Fct Martensitic Transformation Research Articles

Thermodynamic study of fcc-fct-fco multi-step structural transformation in Mn-Ni antiferromagnetic shape memory alloys

Shape memory effect in Mn-Ni antiferromagnetic alloys is closely related to the structural transformations that happen in this system, especially the fcc→fct→fco multi-step structural transformation. In order to discuss the change of Gibbs free energy of this alloy system during the multi-step transformation process, thus study the mechanism of shape memory effect from the standpoint of thermodynamics of fcc→fct→fco transformation, the unknown thermodynamic parameters of fct and fco phases in Mn-Ni alloys must be determined. In this work, based on the application of subregular solution model of the binary system and equilibrium relationships between different phases given in the Mn-Ni phase diagram, these unknown thermodynamic parameters were determined by solving overdetermined equation systems, and dependencies of Gibbs free energies of every phase in Mn-Ni antiferromagnetic alloys on temperature were obtained. The results of calculations show that the critical driving force of fcc→fct martensitic transformation is about 10 J/mol and higher than that of fct→fco transformation (about 1 J/mol), while the enthalpy, entropy and heat capacity of fcc→fct transformation are several times higher than the equivalent parameters for the fct→fco transformation. The dependences of critical chemical driving force, enthalpy, entropy and heat capacity on the alloy composition and temperature were calculated and discussed for the fcc→fct→fco multistep structural transformation. Based on these results, the orders of the two transformations in the fcc→fct→fco multi-step transformation happening in this system were compared with that of the single-step phase transformation, and were found to be all of first-order and to be weakened during the multi-step transformations.

FCC → FCT martensitic transformation in two-phase Mn–Cu alloys

The fcc → fct martensitic transformation in Mn–Cu alloys, which contain two isomorphous fcc phases with different manganese contents, is considered. The influence of the ratio of contents of these phases, the phase dispersity, and the state of interphase boundaries on the development of the martensitic transformation in the volume of the alloys is described. As a result of these factors, the martensitic transformation can cover the entire volume of an alloy, including particles with the manganese content that is lower than the critical content required for the martensitic transformation to occur in single-phase solid solutions, and can take place only in manganese-rich particles. In the first case, a general tetragonal structure modulated with respect to lattice parameter c forms in the volume of a two-phase alloy.

Thermodynamics of fcc–fct martensitic transformation in Mn–X(X = Cu,Fe) alloys

Mn-based antiferromagnetic alloys, e.g. Mn–Fe and Mn–Cu, are widely used for their magnetic shape memory effect and reversible shape memory effect, which are closely related to their fcc–fct martensitic transformation. However, due to a lack of thermodynamic parameters of fct martensite phase, the thermodynamics of such a transformation has not yet been well established. In this work, a method was proposed to solve this question for the first time and could be applied to other systems. By using the sub-regular solution model and thermal equilibrium condition, the Gibbs free energy of fct phase is calculated for the first time and expressed as a function of temperature and alloy composition. Furthermore, the dependences of chemical driving force on temperature for a given composition, that of critical chemical driving force on composition, and that of other thermodynamic quantities, i.e. entropy, enthalpy and heat capacity, on temperature were investigated for the first time. Finally, the influences of the calculating method and the assumption on the calculation were discussed, and the order of fcc–fct martensitic transformation was assessed from the viewpoint of thermodynamics semi-quantificationally, as well as the order of paramagnetic–antiferromagnetic transition.

In situ atomic force microscope study of high-temperature untwinning surface relief in Mn-Fe-Cu antiferromagnetic shape memory alloy

The N-type untwinning surface relief associated with the fcc ↔ fct martensitic transformation (MT) was observed in the Mn81.5Fe14.0Cu4.5 antiferromagnetic high-temperature shape memory alloy (SMA) by in situ atomic force microscopy. The measured untwinning relief angles (θα|θβ) at the ridge and at the valley were different, and both angles were less than the conventional values. The surface relief exhibited good reversibility during heating and cooling because of the crystallographic reversibility of thermal-elastic SMAs. Untwinning shear was proposed as the main mechanism of the N-type surface relief. The order of the reverse MT was discussed based on the experimental measurements.

