Aging characteristics of thermomechanically processed Cu-9Ni-6Sn alloy

Cu-Ni-Sn alloys have gathered a considerable amount of interests over the past 20 years, since the strength level of Cu-Ni-Sn alloys with a proper thermomechanical processing equals to those obtained by high-strength, precipitation-hardened Cu-Be alloys (1-3). The aging response of Cu-Ni-Sn alloys is known to be extremely sensitive to aging temperature, aging time and prior processing history (1-4). When Cu-Ni-Sn alloys are aged below 400 degrees C after solution heat treatment, for example, the supersaturated solid solution ct is reported to follow the three stages of decomposition. In the first stage, the modulated structure with Sn-rich and Sn-poor regions forms along the [100]alpha direction as a result of spinodal decomposition. In the second stage, the metastable gamma' precipitates with D0(22) type structure nucleate from the modulated structure, and the highest strength occurs in this stage. In the final stage of transformation, the gamma' phases decompose into the cellular structures, consisted of equilibrium a and gamma phases, and the strength ceases to increase. The effect of cold working prior to aging practice on tensile properties of Cu-Ni-Sn alloys is significant, but still controversial (1,2,5). In the present study, aging characteristics of heavily swaged Cu-9Ni-6Sn alloy, the composition of which is known to have the best combination of mechanical and physical properties, were examined. The hardening behaviors of Cu-9Ni-6Sn alloys, either solutionized and aged or directly aged, are discussed with SEM fractographic studies and TEM micrographic observations.