Transformation-induced Defects Research Articles

On the Importance of Structural and Functional Fatigue in Shape Memory Technology

The present work provides a brief overview on structural and functional fatigue in shape memory alloys (SMAs). Both degenerative processes are of utmost technological importance because they limit service lives of shape memory components. While our fundamental understanding of these two phenomena has improved during the last two decades, there are still fields which require scientific attention. NiTi SMAs are prone to the formation of small cracks, which nucleate and grow in the early stages of structural fatigue. It is important to find out how these micro-cracks evolve into engineering macro-cracks, which can be accounted for by conventional crack growth laws. The present work provides examples for the complexity of short crack growth in pseudoelastic SMAs. The importance of functional fatigue has also been highlighted. Functional fatigue is related to the degeneration of specific functional characteristics, such as actuator stroke, recoverable strain, plateau stresses, hysteresis width, or transformation temperatures. It is caused by the accumulation of transformation-induced defects in the microstructure. The functional stability of SMAs can be improved by (1) making phase transformations processes smoother and (2) by improving the material’s resistance to irreversible processes like dislocation plasticity. Areas in need of further research are discussed.

Electrical resistivity of NiTi with high transformation temperature

For a better understanding of the transformation behaviour on NiTi, the electrical resistivity of Ni 49.5Ti 50.5 was precisely measured by a four-probe potentiometric method. The temperature dependence of the electrical resistivity is linear in the regions of the high- and low-temperature phases, and a drastic change in electrical resistivity is observed in the temperature range of the transformation. The transformation remains of the one-stage between the high- and low-temperature phase even with thermal cycling, although with increasing the number of thermal cycles the electrical resistivity versus temperature curve shifts to the low temperature side and the electrical resistivity increases in the single phase temperature region which is attributable to the generation of transformation-induced defects. The one-stage transformation changes to a two-stage transformation due to the appearance of an intermediate phase after an incomplete thermal cycle, and with subsequent thermal cycling the peak in the electrical resistivity is enhanced to stabilise the intermediate phase. The as-cut Ni 49.5Ti 50.5, of which the surface is damaged, shows a two-stage transformation, even during the first cooling after cutting, and a resistivity-peak due to such a two-stage transformation is also enhanced with thermal cycling. It is thought that the amount of defects plays important roles in the shifts of transformation temperatures and in the transfer from one-stage to the two-stage transformation, and contributes to the stability of each phase.

Irreversibility in transformation behavior of equiatomic nickel–titanium alloy by electrical resistivity measurement

Measurements of the electrical resistivity were precisely performed on shape memory Ni 50Ti 50 alloy in order to reveal the irreversible behavior of the thermoelastic martensitic transformation with thermal cycling. The hump in the electrical resistivity during cooling is enhanced with increasing the number of complete thermal cycles to result in a peak, although no peak in the electrical resistivity is observed on the reverse transformation during heating. The electrical resistivity in the low-temperature phase, of which the temperature dependence is linear, increases with increasing the number of complete thermal cycles. The temperature coefficient of the electrical resistivity in the temperature region of the high-temperature phase increases with elevating the temperature. The transformation is strongly influenced by incomplete thermal cycles to result in a peak in the resistivity even on the reverse transformation after incomplete thermal cycling. It is thought that the anomalous behavior such as enhancement of a resistivity-peak, the increase in the electrical resistivity of the low-temperature phase, and the nonlinear relation between the resistivity and the temperature in the high-temperature phase are attributable to the appearance of an intermediate phase stabilized by transformation-induced defects, the accumulation of the transformation-induced defects, and the electron scattering due to the softening of a phonon mode in the high-temperature phase, respectively. It proved useful to make more accurate measurements of the electrical resistivity in order to investigate the intrinsic behavior of the transformation in NiTi.

