Nathalie Gey

Characterization of the variant selection occurring during the α→β→α phase transformations of a cold rolled titanium sheet

Abstract The fact that a cold rolling prior to an α → β → α transformation sequence causes the sharpening of the inherited α textures is analyzed. A restitution method is used to evaluate the intermediate β textures which allowed us to discuss the occurrence of variant selection in the β → α transformation. It appears that cold rolling prior to the α → β transformation makes the high temperature β texture sharper, with a density reinforcement around the {112}/{11-2} components and further the variant selection occurring in the β → α phase transformation becomes stronger. The local texture analysis obtained by EBSD leads to the same results and is used to characterize the variant selection occurring in the β → α transformation.

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.

Mechanisms of structural evolutions associated with the high current pulsed electron beam treatment of a NiTi shape memory alloy

The aim of this study was to investigate, for the first time, the surface modifications associated with the use the recently developed high current pulsed electron beam technique for modifying the surface of an intermetallic NiTi alloy. Samples were treated with the same electron beam parameters but different numbers of pulses (i.e., five and ten pulses) and the present article concentrates on a detailed characterization of their texture and microstructure modifications. The observation of surface features such as craters, wavy surfaces with protrusions, chemistry modifications, and the development of specific texture components are discussed as the consequence of the combination of surface melting and evaporation mechanisms. It is also shown that in the subsurface, below the melted layer, the martensitic transformation was triggered due to the effects of the thermal stresses and shock waves propagating in the material.

Modeling the transformation texture of Ti-64 sheets after rolling in the β-field

Abstract The variant selection occurring in the β-α phase transformation of Ti-64 material after rolling in the β-field was investigated by using texture transformation modelings. In a previous work published in this journal, we observed that the variant selection was very sensitive to the deformation degree imposed at high temperature prior to the transformation. In this contribution, we assumed that crystal defects formed during glides on the 110〈111〉 and 112〈111〉 β slip systems could favor the growth of plates corresponding to specific variants. Moreover, a detailed study of the β → α phase transformation suggested a link between the selected variants and the slip systems activated at high temperature. Therefore, we developed a texture transformation modeling, assuming that a variant is selected if its corresponding slip system was sufficiently activated in the last step of the hot deformation. The activity of the slip systems was evaluated by a rate sensitive Taylor model. The modeling was applied to a Ti-64 material 75% rolled at 1050°C and the simulated texture was in good agreement with the experimental one.

Texture heterogeneities in αp/αs titanium forging analysed by EBSD‐Relation to fatigue crack propagation

The microstructure and the local texture of a large IMI 834 forging were characterized using the Electron Back Scattered Diffraction (EBSD) technique. Crystallographic domains called macrozones and formed by a majority of primary αp grains with their axes in nearly the same direction were found. They had a band‐like structure, parallel to the axial direction of the forging. The influence of these macrozones on the cold dwell‐fatigue properties was studied. Several samples were tested under cold dwell‐fatigue conditions. The crack initiation and the short‐distance propagation region optically matched a bright region that contained numerous quasi‐cleavage facets. The analysis of the EBSD measurements showed that this bright region was enclosed within a sharp textured region with axes at less than 30° from the loading axis. The crystallographic features of the crack nucleation site and the crack propagation path were also analysed.

The calculation of a parent grain orientation from inherited variants for approximate (b.c.c.–h.c.p.) orientation relations

The orientations of parent β grains are evaluated from several α variants inherited from the same parent during the body-centred cubic (b.c.c.) to hexagonal close packed (h.c.p.) phase transformation. The proposed calculation, based on orientation correlating and orientation averaging, is particularly useful when the inherited variants are not strictly related to the parent orientation by a strict Burgers orientation relation or when the orientations of the inherited volumes vary slightly at different locations of the variant. This method of parent identification from variant orientations is an improvement of a previously published method.

EBSD for analysing the twinning microstructure in fine‐grained TWIP steels and its influence on work hardening

A 22 Mn–0.6 C twinning induced plasticity steel with an average grain size of 2.6 μm was deformed in tension at room temperature. The electron backscattered diffraction technique was used to characterize the twinning structure in relation with the local texture evolution. For nanoscale analysis, additional transmission electron microscopy analysis was performed. Nanotwins were activated in the largest grains from the beginning of the deformation. They interacted with a well‐developed dislocation structure that induced detectable intragranular orientation variations. With increasing deformation, dense bundles of nanotwins preferentially developed in grains oriented close to the <111>//tensile direction fibre (promoted by the deformation) as well as medium to high angle sub‐boundaries. These key features of the twinned microstructure were finally related to the remarkably high strain hardening, which evolved according to different stages.

Elasticity-based model of the variant selection observed in the β to α phase transformation of a Zircalloy-4 sample

Abstract The β→α texture inheritance of a Zircalloy-4 sample has been investigated after an α→β→α transformation cycle. The final inherited α texture has been determined from a crystal orientation map determined by electron back-scattering diffraction, whereas the texture of the high temperature β phase has been reconstructed by a method analysing the orientations and misorientations of α variants. The comparison of the α texture calculated from the parent β texture without variant selection with the experimental sharp α texture shows differences due to a strong variant selection mechanism occurring during the phase transformation at cooling. A model of a variant selection mechanism based on the elastic anisotropy of the parent β phase leads to a simulated inherited α texture with the main characteristics of the experimental texture.

Determination of parent orientation maps in advanced titanium-based alloys.

In the past few years, reconstruction methods have been developed and applied successfully to restore the β microtexture in titanium alloys. This contribution shows how these methods are extended to other transformations often encountered in advanced titanium alloys: the α (hcp)→γ (tetragonal) and the β (bcc)→O (orthorhombic) transformation.

Restitution of high temperature parent textures of bcc and hcp polycrystals by a non-linear positivity method

The evaluations of the parent texture from the inherited texture have been performed by different methods, in the case of an HSLA steel. Comparison of the results shows the great efficiency of the restitution of the parent texture by using a non-linear positivity method. The method has then been adapted to BCC to HCP transformation. An example of the restitution of the parent phase texture is proposed in the case of the transformation of a β metastable Ti based alloy.