Qingping Sun

Micromechanics modelling for the constitutive behavior of polycrystalline shape memory alloys—I. Derivation of general relations

Abstract A MICROMECHANICS constitutive model has been proposed in this paper to describe the pseudoelastic and shape memory behavior of polycrystalline shape memory alloys under various temperatures. The derivation of the model is based on the thermodynamics, micromechanics and microstructural physical mechanism analysis of the material during deformation and it is shown that the inelastic deformation of the material in the mechanical and/or thermal loading processes is associated with some temperature, stress state and loading history dependent yielding surfaces which microscopically correspond to the forward and reverse transformation (or reorientation) processes, respectively.

Transformation-induced plasticity (TRIP)

This review article attempts to explain TRIP generally and its appearance in different materials. Continuum mechanics formulations are mainly used to explain this phase change phenomenon. An overview is given of the published literature which is often not very easily accessible. Technological aspects are presented, such as constitutive equations of materials showing TRIP. Aspects of material selection and future material design are also treated. This article contains 315 references.

Micromechanics Modelling for the Constitutive Behavior of Polycrystalline Shape Memory Alloys II: Study of the Individual Phenomena

Abstract T he constitutive relation for various phenomena of SMA ( superelasticity, rubber-like elasticity, ferroelasticity, elastic anomaly, shape memory effect ) is studied in detail and compared with the available experimental data. It is shown that the micromechanical model developed in Part I can satisfactorily describe the main peculiarities of the macroscopic thermomechanical constitutive behavior in the course of uniaxial mchanical and/or thermal loadings and that the existing phenomenological models are special cases of the proposed theory under proportional loading conditions. Some theoretical predictions and discussions for complex loading paths are also given which are yet subject to experimental verification.

Phase transformation in superelastic NiTi polycrystalline micro-tubes under tension and torsion––from localization to homogeneous deformation

Abstract Recent tension–torsion test results on nano-grained NiTi shape memory alloy micro-tubes are reported in this paper. It is discovered that: (1) during uniaxial tensile loading, the stress-induced transformation in the micro-tubes is realized by the initiation and growth of a macroscopic spiral martensite band with a quite sharp austenite–martensite (A–M) interface; (2) during loading by torsion (pure shear), the stress–strain curve exhibits monotonic hardening, the stress-induced transformation is axially homogeneous throughout the whole tube and the transformation strain is much smaller than that under tension; (3) under tension–torsion combined loading with an increasing shear/tension stress ratio, there is a gradual change in the deformation mode from localization and propagation (under pure tension) to the homogeneous deformation (under pure shear). The tube surface morphology observation indicates that, during this gradual change, the A–M interface thickness increases as its evolution is recorded. The test results demonstrate that there is a strong anisotropy and loading path dependence of the tube responses in both kinematics and kinetics. Possible underlying physical mechanisms are analyzed and implications for future theoretical modelling are also discussed.

The initiation and growth of macroscopic martensite band in nano-grained NiTi microtube under tension

The superelastic behavior of polycrystalline nano-grained NiTi shape memory alloy microtube under uniaxial tension is studied in this paper. The nominal stress–strain curve of the microtube during superelastic deformation is recorded. Both direct surface observation and observation by using a special surface coating show that the deformation of the tube is via the nucleation and propagation of macroscopic stress-induced martensite band. It is also found that the martensite nucleates in the form of a spiral lens-shaped narrow band that inclines at about 33o to the plane of cross section of tube when the stress reaches the peak of stress–strain curve. The spiral band grew via gradual increase in both width and length of the band and finally merged into a single cylindrical band. The subsequent deformation of the tube is realized by the growth of this cylindrical martensite band. Several other deformation features of the tube are also observed and the results are discussed and compared with the theoretical analysis in this paper.

