Stress-induced Phase Transition Research Articles

Investigation of nucleation and propagation of phase transitions in TiNi SMA

The attention of the present paper is focused on the aid provided by infrared thermography, for spectacular investigation of nucleation and further development of the stress induced phase transitions in TiNi shape memory alloy. This is a qualitative analysis aimed to verify the feasibility of further study in the application of IR for studying change phenomena. To this end, the material stress-strain curves, obtained during tension test with various strain rates, were completed by the temperature characteristics. The temperature distributions on the specimen's surface were determined by using an infrared camera. A heterogeneous temperature distribution, related to the nucleation and development of the new martensite phase, were registered and analyzed. A significant temperature increase, up to 30 K was registered during the martensite transformation. Similar effects of the heterogeneous temperature distribution were observed during unloading, while the reverse transformation, austenite into martensite, tooks place. The reverse transformation was accompanied, in turn, by a temperature decrease, of up to 10 K.

New concepts in thermoplastic elastomers: the case of syndiotactic polypropylene, an unconventional elastomer with high crystallinity and large modulus.

Syndiotactic polypropylene (sPP), despite high crystallinity, shows unusual elastic properties, associated with a reversible crystal-crystal phase transition. The mechanism behind the crystal-crystal phase transition involved during cyclic elongation and recovery and the time scale of this process is here investigated, to unravel the origin of the elasticity of such an unconventional elastomer. The structural transformation of fibers of sPP during deformation has been studied by wide-angle X-ray diffraction measurements, using the high flux available on the beamline ID11 at the European Syncrotron Radiation Facility (ESRF), Grenoble, France, performing a time-resolved analysis of structural changes occurring while cyclically stretching and relaxing sPP fibers. This analysis indicates that the stress-induced phase transition is a martensitic transformation. As in martensitic phase transitions it occurs readily and directly, supporting the idea that elasticity in sPP is partially of enthalpic nature.

Stroboscopic neutron diffraction from spatially modulated systems

Abstract The combination of stroboscopic techniques and neutron diffraction can be used to study the kinetics of structural changes in condensed matter on a microscopic level. Transient states may be identified and characterized on time-scales down to the microsecond regime. Hence, valuable information about the underlying mechanisms can be obtained from time-resolved experiments. Particularly interesting subjects for this type of investigation are spatially modulated systems which undergo phase transitions or phase separation. The potential of stroboscopic neutron diffraction is demonstrated using three different examples in which samples are periodically perturbed by the variation of temperature, mechanical stress or electric field and their structural response is characterized by time-resolved diffraction. Spinodal decomposition in ionic crystals of the silver-alkali halide type is shown to be dominated by two different processes on different time-scales. The stress-induced phase transition into the incommensurate phase of quartz involves relaxation processes which are reflected by different kinetic behaviours of Bragg peaks and satellite reflections, respectively. Finally, metastable transient states are observed during the field induced lock-in transition in ferroelectric Rb2ZnCl4 which are most probably due to strain fields and pinning effects.

Stress-Induced Reversible Phase Transition of Poly(tetramethylene naphthalate)

Crystalline modifications α and β of uniaxially oriented poly(tetramethylene naphthalate) [PTMN] sample have been found to show a reversible phase transition, which is induced by an application of a tensile force along the draw axis, as clarified by the measurement of polarized infrared spectra under tension. The transition behavior was found to be essentially the same with that observed for uniaxially oriented poly(tetramethylene terephthalte) [PTMT] sample. The β phase fraction, evaluated by quantitative analysis of the infrared spectra, was found to be linearly proportional to the strain and increase dramatically beyond a critical stress. The stress−strain curve measured for the bulk PTMN sample showed a plateau region, in which the α−β transition occurred. The plateau length became longer by raising the temperature of heat treatment of the samples or with increasing the degree of crystallinity. All these characteristic features are similar to those observed for PTMT. But, in the PTMT case, the plateau stress was almost constant irrespective of the annealing temperature. The stress−strain curve of the PTMT sample could be interpreted reasonably on the basis of a mechanical model in which α crystal and amorphous region are connected in a series mode. On the other hand, the plateau region in the stress−strain curve of the PTMN sample was found to shift apparently toward higher stress side and became more difficult to detect for the sample annealed at lower temperature. This was due to higher residual strain or β fraction remained in the sample. High-temperature annealing accelerated a stress relaxation, causing a recovery of the original α form and making the plateau part clearer. All these behaviors could be interpreted reasonably by employing a mechanical model in which a serially connected part of α crystal and amorphous region is combined in a parallel mode with a serial part of β crystal and amorphous region. The contribution of the β crystal is higher for the sample prepared at a lower temperature.

