Point Defect Mechanisms in Deformation and Phase Transformation of Titanium Aluminide Alloys
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Point Defect Mechanisms in Deformation and Phase Transformation of Titanium Aluminide Alloys Fritz Appel, Jonathan D.H. Paul and Ulrich Fröbel Institute for Materials Research, GKSS-Research Centre Geesthacht, Max-Planck-Str. 1 D-21502 Geesthacht, Germany ABSTRACT Point defects in intermetallic compounds are subjected to significant constraints due to the directional bonding, long-range order and off-stoichiometric deviation of the materials. This leads to a variety of defects with distinct differences in morphology, concentration and mobility. In the present study the implications of these defect characteristics on the mechanical properties of two-phase titanium aluminide alloys will be investigated. The major areas of the study are: (i) deformation induced point defects that contribute to work hardening; (ii) dislocation locking due to the formation of defect atmospheres; (iii) transport processes involved in phase transformation and recrystallization occurring during long-term creep. The applied methods include mechanical testing, static strain aging and structural characterisation by high resolution electron microscopy. INTRODUCTION There is undisputed evidence that point defect mechanisms play an important role for the processing and performance of high-temperature structural materials. This holds certainly also for TiAl alloys, where such processes may be involved, to varying degrees, in annealing, homogenisation, phase transformation, deformation, recovery, and recrystallization. In many intermetallics including γ(TiAl) vacancies are predominant over interstitials [1], because their formation energy is significantly lower. When compared at homologous temperature the diffusivity of Ti in stoichiometric γ(TiAl) agrees relatively well with that of disordered f.c.c metals [2]. Thus, diffusion phenomena in γ(TiAl) are often associated with thermal vacancies. Alloys of technical significance are lean in Al and contain the intermetallic phases γ(TiAl) and α2(Ti3Al). The directional bonding and ordered crystal structure of these compounds impose severe constraints on the nature and state of point defects. There is general agreement that in γ(TiAl) no structural vacancies are formed [1,3,4]; the dominant point defects in offstoichiometric alloys are substitutional antisite defects on the relevant sublattice. Antisite atoms along with the atoms of the same type on regular sites can form a percolating network, which permits atoms to undergo long-range diffusion without disordering [2]. Thus, atomic transport processes may not only depend on temperature and thermal history of the material, but can also be affected by off-stoichiometric deviations, phase constitution and the partitioning of alloying elements on the different sublattices of the ordered phases. The main objective of the present study is to link deformation phenomena with that what is known about point defects in titanium aluminide alloys. Particular attention is paid on work hardening, strain aging, and phase transformations associated with creep.
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