Study on the Microstructure Control of Tic_x in Al Melt and Its Strengthening & Toughening Mechanisms for Al Alloys
Author:Yang Hua Bing
Supervisor:liu xiang fa
Grain boundary strengthening is an important strengthening mechanism for A1 alloys,which can also increase the ductility and formability.TiC and a-Al are both face-centered cubic lattice structures,and they have nearly equal lattice constant.Thus,TiC can act as heterogeneous nucleating substrate for a-Al.However,the grain refinement effect of Al-Ti-C refiner is not satisfied at present,which is due to the insufficient understanding about a-Al nucleation mechanisms.Nucleation interface plays an important role in grain refinement effect.TiC is a non-stoichiometric compound,and the chemical compositions can be written as TiCx(0.48≤x≤0.98).Flexiblity of the x value provides possibility for the nucleation interface regulation.In addition,grain refinement shows limited potential for improving mechanical properties of Al alloys.TiCx can serve as reinforcement particles to further strengthen Al alloys.Particle size and distribution usually influence strengthening and toughening effects of reinforcement particles a lot.On the basis of the background,the strengthening and toughing behavior of TiCx particles for Al alloys were investigated by controlling the x value,particle size and distribution.(1)Relation between x value of TiCx and its nucleating capabilityIt was found that the grain refinement effect and anti-fading capability of Al-Ti-C master alloys depended closely on the C/Ti stoichiometry of TiCx,i.e.the x value.The lower the x value was,the better the grain refinement effect and anti-fading capability exhibited.After addition of Al-Ti-C master alloys to Al melt,the Ti-rich zones around TiCx particles were formed,which was necessary for the nucleation of a-Al on TiCx surface during solidification.According to thermodynamic research,TiCx was not stable in Al melt at 720℃,and the x in TiCxtended to increase(TiCx→TiCy,0.48<x<y<0.92)by releasing Ti atoms to Al melt.The released Ti atoms contributed to the formation of the Ti-rich zones,which facilitated the nucleation of α-Al.What’s more,the lower the x was,the higher the thermodynamic driving force of releasing Ti atoms showed.Thus,the TiCx with lower x value exhibited stronger nucleating potency and anti-fading capability.Zr element in Al alloys can degrade the grain refinement effect of refiners evidently,i.e."Zr-poisoning".Generally speaking,"Zr-poisoning" is resulted from the segregation of Zr atoms to the Ti-rich zones.This increases the lattice misfit between nucleation phase and substrates,which decreases the nucleating capability.The chemical reaction tendency(△G<0)between solute Zr and TiCx was one of the driving forces for Zr segregation.By doping TiCx with B element,the reaction tendency was eliminated(△G>0),which reduced the Zr segregation.Thus,B-doped TiCx exhibited anti "Zr-poisoning" capability.(2)Regulation of TiC nanoparticles for microstructures and properties of AI-Cu alloysOn the basis of grain refinement strengthening effect of TiC nanoparticles for AI-Cu alloys was investigated to further improve mechanical properties.An Al-14TiC master alloy was first prepared by melt reaction method,and the synthesized TiC particles have a size of 107 nm on average.Introducing the Al-14TiC master alloy to Al-Cu melt,three nanocomposites Al-4.5Cu-xTiC,x=0.5,1 and 1.5 were fabricated via casting process.The TiC nanoparticles dispersed in matrix uniformly in the Al-4.5Cu-0.5TiC,and with TiC content increased to 1.5%,i.e.in the Al-4.5Cu-1.5TiC some TiC were gathered along α-Al grain boundaries.Mechanical properties of the AI-4.5Cu-xTiC at both room and high temperatures(350℃)have been tested.It was found that both the dispersed and gathered TiC nanoparticles contributed to the yield strength at high temperature,and just the dispersed TiC nanoparticles played an efficient role in improving the yield strength at room temperature.Since grain boundaries are strong at room temperature,further reinforcing grain boundaries showed limited contribution for strength.However,grain boundaries are weak at high temperature.Thus,reinforcing grain boundaries by TiC nanoparticles improved the strength efficiently.The TiC nanoparticles also exhibited positive effect for the aging process of Al-Cu alloys.During aging at 180℃,the peak-aging time for Al-4.5Cu alloy was about 20 h,while,it was reduced to about 12 h for the Al-4.5Cu-1.5TiC.After aging,the size of 0’ precipitate in the A1-4.5Cu-1.5TiC was smaller and the number density was higher than that in the Al-4.5Cu alloy.This enhanced the age-hardening effect.Increment of yield strength before and after aging for the Al-4,5Cu was 84 MPa,and it reached to 113 MPa for the Al-4.5Cu-1.5TiC.This attributed to the high dislocation density in the Al-4.5Cu-1.5TiC,produced by the difference of coefficient of thermal expansion between TiC and a-Al.In addition,lattice distortion of a-Al close to TiC/Al interface also promoted the precipitation of θ’ phase.(3)Design for TiC nanoparticle architecture and its Strengthening&toughening mechanisms for 2024 Al alloysThe TiC nanoparticles showed satisfied strengthening effect for Al-4.5Cu alloys.However,particle agglomeration is nearly inevitable with the increase of particle volume fraction in composites.The agglomerated particles decreased strengthening effect obviously,and even brought about negative effect.By tailoring spatial distribution of TiC nanoparticles,the negative influence was avoided,and further enhanced the strengthening effect.A 2024-1.5TiC nanocomposite was prepared by casting process.In the 2024-1.5TiC,TiC nanoparticles were distributed along a-Al grain boundaries,forming network architecture.The TiC network architecture improved alloy strength efficiently.Compared to 2024 alloy,The TiC network enhanced ultimate tensile strength of the 2024-1.5TiC from 105 MPa to 151 MPa at 350℃,and the strength for 2024-1.5TiC with random distribution of TiC nanoparticles was just 118 MPa.It was found that just a portion of the matrix was deformed for the 2024 alloy during tensile test,i.e.local deformation mechanism.Construction of the TiC network transformed deformation mechanisms of a-Al matrix from local deformation to overall deformation,which developed the strengthening effect of overall matrix and strengthened the 2024-1.5TiC nanocomposite evidently.Special threadiness arrays of TiC nanoparticles were constructed in 2024-1.5TiC nanocomposite by casting and hot extrusion processes.Compared with 2024 alloy,strength and ductility of the 2024-1.5TiC nanocomposite have been improved simultaneously at room temperature.The ultimate tensile stress was improved from about 430 MPa to 550 MPa,and the elongation was increased from about 9.8%to 15.5%.Introduction of the TiC nanoparticles reduced a-Al grain size from 9.7 μm to 6.8 μm,which contributed to the mechanical properties.In addition,the TiC threadiness arrays were almost not deformed during tensile test,but the matrix was subjected to severe deformation.Thus,the TiC threadiness array can be taken as one strengthening unit,like a short fiber.Since the TiC threadiness showed high aspect ratio,36.6 on average,the load bearing effect appeared evident.This enhanced the strength of the 2024-1.5TiC apparently.What’s more,due to load bearing of the TiC threadiness arrays,the 2024-1.5TiC matrix is softened relatively.It improves the stress release capability for crack tips,which impedes crack propagation,and then led to high ductility.