Martensitic Transformation Process Research Articles

Variation of martensite phase transformation mechanism in minor-stressed T91 ferritic steel

The effects of compressive stress applied at different temperatures on martensite transformation process of the T91 steel were studied by high-resolution differential dilatometer. The stress applied above 850 °C exhibits no influence on the martensite formation. The stress applied below 850 °C not only facilitates the formation of martensite, but also enhances the onset temperature of the martensite transformation. There are two different transformation mechanisms occurring: when the compressive stress is applied at high temperature, the mechanism of strain-induced martensite transformation takes place, as a result, the microstructure tends to be refined with irregular grain boundary. When the compressive stress is applied at low temperature, the stress-induced martensite transformation occurs, and its morphology is similar to that of thermal-activated martensite. In addition, it is summarized that 200 MPa is the critical stress and 440 °C is critical temperature for the onset of the stress-induced martensite transformation for the investigated T91 ferritic steel.

BIFURCATION AND STABILITY OF MARTENSITIC TRANSFORMATION DYNAMICS

In this paper, we have studied the mathematical properties and their physical implications of a system of nonlinear ordinary differential equations with two variables and six parameters, which was proposed to model the martensitic transformation of shape memory alloys. While the system could not have limit cycles, bifurcation to hysteresis was found in load and/or temperature controlled processes with large enough interfacial energies. This agrees qualitatively quite well with the experimental observation and theoretical understandings of the stress-strain-temperature hysteresis of the alloys. A three-dimensional bifurcation diagram was identified. Nonhyperbolic equilibrium points were found as saddle nodes and high order node points. The local behavior was studied and the phase portrait of the system was obtained for the load and temperature parameters. Accordingly, a stable node represents the stable martensitic or austenitic phase, a saddle stands for the unstable phase mixture, and a saddle node corresponds to the beginning or the end of the transformation. Therefore, for a thermal-mechanical loading path, the martensitic transformation process accords with the qualitative changes of the equilibrium points.

Procedure of numerical simulation of martensitic phase transformation processes in small volumes of materials when deformed in diamond anvils

A model, algorithm, and software for a finite-element simulation of the martensitic phase transformation in a small volume of a sample material in the deformation under quasihydrostatic conditions (in a gasket without a pressure medium) in diamond anvils of a superhigh-pressure apparatus have been developed. The model is based on the thermomechanical theory of the phase transformations in inelastic materials formulated by Prof. Levitas. The algorithm has been proposed of the simulation of the phase transformation kinetics that consists of the cyclic solution of a contact elastoplastic problem, estimation of the possibility for the phase transformation, and of the repeated solution of the problem with the allowance made for the phase deformation.

Characteristics of stress-induced transformation and microstructure evolution in Cu-based SMA

The mechanical behavior of shape memory alloys (SMAs) is closely related to the formation and evolution of its microstructures. Through theoretical analysis and experimental observations, it was found that the stress-induced martensitic transformation process of single crystal Cu-based SMA under uniaxial tension condition consisted of three periods: nucleation, mixed nucleation and growth, and merging due to growth. During the nucleation, the stress dropped rapidly and the number of interfaces increased very fast while the phase fraction increased slowly. In the second period, both the stress and the interface number changed slightly but the phase fraction increased dramatically. Finally, the stress and the phase fraction changed slowly while the number of interfaces decreased quickly. Moreover, it was found that the transformation could be of multi-stage: sharp stress drops at several strains and correspondingly, the nucleation and growth process occurred quasi-independently in several parts of the sample.

Magnetic Field-Induced Martensitic Transformation of Heusler-Type Ni<sub>2</sub>MnGa System

Magnetic field effect on a Heusler-type Ni2MnGa off-stoichiometric alloy having a martensitic transformation temperature around room temperature which is coincident with a Curie temperature has been investigated. The process of martensitic transformation of a single crystal was investigated by neutron diffraction under the magnetic field up to 8 [T]. It was found that the magnetic field, which is applied at a temperature near the transformation temperature, causes the martensitic transformation. The process of the transformation caused by the increase in magnetic field is quite similar to the process caused by the decrease in temperature.

