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11PREPARATION OF POROUS NIOBIUM-ALUMINUM INTERMETALLIC BY COMBUSTION SYNTHESIS IN THERMAL EXPLOSION MODE
X. Cai" and P. Feng*"
aSchool of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, 221116 P. R. China *e-mail: pzfeng@cumt.edu.cn
DOI: 10.24411/9999-0014A-2019-10028
Abstract. Porous Nb-Al intermetallic was prepared by thermal explosion (TE) mode of combustion synthesis (CS). The temperature profile, phase composition, open porosity, and oxidation resistance of Nb-Al compact were investigated. The results showed that the significant exothermic reaction occurs, which means that an obvious TE appears during heating process. The volume expansion of 190% was observed in product, and the heated sample exhibited interconnected pores with a high open porosity of 65.7%. The porous NbAb intermetallic show 'pest' oxidation in the temperature range of 500-600°C, and follow parabolic oxidation law at 400°C. Moreover, the highly porous structure makes this material have great potential for separation and heat insulation applications at medium temperature range from room temperature to 400°C.
Keywords: Niobium-aluminum; Intermetallics; Thermal explosion; Sintering
Figure 1 shows the temperature evolution profiles and DSC curves of NbAl samples at a linear heating rate of 10°C/min. As shown in Fig. 1a, the temperature of the sample slowly increases with the furnace temperature at a constant rate. Then, a noticeable temperature change occurs at 669°C: the temperature stops increasing and reaches a plateau for 236 s. During this time of plateau, the heating is continued, and the temperature increases again at the end of this plateau due to the temperature gradient between the sample (669°C) and the furnace (713°C). When the temperature reaches 791 °C, the curve shifts upward and increases to maximum deviation of 925°C. Subsequently, the temperature increases slowly and tends to the furnace temperature (dotted line). From the DSC curve (Fig. 1b), it can be inferred that temperature plateau at 669°C corresponds to the melting of Al, and the profile show a single, sharp exothermic peak at 931 °C, resulting in the temperature of the sample climbed continually, which is indicative of the TE reaction. Moreover, the ignition point of exothermic peak in DSC is consistent with the temperature curve increased markedly at 791 °C.
An interesting feature of Nb-Al system is that TE occurs there at a temperature almost 130°C higher than the melting point of Al (660°C), which is distinct from most other TE reaction system, for instance, Fe-Al [1-5] and Ti-Al [6-11]. Previous study shows that the combustion reaction of Nb-Al is triggered by the dissolution-precipitation process. However, according to the Nb-Al phase diagram, the solubility of Nb in Al is almost near zero and is very limited even in liquid Al. And from the published data, the solubility of Nb in liquid Al is about 0.02 wt % at 700°C, increasing to 0.06 wt % at 800°C. These data indicate that the higher temperature will be required for the ignition of thermal explosion in Nb-Al system. Moreover, the difficulty in wetting the Nb particle surface by liquid Al could be other factor hinder the reaction.
The green compact was characterized by the silver-gray luster of metals, indicates that the Al and Nb powders become a compact with certain strength by the uniaxially pressed process, as shown in Fig. 2a. After sintering, compare with the green compact, the volume of the sintered disc increased significantly (Fig. 2b) and the expansion ratio were measured to 190%, meanwhile, the sample kept in original shape without deformation, crack and losing angle.
ÏSHS2019
Moscow, Russia
Figure 2c shows the XRD patterns of the green compact and as prepared products. Peaks of the pure crystalline Nb and Al are detected in the green compact, and for the sample heated to 1000°C for 1 h, single phase and well crystallized tetragonal NbAb (JCPDS no. 13-0146) with the major peaks at 29 = 20.59, 25.35, 39.17, 41.95, 47.31, 65.03° corresponding to the diffraction of the (002), (101), (112), (004), (200), and (204) planes are synthesized.
