X. Zhao

Microstructure and magnetocaloric effect of melt-spun Ni52Mn26Ga22 ribbon

Microstructural features and magnetocaloric properties of Ni52Mn26Ga22 melt-spun ribbons were studied. Results show that there are four types of differently oriented variants of seven-layered modulated (7M) martensite at room temperature, being twin-related one another and clustered in colonies. Due to the coupled magnetic and structural transformations between parent austenite and 7M martensite, the melt-spun ribbons exhibit a significant magnetocaloric effect. At an applied magnetic field of 5u2009T, an absolute maximum value of the isothermal magnetic entropy change of 11.4u2009J kg−1 K−1 is achieved with negligible hysteresis losses.

Determination of microstructure and twinning relationship between martensitic variants in 53 at.%Ni–25 at.%Mn–22 at.%Ga ferromagnetic shape memory alloy

A recent study by high-resolution neutron powder diffraction provided accurate crystallographic information for the newly developed ferromagnetic shape memory alloy 53u2005at.%Ni–25u2005at.%Mn–22u2005at.%Ga. This made it possible to study by high-resolution electron backscatter diffraction the local microstructures and the twinning relationships between martensitic variants. The twin interfaces were also investigated and they are found to be coherent on the {112} planes.

New insights into crystallographic correlations between ferrite and cementite in lamellar eutectoid structures, obtained by SEM–FEG/EBSD and an indirect two-trace method

Four different ferrite/cementite orientation relationships (ORs) in near-eutectoid steel are derived using SEM–FEG/EBSD (scanning electron microscopy–field emission gun/electron back-scatter diffraction) and an indirect two-trace method. They show a common feature of close-packed plane parallelism between ferrite and cementite. Their crystallographic compatibility with habit planes shows a variety of possible habit planes and excludes the existence of the exact conventional Bagaryatsky and Pitsch–Petch ORs. Each of these new ferrite/cementite ORs is correlated with a different edge-to-edge matching condition between austenite and pearlitic ferrite, and between austenite and pearlitic cementite, and possesses specific morphological features. The present results may give deep insight into the crystallography of pearlitic transformation and provide useful information for materials design through interface tailoring in steels.

Crystal structure and phase transformation in Ni53Mn25Ga22 shape memory alloy from 20Kto473K

The crystal structures, magnetic structures, and phase transformation of the off-stoichiometric Ni53Mn25Ga22 were studied by neutron powder diffraction at different temperatures. It is shown that Ni53Mn25Ga22 has a tetragonal I4/mmm structure from 20Kto403K. An abrupt jump in unit-cell volume around room temperature, corresponding to an endothermic peak in the differential scanning calorimetry curve, was observed. This indicates a pretransformation in the martensitic phase of Ni53Mn25Ga22, which is completely different from the phase transformation in the stoichiometric Ni2MnGa. The sequence of structural transformation in Ni53Mn25Ga22 is closely related to its intrinsic temperature-dependent magnetic structure.

Indirect two-trace method to determine a faceted low-energy interface between two crystallographically correlated crystals

An indirect two-trace method to determine interfaces between two crystals with a uniquely defined interface plane is proposed. It involves preparation of only one sample surface, instead of the two perpendicular sample surfaces required by the traditional two-trace method. By making use of two independent interface trace vectors from one uniquely defined interface and the orientations of their adjacent crystals, the interface plane normal and thus the interface plane in the two correlated crystal systems can be determined through numerical calculations concerning coordinate system change. The new method is widely applicable to identify any uniquely defined and reproducible interfaces between crystallographically correlated crystals, even slip systems, provided that they are microscopically visible and crystallographically of the same nature. It can simplify the experimental procedure, increase the determination accuracy and broaden the scope of interface studies.

