The Orientation Relationships and Crystal Habits in the Martensite Transformation of a Cu-Al Alloy

Abstract
The orientation relationships in a Cu-11.9% Al alloy have been studied with thin foils containing β' martensite (orthorhombic lattice) and β1 phase (ordered B. C. C.) produced by heating in an electron microscope. The crystal habits and other related phenomena have also been observed. On heating β' martensite plate, β1 crystals nucleate in the inner region of the martensite at about 400°C and they grow very straight and all parallel. The growth is not jerky but continuous. Stacking faults in the β' martensite are not annealed out even at such high temperatures. It is found that the orientation relationships are (110)β1 //(128̄)β', [11̄1]β1 //[21̄0]β'. The boundary plane between two variants of martensite is (128̄)β', which is parallel to (110)β1 in the parent phase.