Temperature-induced Martensite Research Articles

Reverse ε→γ transformations of temperature-induced and stress-induced martensites in Fe–Mn–Si alloys with shape memory effect

In this work we argue that the reverse martensitic transformation (MT) of the stress-induced martensite (SIM) of Fe–Mn–Si-based alloys for moderate prestrains occurs in essentially thermoelastic-like way, in contrast to non-thermoelastic MT of temperature-induced martensite (TIM). We compare, using a number of complementary experimental techniques, the temperatures and the kinetics of the reverse ε→γ martensitic transformation of SIM and TIM for commercial and model Fe–Mn–Si alloys after various thermomechanical treatments. We claim that two studied phenomena are generic in Fe–Mn–Si-based alloys of different compositions and microstructure: first, opposite to the conventional effect of stabilization of martensite by prestrain, the reverse MT during heating of alloys prestrained in bending and tension starts well below (up to 100 K) the start temperature of the reverse MT for the TIM of the same alloy. The difference between engineering temperatures of the start of the reverse transformation, As, for SIM and TIM reaches ca. 50 K, pointing to a need to consider different As for TIM and SIM, AsTIM and AsSIM. Second, the reverse transformations of TIM and SIM show very different kinetics. The reverse MT of SIM is diffuse over the temperature range exceeding 100 K. The reverse MT of TIM, on the contrary, is well localized within the temperature range of ca. 20 K. The experimental observations can be interpreted assuming that, due to the differences in the formation mechanisms of SIM and TIM, the latter shows non-thermoelastic behaviour, whereas the SIM shows similarities with the thermoelastic martensites stabilized by prestrain.

Molecular dynamics simulations on one-way shape memory effect of nanocrystalline NiTi shape memory alloy and its cyclic degeneration

The one-way shape memory effect (OWSME) of nanocrystalline NiTi shape memory alloy (SMA) and its cyclic degeneration under thermo-mechanically coupled cyclic loading conditions were investigated by molecular dynamics (MD) simulations. Firstly, the critical temperatures of martensite transformation were obtained by carrying out the MD simulations of temperature-induced martensite transformation; then, the microscopic mechanisms of martensite reorientation-induced plasticity, the deformation during the subsequent heating and cooling, and the cyclic degeneration of OWSME were investigated based on the correspondent MD simulations. The results show that the martensite reorientation of nanocrystalline NiTi SMA occurs via the migration of the interfaces between martensite variants, the growth of martensite variants with favorable orientations and their swallowing to the variants with unfavorable orientations under specific mechanical loading conditions; the cyclic degeneration of OWSME in nanocrystalline NiTi SMA is caused by the interaction among the martensite reorientation, martensite transformation, martensite reorientation-induced plasticity and martensite transformation-induced plasticity occurred in the martensite region and at the interfaces between martensite variants. The martensite reorientation in each OWSME cycle is reflected by the migration of martensite variant interfaces and the growth and swallowing of martensite variants; the martensite transformation in each OWSME cycle is the nucleation and growth of martensite phase; and the accumulation of plastic deformation is caused by the migration of martensite variant interfaces and the dislocation slipping at the martensite variant interfaces which leads to the cyclic degeneration of OWSME. The MD simulations are verified further by comparing them with the available experimental results of NiTi SMA.

Transformation Mechanism of Temperature-Induced Martensite in Ti-Ni Shape-Memory Alloys by In-Situ XRD

Transformation Mechanism of Temperature-Induced Martensite in Ti-Ni Shape-Memory Alloys by In-Situ XRD

Ferrite, martensite and supercritical iron: A coherent elastochemical theory of stress-induced carbon ordering in steel

A mean-field model based on the elasticity theory of point defects has been developed to investigate the role of uniform stress fields on the long-range ordering of carbon atoms in bct-iron. From an analysis of the thermodynamic equilibria, composition – temperature – stress state diagrams are derived. We demonstrate that ferrite, martensite and supercritical iron are various instances of the same bct-iron phase region. A coherent mapping of the phase transitions is drawn, identifying (i) continuous transitions such as ferrite ordering, martensite enhanced ordering and ferrite – martensite transformation, and (ii) discontinuous transitions such as temperature-induced martensite and stress-induced martensite. Our analysis is supported by rigid-lattice Monte Carlo simulations. Recently published experimental results on highly-drawn perlitic wires are re-interpreted in terms of supercritical iron, rather than strain-induced martensite. Novel low-temperature thermomechanical treatments of supersaturated ferrite are suggested, for improved nanostructure design of martensitic steels.

