Deformation Twins in Ni 3 Nb Single Crystals with D0 a Structure
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Deformation Twins in Ni3Nb Single Crystals with D0a Structure Kouji Hagihara, Takayoshi Nakano and Yukichi Umakoshi Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-Oka, Suita, Osaka 565-0871, Japan ABSTRACT
Plastic deformation behavior and substructures in Ni3Nb single crystals were examined focusing on the operative deformation twin systems. Three twinning systems were operative depending on crystal orientation. The twinning behavior was discussed comparing with that in pure hcp metals. Microcracks initiated at the twin intersection. A large number of twin intersections were analyzed and classified into several types.
INTRODUCTION Ni3Nb with the D0a structure has been of considerable interest for a strengthening phase in unidirectionally solidified eutectic alloys of Ni(γ)/Ni3Al(γ’) -Ni3Nb(δ) (for example, see [1],[2]). Recently, our group reported the deformation mode and plastic behavior in Ni3Nb single crystals [3]. The D0a structure of Ni3Nb is crystallographically classified as an orthorhombic system because Nb atoms are arranged in rectangular fashion on the close-packed (010) plane. However, since the atomic arrangement and stacking sequence of the (010) plane are identical to those on the (0001) basal plane in the hcp lattice, the D0a structure can be also regarded as an ordered structure based on the hcp lattice. As pyramidal slip (c+a dislocation in the hcp notation) is not operative, deformation twins are essential to provide strains along the b-axis in Ni3Nb. On the other hand, deformation twins are known to induce fracture under some circumstances. The role of deformation twins on plastic behavior in pure hcp metals has been extensively investigated (for example, see [4]), but little information on intermetallics is available [5]. In this study, plastic deformation behavior and substructures in Ni3Nb single crystals were examined focusing on the operative deformation twin systems.
EXPERIMENTAL PROCEDURE Master ingots with a composition of Ni-25.0 at.% Nb were prepared by melting high purity Ni and Nb in a plasma arc furnace in high purity Ar gas. Single crystals were grown by the floating zone method using our NEC SC-35HD furnace at a growth rate of 2.5 mmh-1 under an Ar gas flow. Rectangular specimens for compression tests were prepared by the electro-discharge machining from the as-grown single crystals. Detailed preparation method should be referred to in our previous paper [3]. Six orientations of [302]*, [205]*, [111]*, [270]*, [011]* and [012]* were chosen for the loading axes, where asterisk (*) indicates the reciprocal space representation. Compression tests were performed on an Instron-type testing machine at a nominal strain rate of 1.7x10-4 s-1 at temperatures between room temperature (RT) and 1000Û& in a vacuum. Deformation markings were observed using an optical microscope with Nomarski interference contrast. Deformation substructures were observed in a transmission electron microscope (TEM, Hitachi H-800) ope
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