Abstract
Stress-induced acicular martensites in Cu-14.2Al-4.3Ni (wt%) alloy were investigated by means of transmission electron microscopy and selected area electron diffraction method. As a result acicular γ′ martensites were found beside numerous β1′ martensites reported previously, although the occurrence was quite rare. This martensite is different from the spear-like martensite thermally formed in the same alloy in several respects such as in shape, lattice invariant shear, habit plane and orientation relationship, even though both have the same crystal structure and lattice parameters. The lattice invariant strain of the martensite was found to be stacking faults on the basal plane, and a defectless region was observed at the middle of the martensite, which is in sharp contrast to the midribs in ferrous twinned martensites. The habit plane was roughly located between (32\bar1)β1 and (32\bar2)β1. The orientation relationship was found to be (110)[1\bar11]β1⁄⁄(121)[0\bar12]γ′.
