Claude Esling

Modelling and prediction of mechanical properties for materials with hexagonal symmetry (zinc, titanium and zirconium alloys)

Abstract In this work we have modelled various mechanical properties for hexagonal materials having various textures and/or deformation mechanisms. The main purpose of this work was to determine with great accuracy the active deformation mechanisms and to evaluate the corresponding CRSS ratios. This study—carried out by optical and electron microscopy—is based on the statistical data obtained for the deformation mechanisms (frequency of occurrence) applying to each alloy. Though more sophisticated models are available, we used—in a first approach—the relatively simple Taylor model (constrained and relaxed variants) with the fairly reliable CRSS ratios we had previously assessed (accuracy around 10%) and used earlier for the modelling of the texture evolution. With these values, we then modelled the variation in the sheet plane of the yield stress, the plastic strain ratio, and the yield loci. The predicted curves were then compared with the experimental ones that had been drawn, including the margin of experimental error. In the case of TA6V we made a distinction between pyramidal 〈c + a〉+ and 〈c + a〉2− slip directions. The predicted yield loci of TA6V show the same asymmetry in tension and compression as the experimental curves.

Modelling of texture evolution for materials of hexagonal symmetry. II: Application to zirconium and titanium α or near α alloys

Abstract This work describes the evolution of texture and microstructure during cold rolling of different Ti and Zr alloys. These alloys accommodate deformation with prismatic glide and with “secondary” mechanisms (gliding and/or twinning) which are different according to the type of alloys and may vary with deformation degree. We have modelled texture evolution during cold rolling of two Ti and Zr alloys, using a Taylor theory. The choice or relevance of the model variant (FC = full Constrained, RC = Relax Constrained) are discussed. In order to account for the changes in the secondary systems during deformation, we have decided to work by steps, since there are no defined hardening laws accepted for each system. The results of the modelling, both for the Pole Figures (PF) and for the Orientation Density Function (ODF), agree well with experiment in the range from 0 to 80% reduction. Therefore, a good knowledge of the microstructure evolution and of the deformation mechanisms is required.

Determination of a Mean Orientation from a Cloud of Orientations. Application to Electron Back‐Scattering Pattern Measurements

The purpose of this work was to determine the orientations of grains in polycrystals using electron back-scattering patterns (EBSP). In order to lower the degree of statistical uncertainty, the orientation of the same grain was measured several times. The orientation of the corresponding grain was assumed to be the mean of the orientations measured. From a theoretical point of view, the way to calculate a mean orientation from several orientations was solved on the basis of the properties of quaternions. The accuracy of the measurements, including the mean and maximum deviation of the orientations established, could be evaluated. The orientation of α plates of Ti-64 products was determined and it was possible to show that the metallurgical state of the parent β phase prior to the β–α phase transformation is likely to influence the orientation spread of the variants inherited from the same β grain, belonging to the same orientation family.

Crystalline, electronic, and magnetic structures of θ-Fe3C, χ-Fe5C2, and η-Fe2C from first principle calculation

First principle calculations have been performed to study the crystalline, electronic, and magnetic structures of three iron-carbide systems: θ-Fe3C, χ-Fe5C2, and η-Fe2C. The Kohn-Sham equations were solved by applying the full-potential linearized augmented plane wave method. The generalized gradient approximation in the Perdew-Wang formalism was used to the exchange and correlation energy functional. The internal positions of atoms within the unit cell were optimized and the ground state properties such as lattice parameter and bulk modulus were calculated. The results are compared with experimental data when available. Comparison of the two metastable systems χ-Fe5C2 and η-Fe2C shows that the last one has lower formation energy; this is corroborated by the formation sequence observed during tempering. The electronic structures of the three carbides were then studied and the magnetic moments calculated by means of electronic spin-resolved density of state calculations at their equilibrium lattice consta...

Giant magnetocaloric effect in melt-spun Ni-Mn-Ga ribbons with magneto-multistructural transformation

Magnetic refrigeration based on the magnetocaloric effect (MCE) may provide an energy-efficient and environment-friendly alternative to the conventional gas compression/expansion cooling technology. For potential applications, low-cost and high-performance magnetic refrigerants are in great need. Here, we demonstrate that giant MCE can be achieved in annealed Ni52Mn26Ga22 ribbons with magneto-multistructural transformation. It yields a maximum magnetic entropy change of −30.0 J kg−1 K−1 at the magnetic field change of 5 T, being almost three times as that of initial melt-spun ribbons and comparable to or even superior to that of polycrystalline bulk alloys.

Microstructure, texture, grain boundaries in recrystallization regions in pure Cu ECAE samples

By means of equal channel angular extrusion (ECAE), pure Cu single crystal samples were processed down to the submicron scale. In some parts of the samples, recrystallization occurs at room temperature. The recrystallization mechanism was analyzed by SEM-EBSP and SEM-ECC techniques. The grain boundary character distribution (GBCD) and orientation distribution function (ODF) of the regions undergoing recrystallization were computed. The results show that the nuclei can be formed at the intersections of two different shear bands, and the microstructures and the grain boundary characters in these locations contribute to growth of recrystallized nuclei. The recrystallized grains had grown according to the Felthams mechanism

Classification of the critical resolved shear stress in the hexagonal-close-packed materials by atomic simulation: Application to α-zirconium and α-titanium

We have studied the hierarchy of the activation of dislocation glide in zirconium and titanium alloys and presented experimental results in zirconium alloys. We have compared the experimental results with simulations obtained by two different approaches. The first is by using the stacking fault energy maps (γ surfaces) obtained by molecular dynamics (MD) and by ab initio approaches. A good agreement was observed between the two approaches and with recent published work. The second is to compare the experimental critical resolved shear stresses (CRSS) with those determined by MD simulations based on embedded atom method (EAM) potentials. The CRSS for slip in the -direction for the basal, prismatic (type 1) and pyramidal (type 2) planes for edge dislocations are obtained. Finally, we discuss the hierarchy of the glide systems with the energy criterion of the γ surfaces and with the CRSS values and we compare with both experimental and modeling data.

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 5 T, an absolute maximum value of the isothermal magnetic entropy change of 11.4 J 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 53 at.%Ni–25 at.%Mn–22 at.%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.

Crystallographic, magnetic, and electronic structures of ferromagnetic shape memory alloys Ni2XGa (X = Mn,Fe,Co) from first-principles calculations

The crystallographic, magnetic and electronic structures of the ferromagnetic shape memory alloys Ni2XGa (X=Mn, Fe, and Co), are systematically investigated by means of the first–principles calculations within the framework of density functional theory using the VIENNA AB INITIO SOFTWARE PACKAGE. The lattice parameters of both austenitic and martensitic phases in Ni2MnGa have been calculated. The formation energies of the cubic phase of Ni2XGa are estimated, and show a destabilization tendency if Mn atom is substituted by Fe or Co. From Ni2MnGa to Ni2CoGa, the down spin total density of states (DOS) at Fermi level is gradually increasing, whereas that of the up spin part remains almost unchanged. This is the main origin of the difference of the magnetic moment in these alloys. The partial DOS is dominated by the Ni and Mn 3d states in the bonding region below EF. There are two bond types existing in Ni2XGa: one is between neighboring Ni atoms in Ni2MnGa; the other is between Ni and X atoms in Ni2FeGa and ...