Christophe Schuman

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.

Multiple twinning in pure hexagonal close-packed titanium

Commercial pure titanium (T40) was deformed in channel die compression by means of the split-sample technique in order to study multiple twinning. Particular attention was paid to the twin variant presentation and selection during multiple twinning. All possible misorientations, corresponding to the multiple twins arising from the combination of the {1 1 {\overline 2} 2} compression (C) twin, the {1 0 {\overline 1} 2} tension twin (T1) and the {1 1 {\overline 2} 1} tension twin (T2), were calculated with respect to the crystal basis of the matrix grain. All the multiple twin variants are partitioned into ten classes with the same crystallographically equivalent misorientation angle and axis. However, when the influence of twinning order is taken into account, the multiple twin variants are partitioned into 15 classes. Experimental results prove that the selection of twin variants (primary and secondary) is mainly governed by their macroscopic Schmid factor (SF). The normalized SF is more efficient at predicting variant selection. A twin formed in one grain can activate another twin in a neighbouring grain, provided that the angle between the two twinning planes does not exceed 20°.

A method to identify dislocations in a known crystal structure by transmission electron microscopy

This paper proposes a method to identify the type and the Burgers vector of dislocations visualized via transmission electron microscopy (TEM). The first step is to determine experimentally the orientation, with respect to the sample holder, of a grain of known crystal structure whose dislocation slip systems have been previously reported. With this determined orientation of the grain, the method calculates the orientation of the projections of the possible dislocation line vectors in the transmission electron microscope screen coordinate system and then compares them with the observed dislocations to identify their type and the Burgers vector. The coordinate transformations underlying the method are outlined, and its validity is demonstrated using TEM measurements on a titanium sample. The method is expected to simplify the related TEM determination work.

Crystallographic characters of {11¯22} twin-twin junctions in titanium

Abstract contraction twins that are commonly activated in α-titanium interact to each other and form three types of twin–twin junctions (, , TTJs) corresponding to the crystallography of six twin variants (i = 1,2, … , 6). We detected 243 TTJs in rolled pure α-titanium sheets. Electron backscatter diffraction analysis reveals that TTJs are profuse, 79.8% among three types while and TTJs take up 17.7 and 2.5%. Twin transmission does not occur. Consequently, boundaries associated with twin–twin interactions block twin propagation and influence twin growth. We explain structural features of TTJs according to the Schmid factor analysis and the reaction mechanism of twinning dislocations. The knowledge regarding TTJs provides insight for improving the predictive capability of meso/macro-scale crystal plasticity models for hexagonal metals.

Role of glide systems in channel die compression of commercially pure titanium alloy

Abstract Plastic deformation process of Ti alloys depends on the competition of active deformation modes: slip and twinning. The specific deformation mechanism depends on the availability of the specific deformation modes as well as on the critical resolved shear stress for slip and the twin activation stress. Effort has been made to provide information on the different slip systems other than prismatic slips, which get activated during the channel die compression process. It is found that, though at the initial stage of deformation, prismatic slip is prevalent, but other slip systems, especially pyramidal types, also appear at the later stage of deformation. Transmission electron microscopy results confirm the presence of pyramidal slips beside prismatic ones. It is also observed that twining does not take place in those grains having the most stable orientation as per the hexagonal close packed system. Information on the critical resolved shear stresses of different glide systems for T40 is also provided.

Contribution of the Nanoindentation to the Study of HCP Metals

This study combines nanoindentation experiments, electron backscatter diffraction (EBSD) and atomic force microscopy (AFM) topographic measurements to investigate the material anisotropy contribution to the indentation behaviour of individual grains of various hexagonal-close packed (HCP) polycrystals with different axial ratio (zinc, magnesium and titanium). The grain size was much larger than the indents size to ensure quasi-single-crystal indentation and when, combined with an EBSD mapping, a wide variety of crystal orientations can be probed, which provides mechanical characterization of materials at the micro/nanoscale. Experimental curves can be used to determine the mechanical properties of the indented material. Furthermore, by using data issued from AFM topographic measurements, one can analyze the dislocations arrangements below and around the indentation print, and thus characterize the most probably activated deformation systems.

New Model for Twin-Variant Selection in Hexagonal Alloys

A new selection criterion to explain the activation of the twinning variant is proposed. This criterion is based on the calculation of the deformation energy to create a primary twin. The calculation takes into account the effect of the grain size using a Hall-Petch type relation. This criterion allows to obtain a very good prediction for the variant selection. The calculations are compared with the experimental results obtained on T40 (ASTM grade 2) deformed by Channel Die compression.