Orientation and structure of planar facets on the massive phase γ m in a near-TiAl alloy

Abstract

The orientation and structure of planar facets on the ␥ m massive phase formed in a Ti-46.5 at. pct Al alloy have been characterized using a combination of transmission electron microscopy (TEM) and electron diffraction. The planar ␥ m /␣ 2 interfaces are irrational with respect to both the ␣ 2 matrix phase and the ␥ m phase, and there is neither evidence of a rational orientation relationship across such facets, nor resolution of a linear defect structure within the interface planes in conventional electron diffraction patterns and amplitude-contrast images, respectively. However, when imaged parallel to a particular direction in the interface, these irrationally oriented interfaces are invariably parallel to the Moiré plane, which is defined by the intersection between two sets of closely-packed planes in the ␥ m and ␣ 2 phases. The relationship is such that there is an effective continuity of these lattice planes across the interface and a one-dimensional coherency within the planar interface. This is interpreted to imply that such interface facets adopt a configuration of reduced energy and that they are not random and fully incoherent, as often described. It is suggested that these planar facets may migrate in a glissile manner normal to the interface plane by nucleation and rapid lateral movement within the interface plane of interfacial defects that have the form of Moiré ledges, which are defined by the spacing of the Moiré pattern that may be formed by overlap of the relevant crystal planes across the interface.

Microstructure Evolution of a Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y Alloy during Massive Transformation and Subsequent Annealing

“…The results are shown in Figure 4. Similar to that reported in other TiAl alloys [27,32], the massive γ phase is full of various defects, including numerous stacking faults, anti-phase boundaries, and dislocations, as displayed in Figure 4a. The density of defects in some regions of massive γ is so high that it is difficult to find a local area free of defects.…”

“…Although it is not clear whether the massive γ phase can nucleate at the α/β interfaces, the results shown in Figure 1c indicate a negative answer. It is agreed that the nuclei of the massive γ phase normally follow the orientation relationship, with an α grain on one side of the grain boundary that grows into the grain on the other side [19,27]. If a massive γ nuclei appeared at the α/β interface, the growth of γ phase into the neighboring β phase would encounter a high energy barrier due to the lattice structure and composition.…”

Precipitation and Hardening in Magnesium Alloys

“…The migration of this interface in its normal direction seems to involve the formation and later gliding of moire´ledges within the interface plane. [32,33] These observations suggest the existence of commensurate matching of 1 " 100 À Á a and 0 " 33 À Á b planes [29,33,34] in the major facet interface, and of 10 " is the near closest-packed plane in the magnesium lattice, suggests that the major and minor side facets of each b plate have relatively low interfacial energies.…”

“…This diagram demonstrates the coherent matching of the two sets of lattice planes within the moire´plane. [34] from the overlapping of the 1 " 100 À Á a and 0 " 33 À Á b planes (Figure 2(c)), and it contains some ledges whose unit height is defined by the interplanar spacing of the moiref ringes. The migration of this interface in its normal direction seems to involve the formation and later gliding of moire´ledges within the interface plane.…”

A comment on ‘Crystallography and migration mechanisms of planar interface boundaries’ Acta Materialia, 52, 795–807 (2004) by J. F. Nie

“…Edge-to-edge matching of lattice planes has been considered to be a crystallographic feature of interphase interfaces or habit planes in various phase transformation systems [1][2][3][4][5]. In a recent publication [4], Nie presented a set of analytical formulae to describe the geometry of planar interfaces in terms of plane edge matching, and claimed that many existing theoretical models were incapable of describing the interface, including the O-lattice model [6,7] which is well accepted as the most general geometrical theory for interfaces [8].…”