Relationship between acoustic emission energy and the kinetics of martensite formation in plain carbon steels

The kinetics of the martensitic transformation in Fe-0.80C determined on the basis of dilatometry data is compared to the acoustic emission (AE) energy accompanying the transformation in the same steel reported in a previous study. The discrepancy between the AE energy and the volume fraction of martensite indicates that the mechanism for the generation of AE during the martensitic transformation is not solely dependent on the kinetics and the associated moving interfaces as suggested in previous studies. During the growth of martensite, slip takes place in order to relieve internal stresses, and dislocations are thought to be mainly introduced in the relatively soft austenite matrix. The quantitative analysis in this study demonstrates that the AE energy generated per unit time is a function of both the transformation kinetics and the volume fraction of remaining austenite. This strongly indicates that the moving dislocations associated with the plastic deformation of the austenite surrounding the as-formed martensite are the dominant sources of the generated acoustic waves. This improved AE source model is consistent with the well-accepted mechanism of AE during conventional plastic deformation due to an external load.     

Reversible and irreversible martensitic transformations in Fe-Pd and Fe-Pd-Co alloys

Ferromagnetic Fe-Pd shape memory alloys (SMA) undergo a martensitic phase transformation during cooling from a parent FCC phase to a tetragonal FCT martensite. This transformation is thermoelastic and reversible giving rise to the shape memory effect. On further cooling an irreversible FCT to BCT transformation occurs that makes impossible the memory effect. Nevertheless, the transformation from reversible to irreversible phase has been not complete since a volume fraction of reversible phase in the alloy is retained even after cooling below the temperature of appearance of the irreversible phase. The addition of Co lowers the temperature of the reversible and irreversible phase transformations but also reduces the amount of transformed irreversible martensite after cooling to 10 K.     

Effects of Cu on the martensitic transformation and magnetic properties of Mn_(50)Ni_(40)In_(10) alloy

The structures, the martensitic transformations, and the magnetic properties are studied systematically in Mn50Ni40-xCuxIn10, Mn50-xCuxNi40In10, and Mn50Ni40In10-xCux alloys. The partial substitution of Ni by Cu reduces the martensitic transformation temperature, but has little influence on the Curie temperature of austenite. Comparatively, the martensitic transformation temperature increases and the Curie temperature of austenite decreases with the partial replacement of Mn or In by Cu. The magnetization difference between the austenite phase and the martensite phase reaches 70 emu/g in Mn50Ni39Cu1In10; a field-induced martensite-to-austenite transition is observed in this alloy.     

Dislocation density crystalline plasticity modeling of lath martensitic microstructures in steel alloys

A three-dimensional multiple-slip dislocation density-based crystalline formulation, specialized finite-element formulations and Voronoi tessellations adapted to martensitic orientations were used to investigate large strain inelastic deformation modes and dislocation density evolution in martensitic microstructures. The formulation is based on accounting for variant morphologies and orientations, retained austenite and initial dislocation densities that are uniquely inherent to martensitic microstructures. The effects of parent austenite orientation and retained austenite were also investigated for heterogeneous fcc/bcc crystalline structures. Furthermore, the formulation was used to investigate microstructures mapped directly from SEM/EBSD images of martensitic steel alloys. The analysis indicates that variant morphology and orientations have a direct effect on dislocation density accumulation and inelastic localization in martensitic microstructures, and that lath directions, orientations and arrangements are critical characteristics of high strength martensitic deformation and behavior.     

Crystallographic variant selection of martensite at high stress/strain

The phenomenological theory of martensitic transformation is well understood that the displacive phase transformations are mainly influenced by the externally applied stress. Martensitic transformation occurs with 24 possible Kurdjomov-Sachs (K-S) variants, where each variant shows a distinct lattice orientation. The elegant transformation texture model of Kundu and Bhadeshia for crystallographic variant selection of martensite in metastable austenite at various stress/strain levels has been assessed in this present research. The corresponding interaction energies have also been evaluated. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed at different stress/strain levels. It has been investigated that the mechanical driving force alone is able to explain the observed martensite microtextures at all stress/strain levels under uniaxial tensile deformation of metastable austenite under low temperature at a slow strain rate. The present investigation also proves that the Patel and Cohen’s classical theory can be utilized to predict the crystallographic variant selection, if it is correctly used along with the phenomenological theory of martensite crystallography.     

