Effect of Additives on Eutectoid Reaction and Mechanical Properties of Nb-Nb 5 Si 3 Alloys
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Table I. A portion of Pettifor maps with Nb3Si phase area.
A3B
W
184
W
IN
IV
IN
IN
LI0
P Sn 1"0*"'*'$",.,:.::::.ý:WA,*,:ý::::::::::lm O-- -O A As
Sb
57 •lVii -- O 56M0 OO 55 w 4
O .A
-0-?
?
@010
1_
52 Ti 0 A 50 Hf4 ............ -9 49
• iill
ii• ;.ili
0O0-
-
-
ii
-
-000-
-
h ii
A
.:... i ii]
ii
A PNia O Cr3Si * other structure - No stable AMBtype crystal structure ? No data available
J
F igure 1. Microstructure of Nb-25Si-10Ti a Hoy ingots. Number and size of voids irncreases with increasing heat-treatment p eriod.
10 ascast
10Ti1650
X4h
Microstructural observation is conducted by SEM and composition of phases are analyzed by energy dispersive X-ray spectroscopy (EDS). Wave length dispersive X-ray spectroscopy (WDS) is also carried out to detect oxygen. Micro-vickers test with a load of 300g is performed for the measurement of mechanical property of alloys. RESULTS AND DISCUSSION Microstructure of Ti-added Ingots
Fig. 1 shows microstructural evolution of Nb-25Si-l0Ti alloy ingots heat-treated at 1350'C. It is obvious that the number and size of voids increase with increasing heat-treatment time and the KK6.9.2
lamellar structure appears and grows with voids. In the previous study [I] the formation of voids in the alloy system has not been reported and much more time was expended before appearing the lamellar structure. As shown in Fig. 1 voids have a tendency to distribute near grain boundaries. Also WDS analysis reveals that both Oxygen and Ti concentrate at the voids, and the absence of void in the previous study is most probably attributed to the high purity of Ar employed for the heat-treatment. Fig. 2 shows the void formation in the same alloy heat-treated at higher temperatures. The formation of voids cannot be suppressed and is still remained.
A
B
1OTi 15501C X20h
1OTi 1450C X20h
Figure 2. Microstructure of Nb-25Si-1 OTi alloy ingots heat treated for 20 h at (a) 1550'C and (b) 1450'C. Microstructure of Ingots with Various Additives
Fig. 3 shows microstructure of alloy ingots with various additives. Alloys with Cr, V, Ta, Zr, Ge or B have Nb 3Si phase in as-cast ingots because the decomposition of Nb 3Si phase is very slow. On the other hand the alloy with Mo has no Nb3Si phase as shown in the SEM image. It seems that the solubility of Mo in Nb 3Si is limited and this can be explained by the position in Pettifor maps. In fact, the intersection of Mo and Si is located not in the vicinity of PTi3 phase region. The alloy with Mo shows larger lamellar structure which is thought to be formed directly from a melt by the eutectic reaction. This feature is useful to form Nb-Nb5 Si3 lamellar by the eutectic reaction as was pointed out previously [3]. An important result is that there is no void in the alloy. The formation of the lamellar structure in the alloy doesn't need Nb 3Si phase and it can be concluded that voids are formed during the eutectoid reaction related to Nb 3Si phase. After the solution heat-treatment at 1650°C, lamellar structure is found in alloy ingots
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