Phase Selection And Microstructure Evolution Of Hypoeutectic Cu-Cr Alloys In The Initial Transeint Zone Under Unidirectional Solidification

The performance of material depends on microstructure. And the final microstructures of solidification have a kind of close historical relationship with the initial transient zone, which has great important affection on the integrity and changing of subsequent solidifying microstructures. In the application background of the widely used Cu-Cr in-situ composite material, based on the study of solidification of Cu-Cr in-situ composite, it is researched on the initial solidification in the course of preparation. On the one hand, the evolution in the initial transient, growth of eutectic, morphologys of interface, characteristics of structure, features of formation of each phase, the formation of each phase through phase diagram and other's characteristics of solidification were discussed in this paper. On the other hand, microstructures in steady state and its nucleation are also investigated in this paper in order to compare them with that in unsteady state;in addition;the competitive growth of the primary phase and eutectic phase is also studied.On the existing basis of achievements on the technology of unidirectional solidification for Cu-Cr hypoeutectic, It is investigated that the features of evolution in different ranges of rates in this paper, and found that there are three steps during the representative processing of microstructure appeared in initial transient zone: the first step: α phase-->the second step: entire eutectic structure ( α+β ) —the third step: "Primary α phase and eutectic". Besides, there would be different in different range of rates. With the increase of drawing velocities, the processing is changing from two transient steps with single α phase precipitating firstly and the following whole eutectic, to only one transient step with only single α phase. Analyzing by theory, we can find out that: On the one hand, the microstructure of eutectic alloys in the processing of solidification would be failed to keep steady as the growth of primary phase, and it is detruded the critical velocity (V₁) of eutectic alloys which deviates from the eutectic point. On the other hand, eutectic is created in the structure as soon as the primary phase is also easy to be unsteady. Primary phase is much easier to beunsteady when C, = CE in the range of lower rates .When drawing velocity is getting much higher, it is failed to be unsteady at C,