Phase Transformations in β-Phase Shape Memory Alloys Studied by Hraem, Alchemi and X-ray Diffraction

Abstract

Recent research results on the microstructures and phase transitions in some β-phase shape memory alloys are reviewed, which have been obtained by the author’s research group by examining with high resolution analytical electron microscope (HRAEM), atom locations by channeling enhanced microanalysis (ALCHEMI) and X-ray diffraction. The HRAEM and ALCHEMI are first outlined, because the former is specially designed new one and enables us to obtain high resolution transmission electron microscope images, sharp electron diffraction patterns and non-contaminated energy dispersive X-ray spectra from very small areas less than 5 nm in diameter, and the latter is able to determine the atom location of a third element of ternary ordered alloys or compounds.

Then, bainitic transformations in Cu-29.1Zn-6.7A1 (at%) and Ag-45.0Cd (at%) shape memory alloys were examined by the HRAEM as to the microstructural, crystal structural and compositional changes. Ordering process and subsequent martensitic transformation in a Cu-Au-Zn shape memory alloy with the stoichiometric composition CuAuZn₂ have also been examined by the ALCHEMI and X-ray diffraction aiming at the atom configuration. The results obtained from the examinations are summarized, and brief discussion is made in relation to the mechanisms of phase transformations and shape memory characteristics.