Crystal Structure, Phase Stability and Plastic Deformation Behavior of Ti-rich Ni 3 (Ti,Nb) Single Crystals with Various
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Crystal Structure, Phase Stability and Plastic Deformation Behavior of Ti-rich Ni3(Ti,Nb) Single Crystals with Various Long-Period Ordered Structures. Koji Hagihara, Tetsunori Tanaka, Takayoshi Nakano and Yukichi Umakoshi Department of Materials Science and Engineering & Handai Frontier Research Center, Graduate School of Engineering, Osaka University, 2-1 , Yamada-oka, Suita, Osaka 565-0871 , Japan ABSTRACT In Ni-Ti-Nb ternary system, there are some geometrically close-packed (GCP) phases with long-period stacking sequences of a close-packed plane (CPP). Among them, our focus is on the Ni3(Ti0.90Nb0.10) crystals with Pb3Ba-type rhombohedral structure with nine-fold stacking sequence. Compression tests were conducted using the single crystals and the temperature and orientation dependences of plastic deformation behavior were investigated in comparison with those of D024-Ni3Ti crystals with the four-fold stacking sequence. The K-W locking of screw dislocation was found to occur not only in the compounds such as Ni3Al and Ni3Ti with a relatively small unit cell, but also even in complex compounds with longer-period stacking structures by slip on the common CPP in the GCP structures. INTRODUCTION The Ni3X-type intermetallic compounds (X= Al, Ti, Sn, Nb, V, etc.) are known to crystallize in various geometrically close-packed (GCP) crystal structures: L12 for Ni3Al, D024 for Ni3Ti, D019 for Ni3Sn, D0a for Ni3Nb and D022 for Ni3V, respectively. The crystal structures of those GCP phases are composed of the common close-packed plane (CPP), which corresponds to {111}in fcc and (0001) in hcp unit cell. Their crystal structures are therefore characterized by the two features focusing on the CPP; the stacking sequence of CPP and the ordered arrangements of X atoms on the CPP. Among the Ni3X-type compounds, much attention has been paid to the L12-Ni3Al as an important strengthening component in Ni-based superalloys. They show superior mechanical properties including the anomalous strengthening behavior due to the Kear-Wilsdorf (K-W) locking of screw dislocations [1]. Recently, we found that the yield stress anomaly (YSA) appears not only in L12-Ni3Al [1] but also in other GCP compounds with a relatively small unit cell, such as D0a-Ni3Nb [2], D024-Ni3Ti [3] and D019-Ni3Sn [4]. Therefore, they are also expected to act as a strengthening phase in Ni-based superalloys and multi-phase composite alloys [5-7]. Recently, we investigated the phase stability of ternary Ni3X-type compounds in a Ni-Ti-Nb system. We fabricated single crystals of Ni3(Ti, Nb) with various different Nb content by the floating zone (FZ) method, and investigated their microstructures. Figure 1 shows the selected area electron diffraction (SAED) patterns in the various Ni3(Ti1-xNbx) single crystals (x=0, 0.03, 0.05 and 0.10) taken from the [2 1 1 0] direction. All of the SAED patterns are obviously different each other and the periodicity of spots along the c-axis lengthens with the Nb addition. This indicates that their crystal structures changed from
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