Influence of Interfacial Stress on Microstructural Evolution in NiAl Alloys

A phase-field model for the phase transition between austenite and martensite and twinning between two martensitic variants is presented from our previous theory [1] with the main focus on the influence of interfacial stress that is consistent with the sharp interface limit. Each variant-variant transformation can be represented by only one order parameter. Thus, it allows us to get the analytical solution of interface energy and width. Coupled phase-field and elasticity equations are solved for cubic-to-tetragonal phase transformation in NiAl shape memory alloy. The effects of interfacial stress are studied for martensite-martensite interfaces in detail, which was absent in [1]. Additionally, stress and temperature-induced growth of the martensitic phase inside austenite and twining are simulated. Some of the nontrivial experimentally observed microstructures reproduced in the simulations [1] are analyzed in detail. It includes tip splitting and bending, and twins crossing. This theory can be extended for electric, reconstructive, and magnetic phase transformations.

     

Aspects of thermal martensite in a FeNiMnCo alloy

Thermal martensite characteristics in Fe–29%Ni–2%Mn–2%Co alloy were investigated with scanning electron microscopy (SEM) and Mössbauer spectroscopy characterization techniques. SEM observations obviously revealed the lath martensite morphology in the prior austenite phase of examined alloy. As well, the martensitic transformation kinetics was found to be as athermal type. On the other hand, Mössbauer spectroscopy offered the paramagnetic austenite phase and ferromagnetic martensite phase with their volume fractions. Also, the internal magnetic field of the martensite was measured as 32.9 T from the Mössbauer spectrometer.     

Modelling of hysteresis loops taken during the stress- and temperature-induced martensitic transformations

In this article, it has been shown that the process of reconfiguration of the crystal defects system noticeably contributes to the width of the stress–strain and strain–temperature hysteresis loops taken during the stress- and temperature-induced martensitic transformations of the shape memory alloys. It has been demonstrated that the contribution of the defects system to the hysteresis width strongly depends on the alloy temperature and the transformation cycle duration. It has been shown that the hysteresis effect can be observed not only in the course of the first-order phase transition of martensitic type, but also in the course of the gradual deformation of crystal lattice. The obtained results are applicable to the ferroelastic phase transitions in the different crystalline solids.     

Nature of the effect of magnetic fields on the starting temperature of martensitic transformation in iron alloys

The effect of a magnetic field on martensitic transformations, which is satisfactorily described by the Krivoglaz–Sadovskii formula, has been analyzed taking into account the nonequilibrium of the martensitic transformation, the possible adiabatic conditions, and the magnetostriction of the paraprocess in ferromagnetic austenite.     

Kinetics of radiation damage and martensitic transformations in TiNi alloys irradiated with neutrons

The variation of the temperatures of martensitic transformations and the rate of radiation damage in TiNi alloys were studied upon irradiation with reactor neutrons. The irradiation was performed at temperatures of 120 and 335 K. In the process of irradiation, electrical resistance of the alloys was measured continuously and thermal cycling through the temperature range of martensitic transformations was carried out. The transformation temperatures were shown to decrease at different rates with increasing irradiation fluence. The electrical resistance increases linearly with increasing neutron fluence to 6.7×10¹⁸ cm^?2 irrespective of the irradiation temperature. Deviation from a linear dependence is only observed when the irradiation leads to a change in the phase state of the alloy. The rate of the resistance increase only slightly depends on the irradiation temperature. In martensite, it is greater by a factor of 2–4 than that in austenite. Mechanisms of irradiation-induced modification of the structure of TiNi alloys that explain the experimental data obtained are discussed.     

