Phase Transformations and Crystallography of Twins in Martensite in Ti-Pd Alloys
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RESULTS AND DISCUSSION Transformation Behavior Figure 1 shows typical DSC curves of near-equiatomic alloys. Two types of curves are obtained with the composition. Two exothermic and endothermic reactions are observed during heating and cooling, respectively, in Ti-rich alloys as seen in Ti-45 at%Pd alloy. We define that the first and the second peak temperatures during heating are A2* and A1* as indicated in Fig. 1, respectively. In the same way the first and the second peak temperatures during cooling are defined as M1* and M2*, respectively. On the other hand, a single reaction is only measured in equiatomic and Pd-rich alloys as seen in Ti50 at%Pd alloy. The compositional dependence of A2* and Al* is summarized in Fig. 2. The A2* is increased with Pd content up to the equiatomic composition and then overlapped to A1* which is nearly constant with the composition. It is apparent from these results that the successive transformations take place in Ti-rich alloys. The crystal structure of Tirich and equiatomic alloys at room temperature was determined to be B19 by X-ray and electron diffraction experiments. Pd-rich alloys consisted of the B19 martensite and Ti2Pd3 compound. To identify the intermediate products between B2 parent and B19 martensite phases in-situ TEM observation was carried out. However, since large amount of Ti2Pd precipitates was formed during heating, the intermediate phase could not be detected by both the electron diffraction experiments and image observations. The detailed nature of the intermediate phase is now under study and will be reported in due course. Therefore, the intermediate phase is designated as X in the present study as shown in Fig. 2.
lu
0
Ti-50at%Pd
Heating
Cooling • I
300
•
i
350
400
,
.
I
i
450 500 550 Temperature (C)
Figure 1 Typical DSC equiatomic Ti-Pd alloys
curves
[
i
600
650
of near-
650
832
P 6 0 0 ....... -................... t................. .............. ý ~550 ....
S500
Phase Boundary of TiPd Compound From the compositional dependence of transformation temperatures in Fig. 2, the phase boundary of TiPd compound in Pd-rich side is deduced to be near the equiatomic composition. This was supported by aging experiments. In other words, transformation temperatures were not changed in Pd-rich alloys by aging. While the phase boundary of TiPd compound in Ti-rich side is likely extended to 55 at%Ti at 1000"C at least since the A2* is decreased with Ti content. Figure 3 shows the effect of aging temperature on the transformation peak temperatures in Ti-45 and
I
400
-.
...
•-... 50. . . . . . . . ...................... ....... ..........
45
47.5
50
52.5
55
Pd-Composition (at.%) Figure 2 Compositional dependence of transformation peak temperatures during heating in near-equiatomic Ti-Pd alloys quenched from 1000°C. 376
47.5 at%Pd alloys. The all specimens are aged for 360 ks at each temperature. The A2* in Ti45 at%Pd alloy is increased in the specimens aged below 800'C. That in Ti-47.5 at%Pd alloy is increased in the specimens aged
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