(D 13
2 800
(D
CP
E
(D
H 700
b
S 40-
I 20-
» 931 oC
exo
Al melting ' \ Thermal explosion N
„ 668 oC
1000 1500 2000 2500 3000 3500 4000 600 700 800 900 1000
Time (s) Temperature (oC)
Fig. 1. (a) Temperature-time profile and (b) DSC plot of Nb-Al compact heated to 1000°C with a heating rate of 10 °C/min.
a
Green compact
c
1000oC 1 h Î 1 Î NbAl3 *Nb OA! ÎÎ ÎÎ Î ÎÎ Î Î ÎÎÎ
Green compact O A O ♦ O O ♦ ♦ O
10 20 30 40 50 60 70 80 90 2 Theta (degree)
Fig. 2. Macroscopic image and XRD patterns: (a) green compact, (b) sample heated to 1000°C for 1 h, (c) XRD patterns of green compact and product.
a
000
900
600
0
1 cm
Figure 3a shows the secondary electron image of the fracture surface of sample heated to 1000°C for 1 h. A sponge-like and highly porous structure consisting of a large number of agglomerated particles was obtained. Moreover, the enlarged image (Fig. 3b) shows the neighboring particles with size about 3-6 p,m are connected through necks (marked by white arrows), indicating the initial stage of sintering in the sample. The open porosity of the sintered specimen was measured to be 65.7%, which is higher than that of the green compact (19.4%). Besides, the density of the sample is about 1.56 gcm-3 and lower than the theoretical density of NbAl3 (4.54 gcm-3) due to the highly porous structure of the product. Figure 3c shows the EPMA mapping results of as prepared porous intermetallic. As can be seen from the back-scattering electron (BSE) image, single-phase structure was obtained, which is in good agreement with the XRD patterns. The element mapping shows that Nb and Al are homogeneously distributed in the final products, furthering confirmed that each single particle is composed of NbAb phases. Furthermore, during the process of sample preparation for EPMA, the specimen was immersed in an uncured epoxy resin, then the resin was cured. After polishing, it was found that the pores were filled with the resin, showed the feature of interconnected pores (open pores).
Fig. 3. (a, b) SEM image of porous NbAb intermetallic. (c-e) EPMA mapping results of
synthesized porous NbAl3.
To investigate the high-temperature oxidation behavior of porous Nb-Al intermetallic, the mass gain (%) was measured as a function of the oxidation time at 400, 500, 550, and 600°C and plotted in Fig. 4a. The sample has almost no change in mass gain when oxidized at 400°C, as can be seen from the enlarged curve (Fig. 4b), during the oxidation experiments at 400°C, the mass gain of samples shows a continuous increase with the exposure time. In the initial period (I), the weight increases significantly, indicating a higher oxidation rate. In the subsequent stage (II), the oxidation rate decreases and the mass gains of the porous Nb-Al intermetallic are found to follow the parabolic oxidation rate law. The mass gain was only 0.19% when oxidation was for 144 h. However, for the sample oxidized at 500, 550, and 600 °C, the mass gain is two orders of magnitude higher than that of sample oxidized at 400°C, which means that the oxide scale was not protective. Moreover, at the end of oxidation experiments, it was found that the sample was complete disintegration into a powder. The accelerated oxidation of porous Nb-Al intermetallic at 500-600°C can be well interpreted by the 'pest' or catastrophic oxidation: grain-boundary and non-selective oxidation, does not form the protective alumina scale in the sample and producing cracks due to the mismatch in volume expansion between the formed oxide and the base alloy, which causes complete disintegration of monolithic samples into powder. This phenomenon not only had been detected in Nb and its alloys, but also revealed in MoSi2 and high-entropy alloys. In order to improve the oxidation resistance of such materials with 'pest', alloying and coating are effective means for the formation of an adhesive, dense, protective scale on the surface of material.
Oxidation time (h) Oxidation time (h)
Fig. 4. Mass gain vs. time plot of porous NbAb intermetallic after oxidation: (a) 400°C, 500, 550, and 600°C for 144 h, (b) enlarged plot of 400°C in (a).
ISHS 2019 Moscow, Russia
This work was supported by National Natural Science Foundation of China (51574241).
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