Lionel Germain

Interactions between ferrite recrystallization and austenite formation in high-strength steels

Using both experimental and modeling approaches, we give some clarifications regarding the mechanisms of interaction between ferrite recrystallization and austenite formation in cold-rolled high-strength steels. Using different thermal paths, we show that ferrite recrystallization and austenite formation can be strongly interdependent. The nature of the interaction (weak or strong) affects significantly the austenite formation and the resulting microstructure. We show that the kinetics of austenite formation depends intrinsically on both heating rates and the extent of ferrite recrystallization. An unexpected behavior of austenite growth was also seen at high heating rates. A possible explanation is presented based on the nature of the local equilibrium at the ferrite–austenite interface. The microstructure is more heterogeneous and anisotropic when both austenite formation and ferrite recrystallization overlap. A mechanism of microstructural formation is proposed, and this is supported by 2D simulations’ images.

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.

A New Approach to Calculate the γ Orientation Maps in Steels

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.

Determination of parent β-phase orientation from inherited orthorhombic phase in β → O + B2 phase transformation of Ti–22Al–25Nb alloy

We propose and describe a method for determining the orientation of parent body-centred cubic (bcc) β grains at high temperatures from the orientations of the orthorhombic variants observed at room temperature as applied to the case of high-Nb-containing Ti3Al-based alloys. The method is based on knowledge of the orientation relationship between the parent and inherited phase. By averaging, the procedure enables determination of the most probable parent orientation using an approximate orientation relationship. The β → O transformation in Ti3Al-based Ti–22Al–25Nb alloy is very suitable for checking the relevance and effectiveness of the method because, in this case, after certain processing, some β-phase is retained at room temperature.

Banded structures in dual-phase steels ― A novel characterization method

Abstract A simple to implement and original automated method is presented to quantify the intensity and wavelength of “banded structures”, i. e. microstructures presenting irregular and parallel bands enriched in a given phase. This method is based on the analysis of the covariance function of binary images. It is firstly tested on model representative images. It is compared with a conventional method derived from the ASTM E-1268 standard and appears to present a higher robustness with regard to preparation artifacts. Then, it is applied to real microstructures of dual-phase steels, which show more or less regular martensite-enriched bands in a ferrite matrix. The method is capable of efficiently discriminating dual-phase microstructures resulting from different thermo-mechanical routes.

β Microtexture Analysis in Correlation with HCP Textured Regions Observed in a Forged Near Alpha Titanium Alloy

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.

β→αs Variant Selection in Sharp hcp Textured Regions of a Bimodal IMI834 Billet

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.

Current Approaches for Reconstructing the Parent Microtexture from that Inherited by Phase Transformation

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.

Study and Modelling of Some Variant Selections in bcc to hcp Phase Transformations

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.

Assessment of EBSD Analysis and Reconstruction Methods as a Tool for the Determination of Recrystallized Fractions in Hot-Deformed Austenitic Microstructures

The microstructural evolution of hot-deformed austenite during recrystallization was studied in a non-microalloyed low carbon steel and a low carbon steel, microalloyed with niobium and titanium. Double-hit compression tests were carried out to evaluate the isothermal recrystallization behavior. Specific deformation temperatures and interpass times were combined to produce characteristic recrystallization states by quenching the samples to a martensitic microstructure. The reconstruction software Merengue 2 was used to determine the prior austenite microstructure and evaluate the recrystallized fraction in the microstructure. The reconstruction is based on EBSD measurements of the martensitic microstructure. The determination of the recrystallized fraction was realized by evaluating the grain orientation spread. This work compares the results of both methods and estimates their uncertainties. It shows the potential of EBSD reconstruction methods to characterize different recrystallization states of low carbon steels out of EBSD data.