Defect structures and room-temperature mechanical properties of C15 laves phases in Zr-Nb-Cr and Zr-Hf-Cr alloy systems
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I. INTRODUCTION
THE AB2 Laves phase alloys that are known to be topologically close packed structures[1–4] are of the largest class of intermetallic compounds, and generally exist as a line compound with an ideal atomic size ratio, RA/RB of 1.225 (where RA and RB are diameters of A and B atoms, respectively). There are three types of Laves phases, i.e., cubic C15, hexagonal C14, and dihexagonal C36.[1–4] The AB2 Laves phase alloys as well as many other intermetallic alloys, however, are very brittle at low temperatures, although they show apparent compressive plastic deformation at temperatures beyond about two-thirds of melting temperatures.[5] Difficulty in the plastic deformation of the C15 AB2 Laves phase alloys is supposed to be due to the specific stacking sequence of the most closely packed atomic planes (i.e., the {111} planes) of XYZ type,[6,7,8] as shown in Figure 1. Here, each layer of XYZ type consists of four interpenetrating closely packed atomic planes. The atomic stacking consisting of XYZ layers is very dense and therefore the shear deformation along these atomic planes is assumed to be inherently very difficult. Their Peierls stress is therefore very high. Due to such a specific arrangement of atoms on the {111} planes, an alternative deformation mechanism, i.e., “synchro shear” mechanism, has been suggested to be required for plastic deformation of Laves phases.[6,7,8] Among two type of sandwiches consisting of A and c, shear deformation is expected to occur between the latter types of sandwiches because a shorter shear vector than the former type of sandwiches operates. This synchroshear suggests that one layer of the c sandwich shears along the direction of one Shockley partial dislocation, while the next layer simultaneously shears in the direction of another Shockley partial dislocation. The sum of the two Shockley partial
Y. NAKAGAWA and T. OHTA, Graduate Students, Y. KANENO, Research Associate, H. INOUE, Associate Professor, and T. TAKASUGI, Professor, are with the Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan. Contact e-mail: [email protected] Manuscript submitted January 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
dislocations results in necessary twinning shear, leading to plastic deformation of the Laves phase. In fact, mechanical twinning probably via synchroshear mechanism has been confirmed to occur in a two-phase C15 bcc microstructure,[9] in Nb-added HfV2 C15 alloys,[7,8] and in Ta-added HfV2 C15 alloys.[10,11,12] Considering the atomic size and substitution behavior of the ternary element, easier mechanical twinning via the synchroshear mechanism has been expected to occur in these alloys.[7–12] To improve the mechanical properties, particularly lowtemperature fracture toughness, the AB2 Laves alloys reinforced with soft bcc solid solution phase (composed of their constituent elements) have been studied for binary[13–16] and temary alloy systems.[16–22] Micros
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