Claudio Zambaldi

Characterization of order domains in γ-TiAl by orientation microscopy based on electron backscatter diffraction

A new approach to resolve the slight tetragonality of L10-ordered γ-TiAl by electron backscatter diffraction (EBSD) is presented. The phase has a c/a ratio of only about 2% larger than unity. The corresponding EBSD patterns therefore exhibit cubic pseudosymmetry. As a consequence, different order variants cannot be easily distinguished on the basis of their EBSD patterns. Automated orientation mapping results in frequent misindexing. In the past, either this problem was overcome by identifying order domains by relatively laborious transmission electron microscopy, or the order domain structure was ignored altogether by using a generic face-centered cubic structure to solve for the crystal orientations, accepting a significant loss of microstructural information. The presented approach is based on the detection of the minor tetragonal distortion of the diffraction patterns by an accurate measurement of backscatter Kikuchi band positions. To this end an accurate pattern center calibration together with high-accuracy parameters for pattern acquisition and indexing are required. Together with a modified indexing algorithm, the order domains in a lamellar microstructure of Ti–45.9Al–8Nb (at%) could be reliably identified. The occurrence of superlattice reflections in the Kikuchi patterns was used to validate the technique. The developed method was successfully applied to create a crystal orientation map of Ti–45.9Al–8Nb (at%) with a fully resolved domain microstructure.

A Matlab toolbox to analyze slip transfer through grain boundaries

Slip transmission across grain boundaries is an essential micromechanical processes during deformation of polycrystalline materials. Slip transmission processes can be characterized based on the geometrical arrangement of active slip systems in adjacent grains and the value of the critical resolved shear stress acting on the incoming and possible outgoing slip systems. We present a Matlab toolbox which enables quantification of grain boundary slip transfer properties and comparison with experiments. Using a graphical user interface, experimental grain boundary data can be directly exported as input files for crystal plasticity finite element simulation of bicrystal experiments.

Nanometer deformation of elastically anisotropic materials studied by nanoindentation

The reduced modulus, ER , of elastically anisotropic materials (Si, CaF2 and MgF2) was determined for sub-10 nm, several-10 nm and several-100 nm indentation depths, applying the Hertzian and Oliver–Pharr approaches. The ER values determined for Si(100), Si(111), CaF2(100) and MgF2(100) deforming at sub-10 nm indentations (i.e. 135 GPa, 177 GPa, 142 GPa and 168 GPa) are in good agreement with the theoretical unidirectional ER values (i.e. 125 GPa, 173 GPa, 135 GPa, 160 GPa). With increasing penetration depth up to several-10 nm, the ER values gradually deviate from the unidirectional values to the weighted averaged values. ER remains constant for sub-10 nm and several-10 nm penetration depths, performing the same indentation tests on amorphous organosilicate glass. The results of this study indicate that the modulus determined by nanoindentation depends on the size of indentation (ratio between contact radius and tip radius, a/R), especially in the case of elastically anisotropic materials. It is demonstrated for several-10 nm and several-100 nm penetration depth that the phase transformations in Si during the indentation tests strongly affect the ER values.

Crystal plasticity modelling and experiments for deriving microstructure-property relationships in γ-TiAl based alloys

Single-crystals of γ-TiAl cannot be grown for the compositions present inside the two-phase γ/α2-microstructures that show good mechanical properties. Therefore the single crystal constitutive behaviour of γ-TiAl was studied by nanoindentation experiments in single phase regions of these microstructures. The experiments were extensively characterized by a combined experimental approach to clarify the orientation dependent mechanical response during nanoindentation. They further were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behaviour of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. On the length-scale of multi-grain aggregates two kinds of microstructures were investigated. The lamellar microstructure was analyzed in terms of kinematic constraints perpendicular to densely spaced lamellar boundaries which lead to pronounced plastic anisotropy. Secondly, the mechanical behaviour of massively transformed microstructures was modelled by assuming a lower degree of kinematic constraints. This resulted in less plastic anisotropy on a single grain scale and lower compatibility stresses in a 64-grain aggregate. On the macroscopic length scale, the results could possibly explain the pre-yielding of lamellar microstructures.