Transmission electron microscopy investigation of interfaces in a two-phase TiAl alloy

Accurate modeling and evaluation of microstructures in complex materials

Accurate characterization of heterogeneous materials is of great importance for different fields of science and engineering. Such a goal can be achieved through imaging. Acquiring three- or two-dimensional images under different conditions is not, however, always plausible. On the other hand, accurate characterization of complex and multiphase materials requires various digital images (I) under different conditions. An ensemble method is presented that can take one single (or a set of) I(s) and stochastically produce several similar models of the given disordered material. The method is based on a successive calculating of a conditional probability by which the initial stochastic models are produced. Then, a graph formulation is utilized for removing unrealistic structures. A distance transform function for the Is with highly connected microstructure and long-range features is considered which results in a new I that is more informative. Reproduction of the I is also considered through a histogram matching approach in an iterative framework. Such an iterative algorithm avoids reproduction of unrealistic structures. Furthermore, a multiscale approach, based on pyramid representation of the large Is, is presented that can produce materials with millions of pixels in a matter of seconds. Finally, the nonstationary systems-those for which the distribution of data varies spatially-are studied using two different methods. The method is tested on several complex and large examples of microstructures. The produced results are all in excellent agreement with the utilized Is and the similarities are quantified using various correlation functions.

Atomic Structure of Internal Interfaces in A Ti3AI-TiAI Two-Phase Alloy

TiAI/Ti3AI colonies in an as-cast Ti-40 at.%AI alloy have been studied using HREM. Their microstructures consist mainly of alternating parallel lamellae of y (TiAI, L10) and α2 (Ti3AI, DO19) phases. The γ phase is often twinned with extended boundaries {111] parallel to the adjacent Ti3AI-TiAI interphase boundaries. The detailed atomic structures of the two possible (111)-γ σ3a and (111)γ σ3b twin boundaries have been observed as well as the facetted (0001)α2//(11)γ interphase boundaries. The facetted boundaries contain transformation dislocations whose elastic displacement fields conform well to the fields computed from the Somigliana dislocation model.

Deformation twinning in a creep-deformed nanolaminate structure

The underlying mechanism of deformation twinning occurring in a TiAl-(γ)/Ti(3)Al-(α(2)) nanolaminate creep deformed at elevated temperatures has been studied. Since the multiplication and propagation of lattice dislocations in both γ and α(2) thin lamellae are very limited, the total flow of lattice dislocations becomes insufficient to accommodate the accumulated creep strains. Consequently, the movement of interfacial dislocations along the laminate interfaces, i.e., interface sliding, becomes an alternative deformation mode of the nanolaminate structure. Pile-ups of interfacial dislocations occur when interfacial ledges and impinged lattice dislocations act as obstacles to impede the movement of interfacial dislocations. Deformation twinning can accordingly take place to relieve a stress concentration resulting from the pile-up of interfacial dislocations. An interface-controlled twinning mechanism driven by the pile-up and dissociation of interfacial dislocations is accordingly proposed.

Effect of crystal and beam tilt on simulated high-resolution TEM images of interfaces

The effects of crystal and beam tilt on high-resolution transmission electron microscope (HRTEM) images of planar coherent interfaces were investigated by multislice image simulations. It was found that a beam tilt of 0.5 Bragg angle (theta B) was sufficient to introduce detrimental artifacts into most images of interfaces in crystals only 1/8 xi 000 thick, while crystal tilt had a much smaller effect even for crystals 1 xi 000 thick. Effects produced in HRTEM images of interfaces by crystal and beam tilt included the introduction of additional periodicities and loss of compositional detail across a boundary, translation of a boundary from its actual position, and apparent mismatch of atomic planes across a perfectly coherent interface. These results indicate that alignment of the electron beam parallel to the optic axis is critical for reliable HRTEM imaging of interfaces in materials. Techniques for obtaining accurate alignment are also discussed.