Papers by Author: Michel Humbert

Abstract: In this contribution, we investigate a method able to determine the local representative orientation relation occurring in phase transformation of steels. It is based on the analysis of sets of variant orientations and does not require the knowledge the parent austenite orientation. The method which is an optimization scheme employing quaternions allowed us to determine and investigate ORs of different low carbon steel products.

Abstract: In this contribution, we describe the basic principles to reconstruct parent microtextures from inherited ones. The different methods published in the literature are examined. Their various approaches lead to different restitution capabilities and we discuss their advantages and weaknesses.

Abstract: We propose to reconstruct the g orientation maps from the α maps inherited by the bainitic or martensitic γ→α phase transformation. Our approach comprises two main steps (1) identification of reliable fractions of parent grains – each γ fraction is determined from neighbouring variants related to a unique γ orientation with a low tolerance angle; (2) expansion of these fractions by collecting adjacent variants being in orientation relation (OR) with the calculated g orientation - using a higher tolerance around the strict OR. Moreover, this fully automated g map reconstruction is further improved by an additional semi-automated analysis of α/γ maps. This reconstruction procedure was applied to the α microtexture of bainitic steels. The reliability of the calculations was checked by comparing each γ grain orientation and their corresponding α variants. The results show that even with a large spread around OR, the shape and orientation of most of the γ grains are accurately calculated.

Abstract: We propose a new approach to automatically reconstruct the  orientation map from the ’ map inherited by the bainitic or martensitic ’ phase transformation. Our model comprises two main steps (1) identification of reliable fractions of parent grains – each  fraction orientation is determined from neighbouring variants related to a unique  orientation with a low tolerance angle; (2) expansion of these fractions by collecting adjacent variants being in orientation relation (OR) with the  orientation of the initial fraction - using now a higher tolerance around the strict OR. The code was tested on ’ maps we built from reference  maps to control some characteristics of the transformation, i.e. the type of OR used, the spread around OR, the number of inherited variants. The results show that even with a large spread around OR, the shape and orientation of most of the  grains are accurately calculated.

Abstract: A new method for calculation of the diffraction elastic constants, based on the selfconsistent model, is proposed and tested. This method is especially useful in the interpretation of the results of X-ray measurements since the ellipsoidal inclusion near the sample surface is considered. In X-ray diffraction the information volume of the sample is defined by absorption, causing unequal contribution of different crystallites to the intensity of the measured peak. Consequently, the surface grains participate more effectively in diffraction than the grains which are deeper in the sample.

Abstract: The orientations of the inherited tetragonal (resp. cubic) variants are calculated from the parent hcp orientation in the case of a strict orientation relation. The numbering of the variants is proposed, as well as the misorientations between them. Conversely, a method for calculating the parent hcp orientation from a sufficient number of inherited variants is proposed. It is based on orientation correlating and orientation averaging, and it is particularly useful when the inherited variants are not exactly related to the parent orientation by a strict orientation relation or when the orientations of the inherited volumes slightly vary at different locations of the variant. The method is illustrated by considering the a to g phase transformation taking place in TiAl-based alloys

Abstract: One very often observes that the texture inheritance in BCC to HCP phase transformation shows variant selections, even though no external stress field is applied. These variant selections are related to the metallurgical state, the microstructure and the texture of the parent phase. From our own investigations, we came to the conclusion that the variant selections we observed in some phase transformations of various materials were influenced at different degrees by the elastic behaviour of the parent phase. Considering the transformation strain of each variant and the elastic anisotropy of the parent, we have build variant selection models based on energy minimum of elastic strain and assuming different types of interactions. The simulation results of texture transformation of a zircalloy sample show that the elastic characteristics of the parent phases are key parameters involved in the variant selection.

Abstract: A micromechanical model was used to simulate the mechanical behaviour, the transformation kinetic and the texture evolutions of a 304 stainless steel, deformed by tensile tests at T=–60°C. When the transformation strain is calculated with the phenomenological theory, the model does not very well predict the observed transformation rate and the texture evolution of the main γ grain orientations. XRD and EBSD analyses show that the martensitic transformation of γ phase into α’ martensite involves the intermediate ε phase. From these observations, new simulations were performed in which only the γ→ε transformation strain is considered in competition with classical plasticity of austenite. The α’ variants were calculated in a second step, from the ε variants selected in the micromechanical model. Among the 6 potential α’ variants able to nucleate from the same ε variant, the best oriented ones, with respect to the applied stress were selected. Under these conditions, the numerical simulations reproduce the experimental results in a more satisfying way.

Abstract: Regions with sharp local textures, called macrozones, have been characterised in a bimodal IMI834 billet, containing 30% of primary αp grains surrounded by secondary αs colonies. It is shown that the αs colonies have been inherited according to a strong variant selection during the β→αs phase transformation. In each observed macrozone, the favoured variants have in average their c-axes in the same macroscopic direction as the αp grains. A detailed analysis of neighbouring αp grains and αs variants clearly shows that the variants favoured at β/αp boundaries are those able to share their c-axes with a neighbouring αp grain. The sharpness of such a variant selection mechanism is strongly related to the local orientation distribution of neighbouring αp/β grains at high temperature. This explains the differences in variant selection sharpness observed from one macrozone to the other.

Abstract: The presence of hcp regions with grains having relatively close orientations has been reported in commercial near alpha titanium billets (IMI 834, Ti 6246, etc). The size of these textured regions (called macrozones) is significantly larger than the average grain size of the microstructure observed after thermomechanical processing. The elongated shape of these large hcp regions suggests that they are eventually related to large prior b grains that pancaked during the ingot break down process. In this contribution, Orientation Image Microscopy was used to study the relationship between the hcp local microtexture heterogeneities and the prior b orientations. Specifically, the orientations of the primary (equiaxed) ap grains and the secondary (lamellar) as colonies produced after the transformation of the b phase were discriminated from OIM maps. Furthermore, from the as inherited OIM map, it was possible to reconstruct the corresponding b OIM map over large regions. The analysis showed that the large hcp macrozones observed in the as received material are not related to corresponding bcc macrozones. However, within an hcp macrozone, various clusters of b grains with similar orientations can be found. In such coherent regions, randomly orientated b grains were also observed, which could be related to microstructural changes during deformation (continuous dynamic recrystallization) as suggested by hot deformation results.