| Summary: | 博士 === 國立東華大學 === 物理學系 === 105 === Electrical resistivity, the Seebeck coefficient, thermal conductivity, and specific heat measurements on Ti50Ni50-xFex (x = 2.0 – 10.0 at. %) shape memory alloys (SMAs) were measured to investigate the influence of point defects (Fe) on their characteristics of martensitic transformation. It is found that the Ti50Ni48Fe2 and Ti50Ni47Fe3 SMAs have a two-stage martensitic transformation (B2→R and R→B19), while the Ti50Ni46Fe4, Ti50Ni44.5Fe5.5, and Ti50Ni44Fe6 SMAs display a one-step martensitic transition (B2→R). However, the Ti50Ni42Fe8 and Ti50Ni40Fe10 SMAs exhibit the features of strain glass transition (frozen strain order). Most importantly, the induced point defects significantly alter the martensitic transformation characteristics, such as the transition temperature and width of thermal hysteresis during transition. This can be explained by the stabilization austenite B2 phase upon Fe substitution, which eventually leads to a reduction in transition temperature as well as a decrease in enthalpy that associated to the martensitic phase transition. With increasing Fe substitution, the evolution of phase transformation observed in the TiNi shape memory alloys is presumably caused by the changes in local lattice structure via the induced local strain fields as a result of Fe substitution. In addition, we also compared the physical properties between Ti50Ni44Fe6 and the Ni-rich Fe-substituted SMA Ti49Ni45Fe6 to determine the influence of excess Ni on the transition characteristics. Remarkably, it is found that a small amount of excess Ni in Ti50Ni44Fe6 transformed the SMA from the long-range R-martensite ordering to a short-range strain ordering. Such a fining suggests that the excess Ni also play a key role in the transition characteristics.
We also studied the influence of point defects (Fe) and precipitates (Ti3Ni4) on the characteristics of R-phase martensitic transformation by comparing the transport and thermal properties of as-quenched Ti50Ni46Fe4 and aged Ti48.7Ni51.3 shape memory alloys. Both alloys undergo a weak first-order R-phase transformation with a small thermal hysteresis (less than 7 K), suggests that the introduction of point defects and precipitates lead to a stable R phase in these alloys due to the defects induced local lattice deformations. Furthermore, our study revealed that the transition temperature, transformation width, and transformation strain of the investigated R-phase TiNi-based alloys are strongly affected by the induced defects. As a result, the aged Ti48.7Ni51.3 has a higher transition temperature than that of Ti50Ni46Fe4, as expected.
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