Comment on “A new type of twinning in TiAl-2wt% Mn intermetallic compound”

Abstract

A recent letter [1] by Hanamura and Tanino suggested that a new type of twin formation occurs in TiA1 with 2 wt % manganese. While the authors have presented interesting evidence to support their suggestion, we believe that several points in their letter are confusing and require clarification. In Fig. 1 a and b of their paper, the authors suggest that the alternating bright and dark bands represent twins. However, observations of alloys with similar 50 : 50 compositions by other investigators have suggested that the alternating bands represent a twophase TiA1/Ti3A1 lamellar microstructure [2, 3]. This point will have considerable bearing on the results of their letter because, in the event of a lamellar microstructure, twin-related [4], rather than twinned regions, may play an important role in the interpretation of the results. In their letter, Hanamura and Tanino present an analysis of the dislocation reactions observed in Fig. 3 using the relationships between the Burgers vectors and the g vectors as shown in Table I. However, the values in the table and in the figure do not correspond to each other. In addition, the use of the g vectors given in Fig. 3 is not sufficient to determine the Burgers vectors of the dislocations. Clearly, additional information concerning the dislocations will be needed to ascertain the nature of the interactions which were observed. The analysis of the twin relationships given by the authors in their Figs 5 and 6 suggests that the preexisting twin boundary was { 1 1 1 } and that the crossing twin boundary was {1 1 0}. However, the authors have not identified the dislocations involved in the interaction on the twin planes. Inspection of their micrographs suggests that these may be screw dislocations. If so, the dislocations with Burgers vectors of 1/217 1 0] and [1 0 1], which are given in Fig. 4, cannot co-exist in the (l 1 0) planes. And, if these dislocations are not of screw character, at least one of these two dislocations cannot glide on the (1 1 0) planes. Without clear information concerning a dislocation which exists in both planes, it must be assumed that the two twin boundaries may have crossed but did not interact. Because the {110} planes are not favoured slip planes in the f c c structure, the observations of the authors could represent a special case where the addition of' manganese could promote slip on this plane through a reduction in the energy requirements. However, it is also possible that the atomic-level defect production by the 1 MeV electron radiation in the electron microscope used for the observations could have contributed to the activation of the {1 1 0} dislocation slip, either positively or negatively, without respect to the presence of manganese. In the absence of more complete details concerning the exposure received by the specimen in the microscope, the relative extent of the contributions of these two possibilities cannot be ascertained.