Martensitic transformation in amorphous-crystalline Ti-Ni-Cu and Ti-Hf-Ni-Cu thin ribbons

Martensitic transformations and strain variation in amorphous-crystalline thin ribbons of Ti₅₀Ni₂₅Cu₂₅, Ti_40.7Hf_9.5Ni_44.8Cu₅ and Ti_40.7Hf_9.5Ni_39.8Cu₁₀ alloys were investigated. The amorphous-crystalline state in the samples was produced by an interruption of the isothermal crystallization in DSC apparatus. The volume fraction of the crystalline phase V_cryst in the samples was varied from 10 to 100%. Transformation temperatures in amorphous-crystalline thin ribbons of Ti₅₀Ni₂₅Cu₂₅ alloy depended only slightly on the crystalline volume fraction. On the contrary, the strong dependence of the martensitic transformation temperatures on the volume of crystalline phase was obtained in Ti_40.7Hf_9.5Ni_44.8Cu₅ and Ti_40.7Hf_9.5Ni_39.8Cu₁₀ alloys. Moreover, it was found that the intervals of the direct transformation are extremely narrow, whereas the intervals of the reverse transformation are unusually wide. As the crystalline volume fraction rises, both the direct transformation interval and the reverse one decrease, and the transformation temperatures increase. It was shown that increase of the Cu-content in Ti_40.7Hf_9.5Ni_{49.8 -x}Cu_x (x = 5, 10%) alloys results in decrease of the transformation temperatures, of the temperature interval of the reverse transformation on heating and of the hysteresis, while the temperature interval of the direct transformation on cooling widens     

Strain-induced martensite to austenite reverse transformation in an ultrafine-grained Fe–Ni–Mn martensitic steel

Research was conducted to evaluate the effect of heavy cold rolling on microstructural evolution in an Fe–10Ni–7Mn (wt.%) martensitic steel. The chemical driving force for the strain-induced martensite to austenite reverse transformation was calculated using thermodynamic principles and a model was developed for estimating the effect of applied stress on the driving force of the martensite to austenite reverse transformation through heavy cold rolling. These calculations show that, in order to make a reverse transformation feasible, the applied stress on the material should supply the total driving force, both chemical and non-chemical, for the transformation. It is demonstrated that after 60% cold rolling the required driving force for the reverse transformation may be provided. Experimental results, including cold rolling and transmission electron microscopy images, are utilized to verify the thermodynamic calculations.     

Mössbauer studies of medium-carbon,high-chromium martensitic steels

⁵⁷Fe Mössbauer effect spectroscopy is employed to determine the relationship between the microstructure and the mechanical properties of martensitic steels with base composition Fe-10wt%Cr-0,26wt%C. The microstructure consists predominantly of two phases: martensite and austenite. The effect of low concentrations of both Mn and Ni on the structure and the mechanical properties of these steels is studied. The results indicate that Mn and Ni additions are equally effective in increasing the fraction of retained austenite. The austenite is an important phase since it is considered to be beneficial to the toughness of steel. However, we find that the impact toughness first decreases and then increases as a function of the fraction of austenite.     

Martensitic transformations and magnetic properties of nonstoichiometric alloys of the Ni-Mn-In system

Martensitic transformations and magnetic properties of Ni_89-x Mn _x In₁₁ (42 ≤ x ≤ 44) alloys have been investigated. Critical temperatures of magnetic and structural phase transitions in the studied alloy system have been determined. It has been shown that the martensitic transformation induced by the magnetic field is observed in all alloys. Temperature dependences of the spontaneous magnetization of austenite and martensite as well as the magnitude of the critical field, in which martensitic transformation occurs, have been determined.     

