Study On Process Optimization And Microstructure-Properties Of High-Entropy Alloy/Aluminium Composites Fabricated By Deformation-Driven Metallurgy

Under the present condition of national energy-conservation and emission-reduction as well as constructing a powerful manufacturing country,developing aluminium（Al）matrix composites with high performances is the right direction.High entropy alloys（HEAs）have the great potentials as the ideal reinforcement of Al matrix,due to their high strength,hardness,outstanding anti-corrosion properties and good interfacial compatibility with metallic matrix.Fabrication of HEA/Al composites with high strength and toughness is based on grain refinement of Al matrix,dispersion of HEA reinforcement and the inhibition of the brittle intermetallic compounds formed at the HEA/Al interface.Combining the severe plastic deformation process of friction stir processing and powder metallurgy process,we proposed a new powder metallurgy technique utilizing high-energy ball milling（HEBM）and deformation-driven metallurgy（DDM）,of which Al_0.8Co Cr Fe Ni was utilized as the reinforcement and pure Al was utilized as matrix to fabricate HEA/Al composites.During DDM process,severe plastic deformation under relatively low temperature can realize the dispersion of reinforcement,grain refinement of matrix and also the inhibition of interfacial reaction,which all contribute to the high performances of HEA/Al composites.This thesis provides supports of tecniques and theory in developing metallic matrix composites with outstanding performances.The feasibility of DDM in directly fabricating dual-phase metallic matrix composites and the effect of the incorporated HEA phases on the microstructure and mechanical properties of HEA/Al composites were researched.DDM was directly applied to the fabrication of 3vol.%HEA/Al composites,during which grain refinement occurred and the incorporated HEAs distributed uniformly in HEA/Al composites.Yield strength of 188 MPa and the tensile strength of 260 MPa for the DDMed 3vol.%HEA/Al composites were obtained,which increased by 91.2%and94.0%than the fabricated pure Al,meanwhile its elongation still reached to 16.3%.Linear diffusion occurred at the HEA/Al interface and Cr in HEAs segregated into Al matrix with the formation of Al₇Cr phase.The strengthening mechanisms in3vol.%HEA/Al composites were fitted and calculated,which indicated that dislocation strengthening contributed to yield strength enhancement mostly.HEBM was utilized as the pre-treatment procedure of DDM in order to further realizing the refining and uniform dispersion of HEA reinforcements,and the effect of HEBM processing parameters on the particle morphology,microstructure of Al-HEA mixed powders and the strengthening effect of HEA/Al composites have been researched.The effect of particle crushing and refinement in Al-HEA mixed powders was enhanced with increasing the proportion of HEA particles.The effect of crushing,refinement and uniform distribution of HEAs in Al-10%HEA mixed powders was better after HEBM due to the weakened effect of cold welding.The HEA size was further reduced and the HEA/Al interface was weaken in the HEBM-DDMed 3vol.%HEA/Al composites.The proportion of recrystallization in 3vol.%HEA/Al composites was increased together with the enhanced yield strength and tensile strength,while the elongation of 3vol.%HEA/Al composites decreased.The yield strength and tensile strength of 3vol.%HEA/Al composites HEBMed for 25 hrs increased to 282 MPa and 341 MPa,which increased by 50%and 31.2%in strength than directly-DDMed 3vol.%HEA/Al samples.Meanwhile,the elongation of the HEBM-DDMed 3vol.%HEA/Al composites still remained around 12.4%.In order to regulate and control the strength-ductility of HEA/Al composites fabricated combining HEBM and DDM,the effect of HEAs’volume fraction and DDM processing parameters on the microstructures and mechanical properties of HEA/Al composites has been researched.With increasing HEAs’volume fraction,more dislocations and internal stresses were incorporated in HEA/Al composites,the grains were refined,the proportion of high angle grain boundaries（HAGBs）and substructures increased,which all contributed to the enhancement of yield strength and tensile strength.The yield strength and tensile strength for 5vol.%HEA/Al composite reached 336 MPa and 406 MPa,which increased by 282%and 247%than the fabricated pure Al.HEBM process enhanced the interfacial diffusion between HEA and Al matrix,for which the diffusion layers of core-shell structures with concentration gradient and Al₉（Co Cr Fe Ni）₂-type intermetallics formed at the interface.The dynamic recrystallization continued with increasing holding time during DDM,for which the grain size increased,together with the proportion of recrystallization and HAGBs.The yield strength of 284 MPa,tensile strength of 387MPa and the elongation of 9.2%for 3vol.%HEA/Al composite were obtained when the holding time during DDM was 30 s.More shearing deformation was incorporated and the dislocation density was enhanced in Al matrix with increasing rotating velocities during DDM,during which the proportion of recrystallization and HAGBs decreased while the substructure’s ratio increased.3vol.%HEA/Al composites fabricated under the rotating velocities of 400 rpm and 500 rpm possessed relatively better mechanical properties.Aiming at the comprehensive demand of strength-ductility and anti-corrosion properties of the fabricated HEA/Al composites,the responses of 10vol.%HEA/Al composites under corrosion tests have been researched and the underlying mechanisms of anti-corrosion with incorporating HEAs in Al matrix have been revealed.As the progress of corrosion,dense and double-deck corrosion layers that caused by the galvanic corrosion between HEA reinforcements and Al matrix were formed at the surface of 10vol.%HEA/Al composites,and the passive films formed by the element diffusion in HEA could prevent Cl^-from dissolving the corrosion layers.Because the corrosion products formed by the galvanic corrosion between HEA and Al were dense and capable of covering the corrosion surface completely and keeping stable during the progress of corrosion,the formed corrosion layers could reduce the corrosion rate of 10vol.%HEA/Al composites effectively.