INTERPHASE BOUNDARIES IN THE α→γ_M MASSIVE TRANSFORMATION IN TI-AL ALLOYS

摘要

The massive-parent phase orientation relations and interphase boundary structures associated with the a→γ_M massive transformation in a Ti-46.5 at.% Al alloy were studied using a combination of electron back scatter diffraction (EBSD) and electron diffraction, and conventional and high resolution transmission electron microscopy (HRTEM). The results reveal that the γ_M nucleates at grain boundaries generally with a low-index Burgers orientation relation and coherent interface with one parent grain, but grows into the adjacent grain with a high-index/irrational orientation relation. The growth interfaces between the two phases are generally free of misfit dislocations or other defects and consist of curved parts as well as planar facets whose macroscopic habit planes are of generally high-index/irrational orientation and deviate substantially from the close-packed planes. On an atomic scale the growth interfaces are sometimes found to be faceted along {111} planes, as well as along other planes, with closely spaced steps, but are concluded to be incoherent with respect to the parent grain into which growth occurs. The nature of the interphase boundaries and their relation to the nuclea-tion and growth mechanisms associated with the α to γ_M massive transformation is discussed.