Characterization of transformation behavior in NiTi(Si)

For a better understanding of the transformation behavior in NiTi alloy containing silicon, the electrical resistivity and the thermal property in NiTi and NiTi(Si) were precisely measured by a four-probe potentiometric method and differential scanning calorimetry, respectively. The transformation behavior depends on the thermal cycles with the repetition of the transformation, and the thermal cycles tend to stabilize the intermediate phase during cooling because of the enhancement of a peak in the electrical resistivity. The peak in the resistivity of NiTi with increasing thermal cycles is markedly enhanced in comparison with NiTi(Si). The reverse transformation during heating remains a one-stage transformation from the low-temperature phase to the high-temperature phase in both NiTi and NiTi(Si). Although a peak in the resistivity during heating is detected in NiTi(Si), it is evaluated to be a seeming peak because the resistivity of the low-temperature phase is larger than that of the high-temperature phase near the reverse transformation temperature. The increase in the resistivity due to the transformation-induced defects is very large for the transformation of NiTi(Si), and NiTi(Si) is useful for examining the formation of defects introduced by a repetition of the transformation. It proved necessary to measure not only the electrical resistivity but also the thermal property in order to characterize the transformation behavior and the effect of the electron scattering, which are altered by an additional element.

Enhancement of a peak in electrical resistivity of NiTi alloy in early stage of transformation cycles

For a better understanding of the transformation behavior on the equiatomic NiTi alloy, the measurement of electrical resistivity was performed in the early stage of transformation cycles, and was compared with the calorimetric measurement. A faint peak in electrical resistivity is observed during the first cooling, at which the temperature is much higher in comparison with the exothermic peak. With increasing transformation cycles, the peak in electrical resistivity is enhanced, at which the temperature is close to the start temperature of the transformation from the intermediate phase to the low-temperature phase. The transformation and the reverse one show a thermal hysteresis of 30 K, and the low-temperature phase is transformed directly to the high-temperature phase, even with increasing transformation cycles. An increase in electrical resistivity with the transformation is detected in the temperature range of the low-temperature phase, which is attributable to the defects induced during the transformation. It is thought on the transformation in the early stage of transformation cycles that the transformation-induced defects play complicated roles such as the electron scattering and the stabilization of the intermediate phase to result in a peak in electrical resistivity.

Effects of transformation-induced defects in cobalt and Ni 48Ti 52

The transformation behavior of pure cobalt and Ni 48Ti 52 were investigated by using a calorimetric technique in order to reveal the influence of transformation cycles. These transformations show a hysteresis with a small thermal width. The FCC–HCP transformation temperature of pure cobalt, during cooling, decreases considerably with increasing transformation cycles at the initial stages and increases gradually for transformation cycles in excess of about 50. The transformation temperature during the direct and the reverse transformations of Ni 48Ti 52 decreases with increasing thermal cycles. These behaviors on the transformation cycles are attributable to the formation of transformation-induced defects. It is thought that the amount and the microstructure of defects play an important role in the shifts of the transformation temperatures, and contribute to the non-chemical free energy and the stability of each phase.

Variation in transformation hysteresis in pure cobalt with transformation cycles

Details of the transformation in pure cobalt were measured by a calorimetric method in order to clear the transformation cycle dependence of the transformation behaviour. Effects of transformation cycles are discussed on the basis of the generation and rearrangement of transformation-induced defects.

On the existence of transformation-induced antiphase boundaries in the γ-phase of two-phase TiAl-based alloys

Abstract The existence of transformation-induced γ antiphase boundaries (APBs) is discussed in TiAl-based alloys in which two phases, γ and α2, are in equilibrium at low temperatures. While such APBs are frequently observed in the metastable γ-phase after the massive transformation α → γ, they are rarely observed within the γ lamellae of the so-called lamellar structure. In both cases (massively transformed structure and lamellar structure), however, other possible transformation-induced defects, such as twin and pseudo-twin boundaries as well as order-related domain boundaries, are found in abundance in the γ-phase (ordered tetragonal L10). The possibility of coupling of APBs with the other boundaries is examined in order to explain the above-mentioned experimentally observed difference regarding the γ APBs.

Transformation behaviour of NiTi in relation to thermal cycling and deformation

The transformation behaviour of an equiatomic NiTi alloy was investigated by measurement of the electrical resistivity and by differential scanning calorimetry. The temperature region of transformation depends on the number of thermal cycles and the intermediate phase appears progressively during cooling with increasing thermal cycles, resulting in a two-stage transformation, while the reserse transformation during heating remains a one-stage transformation. The temperature region of transformation is broadened by plastic deformation, and with subsequent annealing the intermediate phase is enhanced, depending on the annealing temperature. Effects of thermal cycling and annealing after deformation on the transformation of NiTi are attributable to transformation-induced defects as dislocations, and it is concluded that the defects make it possible to stabilize the intermediate phase.

