Planar defects in massively transformed Ti-Al alloys

Abstract

The microstructure of massively transformed Ti-Al alloys has been studied by conventional transmission electron microscopy and high-resolution electron microscopy (HREM). A high density of planar defects, namely complex antiphase domain boundaries (CAPDBs) and thermal microtwins (TMTs) have been observed. Consistent with previous diffraction contrast studies, HREM confirmed that two antiphase-related gamma-matrix domains are generally separated by a thin layer of a 90o domain, for which the c axis is rotated through 90o over a common cube axis with respect to those of the gamma-matrix domains. Two crystallographically distinct types of 90o domains have been observed in association with the CAPDBs. Furthermore, interactions between the CAPDBs and TMTs have been investigated. In regions where TMTs intersect the generally morphologically wavy noncrystallographic CAPDBs, the latter have been observed to facet parallel to the {111}-twinning planes of the TMTs. This local interaction between CAPDBs and TMTs is associated with the transfer of true twinning shears of the type 1/6<112] from the matrix to the thin 90o domain generating a pseudotwinned region where the corresponding shears are of the type 1/6<121]. Diffusion associated with the local rearrangement of the atomic stacking is expected for the accommodation of the relaxation of the CAPDBs by facetting on the low-energy {111} planes. Possible mechanisms for the genesis of these complex planar defects are discussed.