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2.
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. 相似文献
3.
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. 相似文献
4.
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. 相似文献
5.
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. 相似文献
6.
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. 相似文献
7.
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. 相似文献
8.
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. 相似文献
9.
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. 相似文献
10.
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. 相似文献
11.
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 6.7×10 18 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. 相似文献
12.
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. 相似文献
13.
Martensitic transformations and strain variation in
amorphous-crystalline thin ribbons of Ti 50Ni 25Cu 25,
Ti 40.7Hf 9.5Ni 44.8Cu 5 and
Ti 40.7Hf 9.5Ni 39.8Cu 10 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
Ti 50Ni 25Cu 25 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 44.8Cu 5 and
Ti 40.7Hf 9.5Ni 39.8Cu 10 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 -xCu 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 相似文献
14.
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. 相似文献
15.
Martensitic transformations and magnetic properties of Ni 89-x Mn x In 11 (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. 相似文献
16.
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. 相似文献
17.
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. 相似文献
18.
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. 相似文献
19.
The effects of cyclic stress loading on the microstructual evolution and tensile properties of a medium-carbon superbainitic steel were investigated. Experimental results show that the cyclic stress can induce the carbon gathering in austenite and phase transformation from film-like retained austenite to twin martensite, which will obviously enhance the tensile strength and the product of tensile strength and ductility. The higher the bainitic transformation temperature, the lower the transformation rate of the retained austenite. The amount and thickness of the film-like retained austenite play an important role during the cyclic stress induced phase transformation. 相似文献
20.
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. 相似文献
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