Growth of Ge films by cluster beam deposition

Ge epitaxial layers with reasonable quality were grown on the Si(1 1 1) substrates by cluster beam deposition (CBD) process. The growth temperature plays a dominant role in the epitaxial growth of Ge films. The substrate temperature for epitaxial growth is about 500 °C, which is lower than the reported critical temperature of Ge epitaxial growth by MBE and CVD. A stress induced phase transition of Ge lattice from cubic to tetragonal is also observed in the CBD process, and the mechanism is discussed.

Positron Annihilation Study on Defects and Phase Transition of FeRh Alloys Subjected to High-Speed Deformation

The relations between vacancy-type defects in Fe-40, 45, and 50 at%Rh alloys and stress-induced phase transitions from a B2 phase have been examined by XRD, TEM, and positron annihilation experiments. The specimens were deformed at room temperature by a high-speed compression machine, and the natures of related vacancies and vacancy clusters were examined. The results reveal that structural Rh vacancies exist in non-stoichiometric FeRh alloys, and that lifetimes of the long-lifetime components of the introduced defects change in various manners, depending upon the deformation conditions.

Beta transition and stress-induced phase separation in the spinning of spider dragline silk

Spider dragline silk is formed as the result of a remarkable transformation in which an aqueous dope solution is rapidly converted into an insoluble protein filament with outstanding mechanical properties. Microscopy on the spinning duct in Nephila edulis spiders suggests that this transformation involves a stress-induced formation of anti-parallel beta-sheets induced by extensional flow. Measurements of draw stress at different draw rates during silking confirm that a stress-induced phase transition occurs.

Surface mobility and slip of polybutadiene melts in shear flow

Surface mobility and wall slip of entangled polybutadiene melts were studied with attenuated-total-reflectance infrared spectroscopy at stresses characteristic of the sharkskin, spurt, and melt-fracture regimes. Small-scale slip, accompanied by an apparent decrease in transverse mobility, occurs in the sharkskin regime, but at a stress above the visual onset of sharkskin in capillary viscometry. Simulations cannot distinguish between a cohesive mechanism and a lubrication mechanism that might follow from a stress-induced phase transition, but an adhesive failure seems to be excluded. The near-surface length scale is of the order of four to six times the equilibrium root-mean-square end-to-end distance, and the estimated slip velocity is insensitive to molecular weight. Strong slip occurs in the spurt regime, either at the wall or within one radius of gyration. Substantial apparent slip occurs with a fluorocarbon surface, but the mechanism does not appear to be an adhesive failure; there seems to be a substantial decrease in the friction coefficient of chains over a distance of order 300 nm or more from the fluorocarbon surface, and the transverse chain mobility in this region appears to be enhanced rather than retarded. Overall, the results of this study indicate that the influence of the wall extends farther into the sheared melt than would be expected from the chain dimensions, except in the case of strong slip.

The rôle of crystallinity in the deformation of main-chain liquid-crystalline copolyesters

The importance of crystallinity in the deformation processes of main-chain liquid-crystalline copolyesters is examined. Two materials are studied: one a copolymer of hydroxybenzoic acid (HBA) with 2-hydroxy-6-naphthoic acid (HNA); the other a copolymer of HBA with isophthalic acid (IA) and hydroquinone (HQ). In both cases it is observed that the transverse compressive yield stress and modulus of the polymer are optimised by annealing at about 20°C below the melting temperature. In the case of the copolymer containing HBA and HNA, it is found that deformation of unannealed samples results in a pronounced loss of crystallinity. However, the effect is less marked in annealed samples, and the crystallinity is found to recover after modest annealing close to the glass transition temperature. In both polymers, evidence is found for a stress-induced solid-state phase transition, which may be similar to the martensitic phase transition proposed for polyethylene.

