Study On Mechanisms Of Nanoparticle-induced Grain Refinement Of Pure Aluminum And Aluminum-silicon Alloy

The control over the microstructure of aluminum alloys and their composites is a pivotal way to improve the mechanical properties of materials.In aluminum casting,a refined microstructure is crucial in yielding improved mechanical properties,reduced porosity and hot tearing,and dispersive distribution of secondary phases.Grain refinement by inoculation involves addition of particles which can act as substrates for heterogeneous nucleation.Inoculation is particularly widely practiced in the aluminum industry due to its easy operation and low cost.Although many efforts have been concentrated on grain refinement in the past few decades,there remain substantial questions and challenges.Firstly,as for Al-Si alloys inoculated by Al-Ti-B alloys,when the silicon concentration exceeds 3 wt.%,a significant increase in grain size is observed,termed silicon poisoning,which is mainly due to the formation of Ti₅Si₃ that has a poor crystallographic matching with the aluminum matrix.To address it,Al-B,Nb-B and low Ti,B-rich alloys have been developed.However,the lack of titanium in solution acting as a strong grain restrictor can adversely affect the refining effect.Secondly,studies show that the grain refining efficiency is markedly low,with only less than 1%of the nucleating particles becoming active for nucleation of?-Al grains.Also,with increasing the particles,the grain size varies slightly at high addition levels,typically for additions higher than 0.3 wt.%.,indicating the saturation of grain refining efficiency.Thirdly,according to the empirical relationship between the growth restriction factor Q and the grain size d:???,where Q=C₀m?k-1? is the nominal alloy composition,m is the liquidus slope,k is the equilibrium partition coefficient,a is related to the number of active nucleating particles,and b is relevant to the efficiency of the nucleating particles,even though the number of active nuclei approaches to infinity,i.e.a?28?0,which is almost impossible in industrial practice,the barrier set by Q for grain refinement is dominant,given the very limited value of Q.Therefore,overcoming the inherent deficiencies of inoculation,breaking bottlenecks of grain refinement,exploring a robust grain refinement method available for all types of aluminum alloys and their composites,studying the interrelations among the refining treatment process,the microstructure of materials and their mechanical properties,developing high-performance aluminum alloys and their composites are of great significance for the development of aluminum industry.This work is focusing on as-cast pure aluminum and Al-Si alloys.Aided by the ultrasonic treatment,TiCN nanoparticles?NPs?were incorporated into aluminum matrix to achieve the control over the microstructure of aluminum alloys and thus to develop nanoparticles reinforced aluminum matrix composites with high performance.For pure aluminum system,the effect of NPs on microstructure,the mechanism of NP-induced grain refinement and that of strengthening and toughening were researched.For Al-Si system,the effects of NPs on primary and eutectic microstructure,the key factors determining the NP distribution and the mechanism of strengthening and toughening were studied.For Sr-modified Al-Si system,the effects of NPs on primary and eutectic grain sizes,the mechanisms of NP-induced primary and eutectic grain refinement and that of strengthening and toughening were investigated.The microstructural evolution of pure Al and Al-Si alloy was investigated systematically.It is demonstrated that TiCN nanoparticles can induce the refinement of?-Al spherical grains in pure Al and equiaxed dendrites in Al-Si alloy.Even for a high NP addition level of 2.0 vol.%,the?-Al grain size decreases phenomenally.Also,the addition of Sr modifier has no detrimental effect on the grain refinement induced by NPs.The mechanism of NP-induced grain refinement of composites was discussed.It is found that NPs prefer to assemble onto the interface of?-Al grain and then form a uniform and densely-packed NP layer as a contact inhibitor that is effective in impeding the transport of solute atoms onto the solid-liquid interface of?-Al.As a result,the growth of?-Al grains is restricted physically.Based on“the irreversible adsorption model”,two analytical models,i.e.NPs-inhibited spherical grain growth model and NPs-inhibited dendritic grain growth model,were first established to quantitatively describe the growth kinetics of spherical and dendritic crystals at various cooling rates,respectively.For?-Al spherical grains,???;for?-Al equiaxed dendrites at low cooling rates,???;for those at high cooling rates,???.Two numerical models were established to simulate the grain growth restriction induced by the NP-layers coating on the?