High-Temperature Phase Transformation in Cr Added TiAl Base Alloy
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EXPERIMENTAL PROCEDURE An alloy used in the present work is Ti-48AI-3.5Cr (in at.% : hereafter denoted as TiAl-Cr alloy). The ingot of about 120g was prepared by argon arc melting of pure Ti, Al and Cr. The ingot was cut into small pieces with the size of about 10xl0x8mm. The pieces were wrapped with Ta foils and sealed in quartz tubes with Ar gas, and then annealed at 1673K for 1.8ks, followed by cooling at various rates. Thin foils for TEM were prepared by cutting the sample into about 300gm slices, grinding to the thickness of about 100gm, and, finally, by standard twin-jet electropolishing using a 10% perchloric acid-ethanol solution at 253K. TEM observations were made on a 200kV electron microscope (JEM-2000FX). EXPERIMENTAL RESULTS Evolution of the feathery structurein the TiAl-Cr alloy
Figure 1(a)-(c) show a microstructural change of the TiAl-Cr alloy, which have been evolved during continuous cooling from the x phase. As seen in the air-cooled sample (Fig. 1(a)), a typical feathery structure is uniformly formed, which was nucleated at the region indicated by arrows and grew into the feathery fashion. On the other hand, the massive structure covers whole of the waterquenched sample (fig. l(c)), confirming that the sample was in the a single phase region. It is interesting to note that, when the cooling rate is changed during continuous cooling, the feathery structure seem to change continuously to the massive as shown in fig. 1(b). That is, growth directions of these structures with a radial morphology seem to be maintained macroscopically.
Fig. 1. Optical microstructures of a Ti-48AI-3.5Cr (at.%) alloy heated at 1673K for 1.8ks, followed by (a) air-cooling (b) air-cooling for 30s, followed by water-quenching (c).
In order to discuss a formation process of the feathery structure, cooling rate controlling experiment was made. The conditions are shown in figure 2, together with the vertical section of Ti-Al-Cr phase diagram with 3.5at.%Cr [7]. After holding at 1673K for 30min, the samples were cooled at approximately 1K/s (0.5K/s and 0.1 K/s were also performed [8]) to several temperatures shown in fig. 2, followed by water quenching. A cooling rate 1K/s was achieved by switching off the furnace (socalled furnace cooling). KK2.10.2
1673K/30min Clooling Rate :1K/s
1'
%1643K UN
1:
at% Al Fig. 2. Heat treatment and cooling conditions used for the present study. The vertical section of the Ti-Al-Cr ternary phase diagram with 3.5at.% Cr was proposed
by Miura et al. [7]. Figure 3(a)-(d) show the feathery structure evolution during continuous cooling with 1 K/s from 1673K to 1523K. From fig. l(a), it is known that the cooling to 1643K produces approximately the 50% feathery structure in volume. Further cooling to 1623K and 1573K lead to 70% and 90% volume fractions of the feathery region, respectively. It is interesting to note that, when the sample is cooled down to 1523K, Widmannstatten-type laths appear within the feathery region. From these results, it is evident that the formation of the feathery has bee
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