PRECIPITATION SEQUENCE FOR SIMULTANEOUS CONTINUOUS AND DISCONTINUOUS MODES IN CU-BE BINARY-ALLOYS

The precipitation sequence of the discontinuous and continuous modes i n Cu-0-9Be and Cu-1.4Be binary alloys (compositions in wt-%) has been studied primarily via metallographic observations and hardness measure ments. A single C curve and triple C curves separated by two temperatu res T1 and T2 (T1 > T2) are obtained for the precipitation of disconti nuous and continuous modes, respectively. The triple C curves correspo nd to the formation of gamma, gamma', and Guinier-Preston (GP) zones. The precipitation sequence of these two precipitation modes and the ce ll growth behaviour are different for the following three temperature ranges defined by the triple C curves: (i) above T1, discontinuous pre cipitation occurs before the continuous precipitation of gamma phase, and the cell growth rate G decreases abruptly; (ii) between T1 and T2, the precipitation sequence is similar to that above T1, but G decreas es and interlamellar spacing increases progressively with aging time; (iii) below T2, GP zones, fine gamma'', and/or fine gamma' form before cell growth but the associated variation in G is not observed. The ef fect of continuous precipitation on the cell growth behaviour is discu ssed.