Interfacial Structure and Processes in Lamellar Titanium Aluminide Alloys

University Of Connecticut, Storrs CT

Investigators

Abstract

0072721 Aindow This research develops a fundamental understanding of the effects of interfacial structure on the evolution of the lamellar microstructure in TiAl-based alloys and on the subsequent mechanical behavior of alloys that exhibit this microstructure. The lamellar gamma/alpha2 interfaces have been studied in great detail both by conventional transmission electron microscopy (TEM) and by high-resolution electron microscopy (HREM). In a recent HREM study of a low-misfit lamellar alloy, this investigator has shown that the steps on these interfaces can exhibit a greater diversity of heights and Burgers vectors than has been reported previously. Moreover, he found that the Burgers vectors exhibited by the steps varied with both the height and sense of the step. In each case these Burgers vectors were consistent with the steps being perfect interfacial disconnections as described by Pond's topological theory of interfacial defects, not Shockley partial dislocations as proposed previously. These observations have wide-ranging consequences for any defect-mediated interfacial processes in such alloys since one would expect the behavior of perfect disconnections to be radically different from that of Shockley partials in the same interface. In this research, defect-mediated interfacial processes in lamellar TiAl-based alloys are investigated by measuring changes in the interfacial defect configurations as a function of thermomechanical history. The character of the defects are determined by using the topological theory of interfacial defects to interpret detailed TEM and/or HREM observations, and the associated flux analysis are used to identify the factors controlling line defect motion. This approach addresses four key topics: (1) The mechanism for the formation of gamma lamellae; (2) The interaction of interfacial step defects; (3) Slip transmission through gamma/alpha2 interfaces; and (4) Introduction of misfit dislocations at gamma/alpha2 interfaces. %%% It is anticipated that by elucidating the mechanisms for the defect-mediated interfacial processes in TiAl intermetallic alloys the program will have broad technological impact for high temperature structural applications, particularly in the aerospace sector. This understanding is essential in determining alloy development strategy for the next generation of lamellar TiAl-based alloys. This program is the first instance where the topological theory is used to interpret detailed TEM/HREM data from engineering alloys. Thus, it is anticipated that the work will have broader scientific impact in that it will constitute a model study to demonstrate the efficacy of the approach. This program provides integrated training for two graduate students and combines the fundamental rigor of crystallographic analyses with transferable experimental skills. It also provides the students with the opportunity to participate in an international collaboration with scientists in England. ***