| 7055Al alloy has been widely applied in aerospace,rail traffic,national defense and military fields,new energy automobile,etc,due to high specific strength,high specific stiff,excellent toughness and stress corrosion resistance.With vehicles continuing to purse light weight and operational reliability and life under the servicing conditions of high temperature and high load,the higher demands of high temperature properties,especially high temperature creep properties,of the key structural components of deformed aluminum alloys are required.However,the loss of mechanical properties of 7055Al alloys is close to 50%above 473 K and the high temperature creep properties are poor.In-situ nano particles reinforced aluminum metal matrix composites with the advantages of high specific strength,high specific stiffness,excellent high temperature properties,etc,are a new kind of lightweight,ultrahigh-strength and heat resistant metal materials.Whereas,with the increasing particles,the nano particles aggregating phenomenon restrains the properties enhancement and scale applications of composites.In this article,7055Al alloy commonly used in aerospace has been selected as matrix alloy and the heat resistant ZrB2 and Al2O3 ceramic particles reinforced 7055Al composites have been in-situ synthesized under the regulation of external fields,including electromagnetic,ultrasonic and pressure fields.The effects of external fields on the particles distribution and microstructures of composites have been investigated.The effects of particle contents and external fields on microstructures and mechanical properties of composites before and after hot rolling have been studied and the strengthening mechanisms have been discussed.The high temperature creep behaviors of composites have been analyzed and the creep resistance mechanisms have been revealed.The microstructures of as-cast（ZrB2+Al2O3）np/7055Al composites showed that:the average sizes of ZrB2 and Al2O3particles are approximately 87.2 nm and 56.1 nm,respectively.The lattice relationships between ZrB2,Al2O3 andα-Al matrix are semi-coherent and coherent,respectively.The grains are significantly refined from 106.8μm to 36.2μm under the common effects of heterogeneous nucleation sites provided by nano particles and the pushing effects of solidification interfaces.With the regulation of electromagnetic and ultrasonic fields,the grains and particles clusters are refined from 36.5μm and 13.9μm to 30.1μm and 12.2μm,respectively.In-situ ZrB2 and Al2O3are refined to approximately 81.9 nm and 48.3 nm,respectively,and the particle edges are passivated.The grain sizes and particle cluster sizes are further decreased under the regulation of electromagnetic,ultrasonic and pressure fields.When the pressure is 40MPa,the average grain size and particle cluster size are 21.9μm and 9.6μm.The regulation mechanisms of external fields are that:the distribution of reactive salts and in-situ reactions are improved by electromagnetic fields and the in-situ synthesized particles distribute uniformly.The particle clusters are broken by the instant high temperature and pressure induced by ultrasonic cavitation effect and the broken particles are dispersed in the melt under the acoustic streaming effects.On the one hand,the melt supercooling and nucleation rates are increased and the diffusion coefficients of solute atoms are decreased under the pressure fields.The fine grains separate particles uniformly and the particle aggregations near interfaces are alleviated.On the other hand,pressure increases the contact areas and accelerates heat transfer between melt and mold wall.The solidification rates of melt are accelerated and the more particles are engulfed by solidification interfaces.The microstructures of hot rolled（ZrB2+Al2O3）np/7055Al composites showed that:the particle clusters and coarse secondary phases distribute as the form of bands under the shear forces after hot rolling.with the increasing of hot rolling temperature and strains,the grains are refined continuously.Nevertheless,with the excessive hot rolling temperature,the recovery behaviors are enhanced and amounts of elongated grains are retained.With the excessive hot rolling strains,the dislocation densities inside materials are increased and the recrystallized grains significantly grow during heat treatment.When the hot rolling temperature is 673 K and strain is 83.3%,the microstructures of composites are homogeneous,the grains are fine and the significant growth of recrystallized grains do not occur during heat treatment.The nucleation and growth of dynamic recrystallized grains are promoted by particle simulation nucleation mechanism（PSN）of particle clusters during hot rolling.The movements of dislocations and grain boundaries are inhibited by dispersed particles during T6 treatment,which restrains the occurrence of recovery behaviors and the growth of recrystallized grains.With the increasing particles and under the regulation of external fields,the number of dispersed particles is increased and the inhibiting effect of particles on recrystallization is promoted,causing grains further refined.The tensile properties of hot rolled（ZrB2+Al2O3）np/7055Al composites at room temperature and high temperature showed that:（1）the strength of composites at room temperature are increasing with particle volume fractions after hot rolling and T6treatment.When the particle volume fraction is 5 vol.%,the yield strength,ultimate tensile strength and elongation are 581.6 MPa,662.6 MPa and 8.0%,respectively.The yield strength and ultimate tensile strength of 5 vol.%composite are larger by 3.3%and5.6%than those of 7055Al alloy（562.8 MPa,627.8 MPa and 9.2%）,but the elongation is lower by 13.0%.Under the regulation of external fields,the yield strength,ultimate tensile strength and elongation of 3 vol.%composite are 594.6 MPa,684.0 MPa and9.2%,respectively,which are increased by 3.6%,5.9%and 12.2%in comparison to the counterpart without regulation.Compared with 7055Al alloy,the yield strength and tensile strength of 3 vol.%composite regulated with external fields are increased by5.7%and 9.0%and the elongation is retained.The refined grains strengthening,Orowan strengthening and CTE strengthening are the main strengthening mechanisms of composites.（2）the strength of composites at high temperature are increasing firstly and decreasing subsequently with the increasing particle volume fractions.When the particle volume fraction is 3 vol.%,the high temperature strength reaches to peak and is 124.9 MPa at 573 K.Under the regulation of external fields,the high temperature strength of 3 vol.%composites is further improved and is 136.0 MPa at 573 K,increased by 8.9%and 46.6%in comparison to the counterpart without regulation and7055Al alloy（92.8 MPa）,respectively.The strengthening mechanisms of composites at high temperature are resulted from suppressing effects of particles on softening behaviors of matrix.The high temperature creep properties of hot rolled（ZrB2+Al2O3）np/7055Al composites showed that:after hot rolling ad T6 treatment,the steady creep rates of composites are decreasing with the increasing particles.When the particle volume fraction is 5 vol.%,the steady creep rates of composite are 1.1×10-10～1.7×10-6 s-1,4.7～18.8 times lower than those of 7055Al alloy(1.9×10-9～8.2×10-6 s-1).The apparent stress exponent na,apparent activation energy Qaand threshold stressσth are increasing with particles volume fractions.Under the regulation of external fields,the steady creep rates of 3 vol.%composite are 8.0×10-11～1.6×10-6s-1,1.2～2.3 times lower than those of the counterpart without regulation(1.8×10-10～1.9×10-6 s-1)and 5.1～23.8 times lower than those of 7055Al alloy.The creep mechanisms analyses of（ZrB2+Al2O3）np/7055Al composites reveal that the creep rates of both matrix alloy and hot rolled composites are controlled by dislocation climb.The outstanding creep resistances of composites are attributed from particle hindering dislocation movements and grain boundaries slip. |
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