Research On The Refining Mechanism Of Al₄C₃ On Mg-Al Based Alloys And Its In-situ Synthesis

Mg-Al based alloys meet the lightweight and energy-saving requirements industrially.However,the applications are limited by the low strength and poor plastic deformation capacity.Therefore,the grain refinement and the preparation of grain refiner of Mg-Al alloys have been the researching hotspot at home and abroad.Except for refinement,particle reinforcement is also an important method to improve the mechanical properties.In this paper,a novel Al-3C master alloy was prepared.The microstructure,as well as the growth mechanism and morphological evolution of Al₄C₃ were studied.The refining effects of Al-3C on AZ91,AZ63 and AZ31 alloys and its influence on the mechanical properties of AZ91 were researched.To clear the carbon-inoculation mechanism,the a-Mg nuclei in refined Mg-9Al and AZ91 alloy were analyzed.Mn-riched phases in AZ91 alloy were analysed by means of phase extraction method to make clear the influence which Mn plays on the carbon refining mechanism.The in-situ nano-Al₄C₃ particles were synthesized in Mg-Al alloy by diffusion controlled reaction method.The main research results are as follows:（1）Preparation of Al-3C master alloy and its applications in Mg-Al alloyA novel Al-3C master alloy was prepared by liquid-solid reaction method,which overcomes the poor wettability of aluminum and graphite.The Al₄C₃ particles distribute in the A1 matrix homogeneously,mainly with a micrometer size between 1～4μm.The close-packed planes of Al₄C₃ are {0001},{1010} and {1011}.Since the{1011} plane have the highest growth rate,{0001} and {1010} faces are retained as the exposed surface of Al₄C₃ crystal.The growth of Al₄C₃ crystal undergoes the morphological evolution of spherical→polyhedron→hexagonal block.Al-3C exhibits excellent refinement effect on AZ91,AZ63 and AZ31 alloys and the refining behavior follows three characteristics:1)high-efficiency:when holding for 15min,the grain size reduces to～1/3 of the original size;2)fading-resistance:refining effect can maintain about 180min without fading;3)optimal effects are obtained when holding for 60min.With 0.5%Al-3C addition,mechanical properties of AZ91 were significantly improved:Brinell hardness was increased by 10.95%,ultimate tensile strength was increased by 8.59%,while elongation was also increased by 42.56%.（2）Characterization of nucleating substrate in carbon-inoculated Mg-AI（-Mn）alloysFIB method was used for sample preparation,and EBSD analysis was used to identify the phase of a-Mg nucleus.It indicates that the a-Mg nucleus is a cluster of Al₄C₃ particles rather than an individual Al₄C₃ particle.Al₄C₃ can maintain the original crystal structure,morphology and size at 720℃ for 24h in Mg-Al based alloy melt,which is the basic reason for the fading-resistance refining effect.For Mn-containing AZ91 alloy,the attachment behavior of Al₈Mn₅ to Al₄C₃ reduces the nucleating efficiency.Nevertheless,Al₄C₃ which are not attached by Al₈Mn₅ within a nucleating cluster can still have effective refining efffect on a-Mg.Thus,it is deduced that Al₄C₃ particle clusters are beneficial in overcoming the hindering effect of Mn.（3）In-situ nano-Al₄C₃ paticles synthesized in Mg-Al alloyThe in-situ nano-Al₄C₃ paticles were synthesized in Mg-Al alloy by diffusion controlled reaction method using the diffusion couple composed of Al-C prefab and pure Mg,homogeneously distributed on the substrate in terms of spiral ring or streamline,and the size were mostly in range of 50-200nm.The microhardness of Al₄C₃/Mg-Al is 360.9HV,215%higher than that of the matrix.There is no the change of microstructure and phase composition of Al-C prefab after treatment at 550℃ for 3h,while a large amount of nano Al₄C₃ have obtained in the diffusion couple composed of Al-C prefab and pure Mg.It is because the chemical potential gradient at both sides of the interface act as the driving force for the downhill diffusion of Al and Mg atoms,which promotes the interface welding,changes the alloy composition and promotes the appearance of liquid phase.Thus,the reaction path and kinetics between Al and C are changed and formation of Al₄C₃ is promoted.