| Aluminum alloy had attract the attention of researchers all over the world, because ofits low density and high strength-strain and many other advantages, but its room andelevated temperature mechanical properties, and creep resistance can not meet the rapiddevelopment of aerospace, transportation and military on the material properties. Therefore,its comprehensive performance needs to be further improved. Inoculation treatment is thecommon method to improve the strength and strain simultaneously. In previous studies, theinoculation used to treating the aluminum alloy is crystallized, and the inoculationmechanism with using amorphous is still unclear. In this thesis, the study on the effects ofamorphous on the α-Al dendrites, also the number, size and distribution of Al2Cuprecipitate were discussed. The solidification microstructure evolution process andmicrostructures were optimized and controlled of the casting Al-Cu alloys, to establish therelationship between the microstructures and properties. This paper provides a theoreticalbasis and builds the application foundation for the improvement and new preparationtechnology of the aluminum alloy and other system alloys. The main conclusions are asfollows:(1) It reveals that the characteristic temperature of Ni60Nb25Ti15amorphous under theheating rate of20K/min, the glass transformation temperature(Tg) is840K,crystallization temperature(Tx) is884K, melting point(Tm) is1324K and supercooledliquid region(△T) is44K, respectively. The main crystallization phases are NiTi,Nb0.1Ni0.9and Nb0.03Ni3Ti0.97of Ni60Nb25Ti15amorphous after annealing and ballmilling. The apparent activation energy is calculated24.3KJ/mol by JMA equation. Theactivation energy Egand Exare809.4KJ/mol and625.4KJ/mol calculated by Kissingerequation, which is corresponding to the result by Ozawa method.(2) The effect regularity of the amorphous on the α-Al dendrites, θ′precipitated phase,room-temperature mechanical properties are as follows:①Compared with the unmodified alloy, the Ni-Nb-Ti amorphous modified alloys havesmaller and more developed α-Al dendrites. The dendrites size of α-Al of the unmodified alloy is about100-200μm, the dendrites size of α-Al of the modified alloydecreases first and then increases with the amount of Ni-Nb-Ti amorphous increases.The smallest size,39μm, is gained at0.05wt.%Ni-Nb-Ti amorphous.②It is found that θ′precipitates become finer and more uniform in the modified alloys.The average length of θ′phase reduced from100-300nm to20-150nm，thickness isreduced from5-20nm to4nm, respectively.③It is found that the room-temperature mechanical properties of the modified alloysincreases first and then decreases with the increase of the amount of Ni-Nb-Tiamorphous. The best comprehensive performance is gained at0.05wt.%Ni-Nb-Tiamorphous,30min, the tensile strength and fracture strain are534.3MPa and13.36%,increased by13.2%and99.49%than the unmodified alloy, respectively.④It is proposed that the strengthening mechanisms of the modified ZL205A alloy byNi-Nb-Ti amorphous at room-temperature are depending on the refined α-Al dendrites,the finer and more uniform θ’ precipitates.Fine grain strengthening: The average grain size of ZL205A alloy modified by0.05wt.%Ni-Nb-Ti amorphous is reduced from160μm to39μm. The smaller the grainsize is, the larger the grain boundary density. Therefore, the hindrance effects on thedislocation movement are stronger, and alloy can bear greater plastic deformationbefore fracture. The mechanical properties of the alloy are improved.Precipitation strengthening: The average length of θ′precipitates is reduced from100-300nm to20-150nm, thickness is reduced from5-20nm to4nm, respectively.The finer and more uniformly distributed θ′precipitates make the Orowanstrengthening effect more obvious.(3)The inoculation mechanism of ZL205A alloy modified by Ni-Nb-Ti amorphous: Whenthe Ni-Nb-Ti amorphous is added to the melted alloy, it crystallized to fine NiTi,Nb0.1Ni0.9and Nb0.03Ni3Ti0.97phases rapidly, then these crystallization phasesdistributed uniformly in the melt. NiTi phase act as the heterogeneous nuclei of α-Alphase in alloy melt during solidification process, increase the nucleation rate, so theα-Al dendrites of the modified ZL205Aalloy are thinned obviously.(4) It is revealed that the Ni-Nb-Ti amorphous alloy has a significant increase in tensileproperties of453K than the tensile properties of unmodified alloys. At the sametemperature, strain rate is high, the higher the tensile strength of modified alloy; strainrate is constant, with the increase of temperature, the mechanical properties of themodified alloys decrease. The best comprehensive performance under453K and10-1s-1conditions is gained at0.05wt.%Ni-Nb-Ti amorphous, the tensile strength and elongation are406.3MPa and17.60%, respectively, increased by34.4%and182.1%than the unmodified alloy under same experimental conditions.(5) The strengthening mechanisms of the modified ZL205A alloy by Ni-Nb-Ti amorphousat elevated temperature are still due to fine grain strengthening and precipitationstrengthening.Fine grain strengthening: The average grain size of ZL205A alloy modified by0.05wt.%Ni-Nb-Ti amorphous is reduced from160μm to39μm. The smaller the grainsize is, the larger the grain boundary density. Therefore, the hindrance effects ondislocation movement is stronger, only increase the force can make the dislocationmovement continue. The strength of the modified alloy can be further improved. Butwith the temperature increasing, the grain boundary will soften, thereby weakening theeffect of grain boundary strengthening.Precipitation strengthening: the finer and more uniform θ’ precipitates. Under533K、10-4s-1conditions, the average length of θ′precipitates in the modified ZL205A isreduced from220nm to130nm, the average thickness reduced from10-30nm to8-25nm, respectively. A large number of fine θ′precipitates obstacle to dislocationmotion, strengthen the interaction between precipitates and dislocations, therefore, theelevated-temperature mechanical properties of modified alloys are improved. |
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