Role of interface boundaries in the deformation behavior of TiAl polysynthetically twinned crystal: In situ transmission
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Nack J. Kima) Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea (Received 7 November 2004; accepted 18 April 2005)
To understand the role of boundaries in the deformation behavior of TiAl, in situ straining experiments in transmission electron microscopy have been performed on thin foils of polysynthetically twinned (PST) crystal of Ti–49.3 at.% Al. The deformation behavior of PST TiAl is anisotropic, depending on the angle between the lamellar boundaries and the straining axes. For L-orientation, deformation twins and ordinary dislocations transmit across the true-twin (TT) boundaries but are reflected at the pseudo-twin (PT) and rotational order-fault (RO) boundaries. For transverse (T) orientation, deformation twins are transmitted across all TT, PT, and RO boundaries. For I-orientation, shear deformation occurs parallel to the lamellar boundaries. There is a transmission of deformation across the interphase (IP) boundary in longitudinal orientation, but deformation is blocked and reflected at the IP boundary in T-orientation. The role of the various types of boundaries in localized deformation behavior was evaluated by considering Schmid factors and geometric compatibility factors.
I. INTRODUCTION
In recent years, there has been a considerable amount of research on TiAl alloys, which has several interesting characteristics.1–4 They possess high specific strength and specific modulus, and high resistance to oxidation. However, TiAl alloys suffer from low ductility and fracture toughness at low temperatures (i.e., below 600 °C). Recently, it has been shown that the careful selection of alloying elements and the careful control of the microstructure can improve the ductility and fracture toughness of TiAl alloys.5–14 TiAl alloys can be classified into Ti-rich and Al-rich alloys. The microstructures of Ti-rich alloys consist of Ti3Al (␣2) and TiAl (␥). Emphasis is currently placed on Ti-rich TiAl alloys because (␣2 + ␥) two-phase alloys generally have better ductility and toughness than ␥-single-phase alloys.1,15 Several alloy systems have been developed based on the (␣2 + ␥) twophase constitution. These alloys can be classified into two systems based on microstructure. One is the duplex structure, consisting of single-phase ␥-grains and (␣2 + a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0235 1888
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 7, Jul 2005 Downloaded: 14 Mar 2015
␥) lamellar grains. The other structure is a fully lamellar morphology. Derivatives of fully lamellar structures and near-fully lamellar structures have also been developed. In general, duplex structures have a much finer grain size than fully lamellar structures and thus show better ductility. On the other hand, fully lamellar structures have better fracture toughness and higher creep resistance than duplex structures. Hence, fully lamellar TiAl alloys receive more attention than duplex TiAl alloys. Despite recent
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