In-Situ TEM Observations of Strain-Induced Interface Instability in TiAl/Ti3Al Laminate Composite

Hsiung, L L

Lawrence Livermore National Laboratory

关键词: Solutes;    Plasticity;    Electron Microscopes;    Ambient Temperature;    36 Materials Science;   

DOI  :  10.2172/15007359
RP-ID  :  UCRL-CR-152732
RP-ID  :  W-7405-ENG-48
RP-ID  :  15007359

美国|英语

来源: UNT Digital Library

【 摘 要 】

The stability of interfaces in lamellar TiAl (or TiAl/Ti{sub 3}Al laminate composite) by straining at ambient temperatures has been investigated using in-situ staining techniques performed in a transmission electron microscope in order to obtain direct evidence to support the previously proposed creep mechanisms in refined lamellar TiAl based upon the interface sliding in association with the cooperative motion of interfacial dislocations. It has been reported previously that the mobility of interfacial dislocations can play a crucial role in the creep deformation behavior of refined lamellar TiAl [1,2]. Since the operation of lattice dislocations within refined {alpha}{sub 2} and {gamma} lamellae is largely restricted, the motion of interfacial dislocations becomes the major strain carrier for plasticity. Results of ex-situ TEM investigation have 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 {gamma} lamellae is the predominant creep mechanism in HS regime. Stress concentration resulted from the pileup of interfacial dislocations is suggested to be the cause for the interface-activated deformation twinning. Accordingly, the creep resistance of refined lamellar TiAl is considered to depend greatly on the cooperative motion of interfacial dislocations, which in turn may solely be controlled and hindered by the interfacial segregation of solute atoms (such as W) or interfacial precipitation. Furthermore, through the in-situ TEM investigation, we also found that the lamellar interfaces could migrate directly through the cooperative motion of interfacial dislocations. That is, the {gamma}/{gamma}and {gamma}/{alpha}{sub 2} 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 refined lamellar TiAl is employed for engineering applications. Although it is anticipated that interface sliding and migration are prevalent at elevated temperatures, the present in-situ straining study reveals the instability of lamellar interfaces at ambient temperatures.