Quasi-equlibrium description of acoustic emission in martensitic transformations

Acoustic emission is used to study martensitic transformations in metal systems within the framework of the quasiequilibrium theory. In this approach, the dissipative part in the balance equation of driving forces for the matertensitic transformation is represented as the sum of contributions from heat dissipation and “nonchemical” energy dissipation due to acoustic emission. The acoustic contribution is defined as dynamic relaxation which in its turn is related to the transformation kinetics. Examples of events responsible for the production of acoustic radiation are the “microexplosive” nucleation-collapse of martensite crystals, single acts of their stick-slip displacement, and plastic relaxation of elastic transformation stresses. Siberian Physicotechnical Institute at Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 72–79, October, 1999.     

Thermodynamic analysis of ageing-induced multiple-stage transformation behaviour of NiTi

Near-equiatomic NiTi shape memory alloys normally exhibit three martensitic transformations among three phases: the B2 phase, the monoclinic (M) phase and the rhombohedral (R) phase. Some recent work, however, has revealed complex transformation behaviour involving multiple-stage martensitic transformations and multiple-stage R-phase transformations. This paper presents an analysis of these complex transformation behaviours based on thermodynamic concepts of reversible and irreversible energies associated with the transformations. The analysis is successful in identifying all observed transformations and in defining relative positions of various stages of transformations on a temperature scale. It also defines positions of thermodynamically prohibited transformations as well as permitted transformations that have not been experimentally measured. Such identifications enable the determination of actual transformation hystereses that are not directly measured experimentally. Based on the thermodynamic principles adopted, the analysis also renders it possible to identify the possible causes that contribute to the complex multiple-stage transformation behaviour.     

Non-classical austenite-martensite interfaces observed in single crystals of Cu–Al–Ni

Interfaces between austenite and a crossing-twins microstructure consisting of four variants of 2H-martensite are optically observed in a single crystal of Cu–Al–Ni shape memory alloy. It is shown that these non-classical interfaces form during thermally induced transitions from compound twinned 2H-martensite into austenite, which is in agreement with theoretical predictions. Individual twinning systems and martensitic variants involved in the observed microstructure are identified. The corresponding volume fractions are estimated based on the compatibility conditions at the habit plane and the macroscopic geometry of the interface. Miscellaneous topics related to the observed microstructures (formation mechanism and planeness of the interface) are briefly discussed.     

Exploring the origin of variant selection through martensite-austenite reconstruction

An approach of near neighbour correlation, with manual intervention, was developed for reconstructing parent austenite microstructure in a martensitic stainless steel. This was validated in a ferrite-austenite dual structure. Two-hundred and twenty randomly selected austenite grains were reconstructed from the experimental EBSD (electron backscattered diffraction) measurements. From these reconstructions, martensite variant selection was quantified as the number of variants (n_V) and the variant selection index (VSI: a statistical index for the relative area fractions of the variants). For each prior austenite grain, both n_V and VSI appeared to depend on the associated transformation (austenite-martensite) strain. Selection of common variants between two neighbouring austenite grains was related to the presence of 60°<111?>?or Σ3 boundary in the austenite phase and corresponding minimisation of the transformation strain.     

Photoacoustic detection of phase transitions with a resonant piezoelectric scheme with extreme sensitivity to small volume changes

Resonant piezoelectric photoacoustic detection is demonstrated to be a sensitive tool for the determination of phase transitions. A model is presented that describes the changes in the signal expected during phase transitions when resonant detection is used. The technique is applied to the study of first-order martensitic diffusionless transformations in copper-based shape-memory alloys. The model takes into account the signal changes arising from two sources. One behaves like an effective change in the heat capacity, and arises due to the enthalpy of the reaction, and the other can be described as an effective change in the thermal expansion coefficient, and arises from the volume change during the transformation. Due to the relative high frequency used (around 20 kHz), the transformation lags behind the temperature oscillations, yielding a phase shift in the acoustic signal as the transformation temperature is passed. The relative sign of the phase angle and amplitude as the transformation proceeds is an indication as to whether the signal arises from volume changes or heat exchange (enthalpy). Huge signals from very small volume changes (smaller than 0.5%) were observed.