Research On Microstructure Evolution Of Directionally Solidifiied Al-25wt.%Co Peritectic Alloy

Nucleation characteristics of intermetallic compound phases were studied using differential scanning calorimetry (DSC), and microstructure evolution of the alloy during stabilization process before directional solidification and directional solidification process with the rate of 1μm/s¹000μm/s were further investigated in Al-25wt.%Co peritectic alloy in which both primary phase and peritectic phase are intermetallic compound phases. Both the nucleation undercooling degrees of primary phase which crystallizes from liquid and peritectic phase nucleating on the substrate of primary phase were characterized, microstructures in mushy zone before directional solidification and composition distributions in melts of interface front were analyzed, and faced growth characteristics, peritectic reaction mold and volume fraction changes of intermetallic compound primary and peritectic phases during directional solidification were further revealed by the means of optical microscopy (OM) and scanning electron microcopy (SEM). Therefore, nucleation and growth characteristics of intermetallic compound phases, microstructure evolution of the Al-25wt.%Co alloy during directional solidification and peritectic reaction and transformation mechanism were obtained.Both as-cast and directional solidification microstructures were composed of primary phase Al13Co4 and peritectic phase Al9Co2 and residual liquid. Both primary Al13Co4 and peritectic Al9Co2 phases present irregular polygons with typical faceted characteristic, and they process stoichiometric ratio exactly. It is shown that the nucleation undercooling rates of both primary phase which crystallizes from liquid and peritectic phase nucleating on the substrate of primary phase are large, ranging from 32K to 72K. Due to the obvious discrepancy of crystal structure between primary phase and peritectic phase, the role of primary phase as a substrate of peritectic phase is negligible, reflected in the primary phase which was enclosed partly by peritectic phase.A mushy zone which is divided into two sections is created during thermal stabilization process before directional solidification. The two sections are separately composed of Al₁₃Co₄+liquid and Al₉Co₂+liquid. With thermal stabilization time increase, the volume fraction of the liquid in the mushy zone decreases and thus the length of mushy zone decreases. And Co concentration in the melt of S/L interface front decreases, S/L interface becomes plane gradually.The primary phase and peritectic phase grow by stacking during directional solidification of alloys. Peritectic phase nucleate in grain surface of primary phase discontinuously and stackingly, there exist obvious liquid channels between the grains, the channel's existence guarantees the proceed of the peritectic reaction which required triple junction. When the growth rate is lower than 15μm/s, with the solidification distance increases, the volume fraction of primary phase decreases, and finally, peritectic phase replace the primary phase as a leading growth phase. There are three factors, which are the low initial composition of Co, the growth characteristics of intermetallic compound phases, and the growth of the peritectic phase via the peritectic reaction, leading to the change of the leading growth phase. When the pulling speed is greater than 20μm/s, the primary phase has always been a leader in the growth phase, peritectic reaction significantly reduced the consumption of the primary phase. With the increase of the growth rate, both grain dimensions of two phases decrease, meanwhile the volume fraction of the liquid increases. At the same time, a little amount of the peritectic grains directly solidify from the liquid. The microstructure evolution of the different thermal stabilization after pulling a certain distance shows that the peritectic transformation causing the dissolution of the primary phase, will occur only at high temperature with long thermal stabilization length. Thus, the peritectic transformation lead to very little dissolution of the primary phase in the directional solidification.