Spontaneous transformation of metastable β-brass in thin foils

An electron microscope study has been made of metastable β-brass containing 61.0% copper, 39.0% zinc. It was observed that during electrolytic polishing of 0.005 in. thick foil the very thin regions of β at the edges of the foil transformed spontaneously to a face-centred cubic structure α". This transformation is similar to normal martensitic transformations but there is no specific habit plane. The β-phase had an (001) preferred orientation in the plane of the foil and transformed to α" by two different mechanisms to give two orientation relationships: (i) (001)β‖(001)α", [010]β‖[110]α" and (ii) (001)β‖(011)α", []β 5° from [1]α°. The product of the latter transformation was finely twinned. The transformation mechanisms for the two cases have been determined and are related to the surrounding restraints in the foil. It is concluded that the temperature for the start of a martensitic transformation is higher in thin foils than in bulk material.