Nathalie Bozzolo

Misorientations induced by deformation twinning in titanium

The misorientations induced by cold-rolling in commercially pure titanium by twinning and multiple twinning were investigated by electron backscatter diffraction. Three main types of twins occur: compressive twins (64.6° 〈1overline 100〉) which are predominant, followed by tensile twins of type 1 (84.8° [11overline 20]) and a few of type 2 (35.1° 〈1overline 100〉). The main multiple twinning configurations are (i) several variants of a given type of twin inside a given grain and (ii) second-generation tensile twins inside primary compressive twins. All possible misorientations resulting from the combination of two out of the three main twinning systems have been calculated and compared with the experimentally observed twin configurations. This facilitated interpretation of the peak found at 41° 〈5overline 1overline 43〉 in the misorientation distribution as the result of double twinning (tensile twins inside primary compressive twins).

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.

Accuracy of orientation distribution function determination based on EBSD data‐A case study of a recrystallized low alloyed Zr sheet

The question of the statistical accuracy of EBSD data for global texture calculation was re‐explored on the basis of a very large grain population (83 000 grains measured on a recrystallized low‐alloyed Zr sheet). Previous works aimed mainly at identifying and quantifying the main texture components and were based on much smaller data sets. The present work attempts to quantify the accuracy of the complete texture, including low‐density regions of the orientation space. For that purpose, a new statistical parameter, VΔ, based on the calculation of texture difference functions is proposed. This parameter has two main advantages: it is equally sensitive to both high and low peaks of the orientation density function (ODF), and it has a physical interpretation because it is the material volume fraction corresponding to the difference between a given ODF and a reference ODF (considered, or known to be close to the truth). Two main variables were studied: the number of grains taken into account and the peak width ϕ0 of Bunges ‘Gaussian’ model density used as kernel for the actual analysis. The orientation distribution functions were computed by nonparametric kernel density estimation with harmonics up to the order of 34. Minimizing the value of VΔ serves as the objective function for optimizing the peak width ϕ0 as a function of the number of grains. The properties of the VΔ parameter also allows for the definition of a method for estimating the accuracy of a given texture that has been obtained from a limited number of grains, without knowing the true texture of the investigated material.

Strain Induced Abnormal Grain Growth in Nickel Base Superalloys

Under certain circumstances abnormal grain growth occurs in Nickel base superalloys during thermomechanical forming. Second phase particles are involved in the phenomenon, since they obviously do not hinder the motion of some boundaries, but the key parameter is here the stored energy difference between adjacent grains. It induces an additional driving force for grain boundary migration that may be large enough to overcome the Zener pinning pressure. In addition, the abnormal grains have a high density of twins, which is likely due to the increased growth rate.

EBSD coupled to SEM in situ annealing for assessing recrystallization and grain growth mechanisms in pure tantalum

An in situ annealing stage has been developed in‐house and integrated in the chamber of a Scanning Electron Microscope equipped with an Electron BackScattered Diffraction system. Based on the Joule effect, this device can reach the temperature of 1200°C at heating rates up to 100°C/s, avoiding microstructural evolutions during heating. A high‐purity tantalum deformed sample has been annealed at variable temperature in the range 750°C–1030°C, and classical mechanisms of microstructural evolutions such as recrystallization and grain coarsening phenomena have been observed. Quantitative measurements of grain growth rates provide an estimate of the mean grain boundary mobility, which is consistent with the value estimated from physical parameters reported for that material. In situ annealing therefore appears to be suited for complementing bulk measurements at relatively high temperatures, in the context of recrystallization and grain growth in such a single‐phase material.