Effect of deformation on the stress-induced martensitic transformation in (Ni47Ti44)100−xNbx shape memory alloys with wide hysteresis

Martensite in TiNi-based alloys is reported to be thermally stabilized after a moderate deformation. Hence, this paper investigates the effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of (Ni47Ti44)100−xNbx (x=3, 9, 15, 20, 30 at.%) alloys. The stress-induced martensite appears to be stabilized in relation to the thermal-induced martensite that forms on cooling. This observation is confirmed by an increase in the reverse transformation start temperature, during which time the transformation temperature hysteresis reaches about 200°C. Moreover, the Nb content in Ni−Ti−Nb alloy has a great influence on the transformation temperature hysteresis of stress-induced martensite as well as on the process of stress-induced martensitic transformation. The mechanism of wide transformation temperature hysteresis is explained in terms of the microscopic structure of (Ni47Ti44)100−xNbx alloys. Furthermore, the temperature interval of the reverse transformation of stress-induced martensite was found to increase slightly as the strain of the high Nb-content alloy increased, though the value was much smaller than that of the thermally induced martensite. Finally, the paper explains the relation between this unique phenomenon and the elastic strain energy.

Elementary martensitic transformation processes in Ni-rich NiTi single crystals with Ni 4Ti 3 precipitates

The present study shows that multiple-step martensitic transformations can be observed in aged Ni-rich NiTi single crystals. Ageing of solution-annealed and water-quenched Ni-rich NiTi single crystals results in a homogeneous precipitation of coherent Ni 4Ti 3 particles. When the interparticle spacing reaches a critical value (order of magnitude: 200 nm), three distinct transformation processes are observed on cooling from the high-temperature phase using differential scanning calorimetry and in situ transmission electron microscopy. The transformation sequence begins with the formation of R-phase starting from all precipitate/matrix interfaces (first step). The transformation continues with the formation of B19′ and its subsequent growth along all precipitate/matrix interfaces (second step). Finally, the matrix in between the precipitates transforms to B19′ (third step). Elementary transformation mechanisms which account for two- and three-step transformations in a system with small-scale microstructural heterogeneities were identified.

Stress-induced martensitic transformation in (Ni 47Ti 44) 100- xNb x shape memory alloys with wide hysteresis

The effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of the (Ni 47Ti 44) 100- x Nb x ( x=3, 9, 15, 20, 30, mole fraction, %) shape memory alloys was investigated in detail by differential scanning calorimetry (DSC) after performing cryogenic tensile tests at a temperature of M s+30 °C. The results show that Nb-content has obvious effect on the process of stress-induced martensitic transformation. It is also observed that the stress-induced martensite is stabilized relative to the thermally-induced martensite (TIM) formed on cooling, and Nb-content in Ni-Ti-Nb alloy has great influence on the reverse transformation start temperature and transformation temperature hysteresis of stress-induced martensite(SIM). The mechanism of wide transformation temperature hysteresis was fully explained based on the microscopic structure and the distribution of the elastic strain energy of (Ni 47Ti 44) 100- x Nb x alloys.

Evolution of flexural rigidity according to the cross-sectional dimension of a superelastic nickel titanium orthodontic wire

The choice of the most suitable orthodontic wire for each stage of treatment requires estimation of the forces generated. In theory, the selection of wire sequences should initially utilize a lower flexural rigidity; thus clinicians use smaller round cross-sectional dimension wires to generate lighter forces during the preliminary alignment stage. This assessment is true for conventional alloys, but not necessarily for superelastic nickel titanium (NiTi). In this case, the flexural rigidity dependence on cross-sectional dimension differs from the linear elasticity prediction because of the martensitic transformation process. It decreases with increasing deflection and this phenomenon is accentuated in the unloading process. This behaviour should lead us to consider differently the biomechanical approach to orthodontic treatment. The present study compared bending in 10 archwires made from NiTi orthodontics alloy of two cross-sectional dimensions. The results were based on microstructural and mechanical investigations. With conventional alloys, the flexural rigidity was constant for each wire and increased largely with the cross-sectional dimension for the same strain. With NiTi alloys, the flexural rigidity is not constant and the influence of size was not as important as it should be. This result can be explained by the non-constant elastic modulus during the martensite transformation process. Thus, in some cases, treatment can begin with full-size (rectangular) wires that nearly fill the bracket slot with a force application deemed to be physiologically desirable for tooth movement and compatible with patient comfort.

