Morphology and Crystallography of Thermoelastic γ' Cu-Al-Ni Martensite

Thermoelastic γ' Cu–14.2A1–4.3Ni (w/o) martensite has been studied using the techniques of electron microscopy and electron diffraction, in order to explain the morphology on a microscopic scale. A plane of ridge which is frequently observed in a typical "spear-like" martensite has been identified as (121)_γ' twinning plane. Further it has been shown that each half divided by the plane of ridge is internally twinned on {121}_γ', other than the (121)_γ' plane and each internal twin is faulted on each basal plane. A new orientation relationship consistent with the morphology is proposed, that is (110)[1]_β1 //(121)[20]_γ', according to which the two halves divided by the plane of ridge can be regarded as variants of martensite as well as twins to each other. This feature can be satisfied only by the new orientation relationship. Twinning nature of "spear-like" martensite is also discussed.