-Al grains.The theoretical grain sizes match the experimental results well.Model predictions indicate that,with increasing NP additions,a more densely-packed nanolayer can be formed more rapidly on the surface of?-Al spherical/dendritic grain,thereby restricting the spherical/dendritic grain growth more efficiently.Unlike the growth restriction by solute,i.e.Q,which is limited by the chemical composition,phase diagram and the reaction between solutes of the specific alloy system,the NP-induced growth control can break through the inherent limitations of inoculation and apply to any alloy system,which paves a new way for grain refinement.For unmodified Al-Si alloys,NPs can also induce the refinement of eutectic Si and its morphological transformation from a coarse plate-like to a short rod-like structure,whereas for Sr-modified Al-Si alloys,they can promote the nucleation of eutectic grain and the refinement of its cellular substructure,i.e.eutectic cell.Mechanisms of eutectic microstructural evolution induced by NPs in Al-Si composites were discussed.For unmodified Al-Si alloys,nanoparticles can form NP layer on the surface of eutectic Si,impeding the transport of solute atoms,restricting the growth of eutectic Si,and thus refining the size of eutectic Si.Additionally,the NP layer formed on the eutectic Si surface manages to deactivate the growth advantage of the TPRE,and force the growth transition from the preferred growth along<112>Si direction to the isotropic growth.For Sr-modified Al-Si alloys,there is a well-defined orientation relationship between TiCN and Si,with a small misfit of 3.2%,i.e.{200}_N//{111}_S,<001>_N//<112>_S.It means that NPs can act as nuclei to promote the nucleation of eutectic grains and to increase the nucleation density of eutectic grains,which in turn leads to a considerable reduction in the eutectic grain size.The effect of cooling rates on the microstructural evolution of Al and Al-Si matrix composites was investigated.It is demonstrated that the NP distribution type in the matrix varies with the cooling rate.As the cooling rate is increased,the distribution type of NPs in Al matrix composite varies from the intergranular distribution at low cooling rates,i.e..T?20K/s,to the intergranular-intragranular mixture distribution at intermediate cooling rates,i.e.20K/s?T.?40K/s,and eventually to intragranular distribution at high cooling rates,i.e.40K/s?T.?111K/s.For Al-Si matrix composite,the NP distribution in primary microstructure varies from the intergranular distribution at low cooling rates,to the intergranular-intragranular mixture distribution at intermediate cooling rates,and eventually to intragranular distribution at high cooling rates,while that in eutectic microstructure varies from the interphase distribution at low cooling rates,to the interphase-intraphase mixture distribution at intermediate cooling rates,and eventually to intraphase distribution at high cooling rates.The interaction mechanism of nanoparticles with a solid-liquid interface was proposed to quantitatively determine the NP distribution during the solidification of composites.The modeling results suggest that the distribution types of NPs in Al matrix are chiefly determined by the two key factors,i.e.supersaturation and nanoparticle size whereas the distribution types of NPs in Si matrix depend principally on the two key factors,i.e.undercooling and nanoparticle size.It is revealed that NPs can ameliorate the microstructure of composites and thus improve their mechanical properties.Results demonstrate that there is a clear trend of increasing hardness,strength and ductility of composites with increasing NP addition levels.When the addition level reaches 2 vol.%,the hardness,ultimate tensile strength and elongation of TiCN_p/Al,TiCN_p/Al-11Si and Sr-modified TiCN_p/Al-10Si composites are increased to 68.5 HV,96.3 MPa and 43.7%,118 HV,222 MPa and2.5%,132 HV,331 MPa and 6.5%,by 300%,80%and 14%;45%,22%and 108%,39%,29%and 103%,respectively,as compared to the matrix alloys.The strengthening and toughening mechanism of Al and Al-Si matrix composites induced by NPs was discussed and it is found that for Al matrix composite,NPs can dramatically refine?-Al grain size,resulting in fine-grain strengthening.Meanwhile,the NP-induced Orowan and CTE strengthening can lead to the increment in strength.For Al-Si matrix composites,NPs can phenomenally refine the primary grain size and ameliorate the eutectic microstructure,improving the mechanical properties of composites.For Sr-modified Al-Si matrix composites,the primary and eutectic grain refinement induced by NPs can give rise to significant enhancement in strength and ductility of composites.