Simulation of martensitic microstructure

The defects appearing during the process of martensitic transformation from bcc austenite to a microtextured hcp martensite are examined, with reference to molecular dynamics simulations in zirconium. The simulations involve cooling from the high temperature austenitic bcc phase. A variety of geometric defects are identified, with the structure and dynamics being understood in terms of vicinal twin boundaries and dislocations. These defects are classified as either geometrically necessary or nanoscale effects, and the evolution of the material in response to annealing and external stress is described. The observed mechanism for twin boundary motion under applied strees involves a combination of stress concentration by preexisting sessile dislocation, causing nucleation, motion and absorption of defects in the boundary plane.

Temperature memory effect of Ti 50Ni 30Cu 20 (at.%) alloy

After the reverse thermal induced martensitic transformation process of shape memory alloy is arrested at a temperature between the reverse transformation start and finish temperatures ( A s and A f), and then cooled to a temperature below M f, a kinetic stop will occur in the next heat flow curve during the heating process. The kinetic stop is closely related to the arrested temperature. This phenomenon is called temperature memory effect (TME). TME of Ti 50Ni 30Cu 20 (at.%) shape memory alloy with phase transformation between B2 austenite and B19 martensite has been investigated by differential scanning calorimeter in this paper. The results indicate that TME of Ti 50Ni 30Cu 20 alloy only exists in the heating process.

Nano-oxide particle stability of 9–12Cr grain morphology modified ODS steels under neutron irradiation

ODS steels based on a 9–12Cr composition were developed with modified grain morphology and incorporation of recrystallization heat treatment and martensitic transformation process into previous tube fabrication methods. The modifications were for improving strength anisotropy in hoop and longitudinal directions of cladding tubes. Specimens were irradiated in the experimental fast reactor JOYO at temperatures of 670–807 K to 15.0 dpa maximum. TEM and/or HRTEM observation indicated that nano-scale sized oxide particles were finely distributed in each alloy and were relatively stable under neutron irradiation. Quantitative constitution analyses showed that most of the oxide particles were comprised of yttrium and titanium, and that they were in most case non-stoichiometric. Comparing with past reports on neutron-irradiated ODS steels, the main factors determining oxide stability under neutron irradiation are shown to be irradiation temperature and/or type of oxide.

TEM study of the reverse martensitic transformation and decomposition in the NiAlCu intermetallic phase

Reverse martensitic transformation and decomposition processes during heating were investigated in Ni–32Al–5Cu (at.%), by transmission electron microscopy (TEM) and differential scanning calorimetry in samples homogenized and quenched from 1300°C and hot upset at 1150°C. The hot upset sample revealed reverse martensitic transformation when the homogenized sample underwent transformation of 3R martensite predominantly to the Ni5Al3 phase. To explain these differences in situ heating TEM was performed.

436 結晶塑性理論に基づく TRIP 鋼のひずみ誘起マルテンサイト変態過程の有限要素解析

436 結晶塑性理論に基づく TRIP 鋼のひずみ誘起マルテンサイト変態過程の有限要素解析

317 結晶塑性理論に基づく TRIP 鋼のマルテンサイト変態過程の数値シミュレーション

317 結晶塑性理論に基づく TRIP 鋼のマルテンサイト変態過程の数値シミュレーション

Cyclic creep and cyclic deformation of high-strength spring steels and the evaluation of the sag effect: Part I. Cyclic plastic deformation behavior

The cyclic creep and cyclic plastic deformation behavior of two commercial suspension spring steels of high hardness levels, namely, SAE 9259 and SAE 5160, were studied under different testing conditions of cyclic peak stress and cyclic stress ratio. The experimental results indicate that both the cyclic stress ratio and cyclic peak stress have strong, but complicated, effects on the cyclic creep and cyclic plastic deformation behavior of these materials. It has also been found that the addition of silicon can increase the resistance of these steels to cyclic creep and cyclic plastic deformation, although the extent of this increase is also related to other cyclic deformation conditions. A transition in the relationship between the total plastic strain range and the cyclic stress ratio (R) has been detected at approximately R=0.5. The mechanism of such a transition is explained by the operation of cross-slip during the unloading process of cycling. Moreover, a cyclic softening behavior of these spring steels in the quench-tempered condition was also detected and is attributed to the activation and reorganization of obstacle dislocations introduced into the steels during the process of martensitic transformation. More importantly, this study has indicated that parameters such as the cyclic creep strain, the cyclic creep rate in the secondary creep stage, and the total cyclic plastic strain range can better reflect, and should be used to depict and characterize, the sag behavior of spring steels as well as other materials. Finally, the effect of silicon on sag behavior, in comparison with the results from the Bauschinger-effect test, has also been discussed through the influence of Si on carbide formation and distribution.

