Fabrication And Dynamic Mechanical Behaviors Of Amorphous/nano-crystalline Aluminum-based Alloy Composites

The microstructure and properties of atomized Al-based metallic powders,optimization of sintering process parameters during spark plasma sintering(Spark Plasma Sintering,SPS),the dynamic mechanical loading by Split Hopkinson pressure bar(Split Hopkinson Pressure Bar,SHPB)and the simulated protection performance have been systematically investigated in this dissertation.The above studies have theoretical guidance significance for mastering the mechanical properties of alloy materials under high impact load,the evolution law of deformation damage,exploring and improving their mechanical properties,and expanding their engineering application.Al-Ni-Y-Co-La amorphous/crystalline alloy powders are prepared by argon atomization.Most powders exhibit a regular spherical shape.The average diameter of the powder is 35 ?m and the average cooling rate is 105~106 °C/s.The solidification structure of the gas atomized powder changes from the amorphous phase to the amorphous phase,the face-centered cubic phase Al,the orthorhombic phase Al4 Ni Y and the hexagonal phase La7Ni3 as increasing the particle size of the atomized powder.A new crystal structure with face centered orthogonal structure with the lattice parameters of a = 15.92 ?,b = 15.64 ? and c = 4.08 ? has been found in the largest size powder.The thermodynamics(enthalpy)and kinetics(atomic diffusion)determines the structure of the precipitated phase during solidification of the powder.A high strength and highly dense bulk Al-based amorphous/nanocrystalline alloy composites were fabricated by SPS.The effects of sintering parameters such as temperature,pressure,and holding time on the microstructure and mechanical properties of Al-based amorphous/nanocrystalline alloy composites were studied during the sintering process of SPS.The results show that proper temperature,pressure and holding time will help to promote the diffusion and crystallization of the amorphous phase,resulting in better sintered samples.However,the sintering temperature,the sintering pressure and holding time should not be too high in order to avoid overburning,grain growth and mechanical properties decrease.By adjusting the sintering process parameters,the sintered sample exhibits better compressive fracture strength of approximately 1059 MPa and a 4.8 % compressive plastic strain with sintering temperature of 350 ?,sintering pressure of 300 MPa and holding time of 1.5 min.The grain size of the sintered samples is small and the size is about 12 nm with this sintering process parameters.These results indicate that spark plasma sintering is a potential method for the fabrication of Al-based amorphous/nanocrystalline materials by simultaneously controlling the devitrification and consolidation of amorphous powder precursors.The influence of strain rate on dynamic mechanical properties of Al-based amorphous/nanocrystalline alloy composites was studied by SHPB technique(10-5 s-1 ~ 103 s-1).It is found that the unique strain rate sensitivity of aluminum-based amorphous/nanocrystalline alloy composite material is that: the compressive strength of the material is insensitive and the crack expands rapidly when low strain rate is low;and the compressive strength shows the positive strain rate sensitivity when the strain rate is high.The fractal theory is introduced to modeling the fracture behavior.The size distribution of the debris from fractured alloy follows the theory of fractal at all strain rate range.In quasi-static state,the fractal dimension is almost unchanged,corresponding to the large size difference of fragments.When the dynamic loading is applied,the fractal dimension increases,which corresponds to more homogeneous distribution and smaller fragments.The difference in the scale distribution of debris after static and dynamic loading is generated by different loading methods.The formation of multiple microcracks in the dynamic process is the main reason for obtaining small and uniform debris.The effect of temperature on the dynamic mechanical properties of Al-based amorphous/nanocrystalline alloy composites was studied by SHPB technique(-100 ~ 200 ?).The results show that the fracture strength of Al-based amorphous/nanocrystalline alloy composites is the highest at room temperature,and decreases at-100 ? and 200 ? in varying degrees.This is mainly attributed to the stress concentration produced at low and high temperatures.That is,the stress concentration between the increased strength of the amorphous state and invariant strength of crystalline structure at low temperature.Analogously,the strength of amorphous state is decreased for structural relaxation and coarse grain at high temperature.At that time,the stress concentration between crystalline and amorphous became obvious.The distribution of debris after loading at different temperatures also follows the fractal law.The fractal dimension increases slightly with increasing temperature.The numerical simulation was carried out by the hypervelocity impact theory.The debris cloud motion was simulated which characterizes a projectile striking Al-based amorphous/nanocrystalline alloy sheet.The variation of motional characteristics of debris clouds was studied under the influence of V0 and t/D(the ratio of the thickness of the sheet to the diameter of the projectile).The quantitative relationship between normalized velocity of 2 feature points of debris cloud V1,V2 and the impact conditions(V0,t/D)are established.Using the numerical simulation method and the hypervelocity impact theory,the ballistic limit equations(BLEs)of the Al-based amorphous/ nanocrystalline alloy material is studied as a protective structure of Whipple,and it is compared with the BLEs of the common aluminum alloy.The results show that the critical projectile size(6.07 mm in diameter)of the Al-based amorphous/nanocrystalline alloy Whipple structure is 1.24 times that of 2A12 aluminum alloy(4.9 mm in diameter)at the same impact velocity of 6 km/s.Under the same loading speed,the critical energy consumption of Al-based amorphous/nanocrystalline alloy Whipple structure is 1.9 times larger than that of 2A12 aluminum alloy Whipple structure.Compared with common aluminum alloys,the better shield performance is attributed to high fracture strength,high hardness of Al-based amorphous/nanocrystalline alloy composites.