of crystalline phases formed.
Now, in the case of the B2-Al60 Fe40 composition, there is an appreciable decrement of the EET in comparison with the
Al60 Cu23 Fe17 alloy. It is important to point out that both alloys have the same cubic intermetallic structure and the same
aluminum content. This degradation result from this binary alloy suggests the strong relationship between the Al content and
the EET.
Figure 6, also shows the degradation behavior obtained when
Ce, Li, Ni, B, Si, Ti, Co and Cr were added (1 at.%) to AlFe.
This figure illustrates that the addition of these elements affects
the degradation times. Higher degradation times were obtained
with the addition of Ce, Cr, Ni and Li. Therefore, these elements
improve the environmental embrittlement.
5.
ACKNOWLEDGMENTS
The authors would like to thank the Universidad Michoacana de
San Nicolás de Hidalgo, the Universidad Nacional Autónoma
de México and CONACYT for the financial support received
(38410-U).
REFERENCES
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Figure 6: Embrittlement degradation times (EET) from the
Al6 Fe4 alloy with Ce, Li, Ni, B, Si, Ti, Co and Cr minor additions.
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4.
CONCLUSION
The embrittlement times increase with the increment of the crystallographic phase number in the AlCuFe system. The increment of the Al-Al pair in the cubic phase could be related with
the degradation of the material. All the experiments, demonstrate that the embrittlement time under water vapor + O2 was
shorter than that under the other atmospheric conditions. The
addition of Ce, Cr, Ni and Li improves the environmental embrittlement in the AlFe system.
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