日本金属学会誌

J. Japan Inst. Metals, Vol. 34, No. 11 (1970),
pp. 1108-1115

Electron Microscopy Study of the Precipitation Processes in Cu-2 wt%Be Alloy

Ken'ichi Shimizu1, Yasuharu Mikami2, Hiroyasu Mitani3 and Kazuhiro Otsuka1

1Institute of Scientific and Industrial Research, Osaka University, Suita
2Graduate School, Osaka University, Suita. Present address: Sumitomo Electric Industries Ltd., Itami
3Faculty of Engineering, Osaka University, Suita

Abstract:

In order to make clear the precipitation processes in Cu-Be alloy, the structural changes upon ageing of the 2 wt%Be alloy were examined in detail through an electron microscope in particular with the use of the selected-area electron diffraction technique.
The change of electron diffraction patterns with increasing ageing time or temperature was as follows. In the earliest stage the long streaks appeared in the < 001 > directions due to the formation of G.P. zones. Upon prolonged ageing the streaks became shorter and intensity maxima appeared at about 2/3 of {002}α reciprocal lattice vectors. This stage is distinct from the G.P. zones and may be designated as the γ'' phase. The structure of γ'' is considered to be body-centered tetragonal, Cu atom being at the corner and Be atom at the body-center, and is different from that proposed by Geisler. Ageing at higher temperatures caused another superlattice reflection (CuAu (I) type, γ') to appear and 2/3 reflections mentioned above to split. This splitting corresponded to the formation of the γ phase. From these observations, it has been concluded that the ageing sequence in this alloy is α → G.P. zone → γ''→ γ'→ γ .
The structure of the final precipitate γ was identified to be the same as that of the equilibrium phase with the lattice constant 2.71 Å. Then the new orientation relationship between the γ and α has been proposed, such that

{103}γ||{1 13}α,   < 010 > γ|| < 110 > α.
Among several orientation relationships reported so far, only this one was consistent with our experimental results. In spite of the identity in structure, the final precipitates γ were associated with strain contrast around themselves, in contrast to the equilibrium phase. This fact suggests that the transformation γ' to γ might be martensitic in the nucleation stage. The mechanism of transformation γ' to γ   is discussed along this line and from the view point of coherency.


(Received 1970/6/12)

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