High-strength Bulk Nanocrystalline Al-Fe Compositesprepared By Mechanical Alloying And Spark Plasma Sintering

Al–Fe composites have wide potential applications in automobile and aerospaceindustries due to their high specific strength, high specific stiffness, good stability ofmicrostructure and excellent high temperature strength.High-strength bulk nanocrystallineAl–Fe composites were fabricated by mechanical alloying (MA) and spark plasmasintering (SPS). During MA,in order to study the microstructural evolution and solidsolubility extension during the alloying process, the effects of different Fe contents andMA times on the microstructures and morphologies of MA powders were investigated, anda thermodynamic analysis in Al–Fe system was calculated. During SPS,in-situAl13Fe4/Alcomposites and Al/Al13Fe4/Al5Fe2/Fe layer-structure composites were synthesized byadjusting the parameters of MA process, and the detailed microstructure evolution of thelayer-structure was discussed by thermodynamic and kinetic analysis.Starting powders withthe nominal composition Al100-xFex, forx=2.5,3.5,5and10(allcompositions are given in at%unless specified), were prepared byMA.Homogeneous Al(Fe) solid solutions were obtained during the MA process and thesolid solubility increased with the increase of Fe content and milling time. The diffusion ofFe atoms into the Al increased with the increase of milling time. Thermodynamicanalysisshowed that there is a thermodynamic driving force in Al–Fe binary system toform solid solution at all alloy compositions, and solid solution is the most stable phasecompared with intermetallic phase and amorphous phase in thecomposition ranges of~10%Fe.Bulk dense in-situAl13Fe4/Al composites were fabricated by sintering the alloypowders after MA for80h. The sintered samples are composed ofnano and ultrafineneedle-like Al13Fe4phase and angular-shaped Al13Fe4phase with a size range of1~2μmuniformly distributed in the Al matrix. Needle-like Al13Fe4phase originates from theprecipitation of supersaturated Al(Fe) solid solutions, and angular-shaped Al13Fe4phaseoriginates from the reaction of undissolved Fe particles and the Al melt. The hardness ofthe composites varied from1.44to1.97GPa, and the yield strength varied from657to1130MPa, depending on the Fe content. The Al–10%Fe exhibited elastic behavior only, failing without any macroscopic yielding or plastic strain. The lack of plastic performanceof Al–10%Fe is attributed to large amount of undissolved Fe particles reacted with the Almelt to form angular-shaped Al13Fe4, leading to the absence of soft α-Al phase. High yieldstrength (1018MPa) with good plastic strain (14.5%) was achieved in Al–5Fe, which iscurrently the most excellent mechanical property in Al–Fe binary system.Al/Al13Fe4/Al5Fe2/Fe layer-structure composites were synthesized by changing themilling time from0to20h. Bulk samples sintered from0h and10h MA powders werecomposed of Al/Al13Fe4/Al5Fe2/Fe layer structure intermetallic phase and tiny Al13Fe4phase in the Al matrix. However, bulk sample sintered from20h MA powders wascomposed of only relatively small Al13Fe4phase in the Al matrix. Based onthermodynamic and kinetic analysis, the primary phase that formed on the interfacial layerof Al/Fe was Al13Fe4, and then Al5Fe2can be formed by the reaction of Fe and Al13Fe4forthe lower Gibbs free energy of Al5Fe2compared to that of Al13Fe4, leadingto the formationof Al/Al13Fe4/Al5Fe2/Fe layer structure intermetallic phase.