Journal of Materials Synthesis and Processing, Vol. 7, No. 5, 1999
The Effect of Excess Aluminum on the Composition and Microstructure of Nb-Al Alloys Produced by Aluminothermic Reduction of Nb2O5 Alfeu S. Ramos1 and Carlos A. Nunes1,2
Intermetallic aluminides including those phases of the Nb-Al system are of interest for high-temperature structural applications. Through aluminothermic reduction (ATR) of Nb2O5 different alloys of the Nb-Al system can be produced by varying the amount of aluminum (excess aluminum) in the thermit charge. In this work, various Nb-Al alloys were produced by varying Nb2O5 and Al powder blends. The resulting alloys were characterized by chemical analysis (Al, O, and C), X-ray diffraction and scanning electron microscopy. The aluminum content of the alloys increased linearly from 14.5 to 50.4 at% as the excess Al was varied from 10 up to 60% over the stoichiometric amount to reduce the Nb2O5. The carbon content was lower than 300 wt-ppm. The oxygen content decreases with increasing excess Al, reaching 1300 wt-ppm for the alloy produced with 60% excess Al. The inclusion content (Al2O3) decreases significantly as the excess Al is increased. The following metallic phases were identified in the alloys: Nbss (niobium solid solution) and Nb3Al (alloy produced with 10% excess Al); Nb3Al (alloys produced with 15 and 20% excess Al); Nb3Al, Nb2Al, and NbAl3 (alloy produced with 30% excess Al); and Nb 2 Al and NbAl3 (alloys produced with 40, 50, and 60% excess Al). KEY WORDS: Nb-Al alloys; intermetallics; aluminides; aluminothermic reduction.
1. INTRODUCTION
aluminothermic reduction (ATR) of Nb2O5, eliminating the need for pure Nb. In the aluminothermic reduction, it is well established [9, 10] that the excess aluminum dissolves preferentially in the metallic phase, allowing the production of different Nb—Al alloys. These alloys can be used as master-alloy in processes for the production of intermetallic-based parts through either casting or powder metallurgy routes. It should be pointed out that previous investigations on aluminothermic reduction of Nb2O5 were concerned with the production of suitable ATR-alloys for direct electron beam melting to produce pure niobium [11-13].
Intermetallic-based structural materials have been considered for high-temperature structural applications based primarily on the high melting point, good creep, and oxidation resistance of several aluminide and silicide phases [1-4]. In this respect, many studies have focused on evaluating the potential of Nb aluminides, monolithic, and composites [5-7]. Figure 1 shows the Nb-Al binary phase diagram which indicates the existence of three intermetallic phases: Al3Nb, AlNb2. and AlNb3 [8]. The aim of this work was to evaluate the feasibility of producing Nb-Al alloys directly through
2. EXPERIMENTAL PROCEDURE
1
Departamento de Engenharia de Materiais, FAENQUIL, Polo Urbo Industrial, Gleba AI-6, s/n 12600-000, Lorena SP, Brazil. 2 To whom correspondence should be addressed. e-mail:
[email protected].
Thermit charges consisted of Nb2O5 (min. 99 wt%) and Al (min. 99.7 wt%) powder. For each reaction
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Ramos and Nunes
Fig. 1. Nb-Al binary phase diagram [8]. Fig. 2. Al content of the alloys versus excess Al.
300 g of Nb2O5 was used and the aluminum concentration varied from 10 up to 60% excess (10, 15, 20, 30, 40, 50, and 60%) over the stoichiometric amount to reduce the Nb2O5. This stoichiometric amount is that necessary to reduce the whole Nb2O5 to Nb based on the following reaction: 3 Nb2O5 + 10 Al 6 Nb + 5 Al2O3. Thus, the mass of aluminum used in each reaction were 112.2 g (10% excess), 117.3 g (15% excess), 122.4 g (20% excess), 132.6 g (30% excess), 142.8 g (40% excess), 153 g (50% excess), and 163.2 g (60% excess). Before each reaction the reactants were first intimately mixed and then placed into a graphitelined closed reactor, air atmosphere. Niobium strips were used as electrical resistance to trigger the reactions. The reactants were initially at room temperature. After the reaction, the products (ATR-Nb and slag) were kept inside the reactor until reaching room temperature. The ATR-Nb alloys were characterized by chemical analysis (Al, C, O), X-ray diffraction at room temperature (CuKa radiation), scanning electron microscopy/back-scattered electron images (SEM/BSEI), and energy-dispersive spectroscopy (EDS). The aluminum content of each ATR-Nb alloy was measured through induction coupled plasma/atomic emission spectroscopy (ICP/AES).
