JOURNAL OF MATERIALS SCIENCE LETTERS 18 (1 9 9 9 ) 1697 – 1699 A study of precipitation in a Cu-15 wt %Ni-8 wt %Sn alloy V ´ ICTOR M. L ´ OPEZ-HIRATA, ´ ANGEL DE JES ´ US ARIAS-P ´ EREZ, MARIBEL L. SAUCEDO-MU ˜ NOZ Instituto Polit ´ ecnico Nacional, Apartado Postal 75-556, M ´ exico, D.F. 07300 The precipitation sequence in Cu-Ni-Sn alloys has been studied in several works [1–3]. The results of these works have shown that the phase separation occurs spinodally in the early stage of the aging process. The decomposed phases are Sn-rich (α 1 ) and Sn-lean (α 2 ) phases, which have been reported difficult to be ana- lyzed by transmission electron microscope (TEM) [1]. Furthermore, it has been reported that the spinodal decomposition in a Cu-15 wt %Ni-8 wt %Sn alloy occurred only at aging temperatures below 723 K [3]; however, this result was concluded from samples aged for times as long as 10 min. This aging time seems to be too long to study the spinodal decomposition at the early stages, which may take place even during the quenching of solution treated samples. The purpose of this work is to study the early stages of phase separation in a Cu-15 wt %Ni-8 wt %Sn alloy aged at 673, 773 and 873 K for different periods of time, using X-ray diffrac- tion analysis, as well as a field ion microscope, FIM, which has been shown to be an effective tool to study the phase separation at an early stage in different alloy systems. A Cu-15 wt %Ni-8 wt %Sn alloy was prepared by melting in an electric furnace in an alumina crucible under an argon gas atmosphere, using pure copper, tin and nickel. The cylindrical ingot of 13 mm diam- eter was homogenized at 1073 K for 24 h in a vacuum- tubular furnace and then hot- and cold-rolled, to ob- tain bar samples of 10 mm diameter, and finally wire drawn to produce a wire of 0.3 mm diameter. Both bar and wire samples were solution treated at 1093 K for 1 h and subsequently water quenched. The aging treat- ments of samples were carried out at 673, 773 and 873 K for times between 2 and 720 min in a tubular furnace. Solution treated and aged bar samples were analyzed by X-ray diffraction in a diffractometer with Cu K α radiation using a graphite monochromator. FIM sam- ples were prepared by electropolishing of 0.3 mm heat treated wire samples at 6 V (d.c.) with an electrolyte of 670 ml H 3 PO 4 , 100 ml H 2 SO 4 in 300 ml H 2 O. FIM observation was performed at 20 K using a voltage of 8–10kV. The image-forming gas was neon. The X-ray diffractograms (XRD) of the Cu- 15 wt %Ni-8 wt %Sn alloy aged at 673 K for 0 (solution treated), 1500 (25 min), 2700 (45 min), 3600 (60 min), 28800 (480 min) and 432000 s (720 min) are shown in Fig. 1. The XRD of the solution treated sample shows only the existence of a supersaturated solid solution α with an fcc structure. Sidebands were present (shown by arrows in Fig. 1) on the (2 0 0) X-ray diffraction peak in the samples aged at 673 K for aging times from 30 to 28800 s. In addition, the following precipitation sequence: α sss → α 1 + α 2 → α + γ ′ → α + γ → α + δ can be deduced from the XRD of samples aged at 673 K, Fig. 1. This precipitation sequence agrees fairly well with that reported by Miki and Ogino [2]. It is impor- tant to say that the spinodal decomposition process has been reported to occur at aging temperatures lower than 723K in Cu-Ni-Sn alloys [3]; however, the sidebands on (2 0 0) diffraction peaks were detected in samples aged at 773 and 873 K for up to 300 s. This suggests that the spinodal decomposition might occur even at aging temperatures higher than 723 K. This fact seems to agree with the equilibrium Cu-Ni-Sn phase diagram [6], which shows that the alloy composition of this work is inside of the miscibility gap at 973 K. Fig. 2 shows the variation of the wavelength of composition modula- tion, obtained from the sideband analysis, as a function of the aging time for the aging temperatures of 673, 773 and 873 K. It can be observed that the growth kinetics of wavelength is very slow in samples aged at 673 K for aging times shorter than 1500 s. This kind of slow kinetics has been also observed in other alloy systems [4]. Furthermore, it has been pointed out that this type of growth kinetics with a low time exponent in the early stage shows a good agreement with the value of 1/6 pre- dicted by Binder and Stauffer [4] based on a coarsening process by the cluster coagulation mechanism. In the case of samples aged at 673 K for aging times longer than 1500 s and at 773 and 873 K, the time Figure 1 X-ray diffractograms of the Cu-15 wt %Ni-8 wt %Sn alloy aged at 673 K for 0, 1500, 2700, 3600, 28800 and 43200 s. 0261–8028 C  1999 Kluwer Academic Publishers 1697