Micromechanical study of formation and evolution of martensite for Cu–Al–Ni single crystal by moiré interferometry

The basic properties of pseudoelasticity of Cu–Al–Ni single crystal are studied to analyze the morphology associated with the formation and evolution of martensite and the shape memory recovery process at different temperatures. Use is made of a multifunctional macro–micro moiré interferometry measurement system. The β 1 to β 1 ′ phase changes are identified with the stress-induced transformation of a Cu–13.7%Al–4.18%Ni (wt%) alloy. The invariant plane between the martensite and the parent phase is shown directly by fringe patterns. It is found that martensite appears in the shape of bands or thin plates on the surface of the specimen. The formation of martensite is a very rapid process and martensite `jumps' out until the specimen is completely transformed into a single variant. The results reveal the mechanism and process of stress-type and temperature-induced martensitic transformation.

Strain dependence of pseudoelastic hysteresis of NiTi

This work investigated the transformation-strain dependence of the stress hysteresis of pseudoelasticity associated with the stress-induced martensitic transformation in binary NiTi alloys. The strain dependence was studied with respect to the deformation mode during the stress-induced martensitic transformation, which was either localized or homogeneous. It was observed that the apparent stress hysteresis of pseudoelasticity was independent of the transformation strain within the macroscopic deformation range, for the specimens deformed in a localized manner. For specimens macroscopically deformed uniformly, the stress hysteresis of pseudoelasticity increased continuously with increasing strain from the beginning of the stress-induced martensitic transformation. The transformation-strain independence of the stress hysteresis for localized deformation is ascribed to be an artificial phenomenon, whereas the transformation-strain dependence of the hysteresis for uniform deformation is believed to be intrinsic to the process of stress-induced martensitic transformation in polycrystalline materials. This intrinsic behavior is attributed to the polycrystallinity of the materials.

Transformation relaxation and aging in a CuZnAl shape-memory alloy studied by modulated differential scanning calorimetry

The reverse martensitic transformation and aging processes in a polycrystalline Cu-23.52 at. pct Zn-9.65 at. pct Al shape-memory alloy have been studied using the recently developed modulated differential scanning calorimetry (MDSC) technique, and some new findings are obtained. By separating the nonreversing heat flow from the reversing heat flow, MDSC can better characterize the thermodynamic, kinetic, and hysteretic features of thermoelastic martensitic transformations. Two kinds of exothermal relaxation peaks have been identified and separated from the endothermal reverse martensitic transformation: one is associated with the movement of twin interfaces or martensite-parent interfaces, and another is due to the atomic reordering in the parent phase via a vacancy mechanism. The martensite aging processes have been examined, and two stages of the aging process been distinguished: the first stage of aging is characterized by the stabilization of martensite, as manifested in the increase in the reversing enthalpy of the reverse martensitic transformation and in the transformation temperatures, and the second stage is, in fact, the decomposition of the martensite on prolonged aging, accompanied by a decrease in the transformation enthalpy. The results suggest that the mechanisms of the relaxation in the martensite and in the parent phase may be quite different.

On the Three Phase Mixtures in Martensitic Transformations of Shape Memory Alloys: Thermodynamical Modeling and Characteristic Temperatures

We study here the thermally-induced martensitic transformation process of shape memory alloys. Taking the internal energy of phase mixtures as the potential function and introducing coherency energies between martensite and austenite and between different variants of the martensitic phase, we are able to use thermodynamical arguments to obtain hysteresis diagrams which could be measured experimentally. The characteristic temperatures for martensitic transformation, $M_{s,f}$ (martensite start and finish) and $A_{s,f}$ (austenite start and finish) can be identified explicitly and are closely related to the coherency parameters of the coherency energies.