Structurally inhomogeneous fct-martensite in alloys based on the Mn – Cu system: structure, strain mechanism and strength characteristics

The conditions of occurrence of high-temperature fcc →fct martensitic transformation in alloys of the Mn – Cu system containing two isomorphic nanocrystalline γ1 and γ2 phases with high and low manganese content, respectively, are considered. The structure formed as a result is studied. The mechanisms of straining of the inhomogeneous fct-martensite and the factors determining its strength properties are determined.

Concentration dependence of FCC to FCT martensitic transformation in Fe–Pd alloys

Abstract An Fe–Pd alloy exhibits a weakly first order FCC–FCT martensitic transformation. In the present study, we have examined the composition dependence of its transformation temperature by magnetic susceptibility, DSC and X-ray measurements. The susceptibility measured in the cooling process exhibits a sudden decrease in alloys with Pd content of 33.0 at.% and less, which is attributed to the FCC–FCT martensitic transformation. However, the transformation is suppressed in alloys with Pd content of 33.1 at.% and more. The latent heat of the FCC–FCT transformation is about 70 J mol−1 for an Fe–30.0 at.%Pd alloy, and it decreases with increasing Pd content.

The coupling between first-order martensitic transformation and second-order antiferromagnetic transition in Mn-rich γ-MnFe alloy

The fcc → fct martensitic transformation of a Mn-rich γ-MnFe alloy with a (011) martensite twin was investigated using transmission electron microscopy, dynamic mechanical analysis, electrical resistance measurements, differential scanning calorimetry and dilatometry. Experimental results show that the start temperature (M s) of the martensitic transformation and the temperature (T N) of the antiferromagnetic transition in Mn85.5Fe9.5Cu5 alloys are close and almost coincide. Direct and reverse martensitic transformations, normally a first-order transformation, exhibit features similar to those of a second-order continuous transition, which is attributed to coupling between the first-order martensitic transformation and the second-order antiferromagnetic transition.

Magnetic-field-induced strains in polycrystalline Mn1-xCux(0.1≤x≤0.3) alloys

Magnetic-field-induced strain(MFISs) in polycrystalline Mn1-xCux(0.1≤x≤0.3, atomic fraction) alloys are studied by means of X-ray, photomicrograph, DSC, resistance strain gauge method at room temperature. The results show that Mn1-xCux alloys consist of fcc (γ) and fct (γ’) phases after a long period of homogenization because of fcc(γ)→fct(γ’) martensitic transformation in the cooling process. The volume proportion of γ’ phase increases with the increase of Mn content, so that the samples possess much better MFIS performance. At room temperature, the MFIS in Mn0.9Cu0.1 sample reaches 91 ppm in 0.9 T magnetic field.

Shape memory effect of an antiferromagnetic Mn–9.5 at.% Fe–5.0 at.% Cu alloy

This paper mainly presents the shape memory effect in a Mn-rich antiferromagnetic Mn–9.5 at.% Fe–5.0 at.% Cu alloy. The fcc–fct martensitic transformation takes place following an antiferromagnetic transition, and (0 1 1) twin boundaries of fct martensite are formed. When the M s temperature of the fct martensitic transformation and the Néel point of the antiferromagnetic transition approach each other, a coupling between the second-order antiferromagnetic transition and the first-order martensitic transformation occurs. In this case the thermo-hysteresis of the transformation decreases, or even disappears. The temperature-dependent shape memory effect in the alloy was measured with a dilatometer using pre-compression loading followed by heating (from room temperature to 523 K) and cooling and monitoring the associated shape change. The characteristics of the hysteresis-free or very little thermo-hysteresis property are also discussed in relation to thermodynamic properties.

