Microstructure And Phase Selection In The Unidirectional Solidification Of Zn-Cu Peritectic Alloy

Peritectic solidification is a phase transition process of much importance since it is involved in many structural materials and functional materials. In recent years,remarkable diversities were observed in the microstructures of peritectic solidification,which can not be explained by the previous theories for peritectic solidification.Zn-Cu alloy with four different compositions as Zn-2.0wt%Cu,Zn-2.7wt%Cu,Zn-3.0wt%Cu and Zn-4.0wt%Cu were adopted in unidirectional solidification experiments with a temperature gradient of 45K/mm and growth rate range from 2um/s to 6400um/s. The microstructures were analyzed in order to investigate the contributions of composition and solidification velocity to the microstructure formation and phase composition for peritectic alloys,microstructure and phase selection table was constructed based on the experimental results. Numerical model was used to calculate the interface response function of the two phases involved in the solidification process,and the phase selection map were constructed based on the criterion of the highest interface growth temperature under the assumption of abundant nucleation condition. Afterwards the nucleation in the constitutional undercooled zone before the interface of leading phase is considered in order to make amendments to the constructed map. It is discovered that the amended phase and microstructure map shows good accordance with the experimental results. Transformation from cellular to two phase lamellar structure is discovered in Zn-Cu alloys with increasing of the solidification velocity. A typical two phase lamellar structure is observed in the solidification of Zn-2.0wt.%Cu under a temperature gradient of 45K/mm and growth rate higher than 3200um/s,which shows a remarkably different morphology with that of low velocity cellular. A transition from normal peritectic phase diagram to meta-stable eutectic phase diagram occurs in the meta-stable phase diagram obtained by CALPHAD method when the interface nonequilibrium effects and capillary effects under rapid solidification conditions were taken into consideration. The formation mechanism of two phase lamellar structure was explained by the meta-stable eutectic phase diagram,and the lamellar spacing was analyzed based on related theories in eutectic solidification,it is discovered that the calculation model for eutectic lamellar spacing gives gooddescription of the spacing of observed two phase lamellar structure.