Study On The "Immune" Nucleation Mechanism Of α-Al Grain And Its Strengthening And Toughening Behavior In Al-Cu-Zr Alloys

Al-Cu-Zr alloys are widely used in aerospace,national defense and military industries due to their high specific strength,good heat resistance and reliable thermal fatigue properties.However,the wide range of crystallization temperature in the alloys leads to the coarse α-Al dendrites and poor melt fluidity,making it possible to produce the casting defects such as misruns,hrinkage porosity,hot crack and composition segregation,which affect the hot working process and mechanical properties of the alloys.Grain refinement is an important way to improve the casting properties of Al alloys.Neverthless,due to the presence of Zr,traditional grain refiners can not play a normal refining effect in Al-Cu-Zr alloys,which is referred to as Zr-induced refinement "poisoning" phenomenon.Therefore,it is urgent to fundamentally solve the problem of grain refinement and optimize the formability and service performance in Al-Cu-Zr alloys.In this paper,a new Al-TCB master alloy（TCB mean the abbreviation of B-doped TiCx particles）of immune Zr-induced refinement "poisoning" was applied to the Al-Cu-Zr model alloy,as well as the refining behavior of the TCB seeds were investigated in the Al-Cu-Zr alloy,and its immune nucleation mechanism for Zr-induced refinement "poisoning" was revealed.On this basis,effects of the Al-TCB master alloy for the microstructure,formability and mechanical properties of Al-Cu-Zr multicomponent alloys were characterized systematically.In addition,submicron Si₃N₄ particles were introduced into Al-Cu-Zr alloys by gravity casting combined with hot extrusion deformation.A high-strength Si₃N₄/Al-Cu-Zr composite was prepared,and its microstructure evolution and strengthening mechanism were systematically analyzed.The key contribution of this paper is as follows:（1）Grain refining effect of the TCB seeds in Al-Cu-Zr alloys and its "immune"nucleation mechanismTaking Al-Cu-Zr model alloys as the research objects,refining behavior of theα-Al grain in Al-Cu-Zr alloys was researched by adding Al-Ti-B and Al-TCB master alloy in equal amounts.The results showed that the Al-Ti-B master alloy had an obvious Zr-induced refinement "poisoning" phenomenon,and the average grain size of the Al-Cu-Zr alloy was about 206 μm.In contrast,the Al-TCB master alloy exhibited immune Zr-induced refinement "poisoning" capability,making the average grain size of the alloy stable at about 94 μm and there was no obvious decline for its refining capability.Further research found that the heterogeneous nucleation substrate in the Al-Cu-Zr alloy after adding the Al-TCB master alloy originated from the composite particles composed of TCB and TBC（C-doped TiB2 particles）.The TCB seed had the characteristics of evolution in the Al-Cu-Zr melt and its product of evolution was TBC.At the same time,the TCB released Ti,C,and B atoms sustainably,which can be dedicated to the nucleation and growth of TBC on the ｛111}TCB crystal plane.Finally,composite particles containing TBC and TCB were created.The evolved TBC played a dominant role in the nucleation of α-Al,and there was no segregation of Zr elements at the TBC/α-Al interface,which made the Al-TCB master alloy have a immune effect for Zr-induced refinement "poisoning".（2）Effect of the TCB seeds for microstructure,forming and mechanical properties of Al-Cu-Zr alloysZL205A cast Al alloys and 2026 wrought Al alloys are both typical Al-Cu-Zr multicomponent alloys.Taking these two alloys as the research objects,the effect of the Al-TCB master alloy on their microstructure,formability and mechanical properties were researched systematically.The results showed that after adding 0.5%,1.0%,2.0%and 4.0%Al-TCB master alloy to ZL205A alloys,the refining effect ofα-Al grains was gradually improved,and the average grain size was decreased from the original 236.8 μm to 77.7μm,75.5 μm,69.2 μm and 63.9μm,respectively.The grain shape changed from dendritic to nearly spherical.Extending the holding time of Al-TCB master alloy in ZL205A melt,the α-Al grains did not exhibit any obvious refining fading phenomenon.In addition,the melt fluidity of ZL205A alloys were improved when the Al-TCB master alloy was added at 1.0%.The length of the fluidity specimen was raised from 457 mm to 644 mm,an increase of 40.9%.The room temperature tensile properties of ZL205 alloys after heat treatment were tested.The results showed that after adding 1.0%Al-TCB master alloy,the yield strength,ultimate tensile strength and elongation of the alloy increased to 430 MPa,493 MPa and 9.7%,respectively,increasing by 3.1%,3.6%and 70.1%,respectively,especially elongation has been greatly improved.When 0.5%,1.0%,2.0%,and 4.0%Al-TCB master alloy was introduced into 2026 alloys,the average size of α-Al grains was reduced from 481 μm to 302 μm,205 μm,149 μm and 102 μm,respectively.Compared with the traditional Al-Ti-B and Al-Ti-C master alloy,the refining effect of the Al-TCB master alloy was more significant.Through thermal simulation experiments,the flow stress of 2026 alloys in the hot deformation process was tested,and the corresponding microstructure evolution of the alloy was analyzed.At 400℃ and 0.01/s strain rate,the peak flow stress of the 2026 alloy gradually decreased with increasing additions of Al-TCB master alloy,from the initial 50.9 MPa to 48.1 MPa,44.9 MPa,44.3 MPa and 43.8 MPa,respectively.This showed that the grain refinement caused by the application of the Al-TCB master alloy helped to decrease the deformation resistance of 2026 alloys during hot deformation.On this basis,the cast rods with the addition of the Al-TCB master alloy were hot-extruded.The microstructure showed that the grain sizes ofα-Al were correlated with the addition of the Al-TCB master alloy,and their average size gradually decreased with the addition of the Al-TCB master alloy,which was consistent with the grain refining results after solidification.When the addition amount of the Al-TCB master alloy was 1.0%,the mechanical properties of the 2026 alloy reached their best after T6 heat treatment,and its yield strength,ultimate tensile strength,and elongation were increased to 405 MPa,532 MPa,and 14.4%,respectively.（3）Regulation of Si₃N₄ particles for microstructure and mechanical properties of Al-Cu-Zr alloysIn order to further enhance the mechanical properties of 2026 alloys,an Al-Si-N master alloy rich in submicron Si₃N₄ particles was introdcuced into 2026 alloys by gravity casting combined with hot extrusion deformation,and a high-strength Si₃N₄/2026 composite was successfully prepared.Analysis results of the microstructure showed that the Si₃N₄ particles in the composite are mainly distributed along the grain boundaries after solidification.After hot extrusion deformation,the Si₃N₄ particles formed a chain-like structure along the extrusion direction.Due to the hindering effect and particle-stimulated nucleation effect of Si₃N₄ particles,the α-Al grains in the composites were significantly refined after hot extrusion deformation,and the average grain size was reduced from 22.8 μm to 11.6 μm.The preferred orientation of grains in the<111>Al direction is significantly stronger than that of the matrix alloy.In addition,a thin shell composed of nano-AlN particles was found on the surface of the Si₃N₄ particles by TEM observation.The yield strength,ultimate tensile strength and hardness of the Si₃N₄/2026 composite were raised to 454 MPa,586 MPa and 143.5 HBW after T6 heat treatment,which were increased by 15%,12%and 11%,respectively.The improvement of mechanical properties in the Si₃N₄/2026 composite was attributed to the combined effects of load-bearing effects by the Si₃N₄ particles and their chain architecture,grain refinement,high GND dislocation density and Orowan strengthening by the nano-precipitated phases.