Suppression of Discontinuous Precipitation in Cu-Ti Alloys by Aging in a Hydrogen Atmosphere
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AGE-HARDENED Cu-Ti alloys are widely used as conductive materials in small electrical devices such as lead frames and connectors because of their superior mechanical strength, favorable stress-relaxation properties, and good electrical conductivity. However, following the current trend of downsizing and streamlining in electrical product development, it is necessary to improve the mechanical and electrical properties of Cu-Ti alloys. Cu-Ti alloys containing 3 to 5 at. pct Ti are typically manufactured using conventional aging procedures: they are solid solution treated at a high temperature between 1123 K and 1173 K (850 C and 900 C) and then quenched in water, followed by isothermal aging at SATOSHI SEMBOSHI and NAOYA MASAHASHI are with the Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, Miyagi 980-8577, Japan and also with the Department of Materials Science, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka 599-8531, Japan. Contact e-mail: [email protected] YASUYUKI KANENO and TAKAYUKI TAKASUGI are with the Department of Materials Science, Osaka Prefecture University. Manuscript submitted October 2, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
a moderate temperature ranging from 723 K to 773 K (450 C to 500 C). In the early stage of aging, there is continuous nucleation and growth of fine needle-shaped metastable precipitates, often denoted as b¢-Cu4Ti (tetragonal structure, prototype: Ni4Mo, space group: I4/m, lattice parameter: a = 0.584 nm, c = 0.365 nm), within the supersaturated solid solution Cu matrix phase (face-centered cubic (FCC) structure, a= 0.362 nm).[1–7] During prolonged aging, coarse and two-phase cellular components (often referred to as ‘‘nodules’’) comprising a lamellae of solute-depleted terminal Cu and stable b-Cu4Ti (orthorhombic structure, Au4Zr, Pnma, a = 0.452 nm, b = 0.434 nm, and c = 1.292 nm) are heterogeneously formed at the grain boundaries.[4,7–12] Homogeneous dispersions of continuous precipitates (CPs) of fine metastable b¢-Cu4Ti in the matrix grains are favorable for age-induced precipitation hardening. On the other hand, in later aging stages, the hardening behavior is degraded by the development of cellular components containing coarse discontinuous precipitates (DPs) of stable b-Cu4Ti laminates, because of the consumption of the fine metastable b¢-Cu4Ti CPs that favor hardening.[4,7] In addition, the brittleness of the coarsened stable b-Cu4Ti DPs deteriorates the bending workability and fatigue properties.[9,13–15] Therefore, to
ensure appropriate mechanical properties or improve the mechanical properties of Cu-Ti alloys, suppressing the formation of stable b-Cu4Ti DPs is one of the most important guidelines for materials design. There are numerous efforts to suppress the formation of nodule DPs at grain boundaries for many systems such as Al, Cu, Mg, Ni, and Fe-based alloys.[16–20] Focusing on Cu-Ti alloys, it has been empirically shown that the formation of stable b-Cu4Ti DPs is closely related
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