HREM observation and compositional study of microstructure and phase transformation in TiAl-based and Cu-Al-Ni alloys

Characterization of atomic configuration on an atomic scale is required for the development of advanced materials, as well as chemical compositions an d electronic structures. In this paper, by using FEGTEM, studies of phase t ransformations and synthetic characterization of precipitated phase were ca rried out on a nanometer/an atomic scale in TiAl intermetallics and Cu-AlNi shape memory alloy, respectively. In TiAl intermetallics, a deformation-in duced phase transformation at room temperature was observed on the several- ten-nanometer region in the Ti50Al48Cr2 alloy. It is a diffusionless phase transformation and a Ti3Al alpha(2) phase is formed as a metastable phase. On the other hand, a Ti2Al zeta phase was identified in another Al-rich TiA l with a composition of Ti48.3Al51.7 It was formed during the precipitation of Ti3Al alpha(2) plate in the TiAl gamma matrix. It was found that this p recipitation was achieved through a two-step transition and Ti2Al zeta is a n intermediate phase in the phase transformation. Characterization of chi p articles was carried out in the shape memory Cu-Al-Ni alloy containing smal l amount of Ti and Mn. Many small precipitates of 18-100 nm in size were fo rmed in the chi(L) phase. The composition of chi L phase was (NiAl)(2)CuTi, while the small precipitates were almost Cu-3(Al,Ni,Ti,Mn).