| The morphology and size of α-Al and secondary phases and the morphology,size and aspect ratio of eutectic Si in cast aluminum alloys are extremely important to their overall microstructure.The morphology and size of the second phase are also closely related to the overall microstructure manipulation of the alloy and its application fields.Researchers have combined a variety of grain refiners and modifiers to realize the manipulation of various constituent phases on the basis of a single grain refiner and modifier.However,studies have found that grain refiners and modifiers interact and affected each other,and it is difficult to achieve multi-level and high-efficient microstructure manipulation.Therefore,it has important scientific value and practical significance to develop a high-efficient microstructure manipulation agent for cast aluminum alloys to get optimized microstructure configurations.This study comparatively studies the microstructure of aluminum alloys calibrated using three kinds of in-situ nanocrystal crystallized from amorphous alloys,and reveals the influence law and manipulation mechanism of different nanocrystals on the microstructure of various aluminum alloy systems.The characteristics and application scope of different nanocrystals were summarized.The manipulation law and mechanism of different nanocrystals on α-Al,primary Si,eutectic Si and the second phase were studied and revealed.The laws and mechanism of nanocrystals on the microstructure,liquid-solid and solid-state phase transformation of Al-7Si-0.3Mg alloy were studied and revealed.The mechanical properties,fatigue resistance at room temperature and high temperature and high temperature creep behavior of Al-7Si-0.3Mg alloy calibrated using nanocrystals were studied.And the strengthening and toughening mechanism and fatigue resistance mechanism at room temperature and high temperature were revealed.The main innovations of this paper are as follows:1)Based on comparative analysis of the microstructure evolution law and microstructure manipulation characteristics of aluminum alloys manipulated by different systems of insitu nanocrystal,the manipulation mechanism and applicable conditions of in-situ nanocrystals for the main phases in aluminum alloys were revealed.The formation mechanism and difference of in situ nanocrystals:Amorphous alloys immediately crystallize above the crystallization temperature to form nanocrystals.Fe2B and Fe3Si,ZrCu(B2)and Zr2Cu,NiTi(B2),Ni3(Ti,Nb)and NiNb nanocrystals were formed by insitu crystallization of FeBSi,ZrCuAlNi and NiNbTi amorphous alloys,respectively.Fe2B,Fe3Si and NiTi phases are more stable in aluminum alloy melt,thus longer effective manipulation time.The maniulation laws and mechanism of in-situ nanocrystals on α-Al phase,Si phase and second phase:Nanocrystals can act as heterogeneous nucleation sites of α-Al and Si phases in liquid-solid phase transition.Most of the nanocrystals that are not involved in nucleation are adsorbed on the front of the solid-liquid interface,which hinders the growth of dendrites,refines the α-Al and Si phases,and improves the segregation of alloying elements.The refinement efficiency of different nanocrystals onα-Al dendrite and Si phase is:NiNbTi>ZrCuAlNi>FeBSi amorphous alloy.The individual laws and characteristics of the three types of in-situ nanocrystals on manipulating of aluminum alloys:Fe2B and Fe3Si have more significant effects on manipulating α-Al and Si phases,with low price and low addition.Disadvantages:Fe2B and Fe3Si are not efficient in refining second phase and Si.ZrCu(B2)and Zr2Cu have remarkable effects on manipulating α-Al,primary Si,eutectic Si and Al2Cu phases,and are suitable for rapid hot-forming processes such as die casting or casting-rolling or stream inoculation.Disadvantages:the incubation time is too short to precisely control and largescale industrial applications.NiTi and Ni3Nb have the highest efficiency in manipulatingα-Al,Si,Al2Cu and Mg2Si phases,which can obviously refine the Mg2Si phase.The composition phase of common aluminum alloys can be comprehensively calibrated.2)Based on comparative analysis of the rules of nanocrystals on solidification behavior and multi-level microstructure manipulation(α-Al dendrites,eutectic Si and Mg2Si phases)of Al-7Si-0.3Mg alloy,it was revealed that in-situ nanocrystals manipulated the overall microstructure configuration of Al-7Si-0.3Mg alloy.It was found that trace nanocrystals comprehensively manipulated and refined the α-Al,eutectic Si and Mg2Si phases.The grain size of α-Al,the length and aspect ratio of eutectic Si,and the average size of Mg2Si were reduced to 12.7%,47.4%,57.9%and 5.4%,respectively.The microstructure manipulation mechanism was revealed as follows:NiTi nanocrystals act as α-Al heterogeneous nucleation sites to promote the refinement of α-Al grains;nanocrystals hinder the growth of Al-Si eutectic phase,which refines eutectic Si and changes its morphology from needle-like to short-rob like.The grain refinement shortens the diffusion distance of Mg and Si atoms,and the martensitic transformation of NiTi nanocrystals during heat treatment produces higher density of dislocations,which provide nucleation energy and nucleation sites for Mg2Si,induce precipitating smaller and numerous nanosized Mg2Si.3)Based on the research about tensile behavior and mechanical fatigue behavior of Al-7Si0.3Mg alloy manipulated by in-situ nanocrystal,the strengthening and toughening mechanism and fatigue resistance mechanism at room temperature of Al-7Si-0.3Mg alloy calibrated by nanocrystals were revealed.The strengthening mechanisms were as follows:fine grain strengthening,Orowan strengthening and thermal mismatch strengthening.Orowan strengthening contributes the most.The plasticity enhancement mechanisms were as follows:refinement and spheroidization of eutectic Si and grain refinement.Al7Si-0.3Mg alloy calibrated by nanocrystals significantly improved its toughness and fatigue life.At 60 Hz and 20 Hz,the ultimate fatigue strengths(N=107)of the manipulated alloy were increased from 81 MPa and 92 MPa to 102 MPa and 112 MPa,with an increase of 21.7%and 25.9%,respectively.The fatigue resistance mechanisms at room temperature were as follows:grain refinement,eutectic Si refinement and spheroidization,precipitation phase refinement,and reduction of initial movable dislocations.4)Based on the research about tensile behavior,creep and mechanical fatigue behavior of Al-7Si-0.3Mg alloy manipulated by in-situ nanocrystal,the strengthening mechanism and fatigue resistance mechanism at high temperature of Al-7Si-0.3Mg alloy calibrated by nanocrystals were revealed.The high temperature strengthening mechanisms of Al-7Si0.3Mg alloy calibrated by nanocrystals were as follows:precipitation strengthening,short rod-like eutectic Si pinning grain boundary strengthening,and the hindering effect of precipitation on dislocation slip and climbing.The plasticity enhancement mechanism was grain refinement.The creep resistance mechanisms were as follows:short rod-like eutectic Si pinning the grain boundary and hindering the migration of grain boundary,finer precipitation phase pinning dislocation and hindering dislocation movement.Trace nanocrystals significantly improved the creep rupture time and high temperature creep resistance of Al-7Si-0.3Mg alloy.The mechanisms of high temperature fatigue resistance were revealed as following:①α-Al grain refinement and small short rod-like eutectic Si reduce stress concentration and inhibit the formation of crack sources;②Fewer initial movable dislocations in the manipulated alloy improve the microstructure stability;③Under high temperature cyclic stress,the dynamic induced crystallization reduces stress concentration.α-Al grain refinement and fine recrystallized grains make the fatigue crack growth path more tortuous and reduce the growth rate;④The β and β" phases at the crack tip hinder the dislocations climbing and reduce the crack growth rate. |
PDF Full Text Request