Croatian

Shape memory alloys (SMAs) belong to a group of functional materials with the unique property of “remembering” the shape they had before pseudoplastic deformation. Such an effect is based on crystallographic reversible thermo-elastic martensitic transformation. There are two crystal phases in SMAs: the austenite phase (stable at high temperature) and the martensite phase (stable at low temperature). Austenite to martensite phase transformation can be obtained by mechanical (loading) and thermal methods (heating and cooling). During martensitic transformation, no diffusive process is involved, only inelastic deformation of the crystal structure. When the shape memory alloy passes through the phase transformation, the alloy transforms from high ordered phase (austenite) to low ordered phase (martensite). There are two types of martensite transformations. First is temperature-induced martensite, which is also called self-accommodating (twinned) martensite. The second is stress-induced martensite, also called detwinned martensite. The entire austenite to martensite transformation cycle can be described with four characteristic temperatures: Ms – martensite start temperature, Mf – martensite finish temperature, As – austenite start temperature, and Af – austenite finish temperature. The main factors influencing transformation temperatures are chemical composition, heat treatment procedure, cooling speed, grain size, and number of transformation cycles. As a result of martensitic transformation in SMAs, several thermomechanical phenomena may occur: pseudoelasticity, shape memory effect (one-way and two-way SME) and rubber-like behavior. Pseudoelasticity occurs when the SMA is subjected to a mechanical loading at a constant temperature above Af. The second thermomechanical behaviour that can be observed in SMA is the shape memory effect (SME), mainly one-way SME, which is the most commonly used SME. When the sample is subjected to a mechanical loading, the stress reaches a critical value and the transformation of twinned martensite into detwinned martensite begins and finishes when the loading process is finished. When the loading-unloading process is finished, the SMA presents a residual strain recoverable by alloy heating, which induces the reverse phase transformation. As a result, the alloy recovers to its original shape. In this paper, a review of thermomechanical properties of shape memory alloys and general characteristics of martensite transformations is shown.

Temperature-induced martensite in magnetic shape memory Fe2MnGa observed by photoemission electron microscopy

The magnetic domain structure in single crystals of a Heusler shape memory compound near the composition Fe2MnGa was observed during phase transition by photoelectron emission microscopy at Beamline 11.0.1.1 of the Advanced Light Source. The behavior is comparable with recent observations of an adaptive martensite phase in prototype Ni2MnGa, although the pinning in the recent work is an epitaxial interface and in this work the effective pinning plane is a boundary between martensitic variants that transform in a self-accommodating way from the single crystal austenite phase present at high temperatures. Temperature dependent observations of the twinning structure give information as to the coupling behavior between the magnetism and the structural evolution.

Localization Events and Microstructural Evolution in Ultra-Fine Grained NiTi Shape Memory Alloys during Thermo-Mechanical Loading

Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra-fine grained NiTi wires subjected to a broad range of thermo-mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature-induced martensite (formed during cooling at no or relatively small loads), stress-induced martensite, and reoriented martensite (formed under load at low temperatures).

A Correction to the Brinson's Evolution Kinetics for Shape Memory Alloys

Phase diagram based approach is frequently used to investigate the shape memory alloy behavior. Typically, evolution laws for the martensitic phase fraction are defined for different local transformation regions in the stress—temperature phase diagram. In the Brinson's evolution kinetics, under certain conditions, the local evolution law for the formation of martensitic fraction leads to inadmissible phase fraction (>1). This pertains to a case where both stress- and temperature-induced martensite evolve simultaneously. A modified evolution law is presented here to overcome this anomaly.

One-Dimensional Shape Memory Alloy Model Applicable to Any Status of Stress and Temperature

The transformation kinetics formulation is the principal factor underlying the constitutive model of shape memory alloys. Therefore, the transformation kinetics formulation, which is applicable to any status of stress and temperature, is essential for predicting the material behavior of SMAs. In this work, we show that the transformation kinetics of the Brinson model, which is the most widely used 1-dimensional model, has shortcomings under certain stress and temperature histories. In addition, we propose a modified transformation kinetics model that can be used for any stress or temperature conditions. The martensite transformation kinetics is modified so that the transformation from austenite into temperature-induced martensite, due to the decrement of temperature, is coupled with a transformation from austenite or temperature-induced martensite into stress-induced martensite, due to the increment of the stress. Through this modification, the suggested model can simulate the behavior of shape memory alloy materials under arbitrarily changed circumstances at every stress-temperature region.

