Transformation In Iron-based Alloys Research Articles

Giant shape memory effect of the [formula omitted]-oriented FCC CrMnFeCoNi high-entropy alloy single crystals with FCC↔HCP martensitic transformation

For the first time, a giant shape memory effect of 15.7 ± 0.2% was obtained in [1¯44]-oriented single crystals of the Cr20Mn20Fe20Co34.5Ni5.5 at.% high-entropy alloy undergoing FCC↔HCP martensitic transformation, under a tensile stress of 160 MPa. This value amounted to 90% of the theoretical value of the lattice deformation 17.5% for the FCC–HCP martensitic transformation in [1¯44] orientation under tension and is currently the highest for this transformation in iron-based alloys. The physical reason of the giant shape memory effect in the [1¯44]-oriented single crystals of the Cr20Mn20Fe20Co34.5Ni5.5 at.% high-entropy alloy is related to the combination of the high yield strength of the initial FCC-phase with the development of thin HCP-martensite predominantly in one system.

The Degenerate Structure of Transformation Twins and the Monocrystallinity of Part of the Thin-Plate Martensite Initiated by a Strong Magnetic Field

Abstract–In the dynamic theory, the formation of twin martensite crystals is the result of a coordinated propagation of relatively long-wave ( $$\ell $$ -waves) and short-wave (s-waves) displacements. The matching condition is analyzed for the γ–α martensitic transformation in iron-based alloys, taking into account the quasi-longitudinalness of the $$\ell $$ -wave carrying compression deformation. It has been shown for the first time that the previously established single-crystal effect of part of the crystals of thin-plate martensite, which arises upon cooling under the action of a strong magnetic field, can naturally be interpreted as a consequence of the formation of a degenerate structure of transformation twins.

Degenerate Structure of Transformation Twins and Estimation of Dislocation Density in Martensite Crystals

In the dynamic theory of martensitic transformations, the wave mechanism of controlling martensite crystal growth is determined by the superposition of wave beams of quasi-longitudinal (or longitudinal) waves carrying the “tensile–compression” deformation in the orthogonal directions. The wave beam formation is considered to be a result of the formation of excited (vibrational) states. The existence of transformation twins is interpreted as a result of a matched propagation with respect to long-wave (l waves) and short-wave (s waves) shifts. The matching condition is analyzed for the γ–α martensitic transformation in iron-base alloys. It is shown for the first time that the transition to a degenerate twin structure with the allowance for the medium discreteness enables one to estimate the dislocation density in crystals with habit {557}, which agrees with that observed experimentally.

Вырожденная структура двойников превращения и оценка плотности дислокаций мартенситных кристаллов

Abstract In the dynamic theory of martensitic transformations, the wave mechanism of controlling martensite crystal growth is determined by the superposition of wave beams of quasi-longitudinal (or longitudinal) waves carrying the “tensile–compression” deformation in the orthogonal directions. The wave beam formation is considered to be a result of the formation of excited (vibrational) states. The existence of transformation twins is interpreted as a result of a matched propagation with respect to long-wave ( l waves) and short-wave ( s waves) shifts. The matching condition is analyzed for the γ–α martensitic transformation in iron-base alloys. It is shown for the first time that the transition to a degenerate twin structure with the allowance for the medium discreteness enables one to estimate the dislocation density in crystals with habit {557}, which agrees with that observed experimentally.

Martensite Transformation Intergrain and Intragrain Autocatalysis in Fe-Ni alloys

This work presents a consolidated view of the autocatalysis, thermal and entropic effects in martensitic transformation of polycrystalline Fe-31wt%Ni-0.02wt%C, which has been a standard material for the investigation of fundamental aspects of the martensite transformation. The present work is based on the description of classical microstructure descriptors of the transformation and on generally accepted concepts regarding the martensitic transformation in iron base alloys. Present work agrees with the view that the autocatalysis is a means by which the martensite transformation promotes further nucleation and growth. Autocatalysis induces the nucleation and growth of secondary plates in addition to the relatively small number of primary nucleation sites and their corresponding primary plates. We demonstrate that autocatalysis can be factored out into intragrain and intergrain components. The analysis of these factors establishes that intragrain autocatalysis is athermal but intergrain autocatalysis possess an Arrhenius temperature dependence. The reasons for such a behavior are discussed in detail.

A parallelized model for coupled phase field and crystal plasticity simulation

The predictive simulation of materials with strong interaction between microstructural evolution and mechanical deformation requires the coupling of two or more multi-physics models. The coupling between phase-field method and various mechanical models have drawn growing interests. Here, we propose a coupled multi-phase-field and crystal plasticity model that respects the anisotropic mechanical behavior of crystalline materials. The difference of computational complexity and solver requirements between these models presents a challenging problem for coupling and parallelization. The proposed method enables parallel computation of both models using different numerical solvers with different time discretization. Finally two demonstrative examples are given with an application to the austenite-ferrite transformation in iron-based alloys.

