Plasticity and Phase-Transformation of Alkali Metals

Objective of the project was the investigation of fundamental mechanisms of plastic deformation and of martensitic phase transitions in alkali metals. Due to their simple electronic structure, these metals are of essential importance for fundamental metals physics, in particular concerning theoretical modelling and comparison between theory and experiment, though their softness and chemical reactivity does inhibit direct technical applications. Within the project, the slip geometry of potassium between 4K and 70K and the martensitic transformation of lithium were studies in detail. a) slip geometry investigations on potassium First, a quantitative theoretical model describing the complex macroscopic glide properties of b.c.c. metals was developed. It is mainly an extension of the well-established thermodynamic model of kink-pair formation, taking into account additionally kinematic and crystallographic features. The macroscopic "non-crystallographic" glide is described by a composition of "elementary" glide steps on well-defined lattice planes. Experimental investigations on high-purity potassium single crystals were performed by the method of x-ray diffraction analysis combined with in-situ deformation. Evaluation of the experimental results by the above theory clearly showed that the elementary glide step in potassium takes place on (123) planes. A change of the slip geometry at some transition temperature, which had been supposed from indirect experimental indications, was not observed. The project has clearified the slip geometry of the elemental metal potassium in all details. The experimental and theoretical methods developed within this investigation may be applied to the technologically important b.c.c. transition metals (Fe, Mo, W) in the future. b) martensitic phase transition of lithium Martensitic phase transitions are known to change the structure of materials over more than 8 length scales, from atomistic to macroscopic level. In the present project, particular attentention was paid to the interaction between atomistic and mesoscopic processes, which have remained rather unclear up to now. The main experimental methods were optical microscopy (mesoscopic level) and neutron scattering (atomistic level). In-situ optical microscopy was applied for the first time to alkali metals; complex and time-consuming chemical preparation techniques had to be developed since conventional metallographic polishing methods do not apply to the soft and reactive metals. A large number of new and surprising results concerning martensite microstructure, crystallography and kinetics of the phase transition and the nucleation process was obtained. In particular, the low-temperature equilibrium phase of lithium was identified to be a f.c.c. structure, which is observed in the experiment only as an intermediate phase on heating from low temperatures. The rhombrohedral 9R structure which appears on cooling was shown to be a metastable phase. It is formed as a result of geometrical compatibility conditions more favourable for 9R than for f.c.c. This result is in disagreement with ab-initio quantum mechanics calculations on lithium present in the literature, thus demonstrating that the calculation methods even in a metal of very simple electronic structure are still to be improved.