Materials Science and Engineering A 402 (2005) 118–125 Microstructure and properties of a Cu–Ti–Co alloy I.S. Batra ∗ , A. Laik, G.B. Kale, G.K. Dey, U.D. Kulkarni MaterialsScienceDivision,BhabhaAtomicResearchCentre,Trombay,Mumbai400085,India Accepted 11 April 2005 Abstract It is now well established that dilute Cu–Ti alloys with titanium in the range of 2.5–5 wt.% decompose by a spinodal mechanism and, also, that the decomposition begins during the process of quenching itself from the temperature at which the alloy is solution treated. To study the initial stages of the decomposition reaction, the necessary suppression of the decomposition was attempted and achieved either by addition of a small quantity of a third element, namely, cobalt to the binary Cu–Ti alloy or by melt-spinning the binary alloy. Both the approaches were found to be successful. The results obtained through the second approach were recently published. The present work reports the results obtained through the former approach. In this work, the decomposition reaction in the ternary alloy was studied at temperatures of 673 and 723K. The important findings relate to the effect of cobalt on the precipitation process and the properties of the alloy. During aging, the metastable Cu 4 Ti (D1 a ) was seen to evolve in this ternary alloy without the initial 1 1/2 0 ordering in the titanium enriched regions and the formation of the special point N 3 M phase that were seen to precede the formation of the metastable Cu 4 Ti (D1 a ) in the melt spun alloy. Also, the ternary alloy was found, in the aged condition, to be extremely strong and ductile. Moreover, unlike the Cu–Ti binary alloy, the ternary alloy was found to be resistant to over-aging. © 2005 Elsevier B.V. All rights reserved. Keywords: Spinodal decomposition; Order/disorder transformations; Clustering and ordering; Transmission electron microscopy; Cu–Ti alloys; Phase trans- formations 1. Introduction Extensively studied [1–14], age hardenable dilute copper– titanium alloys, with yield strength in the hardened condition exceeding 700 MPa, find applications in fabricating high strength springs, electrical contacts, diaphragms, and in de- veloping corrosion and wear resistant materials. These alloys are considered good substitutes for the toxic binary Cu–Be and, the age resistant, ternary Cu–Be–Co alloys. The mech- anism of their decomposition has, however, been a matter of much debate for about three decades. Whereas Cu–1 wt.%Ti has been shown to decompose essentially by nucleation and growth, somewhat more concentrated alloys (with titanium in the range of 2.5–5 wt.%) exhibit spinodal decomposition [6]. It was also found that it is impossible to suppress de- composition during quenching in the concentrated side-band alloys [1] and, consequently, it is difficult to delineate the ∗ Corresponding author. Tel.: +91 22 2559 5066; fax: +91 22 2550 5151. E-mailaddress: isbatra@apsara.barc.ernet.in (I.S. Batra). sequence in which clustering and ordering occur to form the observed metastable Cu 4 Ti precipitates having Dl a structure [6,7]. To suppress the reaction during quenching, the authors adopted two different approaches. In the first approach, an attempt was made to suppress the onset of the spinodal reaction during quenching by a small addition of a third element to the binary Cu–4wt.%Ti alloy, namely, cobalt. In the second, the alloy was spun into a thin ribbon directly from the liquid state using rapid solidification processing (RSP). Fast cooling rates, in both the solid as well as the liquid state, that accompany melt-spinning, have been known to suppress, in certain systems [15], the early stages of decomposition, which cannot be prevented by a conventional water quench. Both the approaches were found to be successful. The second approach, which obviates the need to add a third element to binary Cu–Ti, formed the subject of the study completed and published recently [16]. The aging response of this melt-spun alloy was studied by transmission electron microscopy with interesting new results, and interpretations 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.04.015