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 . 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@example.com 97 98 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) 99 the cases of water vapor plus O2 or H2 the observed degradation times decreased even more. Past investigations 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). 100 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. 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