Effects And Mechanisms Of Micron/nano-sized Ceramic Particles On The Solidification Microstructure And Mechanical Properties Of Hypoeutectic Al-si Alloys

Controlling the solidification microstructure and precipitation kinetics by introducing ceramic particles into the molten melts,the mechanical properties of the aluminum(Al)alloys could be significantly enhanced.The sizes,types and adding methods of ceramic particles are the key factors that affect the performances of Al alloys.In recent years,on the basis of traditional single particles,researchers have proposed a concept of hybrid particles(bimodal-sized particles and dual-phased particles).Investigations have shown that the hybrid particles,as Al alloy regulators,exhibited superior grain-refining and strengthening-toughening efficiencies to traditional single particles.Additionally,the hypoeutectic Al-Si series alloys are the most commonly used foundry Al alloys.However,the traditional inoculated particles or reinforcing particles are prone to serious"poisoning"problems.In this context,the design and exploitation of new particles have become key factors to further tailor the overall performance spaces of the hypoeutectic Al-Si series alloys.In this paper,dual-phased TiCN-TiB₂ particles with micron and nano-scales and bimodal-sized(micron-and nano-sized)TiB₂ particles were successfully prepared by the combustion synthesis method in the Al-Ti-B₄C-BN system and Al-Ti-B system.The effects of various particle types and sizes on the solidification behavior,microstructure and mechanical properties of commercial pure Al were compared and analyzed using remelting and dilution-assisted ultrasonic vibration technology.The effects of in-situ micron-and bimodal-sized TiB₂particles on the grain refinement and strengthening and toughening of hypoeutectic Al-7Si-4Cu alloy were comparatively investigated.The effects of dual-phased TiCN-TiB₂ particles on the microstructure and room-temperature mechanical behaviors of T6-treated Al-10Si-0.7Mg alloy and as-cast Al-10Si-2.0Mg alloy were investigated.The effects of TiCN-TiB₂particles and TiB₂ particles on the solidification behavior,microstructure and mechanical properties of Al-Si alloy and the mechanism were therefore revealed.The main innovations of this paper are as follows:1)The in situ TiCN-TiB₂ particles were successfully prepared by the combustion synthesis method in the Al-Ti-B₄C-BN system for the first time.The effects and the corresponding mechanisms of various particle types,sizes and solidification conditions on the microstructure and mechanical properties of commercial pure Al were comparatively investigated.i)It was revealed that the micron/nano-sized particles significantly reduced the nucleation undercooling of?-Al,and the recalescence platform characteristics were slowed down.The micron/nano-sized particles served as the heterogeneous nucleation sites for?-Al during the solidification,and the nucleation rates were therefore enhanced.The dual-phased TiCN-TiB₂ particles significantly boosted the grain morphology transition from columnar to equiaxed crystals under steeper temperature gradients,which exhibited higher nucleation potency than the micron-sized TiCN particles.ii)It was revealed that with increasing addition levels of the micron/nano-sized particles, the?-Al grains were continuously and significantly refined.The grain-refining efficiency of micron/nano-sized particles was found:the in-situ particles were stronger than the ex-situ particles;the dual-phased particles were stronger than their single counterparts,and the nanosized particles were stronger than their micron-sized counterparts.The grain-refining mechanisms of micron/nano-particles were found:the micron-sized particles exhibited higher nucleation potency than those nano-sized particles,but nanosized particles could not only serve as the heterogeneous nucleation sites of?-Al but also subsequently hinder their growth;the dual-phased TiCN-TiB₂particles exhibited both micron/nano-sized advantages and therefore possessed higher grain-refining efficiency.iii)It was revealed that with increasing addition levels of the nano-sized SiC and in situ TiC_n-Al₃Ti_m particles,the room-and high-temperature strength of commercial pure Al were continuously and significantly improved,but the ductility continuously decreased;while the room-temperature strength and ductility of commercial pure Al could be simultaneously and significantly improved by incorporating micron/nano-sized TiCN particles or in situ TiCN-TiB₂ particles.It was found that the room-and high-temperature tensile properties of commercial pure Al were significantly improved with high addition levels of micron/nano-sized particles,and the in-situ TiC_n-Al₃Ti_m particles were stronger than the nano-sized SiC particles in terms of efficiency;small addition levels of micron/nano-sized particles could significantly enhance the room-temperature strength and ductility of commercial pure Al,and the in-situ dual-phased TiCN-TiB₂ particles were superior to the ex-situ nanosized TiCN particles,while the ex-situ nanosized TiCN particles were superior to their micron-sized counterparts.iv)The strengthening mechanisms of commercial pure Al by micron/nano-sized particles were revealed:the room-temperature strengthening mechanisms included thermal mismatch strengthening,Orowan strengthening by nano-sized particles and grain-refining strengthening;the high-temperature strengthening mechanism included grain boundary pinning effects and dislocation climb hindering effects by the nano-sized particles.2)The hypoeutectic Al-7Si-4Cu alloy with in-situ micro-sized TiB₂ particles and micron/nano-sized TiB₂ particles addition were fabricated by combustion synthesized master alloys and ultrasonic-assisted stirring casting technology.The effects of different sizes of TiB₂ particles on the grain refinement and strengthening and toughening of hypoeutectic Al-7Si-4Cu were investigated and the mechanisms were revealed.i)The micro-sized and bimodal-sized(micron/nano-sized)in-situ TiB₂ particles could be successfully prepared by adjusting the Al contents in the Al-Ti-B systems.With 50wt.