Implementation strategy for the dual transformation region in the Brinson SMAconstitutive model

The transformation kinetics formulation is the principal factor underlying the constitutivemodel of shape memory alloys (SMAs). Therefore, the transformation kinetics formulation,which is applicable to any status of stress and temperature, is essential for predicting thematerial behavior of SMAs. In this work we show that the transformation kinetics of theoriginal Brinson model, which is the most widely used one-dimensional model,has shortcomings in the case where temperature decreases at low temperature(T<Ms). In addition, we propose a modified transformation kinetics formula that can be used fordual transformation conditions. The martensite transformation kinetics is modified so thatthe transformation from austenite into temperature-induced martensite, due tothe decrease in temperature, is coupled with a transformation from austeniteor temperature-induced martensite into stress-induced martensite, due to theincrease in stress. Through this modification, the suggested formulation can properlydescribe the behavior of martensite fractions in the dual transformation region.

One-dimensional shape memory alloy models for use with reinforced compositestructures

In this paper three models of the shape memory alloy behaviour have beenpresented and re-investigated. The models are attributed to Tanaka, Liang andRogers, and Brinson, and have been used extensively in the literature forstudying the static or dynamic performance of different composite materialstructures with embedded shape memory alloy components. The majordifferences and similarities between these models have been emphasisedand examined in the paper. A simple experimental rig was designed andmanufactured to gain additional insight into the main mechanics governing theshape memory alloy (SMA) mechanical properties. Data obtained fromthe experimental measurements on Ni–Ti wires have been used in thenumerical simulation for validation purposes. It has been found that the threemodels all agree well in their predictions of the superelastic behaviourat higher temperatures, above the austenite finish temperature whenshape memory alloys stay in the fully austenitic phase. However, at lowtemperatures, when the alloys stay in the fully martensitic phase, somedifficulties may be encountered. The model developed by Brinson introducestwo new state variables and therefore two different mechanisms for theinstigation of stress-induced and temperature-induced martensite. This enablesmore accurate predictions of the superelastic behaviour. In general, itcan be recommended that for investigations of the shape memory andsuperelastic behaviour of shape memory alloy components the Brinsonmodel, or refinements based on the Brinson model, should be applied.

Finite Element Modeling of Shape Memory Alloy Composite Actuators: Theory and Experiment

The present paper proposes a non-linear finite element model for the time response of a novel shape memory alloy (SMA) actuator. The actuator has a plane beam configuration composed of a matrix material with SMA sheets or wires embedded in and/or bonded to the matrix part. The finite element model can be used to analyze both active and passive (with or without heating activation) responses of this kind of actuators. Due to large shear stresses and deformations, the model is developed based on a higher order shear deformation beam theory together with the von-Karman strain field. A one-dimensional constitutive equation with non-constant material functions together with sinusoidal phase transformation kinetics is used for the thermo-mechanical behavior of the SMA actuator. The constitutive and phase transformation kinetic equations make distinction between the stress-induced and temperature-induced martensite fractions. To evaluate the model, the fabrication of a prototype SMA composite actuator for conducting experiments is briefly described and the experimental work performed on the prototype actuator is presented. The test results are compared with the finite element numerical results. A good agreement between the finite element and experimental results corroborate the nonlinear finite element modeling approach.

In situ study of stress-induced martensitic transformation in TiNi

By means of synchrotron X-ray diffraction method the stress-induced martensite transformation in TiNi (with two different phase compositions) at room temperature was investigated in situ. It has been shown that in alloys with temperature-induced martensite in the initial state a non-periodic fluctuation of intensity of the X-ray reflexes appear due to anomalous transformation B2+B19′ T→B2→B19′ SI.

In-Situ Investigations of the Martensitic Transformation in TiNi by Synchrotron Radiation

By means of synchrotron X-ray diffraction method the stress-induced martensite transformation in TiNi (with two different phase compositions) at room temperature was investigated in situ. It has been shown that in the alloys with temperature-induced martensite in the initial state a nonperiodic fluctuation of intensity of the X-ray reflexes appeared due to anomalous transformation B2 + B19′T → B2 → B19′Si.