The Cu–Sn binary system provided the first major alloy for man, known as bronze, and has been utilized for a long time. Yet, their phase separation behaviors are often complex, and there remain several unanswered questions. For example, in a systematic study of discontinuous precipitation in Cu-based alloys, Kim et al. pointed out that the reaction rate of Cu–Sn alloys was the slowest among six alloy systems examined (Cu–Ag, Mg, Sb, In, Cd and Sn), and they discontinued the study on Cu–Sn alloys after annealing them at 723K for 11 days. 1) Aaronson and Pande also pointed out the necessity for the investigations on the phase separation behaviors of Cubased systems, including Cu–Sn alloys, from the viewpoints of the formation of cellular morphology. 2) In this regard, the present paper illustrates hitherto unknown precipitation behaviors found in a Cu-rich Cu–Sn alloy. The phase diagram of the Cu–Sn system shows the solubility of Sn increases drastically from about 9mass% at 593 K to the maximum value of 15.8 mass% at 793 K. 3) In the past, this large increase in the solubility within a temperature span of 200K led several researchers to explore possible age-hardening caused by precipitation of Sn-rich phase (s) from quenched Cu–Sn alloys supersaturated with Sn. According to an early report by Haase and Pawlek, who measured hardness changes of 90% cold-rolled alloy upon annealing, the increase in hardness was, however, insignificant in the temperature range of 473–623 K, 4) despite the fact that their optical microscopy (OM) observation on the annealed specimen showed the existence of a number of intragranular precipitates. On the other hand, Bˆhm first reported the development of a cellular morphology around grain boundaries. 5) This type of phase separation phenomena, which are initiated at heterogeneous sites, typically grain boundaries, are generally expressed as discontinuous precipitation, and known to occur in a number of alloy systems. 6)

Later, a metallographic study of Tsubakino revealed that discontinuous precipitation and intragranular precipitation, also known as continuous precipitation, in fact compete each other in such a way that the development of the latter impede the progress of the former. 7) The sluggishness of the phase separation reaction in Cu–Sn alloys was noted by several other researchers using different techniques, such as OM and scanning electron microscopy (SEM), 1) Mˆssbauer spectroscopy, 8) OM and thermal analysis. 9) In addition, in a detailed calorimetric study by Varscheavsky, no thermal events was detected during annealing of a quenched Cu–Sn alloy. 10) Partly because of the slowness of the reaction, several researchers employed mechanical deformation, often cold-rolling, to accelerate phase separation processes. For example, kinetic evaluations of differential scanning calorimetry led researchers to speculate possible existence of a metastable phase. 11)