3. RESULTS AND DISCUSSION The aluminum content of the ATR-Nb alloys as a function of the excess aluminum in the thermit charge is shown in Fig. 2. The Al content increases linearly from 14.5 at% (10% excess Al) up to 50.4 at% (60% excess Al), meaning that aluminum not used in the reduction
reactions dissolves preferentially in the metallic phase. The masses of the produced ATR-Nb alloys were 130.6 g (10% excess), 133.7 g (15% excess), 149.7 g (20% excess), 177.4 g (30% excess), 156.8 g (40% excess), 197.7 g (50% excess), and 173.6 g (60% excess). The maximum Nb yield reached 76% for the reaction using 50% excess Al. Previous experiments on aluminothermic reduction have shown a maximum in metallic yield as a function of excess Al in the thermit charge [9]. For reactions involving larger masses of reactants higher metallic yield values are expected [14]. The carbon content of the alloys were lower than 0.03 wt%, indicating no significant contamination of any ATR alloy by carbon from the lining material. The oxygen content of the alloys as a function of excess Al is shown in Fig. 3, indicating a decrease for higher excess Al. The oxygen corresponds to the total oxygen present, interstitially and Al2O3 inclusions. As will be shown later, higher Al alloys present a lower inclusion content, which decreases the oxygen contribution from this source. In addition, Nb—Al intermetallic phases has a lower solubility for oxygen than pure Nb as shown in the isothermal section of the Nb-Al-O system given in Fig. 4 [15]. Different Al contents of the alloys produced different microstructures. The ATR-Nb alloy produced with 10% excess Al had 14.5 at% Al. Figure 5a shows an SEM/BSEI micrograph of this alloy and Table I results of EDS analysis. These results indicate the presence of Nbss (Nb solid solution) and Al2O3 inclusions in the microstructure. Note in Fig. 5a the significant amount of Al2O3 inclusions in the microstructure of this alloy. The XRD pattern (Fig. 6a) also indicated reflections from the
Nb-Al Alloys by Aluminothermic Reduction of Nb2O5
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Fig. 3. Oxygen content of the alloys versus excess Al.
Nb3Al phase, in addition to those of the Nbss and Al2O3 phases. However, it should be related to some inhomogeneity of the ATR-Nb alloy associated with the solidification process. The XRD result reflects better the phases present in the microstructure of the ATR-Nb alloy since the sample for such experiment was taken from a larger piece of material. The alloys produced with 15 and 20% excess Al presented 21.1 and 24.9 at% Al, respectively. The microstructure of these alloys was essentially single-phase (Nb3Al), which can be inferred from the XRD patterns shown in Figs. 6b and c. A result of EDS analysis from
Fig. S. SEM/BSEI micrographs of Nb-Al alloys produced in this work: (a) 10% excess Al; (b) 50% excess Al; (c) 60% excess Al.
Fig. 4. Isothermal section of the Al-Nb-O system at 1100°C [15].
the alloy produced with 20% excess Al is shown in Table I. In addition, these alloys presented a much smaller volume fraction of Al2O3 inclusions compared to the previous alloy. The ATR-Nb alloy produced with 30% excess Al
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Ramos and Nunes Table I. Results of EDS Analysis of the ATR-Nb Alloysa
Excess Al%
10 20 30 40 50 60
Phase Nbss Nb3Al Nb2Al Nb2Al NbAl3 Nb2Al NbAl3 Nb2Al NbAl3
Nb (at%)
Al (at%)
84.62 76.72 68.77 67.11 28.76 65.16 29.25 60.20 27.07
15.38 23.28 31.23 32.89 71.24 34.84 70.75 39.80 72.93
a
For each condition, the table indicates only the phases which could be probed without interference of other phases.
phase, the major phase in the microstructure; the other two phases were quite small to be probed without interference of the Nb2Al phase. Figure 6d shows the XRD pattern of this alloy. The alloys produced with 40, 50, and 60% excess Al presented 39.8, 44.5, and 50.4 at% Al, respectively. Figures 5b and c show SEM/BSEI micrographs of the alloys produced with 50 and 60% excess Al, respectively. Figures 6e, f, and g show the XRD patterns of the three alloys. The micrographs suggest a very low amount of inclusions in these materials. Results of EDS analysis are shown in Table I. All three alloys presented the same solidification path, i.e., the primary precipitation of the Nb2Al phase followed by eutectic reaction involving the Nb2Al and NbAl3 phases. The amount of primary phase in the microstructure decreases as the Al content increases as expected from Fig. 1. It is possible to observe a significant segregation within the primary Nb2Al phase (Figs. 5b and c), which is associated with the large range of solubility of this phase as shown in Fig. 1.
4. CONCLUSIONS In this work, several Nb-Al alloys were produced through aluminothermic reduction of Nb2O5. The aluminum content of the alloys increased linearly from 14.5 to 50.4 at% as the excess Al was varied from 10 up to 60%. The carbon content was lower than 300 wtppm. The oxygen content decreases for higher excess Al, reaching 1300 wt-ppm for the alloy produced with 60% excess Al. The inclusion content (Al2O3) decreases significantly as the excess Al is increased. The following metallic phases were identified in the alloys: Nbss (niobium solid solution) and Nb3Al (alloy produced with 10% excess Al); Nb3Al (alloys produced with 15 and 20% excess Al); Nb3Al, Nb2Al, and NbAl3 (alloy produced with 30% excess Al); and Nb2Al and NbAl3 (alloys produced with 40, 50, and 60% excess Al).
Fig. 6. XRD patterns of the Nb-Al alloys produced in this work.
presented 32.6 at% Al. Three phases could be identified in the microstructure: Nb3Al, Nb2Al, and NbAl3. Table I presents results of EDS analysis from the Nb2Al
ACKNOWLEDGMENTS The authors acknowledge CONFAB TUBOS S/A for carrying out chemical analysis of the alloys in terms of carbon and oxygen. CAPES—Brazil supports A. S. Ramos.
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