Search

Superelastic NiTi tubes are promising candidates for eco-friendly elastocaloric cooling, but their cyclic stability suffers severely from functional degradation. Herein, we investigate the functional degradation of nanocrystalline NiTi tubes via in-depth analysis and find out that it is beneficial to elastocaloric cooling performance. The results show that the functional degradation accompanies with progressive accumulation of residual strain and significant reduction in both hysteresis loop area (D) and forward transformation stress. The accumulation of residual strain arises from phase-transition-induced dislocations and dislocation-pinned residual martensite. The former separates the original austenite grains to much smaller nanodomains (equivalent to grain size effect) and contributes to the strain hardening during phase transition, leading to the significant reduction of D. The latter induces compressive residual stress in the austenite phase and thus gives rise to the evolutive reduction of forward transformation stress. Consequently, the material coefficient of performance (COPmater) was self-enhanced for 40~104 %. The beneficial effect is mainly because of the cyclically-decreased D. The study might provide a processing route to tailor COPmater and stabilize the mechanical response of NiTi by cyclic compression., Comment: 48 pages, 11 figures, 12 supporting figures.To be submitted to Acta Materialia

The effects of microstructure and its evolution on the macroscopic superelastic stress-strain response of polycrystalline Shape Memory Alloy (SMA) are studied by a microstructure-based constitutive model developed in this paper. The model is established on the following basis: (1) the transformation conditions of the unconstrained single crystal SMA microdomain (to be distinguished from the bulk single crystal), which serve as the local criterion for the derivation of overall transformation yield conditions of the polycrystal; (2) the micro- to macro-transition scheme by which the connection between the polycrystal aggregates and the single crystal microdomain is established and the macroscopic transformation conditions of the polycrystal SMA are derived; (3) the quantitative incorporation of three microstructure factors (i.e., nucleation, growth and orientation distribution of martensite) into the modeling. These microstructural factors are intrinsic of specific polycrystal SMA systems and the role of each factor in the macroscopic constitutive response is quantitatively modeled. It is demonstrated that the interplay of these factors will result in different macroscopic transformation kinematics and kinetics which are responsible for the observed macroscopic stress-strain hardening or softening response, the latter will lead to the localization and propagation of transformation bands in TiNi SMA.

Based on the microstructure-based constitutive model established in Part I, a detailed numerical investigation on the role of each microstructure parameter in the kinematical and kinetic evolution of polycrystalline SMA under axisymmetrical tension loading is performed. Some macroscopic constitutive features of stress-induced martensite transformation are discussed.

A non-invariant plane model for the austenite-martensitic (A-M) interface in CuAlNi single crystal shape memory alloys (SMAs) is proposed in this paper. The model is based on the crystallography of martensitic transformation and the recent uniaxial tensile Moiretests on Cu-14%Al-4.12%Ni (wt%) single crystals. The two types of specimens used have the same tensile axis orientation but have different transition temperatures. One exhibits Shape Memory Effect (SME) and the other exhibits Superelasticity (SE). In the case of SME, the plane invariant nature of the A-M interface is well verified by the Moiretest. On the contrary, the A-M interface in the case of SE was identified to be a non-invariant plane. A crystallography- based model is proposed to explain the formation of this non-plane-invariant A-M interface in SE and is used to predict the resultant interface structure and the transformation strain of the stress-induced b1!b 9 transformation in CuAlNi. The comparison between the theoretical calculations and the experimental results support the proposed model. © 2000 Elsevier Science Ltd. All rights reserved.

The draping behavior of fabric is simulated by using four node quadrilateral thin plate elements with finite rotation. The finite element formulation is based on the total Lagrangian approach. An exact representation of finite rotation is introduced. The strain energy function accounting for the material symmetry is obtained by the tensor representation theory. To avoid shear locking, the assumed strain technique for transverse shear is adopted. The conjugate gradient method with a proposed line search algorithm is employed to minimize energy and reach the final shape of fabric. The draping behavior of a rectangular piece of fabric over a rectangular table is simulated.

The effect of autocatalysis and transformation induced shear strain on the stationary crack tip zone shape and toughening is studied by the finite element method. The numerical analysis is based on the micromechanics constitutive model of transformation plasticity proposed by Sun et al. ( J. Mech. Phys. Solids 39 , 1991). This model is capable of describing the autocatalytic transformation and is used as the foundation for the theoretical and numerical study of the autocatalytic phenomenon during martensitic transformation in ceramics. The results demonstrate that autocatalysis and transformation shear strain play an important role in the anomalous crack tip zone morphology and transformation zone shielding. The “negative shielding effect” for the stationary mode I crack in plane stress is discovered and the narrow elongated crack tip zone shape is first simulated by the constitutive model of Sun et al. (1991).

Numerical study by the finite element method (FEM) is performed to investigate the effect of dual-scale microstructure on the toughness of laminar zirconia composite. The computation is based on the micromechanics constitutive model of polycrystal transformation plasticity developed by Sun et al. [10] where both transformation induced shear and softening effects during autocatalytic transformation are taken into account. The numerical simulation presented in this paper successfully reproduced the experimentally observed two effects of the dual-scale microstructure on the toughness of laminar zirconia composite, i.e., the truncation of the elongated transformation frontal zone that forms in single phase Ce-ZrO2 and the propagation of the transformation zone along the layers normal to the crack plane. Quantitative analysis on the role of microstructure in transformation toughening of laminar zirconia composite is first carried out in the present work which will provide a starting point for the microstructural design of this novel advanced composite in the future.

