Kinetics and Equilibrium of Age-Induced Precipitation in Cu-4 At. Pct Ti Binary Alloy
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INTRODUCTION
AMONG various Cu-based alloys, age-hardenable Cu-Ti alloys are characterized by superior mechanical strength, stiffness, stress-relaxation, and electrical conductivity properties. In particular, Cu-Ti alloys containing 1 to 6 at. pct Ti exhibit effective precipitation-hardening behavior, while alloys with 4.0 to 4.2 at. pct Ti are widely used for commercial manufacturing because of their excellent mechanical properties and good workability. Therefore, numerous fundamental and applied studies of age-hardenable Cu-Ti alloys have been actively reported since the 1970s.[1–18] Alloys are typically manufactured through solid–solution treatment at around 1123 K (850 C) followed by aging at medium temperatures between 673 K (400 C) and 773 K (500 C). The alloy microstructure evolves during isothermal aging via the following subsequent processes: compositional modulation of the parent supersaturated Cu solid solution, continuous nucleation and growth of fine needle-shaped precipitates, and discontinuous precipitation of laminated cellular SATOSHI SEMBOSHI is with the Institute for Materials Research (Trans-Regional Corporation Center for Industrial Materials Research), Tohoku University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka 599-8531, Japan. Contact e-mail: [email protected] SHINTARO AMANO, AKIHIRO IWASE, and TAKAYUKI TAKASUGI are with the Department of Materials Science, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka 5998531, Japan JIE FU is with the Research Organization for the 21st Century, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka 599-8531, Japan. Manuscript submitted May 15, 2016. Article published online January 12, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
components at the grain boundaries.[1–6,12,19] The observed crystallographic features revealed that compositional modulation occurred due to the spinodal decomposition of the a phase of the Cu solid solution, which resulted in the formation of the Ti-lean and Ti-rich fcc disordered regions along the h100i direction.[1–6,12,19–22] The fine needle-shaped precipitates are produced due to the transformation of the Ti-rich fcc disordered regions into coherent metastable ordered b¢-Cu4Ti phases with the orientation relationships between the matrix and the precipitate species [310]a//[100]b¢, and [130]a// of (001)a//(001)b¢, [010]b¢.[1–6,10,12,19–24] In these studies, the b¢-Cu4Ti phase was characterized by a body-centered tetragonal structure (prototype: Ni4Mo, spacing group: I4/m) with lattice parameters of a = 0.586 nm and c = 0.365 nm. The cellular components were composed of the depleted Cu (a¢) solute and stable b-Cu4Ti laminates, which were formed due to the reaction a fi a¢ + b-Cu4Ti at the grain boundaries, with orientation relationships of (111)a//(010)b and [110]a//[001]b.[12,19,25–28] The b-Cu4Ti crystal structure was an orthorhombic one (prototype: Au4Zr; spacing group: Pnma) with lattice parameters of a = 0.453 nm, b = 0.434 nm, and c = 1.292 nm. On the other hand, it has been reported t
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