Austenite-to-ferrite phase transformation in low-alloyed steels

The kinetics of the austenite(γ)-to-ferrite(α) phase transformation in iron-based alloys with low amounts of interstitial and substitutional components has been simulated. The finite mobility and the diffusion of the components determine the γ/α transformation kinetics. Numerical difficulties may occur during simulations of diffusional phase transformations in such systems due to the fact that the diffusion of substitutional and of interstitial components occur on completely different time scales. However, substitutional alloying components (e.g. Mn, Cr, Ni) can be assumed to be immobile, if their amount can be completely dissolved in ferrite and the driving force for the transformation is sufficiently high. For lower, but not too small driving forces the site fractions of the substitutional components remain constant in both phases except a thin concentration spike which occurs at the austenite side of the interface. In a new model the Gibbs energy dissipation due to the diffusional motion of this spike has been considered and the transformation kinetics in the ternary Fe–C–Mn system has been calculated. The simulated transformation kinetics are compared to results obtained by isothermal dilatometer tests on a low-alloyed Fe–C–Mn steel.

Is Homogeneous Nucleation of Martensitic Transformation in Iron-Base Alloys Possible?

The homogeneous nucleation energy has been evaluated on the basis of classic nucleation model, indicating that the energy for homogeneous nucleation of martensitic transformation in iron base alloy is one order of magnitude lower than that available in the thermal fluctuation. From a thermodynamic point of view, homogeneous nucleation is possible in the martensitic transformation of iron-base alloys. Homogeneous nucleation mechanism is compatible to the experimental results concerning the martensitic transformation in iron-based small particles or alloys with fine grains. Recent investigations on the martensitic transformation in ultrafine particles provided further evidences supporting this argument.

Initiation of the ?-? martensitic transformation in iron-based alloys by picosecond pulses

Initiation of the ?-? martensitic transformation in iron-based alloys by picosecond pulses

Fractal Dimension of the Martensitic Transformation

The fractal dimension of field-induced martensitic transformation in iron-based alloys is investigated. The martensitic phase (bcc) appears in the parent austenitic phase (fcc) when a strong magnetic field is applied under suitable temperatures. It is found that microstructural patterns of the grown martensitic domains on a metal surface show a clear fractal nature with a fractal dimension of 1.81±0.02. This result means that the fractal dimension of the martensitic transformation is larger than that of diffusion-limited crystal growth where the dimension is 1.6∼1.7. The result may qualitatively explain that the growing point of the crystal in the diffusion-limited case has a characteristic of a one-dimensional extention, while the martensitic transformation is diffusionless and mainly ruled by two- or three-dimensional stress.

The effect of rapid solidification on the order-disorder transformation in iron-based alloys

The effect of rapid solidification on the ordering reaction in FeSi and FeAl alloys has been reported. It is shown that rapid solidification can influence the ordering reaction in alloys with higher critical ordering temperatures. For ordering reactions at lower temperatures, the effect is similar to that of solid-state quenching. Different factors influencing the ordering reactions and domain structures during rapid solidification of iron-based alloys are discussed.

Effect of alloying elements on the critical driving force of martensitic transformation in iron alloys

Effect of alloying elements on the critical driving force for martensitic transformation in iron base alloys is studied and it is confirmed that the critical driving force is changed by alloy addition. It is shown that there is a qualitative correlation between critical driving force and solid solution hardening.

Crystallography of phase transformations

Microstructural and crystallographic features which are associated with transformation products are generalized on the basis of factors such as interfacial surface energy, strain energy and diffusion. In the case of diffusion-controlled Widmanstätten structures, surface energy factors such as a discrete orientation relationship, a low index habit plane and a tendency for a pseudomorphic or coherent crystal structure are dominant. Factors associated with tolerable strain energy, such as stress relief by recrystallization or concurrent plastic deformation, play only minor roles in diffusion-controlled transformations. On the other hand, it is proposed that in the case of diffusionless martensitic structures the strain energy factors, such as spontaneous deformation by slip and twinning, dominate to modify the factors established by interfacial surface energy. The strain energy factors lead to modified or pseudo-habits and pseudo-orientation relationships in much the same way as surface energy may lead to a pseudomorphic crystal structure. The consequence of spontaneous deformation by slip is illustrated for martensitic structures in iron-base alloys. It is proposed that the (111)A habit and Kurdjumow-Sachs orientation relationship are primitive characteristics of the γ to α transformation in iron-base alloys and that spontaneous slip modifies these characteristics to those observed for bainitic and martensitic structures. The suggestion is advanced that the (225)A habit arises because of rotation of the interface away from (111)A by slip in the austenite whereas the (259)A habit is associated with the dominance of slip in the martensite. Slip in each phase can account for lattice rotations that explain variables of the observed orientation relationships. A generalization that the habit plane tends to be one with indices of the type {ithhl} in a variety of martensitic structures involving cubic phases is explained by spontaneous deformation. On the basis of this viewpoint, shear transformations and martensitic structures are to be regarded as being accompanied by shear rather than generated by shear.