Stress-induced martensitic transformation plastic zones of ceria-stabilized tetragonal zirconia polycrystalline ceramics, under some typical loading conditions, were studied by Moire interferometry. The full-field fringe patterns, including u-fields and v-fields, were acquired. According to these patterns, the transformation zone shape and transformation plasticity distributions of the specimens were obtained. The experimental results show that the stress-induced transformation at room temperature is concentrated in some narrow bands and the transformation plasticity is not uniform within the transformation zone. Experiments also reveal that the transformation zone with a characteristic elongated shape ahead of the notch, in a single-edge notch bending specimen, is obviously different from that resulting from some constitutive relation of transformation. This work provides a significant experimental foundation for establishing the theoretical models of transformation toughening.

A micromechanics analysis on the possibility of designing a two-phase pseudoelastic composite is made for the case where ductile transformable shape memory alloy plastic particles are imbedded coherently in an elastic matrix. It is demonstrated that a pseudoelastic stress-strain loop in a macroscopic loading-unloading cycle can be obtained by microscopically stress induced forward and reverse martensitic transformations in the SMA particles. The relation between the macroscopic stress-strain response and the material parameters of the constituents of this composite is quantified through the micromechanics calculations, which reveals that the best ductility and thus the greatest energy absorption capacity of this novel microstructure can be obtained by the optimum material design.

Transformation plasticity in ceria-stabilized tetragonal zirconia polycrystals due to the stress-induced tetragonal-to-monoclinic martensitic transformation under tension and bending is studied by moire interferometry. The whole fringe patterns includingu fields andv fields are acquired. According to these patterns, the distributions of transformation plasticity in transformation zones are obtained, and the phenomenon of plastic flow localization for transformation is revealed. The above work provides a significant experimental foundation for establishing transformation constitutive relations

A new micromechanics constitutive model for pure dilatant transformation plasticity of structure ceramics is proposed in this paper. Based on the thermodynamics, micromechanics and microscalet→m transformation mechanism analysis of the TZP and PSZ ZrO2-containing ceramics, an analytic expressions of the Helmholtz and complementary free energy of the constitutive element for the case of pure dilatant transformation is derived for the first time in a self-consistent manner. By the analysis of energy dissipation in the forward and reverse transformations, the micromechanics constitutive law is derived in the framework of Hill-Rice's internal variable constitutive theory.

A simple micromechanics constitutive theory is developed to determine the elastoplastic constitutive behavior of ductile particle reinforced brittle materials. The elastoplastic particles, with isotropic or kinematical hardening, are uniformly dispersed in the brittle elastic matrix. The method proposed is based on the Mori–Tanaka's concept of average stress in the composite. The macroscopic yielding condition and the incremental stress strain relation of the composite during plastic deformation are explicitly given in terms of the macroscopic applied stress and the microstructural parameters of the composite such as the volume fraction and yielding stress of ductile particles, elastic constants of the two phases, etc. Finally, the contribution of the plastic deformation in the particles near a crack tip to the toughening of the composite is evaluated.

The transformation toughening effect in mode III crack of Ce-TZP ceramics and the crack growth process of the specimen are investigated by the holographic interference technique. The crack growth path under applied loading and the corresponding displacement field of the specimen were clearly recorded by the holographic interferometer. The experimental results showed that there is an obvious stable crack growth stage before final failure and there is obvious toughening effect (R-curve effect) for mode III crack. It is also shown that the crack does not grow along the original crack plane, but deflects with an angle of about 60 °.

A constitutive Model of transformation plasticity is suggested based on micromechanics and thermodynamics consideration. The model takes into account both the dilatation and shear effects of phase transformation, and is capable of describing the plastic, pseudoelastic and shape memory behavior of structural ceramics during transformation under different temperatures. The constitutive element used, taken as a representative material sample, contains transformed grains or precipitates, as second phase inclusions embedded in an elastic untransformed matrix. Under some reasonable basic assumptions, the expressions of free energy are derived in a self–consistent manner by use of Mori–Tanaka's method, taking account of the interaction between the transformed inclusions. In the framework of Hill–Rice's constitutive theory with internal variables, the transformation criteria and incremental stress strain relations are derived. Theoretical predictions of the suggested constitutive relations are compared with the available experimental data of Mg–PSZ and Ce–TZP polycrystalline ceramics, and good agreement is obtained between theory and experiments. Moreover, the constitutive relations are used to predict toughening effect of transformation for ceramics and lead to excellent coincidence with experiments.

RNA editing exists extensively in the higher plant mitochondria, and is a required step for forming functional proteins. There may be some relationship between RNA editing and cytoplasmic male sterility (CMS), a kind of phenomenon that is attributed to mitochondrial genome mutations. The research materials used are the gametophytic male sterility line (A), maintainer line (B) and F1 hybrid (F1) of HL-type CMS rice. cDNAs and DNAs of atp6 and coxII have been obtained from A, B and F11 by PCR and RT-PCR. Comparing sequences of cDNAs and DNAs, 18 and 15 editing sites were found respectively in the transcripts of atp6 and coxII. A, B and F1 shared the same editing sites. RNA editing improves hydrophobicity and conservation of the predicted protein as compared with other organisms.