Microstructure and Mechanical Properties of Nb-Al-N and Nb-Si-B Powder Compacts Produced by Spark Plasma Sintering
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EXPERIMENTAL PROCEDURE Compositions of Nb-Al-N compacts investigated are plotted on the 1773K isothem of the Nb-Al-N ternary phase diagram [14] (Fig.l). They are in the line connecting the Nb corner and AIN and given as Nb-xAl-xN (x = 0, 8, 10, 12, 16, 22, 36, 50 at %) [15]. Compacts were prepared via KK8.26.1 Mat. Res. Soc. Symp. Proc. Vol. 552 @1999 Materials Research Society
the following three different routes: (i) Nb (99.99% pure, -325 mesh) and AIN (99.8% pure, -300 mesh) powders were blended in a hexane-filled rotational ball mill for 0.5 hours and then consolidated in a SPS furnace at a temperature between 1623 - 1973K for 5 minutes under a consolidation pressure of 49MPa in a vacuum of 6Pa, (ii) two constituent powders were mechanically alloyed in an argon-filled planetary ball mill for 6 hours and consolidated similarly to (i), and (iii) pulverizing compacts prepared via route (i) and consolidated once again similarly to (i). We call compacts prepared via these three different routes (i) blended (BL) powder compacts, (ii) mechanically alloyed (MA) powder compacts and (iii) pre-sintered and milled (PSM) powder compacts, respectively.
N 20
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Fig. 1 Compositions of Nb-Al-N compacts (open circles) plotted on the 1773K isotherm of the Nb-Al-N phase diagram.
20 30
40 60 50
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AIN
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70
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80 20 00 I0
Al
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20
30 A6Nb
40
00
60
70 AINbz
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90
AI•b
Compositions of Nb-Si-B compacts investigated are Nb-37.5Si, Nb-66.7Si, Nb-34Si-1OB, Nb52Si-4B, Nb-18Si-38B, Nb-35Si-26B and Nb-66.7B (at%) in the Nb5 Si3-NbB 2-NbSi 2 triangle (Fig.2[16]). BL, MA and PSM powder compacts with these compositions and dimensions 20 mm diameter and 5 mm thickness were prepared using Nb(99.8% pure, < 10 micron), Si (99.9% pure, 300 mesh) and B (99% pure, -300 mesh) powders. Blending and mechanical alloying of elemental powders was performed for 1 hour and 10 hours, respectively. After sintering, XRD profiles, the density and microvickers hardness at room temperature and the oxidation behavior of compacts at 1523K were examined. Microstructure of each compact was observed by SEM. Compression specimens (2 x 2 x 5 mm 3) were cut from each Nb-AI-N compact and tested at 1473K at a strain rate of 1.7 x 10-4/s in the vacuum of 1.3 x 10-2 Pa.
Nb Fig. 2 Compositions of Nb-Si-B compacts (open circles) plotted on the 1873K isotherm of the Nb-Si-B phase diagram.
NbB Nb3B4 60 NbSj2
NbB2
Si
B KK8.26.2
RESULTS AND DISCUSSION Nb-Al-N system
Figure 3 shows densities of Nb-xAl-xN (x: at %) compacts as a function of x. Both BL and MA compacts with x < 22 at % sintered at 1773K exhibit a bulk density of 98% - 99% of the corresponding theoretical value. For x > 22 at%, sintering has to be made at higher temperatures to obtain dense BL powder compacts. Figure 4 shows SEM photographs of BL and MA powder compacts with compositions of Nb-8AI-8N and Nb-16AI-16N. These two compacts consist of Nb(Al) solid solution and Nb 2N phases and Nb 2A1 and Nb 2N phases, respectively. The constituent phases of these compacts a
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