L12 to do19 structural ordering during the fcc to hcp transformation in a CoCrTa alloy

Abstract

In solid state phase transformations, the evolution of one state into another is realized through the motion of atoms. Such atomic motion may be either diffusional or displacive in nature. Phase transformation are often considered to involve only one type of atomic motion, but there has been mounting experimental evidence and implications above the interplay or the combination of both kinds. In principle, combined diffusional and displacive atomic motion is possible when a phase transformation involves a rearrangement of the crystal lattice as well as the reconfiguration of atoms such as decomposition or atomic ordering. Recently, the authors have conducted an experimental study of a phase transformation of this class (decomposition of a hyper-eutectoid CuBe alloy) and have substantiated diffusional and displacive atomic motion in conjunction with the formation of polytwin precipitate plates. In the present work, they examine the possibility of the transformation between the L1[sub 2] and the DO[sub 19] phase states (ordered derivatives of fcc and of hcp, respectively) through the combination of displacive and diffusional atomic motion. The particular situation they are considering is when the stability of the L1[sub 2] precipitate phase is perturbed by the shear deformation that is responsible for the transformation ofmore » the matrix fcc to hcp. In what follows, they will first introduced experimental findings from recent TEM study of a CoCrTa ternary alloy. They then discuss the implications of these finding by examining the transformation crystallography of the L1[sub 2] ordered structure.« less