This study consists of three major topics in Ni-Ti shape memory alloys ; degradation by cyclic deformation, transformation behavior change by external stress, and the role of preferred orientation on reversible shape memory effect.
The preferred orientation in martensite of Ni-Ti allow which has reversible shape memory effect was obtained by using pole figure X-ray diffraction. In the bending specimen, the tension side had the preferred orientation of $(11\bar{1})_M$ of martensite, while and the compression side exhibited $(020)_M$ of martensite ot the stress axis.
It was also found that $(11\bar{1})_M$ and $(020)_M$ planes were almost parallel to the $(110)_{B2}$ of parent phase. The larger d-spacing of $(11\bar{1})_M$ and smaller spacing of $(020)_M$ than that of $(110)_{B2}$ made the tension side expand and compression side contract during cooling, respectively.
By heating $(11\bar{1})_M$ and $(020)_M$ in both sides returned to $(110)_{B2}$, so the contraction and expansion occurred in the tension side and compression side, respectively.
When Ni-Ti alloy(Nitinol) is strained, phase transformations took place under stressed condition. In this case transformation temperatures shifted remarkably from those under no stressed condition and considerably large strain was accompanied during the phase transformation. In this study, tensile test and electrical-resistance test were performed for annealed $Ni_{50}-Ti_{50}$ wire (specimen 1) and aged $Ni_{50.6}-Ti_{49.4}$ wire (specimen 2) during phase transformation.
$M_s$ and $A_s$ temperatures increased gradually till about 129 MPa and increased rapidly above it in the both specimen 1 and 2, but $T_R$', and $T_R$ temperature increased very slowly with the stress increase in both specimen 1 and 2.
The transformation temperature range $\Delta T(M_s-M_f)$ decreased rapidly from 55 °C to 10 °C up to about 200 MPa and remained a constant value of 10°C at above 200 MPa stress. However, $\Delta T$ in specimen 2 was much smaller than that in specimen 2. The results indicated that R-phase in the specimen 1 was more stable than that in specimen 2.
The permanent strain of $\epsilon_p$ by the cooling and heating under external stress remained constantly low value regardless of stress increase in specimen 1, while $\epsilon_p$ increased greatly from 0.1 to 3.5% at above 240 MPa in specimen 2. This implied that slip deformation by dislocations in the annealed specimen 1 was more difficult than in the aged specimen 2.
The effect of cyclic deformation on the degradation of shape memory effect was investigated for the Ti-rich Ni-Ti alloy. By the cyclic deformation, the $M_s$ and $A_f$ temperatures decreased with the number of cycles, but the $M_s$' remained almost constant. The recovering length and amount of deformation decreased with the number of cycles, especially during martensitic transformation. Transmission electron microscopy revealed that the density of dislocations increased with the number of cycles. According to the above results it can be concluded that the degradation of Ti-Ni shape memory alloys was attributed to the introduction of dislocations during cyclic deformation.