| Magnesium alloys have been widely used in automotive,weapon equipment,and instrumentation applications because of their superior high specific strength and high damping capacity.Due to low strength and poor plasticity of magnesium alloys at room temperature,some reinforcing phases are usually added to improve its mechanical properties.However,reinforced magnesium matrix composites of traditional ceramic particle have problems such as poor plasticity and toughness,poor interface between particles and matrix,and easily formed holes and many scholars have always been pursuing how to improve the strength of magnesium alloys while maintaining good plasticity.As the natural interface between metals is well bonded,researchers select high-entropy alloy particles as the reinforcing phase in the study to solve the contradiction between reinforcement and toughening brought about by traditional ceramic particles,prepare Al Cr Fe Co Nip/AZ91D composites by rotary blowing stirring assisted with ultrasonic treatment,heat treatment,hot extruded,mechanical stirring and squeeze casting,study the influence of particles on the microstructure evolution and mechanical properties of the composite at room temperature,reveal the strengthening mechanism of the composite,further conduct the dynamic compression tests of the as-cast,extruded,and squeeze casting Al Cr Fe Co Nip/AZ91D composites and analyze the dynamic mechanical behavior and micro deformation mechanism.（1）The microstructure and mechanical properties of as-cast,heat-treated,extruded,and squeeze casting Al Cr Fe Co Nip/AZ91D composites at room temperature were investigated.It was found that Al Cr Fe Co Ni particles significantly refine the matrix structure of the composites and effectively improve their mechanical properties of the composites at room temperature.In the composition design of composites,the strength of the four composites at room temperature shows different variation rules with increasing content of Al Cr Fe Co Ni particles.The strength of the extruded composite has clear advantages,with tensile and compressive strength as high as 319.5 MPa and480.9 MPa,respectively.The primary reasons for the increase in strength are that Al Cr Fe Co Ni particles have refined the matrix structure,impeded the movement of dislocation,pinned the grain boundaries,and have a good interface with the matrix.（2）The stress-strain relationship and microstructure under dynamic compression of as-cast,extruded,and squeeze casting Al Cr Fe Co Nip/AZ91D composites were investigated.As the strain rate increases,the peak stress of as-cast,extruded,and squeeze casting composites is found to be clearly correlated with the normal strain rate,and the particles reduce the temperature sensitivity of the peak stress between strain rates of 1000 s-1and 2800 s-1.At 25℃,the particles had a strong influence on the number of twinning in the microstructure;they increase the number of twinning in the microstructure of as-cast and squeeze casting composites,while decrease the average grain size of extruded composites and inhibit the formation of twinning in the microstructure.At 250℃,the particles had a strong influence on the the recrystallization behavior of the matrix structure,they facilitate the realization of the dynamic recrystallization in the microstructure of as-cast and squeeze casting composites,and reduce the grain size and the deformation band size of dynamic recrystallization in the microstructure of the extruded composites.（3）The interface structure and strengthening mechanism of Al Cr Fe Co Nip/AZ91D composite were investigated.It was found that the Al Cr Fe Co Ni particles partially dissolution,and element diffusion occurs in the interface,the Al Cr Fe Co Ni/α-Mg interface is a semi coherent interface with an atomic mismatch of 11.4%,which indicates well-bonded interfaces between them,and there is no intermetallic compound formed.The primary reasons for the increase in strength of as cast,extruded,and squeeze cast composites are that the Al Cr Fe Co Ni particles have refined the grain size and increased the number of dislocation by thermal mismatch.Thus,Hall-Petch strengthening mechanism and CTE strengthening mechanism are mainly strengthening mechanism of as-cast,extruded,and squeeze casting Al Cr Fe Co Nip/AZ91D composites.The increase in strength of the heat-treated composite is primarily the result of the strengthening role of theβ-Mg17Al12phase,as well as the particles of Hall-Petch strengthening mechanism,the Orowan strengthening mechanism,the CTE strengthening mechanism,and the Load strengthening mechanism.（4）The researchers study the dynamic mechanical behavior of as-cast,extruded,and squeeze casting Al Cr Fe Co Nip/AZ91D composites.The results show that the strain hardening rate increases in the strain hardening rate-strain curve of as-cast,extruded,and squeeze casting samples at 25℃,and particles increase the strain hardening rate more significantly as particles hinder dislocation slip and strengthen the strengthening effect of fine grains produced by grain cutting by twinning.The plateau phenomenon appears in the strain hardening rate-strain curve of as-cast and squeeze casting samples at 250℃.Particles make the plateau in the curve of strain hardening rate longer as particles make the thermal softening effect more obvious caused by dynamic re-crystallization and increase the dislocation density of the composite;the strain hardening rate-strain curve of the extruded samples shows an increase in strain hardening rate at 250℃,and particles increase the strain hardening rate more significantly due to the small grain size of the extruded sample,the large number of grain boundaries,and the particle resistance to dislocation slip.At 25℃,the deformation mechanism of as-cast and squeeze casting Al Cr Fe Co Nip/AZ91D composites was mainly base plane slip and twinning,supplemented by non-base plane slip,while the deformation mechanism of extruded Al Cr Fe Co Nip/AZ91D composites was mainly base plane slip and pyramidal<a+c>slip,supplemented by twinning.At 250℃,the deformation mechanism of as-cast and squeeze casting Al Cr Fe Co Nip/AZ91D composites was dominated by non-basal slip and coordinated with recrystallization,while the deformation mechanism of as extruded composites was mainly base plane slip and pyramidal<a+c>slip,supplemented by dynamic recrystallization,and the contribution of dynamic recrystallization increases with the increase of strain rate. |
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