High Strength and High Electrical Conductivity Cu-Ti Alloy Wires Fabricated by Aging and Severe Drawing
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I.
INTRODUCTION
CU-BASED alloy wires, with combined high strength and high electrical conductivity, have attracted extensive attention as they are essential for use in highly mechanically stressed electrical devices, such as conductive lead wires and wires in suspension springs. Cu-Be-based alloys are widely used for such electrical components owing to their excellent combination of strength and electrical conductivity. However, in the recent past, there has been an effort to substitute for these alloys since beryllium is potentially toxic and is also a rare element. Age-hardenable Cu-Ti alloys are considered as candidates owing to their mechanical and physical properties being comparable to those of Cu-Be alloys, although their electrical conductivity is inferior.[1–3] To this end, fundamental and practical efforts to improve not only the mechanical properties but also
SATOSHI SEMBOSHI is 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. NAOYA MASAHASHI is with the Institute for Materials Research, Tohoku University. Manuscript submitted February 20, 2018. Article published online July 10, 2018 4956—VOLUME 49A, OCTOBER 2018
the electrical conductivity of the Cu-Ti alloys have been made.[4–12] Age-hardenable Cu-Ti alloys, which contain approximately 3 to 6 at. pct Ti, are typically prepared using a solid solution treatment above 1123 K (850 C) and then aging at a moderate temperature of ~ 623 K (450 C). During aging, the microstructural evolution has been recognized to progress as follows[1,3,13–19]: (a) The compositional modulation of the parent-supersaturated Cu solid solution in the initial aging stage, (b) the continuous nucleation and growth of fine needle-shaped precipitates of metastable b¢-Cu4Ti (with a tetragonal structure, lattice parameter a = 0.5864 nm, c = 0.3649 nm) in the peak-aged stage, and (c) the discontinuous precipitation of cellular components laminating a terminal Cu solid solution and stable b-Cu4Ti (orthorhombic, a = 0.4523 nm, b = 0.4341 nm, c = 1.2918 nm) plates at the grain boundaries in the over-aging stage. Additionally, it has been identified that alloy strengthening is primarily controlled by the fine needle-shaped precipitates of metastable b¢-Cu4Ti,[1,3,18] and that the electrical conductivity gradually increases with aging time as the amount of solute Ti in the Cu matrix decreases owing to Ti-enriched b¢-Cu4Ti and b-Cu4Ti precipitation.[18–20] This means that, while the highest number density of dispersed fine b¢-Cu4Ti needles in the peak-aged stage may contribute to maximal strengthening, it keeps the electrical conductivity of the alloy low (as shown in later Figure 1). On the other hand, in the
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