Shift of the eutectoid point in the Fe-C binary system by a high magnetic field

The purpose of this paper is to investigate experimentally the shift of the eutectoid point in the Fe–C binary system when applying a high magnetic field. The eutectoid carbon content is observed to shift from 0.77 wt% to 0.83 wt% under a 12 T magnetic field. A practical and complete calculation method is proposed—on the basis of the statistical thermodynamic model—to calculate the Gibbs free energy of the related phases and predict the shift of the eutectoid point due to a magnetic field in both composition and temperature coordinates. The composition values are seen to be in fair agreement with the experimental data. The calculation of both shifts shows that the rise in eutectoid temperature because of the 12 T field is 28.97 ◦ C. The impact of the magnetic field on both eutectoid carbon content and eutectoid temperature is not linear. The rate of the shift of both carbon content and temperature decreases as the magnetic field rises.

Magnetic-field-induced grain elongation in a medium carbon steel during its austenitic decomposition

A 12-T magnetic field was applied during the austenitic decomposition in a medium plain carbon steel at a slow cooling rate. The magnetic field applied promotes proeutectoid ferrite grains to grow along the field direction and results in an elongated grain microstructure. The grain elongation is the result of the opposing contributions from the atomic dipolar interaction energy of Fe atoms and the interfacial energy.

Composition-dependent structural and magnetic properties of Ni–Mn–Ga alloys studied by ab initio calculations

We have revealed the influence of composition doping (Ni2+xMn1−xGa, Ni2+xMnGa1−x, and Ni2Mn1+xGa1−x) on lattice constants and atomic magnetic moments of austenite, 7xa0M and NM martensite, by ab initio calculations. It is demonstrated that Ni-doping decreases the volume, whereas Mn-doping increases it. The total magnetic moment of the three series of alloys is mainly dominated by their Mn content with little phase-state dependence. The perturbation of the magnetic moments by atom substitution is mainly dominated by the Mn environment. This study is expected to provide information on composition-related structure and magnetic properties of Ni–Mn–Ga alloys that could not be obtained by experiments.

Modification of preferred martensitic variant distribution by high magnetic field annealing in an Ni-Mn-Ga alloy

The preferred martensitic variant distribution in Ni53Mn25Ga22 ferromagnetic shape memory alloy (FSMA) samples annealed without and with a high magnetic field of 12u2005T applied during the annealing process was investigated by electron backscatter diffraction. It is revealed that the high magnetic field applied during annealing enhances the regular arrangement of martensitic variants from the morphological point of view and effectively modifies the preferred orientation distribution of martensitic variants without changing the misorientation between them from the crystallographic point of view. Only one texture component, { 1{overline 1}0} langle 33{overline 2}rangle, exists in the sample annealed without a magnetic field, whereas two additional texture components, { 4{overline 6} 3} langle 31{overline 2}rangle and { 1overline 1 0} langle 110 rangle, are developed in the sample annealed in a high magnetic field. The new finding that the preferred martensitic variant distribution can be efficiently modified by introducing a high magnetic field during the annealing process will shed light on the development of high-performance polycrystalline FSMAs via novel processing techniques.

Effect of a High Magnetic Field on Eutectoid Point Shift and Texture Evolution in 0.81C-Fe Steel

A 12u2009T magnetic field has been applied to the annealing process of a 0.81%C-Fe (wt.%). It is found that the magnetic field shifts the eutectoid carbon content from 0.77u2009wt.% to 0.83u2009wt.%. The statistical thermodynamic calculations were performed to calculate the eutectoid temperature change by the magnetic field. Calculation shows that the increase of the eutectoid temperature by a 12u2009T field is 29∘C. Synchrotron radiation measurements were performed to measure the pole figures of the samples and were analyzed by MAUD to determine the bulk texture of the ferrite phase In the field-treated and non field-treated samples. Results show that although there is no specific preferred orientation appearing by applying the magnetic field, slight enhancement of (001) fiber component occurs in both the sample normal direction (ND) and the transverse direction (TD). This effect might be related to the magnetic dipolar interaction between Fe atoms in the transverse field direction.