Abstract
The stress-induced acicular martensites in a Cu-14.2Al-4.3Ni (wt%) alloy were investigated extensively by the method of electron microscopy and selected area electron diffraction. As a result two types of martensites were found; one has the 18R type long period stacking order structure (β1′), and the other the 2H type (γ′). The occurrence of the latter martensite was much less compared to that of the former. In the present paper the structure analysis and crystallographic data of the β1′ martensite are fully described, and the results are summarized as follows. (1) The crystal structure of the β1′ martensite was the 18R type with the stacking sequence of AB′CB′CA′CA′BA′BC′BC′AC′AB′, and its lattice parameters were a=4.382 Å, b=5.356 Å and c=38.00 Å. (2) The internal defects of the martensite was essentially stacking faults on the basal plane. But some striations were sometimes observed along the (\bar128)β1′ plane and they were suspected to be the traces of the internal twins. (3) The habit plane of the β1′ martensite was close to (\bar1\bar55)β1. (4) The orientation relationship between matrix and martensite was consistent with the Kajiwara-Nishiyama relationship found in the thermally formed β1′ Cu–Al martensite. (5) The martensite showed thermoelastic behaviour. (6) The acicular β1′ martensite was stable under stress roughly above the Ms temperature, but the spear-like γ′ martensite was more stable roughly below the Ms temperature. (7) The crystallography of similar martensites formed in thin films was discussed in relation to that in bulk specimens.
