MICROSTRUCTURE EVOLUTION OF Al-Fe ALLOYS PREPARED BY MECHANICAL ALLOYING AND SPARK PLASMA SINTERING

Abstract

Al-Fe alloys have wide potential applications in automobile and aerospace industries due to their high specific strength,high specific stiffness,good stability of microstructure and excellent high temperature strength.However,a wide variety of metastable phases can be formed in Al-Fe binary system,such as Al(Fe) supersaturated solid solution,amorphous and intermetallic phase.In order to better understand the phase formation in Al-Fe alloys,a systematic investigation of microstructure evolution is necessary.In this work,bulk dense Al-5Fe alloys were fabricated by mechanical alloying(MA) and spark plasma sintering(SPS).The phases,microstructures and morphologies of MA powders and the corresponding sintered samples were characterized by XRD,SEM and EDS.Special attention was paid to the effects of different milling times on structural change of phases during MA-SPS process.The results showed that during the MA,the size of alloy powders increased with increasing milling time(0—10 h),and then decreased with further milling time(10—20 h).The(111)_(Al) peaks in XRD spectra of MA powders shifted to higher angles with the increase of milling time,indicating the dissolution of Fe atoms into the Al crystal lattice. Homogeneous Al(Fe) solid solutions were obtained after MA for 20 h.Bulk samples sintered from MA powders of 0 and 10 h contained Al/Al_(13)Fe_4/Al_5Fe_2/Fe layer structure intermetallic phase and tiny Al_(13)Fe_4 phase in the Al matrix.However,bulk sample sintered from MA powders of 20 h contained only relatively small Al_(13)Fe_4 phase in the Al matrix.Based on thermodynamic analysis(effective heat of formation theory) and kinetic analysis(spherical shell model),the primary phase that formed on the interfacial layer of Al/Fe was Al_(13)Fe_4,and then Al_5Fe_2 can be formed by the reaction of residual Fe and Al_(13)Fe_4 for the lower Gibbs free energy of Al_5Fe_2 compared to that of Al_(13)Fe_4,leading to the formation of Al/Al_(13)Fe_4/Al_5Fe_2/Fe layer structure intermetallic phase.The absence of Al_5Fe_2 and Fe phases in sample sintered from MA powders of 20 h were attributed to the complete reaction between relatively small Fe particles and Al melt during SPS process.