Damping mechanism associated with coupling between antiferromagnetic transition and martensitic transformation

To study the relationship between the martensitic transformation and the antiferromagnetic transition and to develop new twin-based high damping materials, the internal friction and modulus in γ Mn-based Mn–Fe and Mn–Ni alloys were measured. Three internal friction peaks were observed between −100 and 400 °C, namely at the Néel temperature T N, the martensitic transformation temperature M S and 0 °C. They are related to the antiferromagnetic transition, the martensitic transformation and twins in the fct phase. The first is accompanyied by a rapid decreases of modulus at T N. The third, high damping peak is similar to the one observed in Mn–Cu alloys, and is attributed to the fcc–fct martensitic transformation or to fine twins in the antiferromagnetic domains.

Martensitic twins and antiferromagnetic domains in γ-MnFe(Cu) alloy

The multi-function characteristics of Mn-base alloys are related to the domains accompanying the antiferromagnetic transition and twins resulting from the fcc–fct martensitic transformation. The morphology and orientation of the twins have been investigated with transmission electron microscopy. Fringe pattern which result from twin boundaries were observed, and they are found to be {0 1 1} type as indicated by electron diffraction. It is proved with Landau theory that the fct twin corresponds to one kind of antiferromagnetic domain as the martensitic transformation is induced by antiferromagnetic ordering.

Magnetic-field-induced strain in an antiferromagnetic γ-MnNi alloy

In the present study, the fcc→fct martensitic transformation and antiferromagnetic transition of the Mn82.2Ni17.8 alloy are studied by electric resistance measurement, dynamic mechanical analyzer, and transmission electron microscopy. The magnetic-field-induced strain associated with antiferromagnetic-order twin martensitic structure of the polycrystalline antiferromagnetic alloy is investigated. In addition, the threshold field of magnetic-field-induced strain of antiferromagnetic materials and the relation of antiferromagnetic domains and martensitic twins are discussed qualitatively.

Typed-new multifunctional Mn-rich antiferromagnetic alloys

The lattice distortion of crystal structure can be formed and following fct martensitic transformation is induced with antiferromagnetic transition in Mn-rich antiferromagnetic alloys. The twin boundaries formed with fct martensitic transformation or tiny twin (antiferromagnetic domain boundaries) with antiferromagnetic transition have mobility and high damping property can be obtained under an alternate stress. With the help of the coupling between the second order antiferromagnetic transition and the first order martensitic transformation, the hysteresis between martensitic transformation and reversible one is decreased or even is disappeared, the martensitic transformation temperature, M s, and its reversible transformation temperature, A s, move closer to antiferromagnetic transition point, T n, the shape memory effect formed by martensitic transformation and its reversible one may be related with a simplex temperature function and a non-hysteresis on temperature can be formed. Because the tiny twin boundaries and the fct twin boundaries have movability under applied magnetic field, the magneto-driven strain and magneto-controlled shape memory effect may be shown. Now the new-typed multifunctional antiferromagnetic materials are being researched and developed in theoretically and practically.

Magnetic shape memory effect in an antiferromagnetic γ-Mn-Fe(Cu) alloy

A magnetic shape memory effect occurring in a polycrystalline antiferromagnetic Mn85.5Fe9.5Cu5.0 (at. %) alloy is reported. The results show that because of a strong coupling between the antiferromagnetic transition and the fcc→fct martensitic transformation with (101) twinning in the alloy, its martensitic transformation start temperature (MS) and reverse transformation temperature (Af) are close to each other and almost coincide with the antiferromagnetic transition temperature (TN). Being driven by an applied magnetic field of ∼10kOe, the recoverable magnetic-field-induced strains reach to −4.8×10−4(ε‖) and +3.0×10−4(ε⊥) in the directions parallel and transverse to the field, respectively, indicating a type of magnetic shape memory material, based on a Mn-rich Mn-Fe composition, which is different from existing ferromagnetic alloys.

Crystallographic analysis of the fcc-to-fct martensitic transformation in an In22.73at.%Tl alloy

Analytical solutions for various crystallographic parameters have been obtained from the application of the infinitesimal deformation (ID) approach to the fcc M fct martensitic transformation with the (101) g [101] g twinning shear as the lattice invariant shear deformation. All crystallographic parameters were calculated for an In-22.73at.%Tl alloy. In order to compare numerical solutions between the ID approach and phenomenological crystallographic theory, the corresponding crystallographic parameters were also calculated using the Wechsler-Lieberman-Read (WLR) theory. Agreement between the two results obtained from the ID approach and the WLR theory is found to be excellent.