Evolution of the microstructure of cobalt during diffusionless transformation cycles

Differential scanning calorimetry and transmission electron microscopy have been used to study thermal fatigue due to diffusionless phase transformation cycling in pure cobalt. Thermal cycling through the allotropic (hcp ↔ fcc) transformation results in a temperature shift of the calorimetric peaks, which means a delay of the transformation. In addition, the transformation enthalpy, which is greater on heating than on cooling, diminishes when the number of transformation cycles increases. This is interpreted as being due to an evolution of the microstructure. Transmission electron microscopy shows the appearance of transformation-induced defects, which are mainly sessile dislocations. We can interpret the calorimetry results (enthalpy evolution and transformation delay) as due to the interactions between interface dislocations and these sessile dislocations.

TEM as a Tool in Study of Carbonate Crystal Chemistry: ABSTRACT

All natural carbonate minerals exhibit crystal defects, either in terms of nonstoichoimetry or structural imperfections. These defects represent a record of the crystallization (or in some places post-crystallization) process and thus provide direct evidence concerning various diagenetic reactions. The characterization of these defects is facilitated in many ways by use of the transmission electron microscope (TEM) and its various modifications. The diffraction contrast mechanism allows certain defects not visible in other microscopes (e.g., domain structures, stacking faults, modulated structures, etc) to be imaged with very high resolution (approximately 20 A). High-resolution electron microscopy (HREM) utilizes a phase contrast mechanism and is capable of imaging individual lattice planes with a resolution better than 2 A. Depending on the particular planes imaged, information about local ordering and stacking defects is obtained, aspects End_Page 538------------------------------ especially useful in studying dolomites and magnesian calcites. The capability for simultaneous electron diffraction from regions less than 1 µm allows for identification of phases and evaluation of short-range ordering. In addition, many modern TEMs have scanning devices allowing scanning-transmission images (STEM) as well as the more conventional secondary-electron images to be formed. The use of energy-dispersive X-ray spectrometers is optimized with such STEM instruments as spatial resolution of roughly 150 A is possible. Recent studies of carbonate minerals using these techniques have revealed a wide variety of defect microstructures characteristic of different growth and diagenetic conditions. In many places, the microstructures are present in such high densities as to significantly influence the stability and reactivity of the mineral. Notable examples include the calcium-rich dolomites, both from ancient rocks and Holocene sediments. The ancient calcian dolomites exhibit a complex modulated structure with growth and possibly transformation-induced defects. Holocene dolomites are structurally distinct, characterized by a high-density domain structure associated with growth faults. Pervasive, complex microstructures have also been found in saddle dolomites, magnesian calcites, and low-Mg calcites. In the latter, the defect structure is thought to be associated with local CO3 group disorder. Since the microstructures are characteristic of different diagenetic growth conditions, their proper characterization and interpretation represents a potential tool for hydrocarbon exploration in carbonate terrains. End_of_Article - Last_Page 539------------

Transformation-induced defects in α-cristobalite

Transformation-induced twins and antiphase domain boundaries have been observed in both lunar and terrestrial cristobalite. The lunar cristobalite is from mare basalts 12038, 12053 and 70017, whereas the terrestrial material is from San Cristobal, Mexico. All of these samples contained defects resulting from the transformation of the high temperature or β polymorph, usually assumed to be face-centered cubic Fd3m, to α- cristobalite, the low temperature form which is primitive tetragonal P41 212 or P43 22 2. The distribution of defects in all samples is quite heterogeneous; the San Cristobal sample had more defects than the lunar samples. In addition, electron-beam damage is considerably slowed in the lunar cristobalite.Two types of defects were present (Fig. 1). The first consists of twin boundaries which form because any of the three cube axes can become the tetragonal c-axis; this has been previously observed optically and by X-ray diffraction. A second defect consists of domain boundaries (Fig. 2) which appeared to have been produced after the twin boundaries were formed.