In order to make clear the precipitation processes in CuBe alloy, the structural changes upon ageing of the 2wt% Be allo y were examined in detail through an electron microscopy in particular by the use of the selected-area electron diffrac-tion technique. The change in electron diffraction patterns with increasing ageing time or temperature was as follows. In the earliest stage, long streaks appeared in the <001> directions due to the formation of G. P. zones. Upon prolonged ageing, the streaks become shorter and intensity maxima appeared at about 2/3 of {002} reciprocal lattice vectors. This stage is diStinct from the G. P. zone formation and may ooƒÎespond to the formation of the ƒÁ" phase. The structure of ƒÁ" is considered to be a body-centered tetragonal, with Cu atoms at the corner and Be atoms at the body-center, and is different from that proposed by Geisler et al. Ageing at higher temperatures caused another superlattice reflection (CuAu(‡U) type, ƒÁ') to appear and 2/3 reflections mentioned above to split. This splitting corresponded to the formation of the ƒÁ phase. From these obervations, it has been concluded that the ageing sequence is ƒ¿ •¨G. P. zone•¨ƒÁ"•¨ ƒÁ'•¨ ƒÁ. The structure of the final precipitate r was identified to be the same as that of the equilibrium pllase with the lattice constant 2.71A. Then a new orientation relationship between the ƒÁ and ƒ¿ has been proposed, such that Among seveTal orientation relationships reported so far, only this one was consistent with our experimental results. In spite of th e identity in structure, the final precipitates ƒÁ were associated with strain contrast around themselves, in contrast to the equilibrium phase. This fact suggests that the transformation ƒÁ' toƒÁ might be martensitic in the nucleation stage. The mechanism of ƒÁ' to ƒÁ transformatien is discussed along this line and from the viewpoint of coherency.

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