Kinetics of the stress induced phase transition in quartz by real-time neutron scattering

Real-time stroboscopic neutron scattering has been used to investigate the time evolution of the phase transition in quartz. Under periodic external stresses relaxation times of the order 1 s are observed and the incommensurate phase exhibits a different stability regime as compared to static conditions.

On a shock-induced martensitic phase transition

A recently developed continuum-mechanical model for stress-induced phase transitions in solids is applied to a transition generated by impact. The role of transition kinetics in determining the macroscopic response to impact is discussed; in addition, the special way that “overdriven” phase boundaries emerge in this model is described. The predictions of the model are compared with experiments involving shock-induced graphite-to-diamond phase transitions.

Thermodynamics of perfect polymer fibers in tension: stress-induced enantiotropic phase transitions

The principles of the thermodynamic fusion theory [Smith Jr, KJ. Comp. Theor. Poly. Sci. 1997;7:139] are applied to solid–solid phase transitions affected by stress. Only perfect fibers of finite molecular weight polymers are considered. Transitions of one crystal form to another involves stretching work as well as that of constant force transition; the necessary equation is cast into an experimentally accessible form. Fiber volume and entropy are shown to pass through, respectively, a maximum and minimum as tensile force increases. Recovery of both the undeformed values is interpreted as the necessary criteria of phase transition under stress. The fundamental, Flory–Gee equation of phase equilibrium is integrated rigorously. Applications to actual fibers are deferred until sufficient empirical data is available.

Application of a new virtual crystal approach for the study of disordered perovskites

The “virtual crystal” (VC) approach is a tractable way of studying configurationally disordered systems; the potentials which represent atoms of two or more elements are averaged into a composite atomic potential. This approach has the advantage that a single configuration with a smaller unit cell represents the disordered system. However, due to the different local environment of the virtual atom, some properties may not be reproduced. In this work, we develop a new virtual crystal approach and apply it to the study of the stress-induced phase transition in Pb(Zr 1/2Ti 1/2)O 3. We compare four averaging algorithms for the construction of the virtual atom pseudo-potentials and we assess the accuracy of each by comparing with a superlattice prediction of the equations of state.

The effect of nonlinear stress and temperature on the ferroelectric properties of perovskites. Part 1: Phenomenological equations

Ferroelectric material properties strongly depend on the external and internal stress and electric fields. This strong dependency is due to the coupling of the nonlinear switching polarization to the nonlinear switching strain of the BO6 octahedra of the ABO3 perovskite structure. Therefore any changes imparted to this atomic octahedral configuration either by external or internal mechanisms would lead in an average sense to changes in the material properties. In the current paper, a first order phase transition Landau model is modified by making the nonlinear dielectric stiffness, α11 temperature, T 3 dependent and retaining the nonlinear c ijk, elastic stiffnesses. Due to these modifications, stress induced phase transition shift is observed as well as shifting of the first order transition to second order transition under the applied stress which agrees well with the experimental observations. The model can also predict critical points where multiple phases coexist, the phase stability, thin film stress effects, grain size effects, doping effects and single domain material properties, which are presented in follow up papers. The free energy equations and the material properties derivation with the necessary justifications are presented in the current paper.

Bifurcation and metastability in a new one-dimensional model for martensitic phase transitions

Materials undergoing stress-induced martensitic phase transitions often form complex twinned microstructures with multiple phase boundaries. They also exhibit hysteretic mechanical behavior. We propose and analyze a one-dimensional model for twinning. We consider two elastic bars coupled by a system of continuously distributed linear springs. One of the bars has a two-well nonconvex elastic energy density that models a two-variant martensitic phase. The other bar is linearly elastic and is meant to model the parent austenite phase. Interfacial energy is modeled by a strain-gradient term. Various types of boundary conditions model parameter-dependent loading. A local bifurcation analysis shows that local energy minima (metastable states) often involve a large number of phase boundaries. This is confirmed by the global-bifurcation diagrams obtained numerically. We observe that this microstructure emerges via both sudden (finite) and gradual (infinitesimal) phase nucleation. We propose an energetic argument that predicts hysteresis in overall load-deformation behavior due to metastability of multiple equilibria. A limiting case with zero interfacial energy is treated analytically, yielding global solution diagrams.

