Crystallographic And Microstructural Study Of As-cast And Heat-treated Sr-modified Eutectic Al-Si Alloys

Al-Si alloys have attracted considerable attention due to their importance to industrial applications.However,the characterization techniques of Al-Si alloys are mainly limited at nano and atomic scale,as the eutectic ?-Al phase and Si phase have similar crystallographic structures,which greatly restrict the further understanding of Al-Si alloy.In the present work,based on the spatially resolved electron back-scatter diffraction(EBSD)technique,the growth characteristics of eutectic Si crystals in both unmodified and Sr-modified eutectic Al-Si alloys have been studied in microscopic scale.The influence of Sr addition and post heat treatments on the microstructural and crystallographic features of the eutectic phases has been systematically studied.The innovative results are summarized as follows:(1)For the first time,the eutectic Si and ?-Al phases were differentiated under SEM/EBSD,with a home-made software ATOM,which represents a prerequisite for the correlated microstructural and crystallographic analyses of the eutectic microstructures at a large scale with statistical reliability.Moreover,this phase differentiation techinique can be applied to other alloys which have the similar problems.(2)For the non-modification case,microscopically,Si crystals are plate-like elongated in one<1 1 0>direction that is not in accordance with the<1 1 2>growth observed at atomic or nano scale assumed by the TPRE model.Based on the TPRE model,a three-dimensional model explaining Si-<1 1 0>preferred growth is proposed.The<1 1 0>extension is realized by paired<1 1 2>zigzag growth on parallel twinning planes,leading to alternative disappearance and creation of 141° re-entrants.This growth manner ensures Si crystals to expose only their low-energy {1 1 1} planes to the melt.(3)Based on the micro structural and crystallographic characterization,the long-term debate on the crystallographic orientation relationship(OR)of the two eutectic phases is clarified.And there is no representative OR between the two eutectic phases in both unmodified and Sr-modified alloys which were crucible slowly solidified,indicating that the nucleation of the two eutectic phases is attributed to the local chemical composition in the melt.Besides,the morphlogical formation mechanism of the plate-like eutectic Si crystals is revealed.(4)For the Sr-modification case,eutectic Si crystals still keep<1 1 0>preferred growth,though substantial changes appear in eutectic Si morphology,indicating the addition of Sr elements doesn't change the growth habit of eutectic Si crystals.Moreover,multi-orientation twin variants,as well as curved or stepped twin boundaries,are formed.The changes in eutectic Si morphology are demonstrated to be mainly associated with the growth process,as a result of the restricted TPRE growth and the IIT growth.(5)In-situ neutron diffraction technique was adopted to investigate the thermal expansion difference between eutectic ?-Al and Si phases in Al-2.7Si-0.04Sr alloy.On this basis,the fracture mechanism of eutectic Si crystal and the recrystallization mechanism of eutectic ?-Al matrix are proposed.The former is mainly due to the large thermal expansion incompatibility between the two phases during the heating stage of the heat treatment.The eutectic Si phase can not be thermally expanded together with the ?-Al phase,and the Si phase breaks under tensile stress.The refinement of the ?-Al occurs concomitantly with the fragmentation and spheroidization of Si and is mainly related to the fracture of the Si crystals due to their limited capacity to accommodate the giant thermal expansion of the ?-Al and the diffusion of Al atoms to the cracks during the heat treatment.The Si fracture generates "capillarity" force that activates the diffusion of Al atoms to the gap of the crack.Due to the substitutional feature of Al diffusion,the migration of vacancies toward the interior of the ?-Al is induced when Al moves to the gaps,thus the voids of the Si fracture are transferred into the ?-Al.In this way,the crystals of ?-Al are distorted and defected.The produced crystal defects,in turn,initiate recovery and even recrystallization of the ?-Al,resulting in grain refinement.(6)Directional solidified(DS)Al-12.7Si-0.04Sr alloys that have characteristic heat flow directions were prepared.And the global and micro-texture evolution of eutectic ?-Al and Si phases with heat treatment have been investigated by neutron diffraction and SEM/EBSD techniques,respectively.Then the relationship between the texture and heat transfer direction is confirmed.The present work has provided new insight into the microstructural and crystallographic nature of eutectic Al-Si alloys,enriched the chemical modification mechanism and extended the crystallographic characterization technique of Al-Si alloys.