Jean-Sébastien Lecomte

Polycrystal orientation maps from TEM

Determination of topography of crystallite orientations is an important technique of investigation of polycrystalline materials. A system for creating orientation maps using transmission electron microscope (TEM) Kikuchi patterns and Convergent beam electron diffraction patterns is presented. The orientation maps are obtained using a step-by-step beam scan on a computer-controlled TEM equipped with a CCD camera. At each step, acquired diffraction patterns are indexed and orientations are determined. Although, the approach used is similar to that applied in SEM/electron back scattered diffraction (EBSD) orientation imaging setups, the TEM-based system considerably differs from its SEM counterpart. The main differences appear due to specific features of TEM and SEM diffraction patterns. Also, the resulting maps are not equivalent. On these generated by TEM, the accuracy of orientation determination can be better than 0.1 degrees. The spatial resolution is estimated to be about 10nm. The latter feature makes the TEM orientation mapping system an important tool for studies at fine scale unreachable by SEM/EBSD systems. The automatic orientation mapping is expected to be a useful complement of the conventional TEM contrast images. The new technique will be essential for characterization of fine structure materials. To illustrate that, example maps of an aluminum sample produced by severe plastic deformation are included.

EP – a program for determination of crystallite orientations from TEM Kikuchi and CBED diffraction patterns

Orientations of individual crystallites are of interest in many areas of research on polycrystalline materials (see e.g., Kocks et al., 1998). Analysis of Kikuchi patterns is one of the oldest and best established techniques of orientation determination. Convergent beam electron diffraction (CBED) patterns have the same geometry as Kikuchi patterns and can be used for determining orientations if a suf®cient acquisition solid angle is available. The program EP is a user interface linked with KiKoCh ± an engine for pattern indexing and for determination of orientations. EP allows diffraction patterns to be loaded from bitmap ®les and the crystallite orientation to be obtained by marking lines with a mouse; in good quality patterns lines can be detected automatically. A similar system has been described recently by Zaefferer (2000).

Plastic deformation of a Ti-6% Al-4% V alloy with a strong transverse-type crystallographic α-texture at elevated temperatures

Abstract The deformation behaviour of ( αβ )-Ti with a very sharp texture is influenced by temperature, strain rate and the sense of stress. In this paper the plastic deformation of ( αβ )-Ti is investigated in a temperature range between 295 and 1073 K in compression and tension along different directions of the sheet plane (0 °, 45 ° and 90 ° from the rolling direction (RD)). For this temperature range, two phenomena are observed: a dynamic strain ageing (DSA) and a asymmetry of the yield stress associated with the movement of 〈c + a〉 dislocations. Electron microscopy of specimens strained in uniaxial tension or compression along [0001] at 570 K shows 〈c + a〉 dislocations moving on 101 and 11–2 planes and an anomalous activity of 〈c + a〉 dislocations is observed on the whole range of orientation. We discuss the effect of interstitial solutes on the activity of the 〈c + a〉 dislocations between 500 and 750 K and the effect of the non-centrosymmetry for the 101 planes on the directional asymmetry of the yield stress associated with the 〈ca〉 movement.

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°.

Determination of surface crystallography of faceted nanoparticles using transmission electron microscopy imaging and diffraction modes

A general calculation method is proposed to characterize the crystalline planes and directions of a faceted nanoparticle using transmission electron microscopy (TEM) imaging and diffraction modes. With the determination of the edge vectors and then the plane normal vectors in the screen coordinate system of TEM, their Miller indices in the crystal coordinate system can be calculated through coordinate transformation. The method is helpful for related studies of the determination of the surface structure of nanoparticles.

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.

Individual Phase Mechanical Properties at Different Temperatures of Sn–Ag–Cu Lead-Free Solders Incorporating Special Pileup Effects Using Nanoindentation

Sn-Ag-Cu (SAC) alloys are considered as good replacements of Sn-Pb alloys which are banned due to the toxic nature of Pb. But, SAC alloys have a coarse microstructure that consists of beta-Sn rich and eutectic phases. Nanoindentation is a useful technique to evaluate the mechanical properties at very small length scale. In this work, continuous stiffness measurement (CSM) nanoindentation setup (CSM Instruments SA, Peseux, Switzerland) is used to determine the individual phase mechanical properties like Youngs modulus and hardness at high temperatures. It is demonstrated that these properties are a function of temperature for both beta-Sn rich and eutectic phases. Loadings starting from 500 mu N up to 5000 mu N are used with 500 mu N steps and average values are presented for Youngs modulus and hardness. The loading rates applied are twice that of the loadings. High temperatures result in a higher creep deformation and therefore, to avoid it, different dwell times are used at peak loads. The special pileup effect, which is more significant at elevated temperatures, is determined and incorporated into the results. A better agreement is found with the previous studies.

Variant selection of twins with low Schmid factors in cross grain boundary twin pairs in a magnesium alloy

Samples of magnesium AZ31 alloys are deformed in compression at room temperature under a strain rate of 1×10−3 s−1. The initial texture with respect to the loading direction is favorable for {10-12} extension twinning during the deformation. At an engineering strain of 2.75%, many extension twins are found to be connected with each other at grain boundaries, forming cross grain boundary twin pairs. Some have low positive or even negative Schmid factors (SFs). The variant selection of them are interpreted in terms of shear accommodations. The observed twin variants require the least or no accommodation through deformation modes with high CRSSs, but the most or more accommodation through those with low CRSSs.

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.