In-Situ Tem Study of Interface Sliding and Migration in an Ultrafine Lamellar Structure
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IN-SITU TEM STUDY OF INTERFACE SLIDING AND MIGRATION IN AN ULTRAFINE LAMELLAR STRUCTURE Luke L.M. Hsiung1 and T.G. Nieh2 1 Lawrence Livermore National Laboratory, University of California L-352, P.O. Box 808 Livermore, CA 94551-9900, USA. 2 The University of Tennessee, Department of Materials Science and Engineering Knoxville, TN 37996-2200. Abstract The stability of interfaces in an ultrafine TiAl-(γ)/Ti3Al-(α2) lamellar structure by straining at room temperature has been investigated using in-situ straining techniques performed in a transmission electron microscope. The purpose of this study is to obtain experimental evidence to support the previously proposed creep mechanisms in ultrafine lamellar TiAl alloys based upon the interface sliding in association with a cooperative movement of interfacial dislocations. The results have revealed that both the sliding and migration of lamellar interfaces can take place simultaneously as a result of the cooperative motion of interfacial dislocations. Introduction It has been reported previously that the mobility of interfacial dislocations can play a crucial role in the creep deformation behavior of ultrafine TiAl-(γ)/Ti3Al-(α2) lamellar alloys [1-4]. Since the operation of lattice dislocations within refined γ and α2 lamellae is largely restricted, interfacial dislocations become the major • strain carriers for plasticity. As shown in Fig. 1 (a), a nearly linear creep behavior [i.e. ε (steady-state creep rate) = kσn, where σ is applied creep stress and n ≈ 1] was observed in low-stress (LS) regime, and power-law break down (n > 5) was observed in high-stress (HS) regime. Results of ex-situ TEM investigation as shown in Figs. 1 (b) and 1 (c) have indirectly revealed the occurrence of interface sliding in low-stress (LS) creep regime and deformation twinning in high-stress (HS) creep regime. These results have led us to propose that interface sliding associated with a viscous glide of pre-existing interfacial dislocations is the predominant creep mechanism in LS regime and interface-activated deformation twinning in γ lamellae is the predominant creep mechanism in HS regime [1, 2]. Stress concentration resulting from the movement and pileup of interfacial dislocations has been suggested to be the cause for the interface-activated deformation twinning. Accordingly, the creep resistance of ultrafine lamellar TiAl alloys is considered to depend greatly on the cooperative movement of interfacial dislocations, which in turn may be controlled and hindered by the interfacial segregation of solute atoms (such as W) or interfacial precipitation [3, 4]. Furthermore, through the in-situ TEM observation, we have also found that the lamellar interfaces can migrate directly through the cooperative motion of interfacial dislocations. That is, the γ/γ and γ/α2 interfaces can migrate through interface sliding and lead to the coalescence or shrinkage of constituent lamellae (i.e. microstructural instability), which results in a weakening effect when ultrafine lamellar TiAl is employ
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