%Al content,the synthesized TiB₂ was in micron scales(0.73?m);while with 20wt.%Al,TiB₂ was in bimodal-sized scales(20 nm?350 nm).ii)It was revealed that the strength and ductility of the Al-7Si-4Cu alloy could be simultaneously and significantly enhanced by the in-situ bimodal-sized TiB₂ particles,which exhibited higher strengthening and toughening efficiency than their micron-sized counterparts.The strengthening mechanisms included?'precipitation strengthening,thermal mismatch strengthening,Orowan strengthening and grain-refining strengthening.The ductility enhancement mechanism included the refinement of primary?-Al grains and eutectic Si structures and the small size of TiB₂particles.iii)Compared with the micron-sized particles,the bimodal-sized TiB₂ particles exhibited higher refining efficiency for the primary?-Al dendrites and eutectic Si.The grain-refining mechanisms were that the heterogeneous nucleation events of?-Al were significantly promoted by the submicron TiB₂ particles,while the nanosized TiB₂particles effectively inhibited the subsequent growth of?-Al grains.The refining mechanisms of eutectic Si were that the considerable refinement of?-Al grains resulted in the formation of dispersedly distributed interdendritic liquid phase microdomains,which restricted the growing spaces of eutectic Si and therefore led to the refinement of eutectic Si;the refining mechanism of??precipitates was that high-density geometrically necessary dislocations induced by TiB₂ particles during quenching process promoted the nucleation and dispersion of the?'precipitates.3)The effects of ex-situ nanosized TiCN and in situ dual-phased TiCN-TiB₂ particles on the microstructure and mechanical properties of the T6 heat-treated Al-10Si-0.7Mg alloys and the corresponding mechanisms were revealed.i)It was revealed that the dual-phased TiCN-TiB₂ particles synthesized by the combustion synthesis method exhibited excellent high-temperature and chemical stability in the Al-10Si-0.7Mg alloys,whereas the ex-situ nanosized TiCN particles and the particles synthesized by the melt reaction method in the Al-Ti-B₄C-BN system led to lots of needle-or block-shaped Al₃Tiphases of large scales.ii)It was revealed that the dual-phased TiCN-TiB₂ particles could significantly refine the primary?-Al and eutectic Si structures of the Al-10Si-0.7Mg alloys.During the solidification process,the submicron-sized TiB₂ particles served as the heterogeneous nucleation substrates for the primary?-Al,while the nanosized TiCN particles were more likely to adsorb at the advancing solid/liquid interface to hinder the growth of the solid grains,and these twofold effects led to the significant refinement of the?-Al grains.In addition,nanosized TiCN particles promoted the heterogeneous nucleation of eutectic Si.Moreover,the considerable refinement of primary?-Al also altered the spatial configuration.Thus,the eutectic Si structures were refined.iii)It was revealed that the comprehensive mechanical properties of the Al-10Si-0.7Mg alloys could be significantly improved by the dual-phased TiCN-TiB₂ particles.Moreover,the ductility was also significantly enhanced by the considerable grain refinement and the substantial reduction of the aspect ratio of the eutectic Si.With the addition of 0.5 wt.%,the product of strength and elongation increased significantly from 3354.7 MPa%to 5433.4 MPa%,indicating an increase of 62.0%.4)A novel as-cast Al-10Si-2.0Mg alloy with high strength and ductility was successfully designed;the effects of dual-phased TiCN-TiB₂ particles and low-temperature stabilization treatment on the microstructure mechanical behavior of the as-cast Al-10Si- 2.0Mg alloy were investigated and the corresponding mechanisms were revealed.i)It was revealed that with increasing Mg contents,the secondary dendrite arm spacing of primary?-Al dendrites,eutectic Si and Mg₂Si compounds were significantly refined,and both the strength and ductility were simultaneously increased.The mechanisms of strength enhancement included solid solution strengthening,solute clusters strengthening,?-Al refining strengthening.The mechanisms of ductility enhancement included?-Al refinement,eutectic Si and Mg₂Si refinement.ii)It was revealed that the microstructure of the as-cast Al-10Si-2.0Mg alloy was significantly refined by the dual-phased TiCN-TiB₂ particles.The size of primary?- Al significantly decreased by 77.4%.The dual-phased TiCN-TiB₂ particles promoted the formation of high-density twins on the surfaces of the eutectic Si particles,and the isotropic growth was encouraged,which led to the refinement of eutectic Si.iii)It was revealed that both the strength and ductility of the as-cast Al-10Si-2.0Mg alloy were simultaneously improved dual-phased TiCN-TiB₂ particles,and further strength enhancement was obtained by the low-temperature stabilization treatment.The dual- phased TiCN-TiB₂ particles not only improved the solid solubility but also accelerated the precipitation kinetics.The yield strength and tensile strength were 184.3 MPa and 296.2 MPa,respectively,while the fracture strain was still as high as 11.8%.Strength enhancement mechanisms included grain-refining strengthening, thermal mismatch strengthening and Orowan strengthening induced by TiCN-TiB₂ particles,GP zones and pre-?''nanoprecipitates strengthening;ductility improvement mechanisms included primary?-Al and eutectic Si particles refinement.