Crystallization of Amorphous Ribbon in NiTi-Cu Shape Memory Alloy
- PDF / 2,243,060 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 27 Downloads / 236 Views
The alloy, with a composition
459 Mat. Res. Soc. Symp. Proc. Vol. 481 0 1998 Materials Research Society
1.0
I
IL723K
718K
72
0702K
R0.6
.2
L
2
I"
78707K 6A97K
0.4
:
.702K
S~697K
692K
0.2
u
692K -0.0
0
20
40
60
80
0
Time (min)
40
80
120
Time (min)
Figure 1. (a) The typical isothermal mode DSC curves, and (b) the fraction of crystallization as a function of time under different isothermal temperatures. temperature with a heating rate of 40-60°C/min depending on the required annealing temperature. Since overshoot is serious as the temperature reaches higher than 4650C, to avoid the overheating effect the maximum isothermal temperature in this work is 4650C. During the testing, the temperature was controlled within 0.5'C. The tests were also conducted in continuous mode in the DSC instrument. The heat flow is recorded as a function of temperature for a given heating rate, T. Specimens for annealing were cut into pieces and encapsulated in a vacuum in quartz tubes. Isothermal annealing was done in a salt bath furnace with a temperature accurate to 0.50 C. The as-solidified and crystallized samples were examined using a SIEMENS D-500 X-ray diffractometer and Philips CM200 transmission electron microscope (TEM). EXPERIMENTAL RESULTS Isothermal Crystallization Kinetics The amorphous nature of the as-casted ribbons was confirmed both by X-ray diffraction and TEM. Figure Ia shows the DSC curves associated with the crystallization of Ti 50Ni 25Cu 25 amorphous ribbon at different temperatures. The isothermal crystallization kinetic curves were calculated by integrating the DSC curves in Figure la and were plotted in Figure lb. It is shown that the start and finish time of the crystallization process are strongly dependent on the isothermal crystallization temperature. The activation energy for crystallization can be evaluated from the data of isothermal crystallization kinetics. By applying the Arrhenius-type of relationship to the time required for a fix amount of transformation at different temperature, the activation energy of this alloy is calculated to be 440 KJ/mol using so called KJMA plot [4]. Microstructural Evaluation by TEM Figure 2 (a)-(d) shows the crystal phase in amorphous matrix in the samples annealed in 4250 C, 430 0 C, 4400 C and 4600 C, respectively. All the crystal phases are spherical in the samples annealed below 440TC. In the samples annealed at 450'C, the crystal phase exists in the mixed
460
Figure 2, (a)-(d). Bright field TEM micrographs of partially crystallized TisoNi•Cu2s alloy under annealing at 425°C, 430°C, 4400C and 460°C, respectively. shape of sphere and needle. As the annealing temperature reaches above 4600C, the formed crystal phase exists in a needle shape. Figure 3 (a)-(d) shows the growth of the crystal phase in the samples annealed at 4250(2. As the annealing time increases, the spheres grow and eventually connect to each other. Note that as the annealing time increases, all the spheres keep their shape unchanged. This means that the sphere is a sta
Data Loading...
