Showing papers on "Ferroelasticity published in 1997"

Neutron Diffraction Observations of Ferroelastic Domain Switching and Tetragonal‐to‐Monoclinic Transformation in Ce‐TZP

Abstract: In-situ neutron diffraction has been used to study the plastic deformation of a tetragonal zirconia polycrystal stabilized with 12 mol% ceria under compressive loads up to 1.6 GPa. The development of significant plastic strain in the ceramic has been found to be due to a combination of ferroelastic switching and the tetragonal-to-monoclinic phase transformation, both beginning at ∼1.2 GPa. Evidence of a strong coupling between the two phenomena is present. Both transitions are partially reversed on removal of the load. The linear elastic response of the a and c crystal axes of the parent tetragonal phase suggests that the ferroelastic switching occurs directly by a shear mechanism rather than via a cubic intermediate state. Anisotropic distortion of the tetragonal unit cell, as the critical stress is approached, gives some insight into the shear transformation mechanisms.

Analysis of the ferroelastic phase transition of (TMMC) by means of x-ray diffraction study

Abstract: The order - disorder phase transition in the linear chain compound (TMMC) was investigated by means of x-ray diffraction. The hexagonal structure at room temperature (space group with Z = 2) is characterized by an orientational disorder of the organic group TMA. A weakly first-order phase transition occurs at 126 K which stabilizes a monoclinic low-temperature phase (space group with Z = 4) characterized by a doubling of the hexagonal unit cell along the b direction. The cell parameters were determined in a large range of temperature including the two phases (from 5 to 300 K) and the extent of lattice distortion was measured in the ordered monoclinic phase. The temperature dependences of both the spontaneous strain component and the intensity of superstructure reflections were analysed by a Landau type free energy expansion involving two coupled order parameters necessary to account for this `triggered' phase transition.

Experimental study of mechanical hysteresis of NiTi during ferroelastic and superelastic deformation

Abstract: Shape memory alloys are known to exhibit a range of novel thermomechanical behaviour associated with the unique thermoelastic martensitic transformation. Such behaviour includes the superelasticity associated with stress-induced martensitic transformation at relatively high temperatures and the ferroelasticity associated with a martensite reorientation process at low temperatures. Both the stress-induced martensitic transformation and the martensite reorientation are energy-dissipative processes, i.e., hysteretic between the forward and reverse processes. This work was aimed at studying the hysteretic behaviour of the ferroelasticity and superelasticity observed in a polycrystalline NiTi alloy by carrying out simple shear deformation tests through both major and subloop cycles. It was found that subloops are always closed and enclosed inside the major loop and that the stress hysteresis of a subloop is only dependent on the strain amplitude of the subloop, regardless of the position of the subloop inside the major loop.

Ferroelastic domain structure of (NH4)3H(SeO4)2 crystal

Abstract: Theoretical and experimental studies of the ferroelastic domain structure of (NH4)3H(SeO4)2 crystal in phases III and IV are discussed. Using the refined structural data the orientations of W- and W1-−type domain walls as well as temperature evolution of the domain structure are investigated. It is shown that despite the fact that the room-temperature phase III is triclinic with small deviations from monoclinic, the domain structure in this phase differs strongly from the structure in the case of mono-clinic symmetry. In the monoclinic phase IV all W1-−type domain walls lie almost parallel to the (001)-plane, which explains the invisibility of W' walls after the III—IV phase transition during observation of domain structure in the (001)-plane.

Ferroelastic Property of LiNH4SO4 Single Crystals

Abstract: We have investigated the ferroelastic properties by stress in LiNH 4 SO 4 single crystals grown by a slow evaporation method. The temperature dependence of the stress-strain hysteresis was also studied. From this result, it was determined that the LiNH 4 SO 4 single crystal undergoes a phase transition near T C = 11.5°C. Also, the shapes of the stress-strain curves in phase III are different from that of the phase II. It was confirmed that LiNH 4 SO 4 crystals have ferroelastic characteristics in phase III by the stress-strain hysteresis and Aizu classification.

Phase transitions and twin domains in (NH4)2SbF5. Dielectric and optical properties

Abstract: The dielectric and optical properties of (NH 4 ) 2 SbF 5 crystals (APFA) have been studied simultaneously by optical observation in the temperature range 80 to 300 K. The existence of two phase transitions is demonstrated at temperature 293 and 169 K. The second order of the first one is confirmed. The continuous character of the lower one is demonstrated and the proportionality between electronic density deformation and NH 1 + orientations seems clear. A notable phenomenon of brilliant domain walls is discovered at about 145 K (when β = 0°).

High Temperature Ferroelastic Phase of LiRbSO4 Single Crystals

Abstract: We investigated the ferroelastic properties of LiRbSO 4 single crystals grown by the slow evaporation method. The temperature dependence of the stress-strain hysteresis was also studied. From these results, it was determined that the LiRbSO 4 single crystal undergoes two phase transitions near T C2 = 200 °C and T C4 = 165 °C. In other words, the phase transition due to an external mechanical stress applied to LiRbSO 4 crystals causes a transformation from one phase to another phase. Also, the shapes of the stress-strain curves in phases I, II, and V are different from those of the phases III and IV. It was confirmed by stress-strain hysteresis and Aizu classification, that LiRbSO 4 crystals have ferroelastic characteristics in phases III and IV.