A micromechanics constitutive model of transformation plasticity with shear and dilatation effect

Abstract B ased on micromechanics, thermodynamics and microscale t → m transformation mechanism considerations a micromechanics constitutive model which takes into account both the dilatation and shear effects of the transformation is proposed to describe the plastic, pseudoelastic and shape memory behaviors of structural ceramics during transformation under different temperatures. In the derivation, a constitutive element (representative material sample) was used which contains many of the transformed m-ZrO2 grains or precipitates as the second phase inclusions embedded in an elastic matrix. Under some basic assumptions, analytic expressions for the Helmholtz and complementary free energy of the constitutive element are derived in a self-consistent manner by using the Mori-Tanaka method which takes into account the interaction between the transformed inclusions. The derived free energy is a function of externally applied macroscopic stress (or strain), temperature, volume fraction of transformed phase and the averaged stressfree transformation strain (eigenstrain) of all the transformed inclusions in the constitutive element, the latter two quantities being considered to be the internal variables describing the micro-structural rearrangement in the constitutive element. In the framework of the Hill-Rice internal variable constitutive theory, the transformation yield function and incremental stress strain relations, in analogy to the theory of metal plasticity, for proportional and non-proportional loading histories are derived, respectively. The theoretical predictions are compared with the available experimental data of Mg-PSZ and Ce-TZP polycrystalline toughening ceramics.

Anomalous relationship between hardness and wear properties of a superelastic nickel–titanium alloy

We have studied the behavior of microwear and hardness of a superelastic nickel–titanium alloy using a triboindenter at various temperatures. Wear resistance was found to anomalously decrease with an increase in hardness. The observations are analyzed based on simple contact theory which suggests that the increase of hardness with the temperature is mainly due to an increase in phase transition stress, while the decrease of wear resistance with the temperature is due to an increase of the austenite elastic modulus and a decrease of the amount of phase transition that can be recovered.

Stress hysteresis and temperature dependence of phase transition stress in nanostructured NiTi—Effects of grain size

Stress hysteresis (H) and temperature dependence of phase transition stress (dσ/dT) are the two signatures of first-order phase transition in shape memory alloys. We studied the effects of grain size on these two properties in polycrystalline superelastic NiTi with the average grain size from 10 nm to 1500 nm. We identified a critical grain size (∼60 nm) below which both H and dσ/dT rapidly decrease, leading to vanishing hysteresis and breakdown of Clausius-Clapeyron equation. The physics behind such grain size effects are the dominance of interfacial energy in the energetics of the polycrystal and the lack of two-phase coexistence at nano-scales.

Micromechanics modeling of composite with ductile matrix and shape memory alloy reinforcement

Abstract A quantitative micromechanics-based analysis on the role of microstructure and constituent properties in the overall behavior of shape memory alloy (SMA) composite is carried out in the present work. The composite consists of ductile matrix and SMA second phase inclusions. The macroscopic constitutive relations of the composite are established by using self-consistent approach where the micro–macro correlation is realized by volume averaging and by introducing the concept of stress and strain concentration tensors. In this micromechanics modeling, the internal stress and strain in both matrix and SMA and their evolution are derived as function of externally applied thermomechanical loading as well as the degree of phase transformation in SMA. As an application of the present theory in the microstructural design of this novel composite, the constitutive response of composites with spherical SMA particulate embedded in two different elastoplastic matrixes under uniaxial tension is calculated. The obtained results demonstrate several interesting deformation features of the new composite, which are expected to have potential applications in the future.

Characterization, thermomechanical behaviour and micromechanical-based constitutive model of shape-memory CuZnAl single crystals

Abstract CuZnAl shape memory alloys are grown as single crystals by the Bridgman technique with a final shape directly suitable for thermomechanical tests (cylinders with tapered heads: 25 mm gauge length, 4 mm in diameter). The four classical transformation temperatures are checked by Differential Scanning Calorimetry and resistivity. The orientation of crystal structure is investigated by X-ray diffraction. Isothermal pseudoelastic tensile tests show that the width of the hysteresis loops and the slope of the stress-strain curves during phase transformation increase as the applied stress rate increases. A micromechanical-based constitutive model allows us to describe this single crystal behaviour.