Effect of aging on martensitic transformation in γ-MnCu alloy

Abstract Effect of isothermal aging was investigated on fcc–fct martensitic transformation behavior of Mn–14at.%Cu alloy. It was revealed that by aging at temperatures ranging from 630 to 690 K, the transformation temperature increases markedly and the residual resistivity decreases in parallel. DSC peak profiles have also been modified significantly by aging. A good linear correlation has been found between the changes in the information temperature and those in the resistivity. It is also shown that the observed transformation temperature increase, resistivity decrease and their linear correlation can be well explained by the rule of mixture assuming the heterogeneous microstructure, which forms as a result of spinodal decomposition as reported in previous studies.

Correlation between fcc–fct transformation behavior and spinodal decomposition in γ-MnCu alloys

Abstract The effect of γ phase aging was investigated on fcc–fct martensitic transformation behavior for Mn–14∼30 at.% Cu alloys. The martensitic transformation in these alloys is ‘quasi martensitic’ as characterized by a small hysteresis and a continuous change in lattice parameters below the transformation temperature. It was shown that by aging at temperatures ranging from 630 to 690 K, the transformation temperature increased markedly and the residual resistivity decreased in parallel. Differential scanning calorimetry (DSC) peak profiles were also modified significantly by aging and the heat of transformation increased. A good linear correlation has been found between the changes in the transformation temperature and those in the resistivity. It is also shown that the observed increase in transformation temperature, decrease in resistivity and their linear correlation can be well explained by the rule of mixture assuming a heterogeneous microstructure, which formed as a result of the spinodal decomposition.

Adaptive phase formation in martensitic transformation.

It is shown that an appearance of an intermediate martensite phase called adaptive martensite may be expected if the surface energy of a boundary between two orientational variants of the normal martensite phase is very low and the typical lattice-mismatch-related elastic energy is high. The adaptive martensite is formed as an elastically constrained phase when the scale of structure heterogeneities induced by the crystal-strain accommodation is reduced to the microscopic scale commensurate with the twin-plane interplanar distance. An example of the cubic\ensuremath{\rightarrow}tetragonal transformation is considered where the adaptive phase has a pseudo-orthorhombic lattice. The crystal-lattice parameters of the adaptive phase are expressed through those of the parent cubic phase and tetragonal normal martensite. It is shown that the (5,2\ifmmode\bar\else\textasciimacron\fi{})7R martensite in \ensuremath{\beta}'NiAl alloys and the intermediate phase recently found just above the temperature of the fcc\ensuremath{\rightarrow}fct martensitic transformation in Fe-Pd are examples of the adaptive martensite. A possible role of the adaptive phase in the thermal nucleation of the martensite is discussed. The nucleation of the normal martensite may be bypassed by nucleation of the adaptive phase, which transforms to the normal martensite during the growth.

Thermodynamic and X-ray studies of fcc to fct phase transformations in In-Tl system

Abstract The In-Tl alloy system undergoes a fcc → fct martensitic transformation on cooling, in the composition range 16.0–21.0 at.-%Tl. The thermodynamic properties of both phases were directly determined in the temperature range 300–410 K, together with the enthalpy of phase transformation. From the X-ray data, the lattice parameters of both phases were determined. The volume changes accompanying the transformation were calculated. The volume changes are directly proportional to the strain-free energy which, in this case, makes the outstanding contribution to the non-chemical free energy change. These volume changes decrease with the increase in Tl content in these alloys. The chemical free energy change necessary to overcome the non-chemical free energy change to allow the transformation to take place, is directly reflected in the enthalpy of phase transformation. This means that the enthalpies of phase transformation should also decrease with the increase in Tl content in these alloys, and this is obs...

Theoretical Analysis of the Crystallography of the BCC to FCT Martensitic Transformation

Theoretical Analysis of the Crystallography of the BCC to FCT Martensitic Transformation