Stress-induced phase transitions in elastic solids

This paper describes a continuum model, developed recently by R. Abeyaratne and the author, for the response of elastic solids capable of undergoing stress-induced phase transitions. Models of the kind sketched here and their generalizations are intended to apply to both quasi-static and dynamic experiments for shape memory alloys and to impact-induced phase changes in ceramics. The present discussion is confined to a purely mechanical theory, omitting thermal effects, so that the natural setting is the nonlinear theory of elasticity. The presentation below is limited to one space dimension.

Rheology and phase behavior of the system PBZT-phosphoric acid-water

Solutions of poly-p-phenylenebisbenzthiazole (PBZT) in phosphorous acid were examined in a rheogoniometer for shear response and relaxation as a function of concentration and temperature based on knowledge of the phase diagram of the system. It was found that the various regions of the phase diagram (i.e., the isotropic, the nematic, and the biphasic regions) and the various kinds of crystal solvates were reflected by their rheological behavior. This shows that rheological properties can be used to map, at least, the broad features of phase diagrams; conversely, the knowledge of the phase diagram is a prerequisite for the appreciation of rheological behavior. At the same time, the stress involved in the rheological tests themselves can modify, to a lesser extent, the positions of the phase dividing lines compared to those in the static system, which at appropriate points in the phase space can lead to stress-induced phase transitions. The principal rheological effect, here used diagnostically, is in itself novel; it is a spontaneous stress buildup in the stationary state of the rheometer after the stresses generated by the preceding shearing have decayed following stoppage of the rotor. The effect itself will be followed up in a separate study.

A Simple Micromechanical Model for Pseudoelastic Behavior of CuZnAl Alloy

A simple micromechanical model for thermoelastic martensitic phase transitions (PT) is developed. It is deduced from the local description of PT in transforming particles with subsequent usage of average procedure, based on a model for elastic three-phase materials (austenite, martensite and new infinitesimal nucleus) under assumption of homogeneity of stresses in each phase. In contrast to known approaches, a new local PT criterion and a corresponding extremum principle for PT with dissipation are used. The macroscopic PT criterion obtained is split into two different equations for description of temperature-induced PT and stress-induced PT. To identify the material parameters of the model and to check its validity, simple one-dimensional experiments were carried out for CuZnAl alloy. The experimental values of martensite start and finish temperatures and austenite finish temperature for temperature-induced PT and the stress-strain diagram for stress-induced direct PT at any fixed temperature have allowed to determine six material parameters of the model for the simplest one-dimensional case. Then model prediction is compared with other independent tests. A good agreement is obtained of the calculated stress-strain curves for reverse PT (martensite-austenite) at 01 = 20C and for direct PT at temperature range of 30—80C with experimental data. Finally, the formula for determination of the transformation heat during temperature-induced PT for the given model is derived. It is shown that the predicted transformation heat is close to the experimental one.

Temperature-dependent fracture mechanisms in gel-spun hot-drawn ultra-high molecular weight polyethylene fibres

Tensile testing of gel-spun hot-drawn ultra-high molecular weight polyethylene (UHMWPE) fibres reveal a ductile-brittle transition temperature. Ductile fracture above the transition temperature is believed to be initiated by a stress-induced orthorhombic-hexagonal phase transition, whereas at lower temperatures brittle fracture occurs in the orthorhombic phase. SEM micrographs of the fracture surfaces of UHMWPE fibres fractured below the transition temperature show fibrillar fracture and cannot be distinguished from fibres fractured in a ductile way above the transition temperature.

Stress-induced formation of high-density amorphous carbon thin films

Amorphous carbon films with high sp3 content were deposited by magnetron sputtering and intense argon ion plating. Above a compressive stress of 13 GPa a strong increase of the density of the carbon films is observed. We explain the increase of density by a stress-induced phase transition of sp2 configured carbon to sp3 configured carbon. Preferential sputtering of the sp2 component in the carbon films plays a minor role compared to the sp2⇒sp3 transition at high compressive stress formed during the deposition process. Transmission electron microscopy shows evidence of graphitic regions in the magnetron sputtered/Ar plated amorphous carbon thin films. Differences in the microstructure of the tetrahedral amorphous carbon (ta–C) films deposited by filtered arc and mass selected ion beam; and those films deposited using magnetron sputtering combined with intense ion plating can be used to explain the different electronic and optical properties of both kinds of ta–C films.