Abstract: Strain fields introduced by coherent Ni4Ti3 precipitates in austenitic Ni-Ti are believed to be a possible origin of why the R-phase transformation is introduced as an extra step before transforming to the B19'. The presence of this strain field was already confirmed in the past by conventional transmission electron microscopy (TEM) techniques and measured quantitatively by high resolution TEM (HRTEM). This time the geometrical phase method is applied on HRTEM micrographs to measure the full 3D strain tensor of the strain fields. Since each atomic resolution micrograph only results in a 2D measurement of the strain, observations in two different zone orientations are combined to retrieve the 3 x 3 strain tensor. In this work observations in a [1-1 1](B2) and [1 0-1](B2) zone orientation are used and this in case of precipitates with a diameter of around 50nm. In a next step the measured strain tensor is compared to the calculated eigenstrain of the R-phase in reference to the B2 matrix. This comparison shows that the introduced strain is very similar to the eigenstrain of one R-phase variant. Since for both structures, Ni4Ti3 and R-phase, four orientation variants are possible, each variant of the R-phase is thus able to accommodate the strain field of one of the Ni4Ti3 variants.

Abstract: Defects in AgG tabular crystals with {111} surfaces are characterised by transmission electron microscopy (TEM) and their influence on the growth process is discussed. In the tabular crystals, twins parallel to the tabular face as well as dislocations along different directions are observed. The twins induce the tabular growth, while the dislocations do not influence the morphology. In 10 to 30% of the crystals that have been characterised, thickness growth is observed and it is shown that in all cases twins on other planes than the tabular ones are present. Two configurations occur more frequently and are analysed in detail. For the first group, twins parallel to the tabular face as well as a microtwin along a non-parallel {111} plane and ending inside the crystal are present. In the crystals of the second group only one extra non-parallel twin occurs giving rise to a bicrystal built up by a tetrahedral shaped part and a flat triangular or trapezoidal part. More complex twin configurations give rise to various, less characteristic morphologies.

Abstract: The alloy Ti6Al4V is known to be prone to the formation of adiabatic shear bands when dynamically loaded in shear. This causes a catastrophic decrease of the load carrying capacity and is usually followed by fracture. Although, the main mechanism is recognized to be the competition between strain hardening and thermal softening, a detailed understanding of the role of microstructural plasticity mechanisms and macroscopic loading conditions does not exist yet. To study strain localization and shear fracture, different high strain rate shear tests have been carried out: compression of hat-shaped specimens, torsion of thin walled tubular specimens and in-plane shear tests. The value of the three techniques in studying shear localization is evaluated. Post-mortem analysis of the fracture surface and the materials' microstructure is performed with optical and electron microscopy. In all cases a ductile fracture is observed. SEM and TEM techniques are used to study the local microstructure and composition in the shear band and as such the driving mechanism for the ASB formation. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ICM11

Abstract: In the present contribution several advanced electron microscopy techniques are employed in order to describe chemical and structural features of the nano- and microstructure of a Ni45.5Ti45.5Nb9 alloy. A line-up of Nb-rich nano-precipitates is found in the Ni-Ti-rich austenite of as-cast material. Concentration changes of the matrix after annealing are correlated with changes in the transformation temperatures. The formation of rows and plates of larger Nb-rich precipitates and particles is described. The interaction of a twinned martensite plate with a Nb-rich nano-precipitate is discussed and the substitution of Nb atoms on the Ti-sublattice in the matrix is confirmed.

Abstract: A single crystal of Cu74.08Al23.13Be2.79 undergoes a martensitic phase transition at 246K and 232K under heating and cooling, respectively. Surprisingly, the martensite phase is elastically much harder than the austenite phase showing that interfaces between various crystallographic variants are strongly pinned and can not be moved by external stress while the phase boundary between the austenite and martensite regions in the sample remains mobile. This unusual behavior was revealed by Dynamical Mechanical Analysis and Resonant Ultrasound Spectroscopy. Transmission Electron Microscopy shows that the pinning is generated by dislocations, which are inherited from the austenite phase. Such dislocations can hinder the movement of stacking faults in the 18R martensite structure or twin boundaries between martensite variants.

Abstract: The properties of the shape memory behaviour of Ni-rich binary NiTi are strongly dependant on the thermal history of the material. In this respect the changing of transformation temperatures of the underlying martensitic transformation and the occurrence of multiple step transformations are the most important phenomena. Part of the explanation is found in the presence of Ni4Ti3 precipitates in the B2 matrix after particular heat treatments. The formation of these precipitates changes the Ni concentration of the matrix and induces a strain field, with both of these aspects expected to be of importance. In this work atomic resolution and analytical TEM (transmission electron microscopy) techniques are used to obtain quantitative information concerning these two main features. Furthermore, the known structure of Ni4Ti3 is refined by a least squares optimization of quantitative electron diffraction data. The high-resolution TEM results show that there are strains up to 2% in the matrix surrounding the precipitates and they gradually increase until a maximum is reached when moving away from the interface. Analytical results reveal a global decrease of Ni content in the matrix when sufficient precipitates are present and a gradient in their close vicinity. The refinement of the structure shows atomic displacements, thereby increasing our understanding of the shrinking of the precipitate lattice with respect to the matrix.