High Resolution Electron Microscopy Investigation of Interfaces in Martensitic Phase Transformations

Using high-resolution transmission electron microscopy a systematic investigation of martensitic phase transformations was carried out. The mechanisms of the atomic re-arrangements at the interfaces and the compensation of the transformation strains were analysed in detail and correlated with the macroscopic properties of the transformation. Martensitic phase transformations are complex structural changes in the solid state. They are of considerable scientific and technological interest. Martensitic phase structures in steels lead to an ultrahigh hardness and good ductility. Shape memory materials showing remarkable thermo-mechanical properties are used e.g. in thermal switches and surgical instruments. The martensitic phase transformations are closely correlated to atomic re- arrangements occurring at the interfaces between the parent phase and the product lattices (the martensitic domains). The atomic movements are leading to transformation induced strains. The energy of the lattice strains, the interfaces and the martensitic interdomain boundaries as well as the grain boundaries of the parent phase determine the transformation. For the first time the interfaces in the interior of single crystals of cobalt-iron were imaged directly by high- resolution transmission electron microscopy and analysed by comparing them with computer simulations based on model calculations. The dynamic mechanisms of the transformations were investigated directly in the transmission electron microscope performing in-situ heating and cooling experiments. In addition, the nanocrystalline nickel- titanium shape memory alloy containing a very high density of grain boundaries was analysed at an atomic scale. In the case of the cobalt-iron alloy the analysis shows the occurrence of cooperative atomic movements at the interfaces leading to the required structural changes. A model of the interface was developed based on the terminology of linear lattice defects (dislocations). The possible arrangements of the interfacial dislocations can be correlated to the macroscopic features of the transformation in a unique way. Two different and competitive modes of the transformation were observed. At the atomic scale an alternating sequence of different transformation dislocations is leading to the most effective compensation of the transformation strains. Contrary to this, like dislocations are leading to high inner strains that can trigger the formation of new martensite domains. Since this occurs repeatedly the whole volume of the crystal transforms almost instantaneously.