Study On The Nucleation Mechanism And Regulation Of Microstructure And Properties Of Al-Si-Mg Alloys Based On Tcb Evolution Effect

Al-Si alloys are widely used in industry,and the sustainable development of lightweight puts forward higher requirements for their comprehensive properties.However,the grains of Al-Si alloys are difficult to be refined,which makes it difficult to solve the problems of uneven microstructure,casting defects and poor strength-plasticity caused by coarse α-Al dendrites.So the optimization of microstructure and the improvement potential of comprehensive properties of Al-Si alloys are restricted.Therefore,it is urgent to solve the problem of grain refinement of Al-Si alloys.In this paper,the dynamic evolution effect of B/C doped particles in Al-Ti-C-B(hereinafter referred to as Al-TCB)master alloy in Al-Si melt was taken as the key breakthrough to study the refining behavior and nucleation mechanism of Al-TCB on Al-Si alloy,and the nucleation activity of the new substrate was improved and maintained by using the structural evolution effect of TCB complex.The coarse grain problem of Al-Si alloy was solved,and excellent fine grain structure was obtained.Based on the effect of grain refinement on the strength-plasticity,the influence mechanism of Al-TCB on microstructure and properties of Al-Si-Mg alloys was studied systematically.The main results are as follows:(1)The nucleation mechanism of α-Al based on TCB complexAl-TCB master alloy contains two types of doped particles:C-doped TiB2(hereinafter referred to as Ti(B,C)2)and B-doped TiC(hereinafter referred to as Ti(C,B)).There is a coherent orientation relationship between(111)Ti(C,B)and(0001)Ti(B,C)2.Ti(C,B)can evolve into Ti(B,C)2 under certain conditions and provide a good substrate for its epitaxial growth.Therefore,these two doped particles in Al-TCB master alloy tend to grow together to form the TCB complex whose inner layer is Ti(C,B)and outer layer is Ti(B,C)2.TCB complex is distributed uniformly in the matrix of Al-TCB master alloy at submicro scale on the whole.The structure of TCB complex is very easy to undergo structural evolution in Al-Si melt,so that the outer Ti(B,C)2 particles are free in the melt,providing a nucleating substrate with more nucleation activity and quantity for α-Al grain.The C atom doped in metastable Ti(B,C)2 tends to diffuse towards the surface and evolve towards stable TiB2.This dynamic evolution process stimulates the activity of Ti(B,C)2/Al interface,enhances the chemical affinity of Ti(B,C)2/Al interface,and reduces the aggregation tendency of Si at the Ti(B,C)2/Al interface.Ti(B,C)2 becomes an effective nucleating substrate for α-Al grain,which has stronger nucleation ability.At the same time,the solute Ti released during the evolution of Ti(C,B)can be adsorbed by Ti(B,C)2,improving the formation capacity of TiAl32DC at the Ti(B,C)2/Al interface,and also can play a solute limiting effect on the growth of grains.Since the structural evolution of TCB in Al-Si melt is explosive at the beginning,while the evolution of Ti(B,C)2 to TiB2 is slow and everlasting,the evolution of TCB complex in melt is continuous and can continue to evolve to the nucleation stage.The heterogeneous nucleation of α-Al grain that based on TCB complex evolution effect,is referred to as evolution nucleation,which makes it efficient and non-decaying for the grain refinement of Al-7Si alloy.(2)Design of novel Al-Si-Mg alloy with high strength-ductilityAl-TCB master alloy refines the grain morphology of Al-7Si-0.4Mg alloy into nearspherical shape,which makes the eutectic Si distribute in a network configuration along the grain boundaries,alleviating the aggregation tendency of brittle eutectic phase and improving the strength and toughness of the alloy,especially the elongation.The refined Al-7Si-0.4Mg aluminum alloy by Al-TCB shows excellent microstructure heritability.The alloy still has good microstructure with refined grains even after remelting.On the basis of Al-7Si-0.4Mg alloy with good plasticity and toughness refined by Al-TCB,the strength of the alloy can be further improved by increasing the number density of nano aging strengthened phase β".Under the synergistic effect of refined α-Al grain and network distributed eutectic Si,the tensile strength,yield strength and elongation of Al-7Si-0.8Mg alloy can reach 386.4 MPa,352.9 MPa and 3.9%,respectively.(3)Microstructure optimization and property regulation of Al-Si-Mg alloy based on TCB evolutionAfter obtaining refined Al-7Si-0.4Mg alloy with good strength and ductility,the influence mechanism of the microstructure configuration on the properties of the alloy was further studied.Firstly,the Ti(C,B)of Al-TCB master alloy transformed into SiC by melt reaction,and the Al-Si-Mg alloy reinforced by SiC and Ti(B,C)2 particles with network configuration was obtained.Then the microstructure configuration was changed to streamline by hot extrusion.It is found that the both Al-Si-Mg alloys with network and streamlined configuration have refined equiaxed grain structure.A large number of nano Al precipitates and Si twins are formed in the eutectic Si,providing good toughness for the designed Al-SiMg alloy.When the reinforced phase of Al-Si-Mg alloy is in network configuration,the alloy can better withstand the stress,improving the deformation resistance of the matrix and resulting in a higher strength of the alloy.The reinforced phase with a streamlined configuration provides favorable conditions for the plastic deformation of the matrix.The cracks caused by tensile are perpendicular to the section direction,which is difficult to connect with each other,leading to better elongation of the alloy.In addition,the reinforced phase with a streamlined configuration provides favorable conditions for electron transport,so that Al-Si-Mg alloy has better electrical conductivity.The microstructure configuration also has a significant influence on the intergranular corrosion sensitivity of Al-Si-Mg alloy.The network configuration of the reinforced phase distributed along the grain boundaries enhances the intergranular corrosion expansion.However,the Al-Si-Mg alloy has the highest intergranular corrosion resistance when the streamlined configuration direction of the reinforced phase is perpendicular to the corrosion expansion direction.(4)Study on microstructure evolution behavior of Al-Si-Mg wrought aluminum alloy by TCBGrain recrystallization often occur in the process of hot deformation and subsequent heat treatment,which makes the grain size and morphology of aluminum alloy significantly different from that of as-cast alloy.It is found that Al-TCB master alloy has an efficient refining effect on the as-cast microstructure of Al-Si-Mg-Mn alloy.However,only when the content of Ti(B,C)2 particles reaches 0.5 wt.%,the grain growth during static recrystallization of the alloy can be effectively inhibited and title refined microstructure can be stabilized.The 0.5 wt.%Ti(B,C)2 particle can not only refine the grain size of Al-Si-Mg-Mn alloy,but also refine the solidified precipitated phases AlFeMnSi,Mg2Si and dispersed phase AlMnSi.At the same time,the streamlined configuration of the precipitated phase in the T6 heat-treated alloy can be changed to a network configuration,which is beneficial to improve the alloy strength.Al-TCB master alloy can also improve the thermoplastic machining properties of homogenized Al-Si-Mg-Mn alloy by promoting dynamic recrystallization.Furthermore,by affecting the geometric necessary dislocation density and changing the grain size and orientation of as-extruded and T6 treated Al-Si-Mg-Mn alloy,the Al-TCB can effectively improve the strength and ductility at room temperature as well as the strength at high temperature of Al-Si-Mg-Mn alloy.