| As a new developing direction for spent fuel storage,dry storage urgently needs neutron absorbing materials that possess both neutron absorption function and high-temperature mechanical properties.However,the domestic research on this kind of structure-function integrated material is basically in a blank state,and there has not been relevant investigations on its component design,preparation process,microstructure characterization and high-temperature strengthening mechanism.In this study,two approaches of introducing reinforcements(in-situ and ex-situ)were used to incorporate nano-Al2O3 particles into B4C/Al composites.The effects of addition routes of nano-Al2O3 particles on the microstructure and mechanical properties of the composites were characterized and analyzed by means of sophisticated characterization and tests,and a series of scientific issues involved in the preparation,forming and friction stir welding(FSW)of the composites were discussed in depth.After optimization of ball milling parameters,nano-Al2O3 particles were uniformly dispersed into Al matrix composites.The room and high-temperature strengths were significantly improved,but the elongation was low.This study proposed a hierarchical,structure scheme to improve the strength and ductility synergy of Al matrix composites.Three ball milling procedures were used to construct uniform ultrafine-grained,conventional bimodal-grained and heterogeneous lamellar-grained structures.The results showed that,compared with the former two,heterogeneous lamellar-grained structure had higher strength and elongation.The results could be rationalized by the back stress strengthening theory.The geometrically necessary dislocations generated at the interfaces between coarse and fine grains were considered to be the main reason for increasing the elongation without sacrificing strength.However,through high-temperature tensile experiments,it was found that this solution could not improve the high-temperature elongation of the composites.The main reason was the lack of dislocation activity at high temperature and the weakening of the grain boundaries,which disenabled the redistribution of stress and strain between the coarse and fine grains.Increasing the specific surface area of flaky Al powders can utilize the surface oxidation to introduce more in-situ Al2O3 into the matrix.In this way,(B4C+Al2O3)/Al composites were prepared.In order to study the effects of Al2O3 distribution on the mechanical properties of the composites and the involved strengthening mechanisms,hot-pressed billets were forged and extruded to obtain composites with different microstructures.It was shown that the forged samples exhibited a lamellar structure,with Al2O3 remaining at the grain boundaries.In the extruded samples,most of the Al2O3 entered into the ultrafine grains.Al2O3 distributed at the grain boundaries could enhance the high-temperature strength by strengthening the grain boundaries and preventing the dislocation from annihilating at the grain boundaries.Therefore,at high temperature,the strength of the forged samples were significantly higher than that of the extruded sample,which could meet the strength demands for structure-function integrated neutron absorption materials.In order to improve the composites’ deformability,ultrafine Al powders were used as the raw material.By controlling the pre-oxidation and sintering process,different forms of Al2O3 were obtained to study its effects.The results showed that the samples sintered at 450℃ exhibited the best strength-elongation balance at 350℃(its tensile strength was 106.2 MPa,and elongation was 9.6%).The addition of B4C particles and the increase in the thickness of Al2O3 could increase the strength of the composite at room temperature,but it exhibited no significant effect on the high-temperature strength.When the sintering temperature of the composites increased from 450℃ to 550℃,the lamellar amorphous Al2O3(am-Al2O3)transformed into granular γ-Al2O3,resulting in a decrease in the strength.Through the analysis of the high-temperature deformation mechanism and strengthening mechanism,it was believed that the lamellar am-Al2O3 could be more effective in pinning the grain boundaries and prevent them from sliding.Therefore,remaining the Al2O3 as lamellar amorphous state was the most important factor to obtain excellent high-temperature strength.Using ultrafine aluminum powders as raw material,the microstructure and mechanical properties of(B4C+Al2O3)/Al composites after forging,rolling and extrusion were analyzed.Since the aluminum matrix and Al2O3 in the ultra-fine aluminum powders had good deformability,they could be coordinated with each other during extrusion,and finally Al2O3 was still mainly located at the grain boundaries.Extruded samples exhibited a netlike structure,and both strength and ductility had obvious advantages,which could determine the best forming method of this composite material sheet was extrusion.The FSW study of extruded plates was carried out at a welding speed of 100 mm/min and rotation rates of 400-800 rpm.Sound joints were obtained under all welding parameters;however,the nugget zones(NZs)of all joints were significantly softened.Therefore,the joints showed decreased strength at both room and high temperatures,with the joints fracturing in the NZs.The rotation rate exhibited no obvious effect on the strength of the joint,but increasing the rotation rate made NZs wider and increased the elongation.Microstructural characterization showed that the thermo-mechanical effect of FSW destroyed the netlike structure in the base material,causing Al grain coarsening,and agglomeration and crystallization of am-Al2O3 which led to weakening of the material.Therefore,avoiding the breakup of the netlike structure and Al2O3 crystallization are the key factors to improve the strength of the(B4C+Al2O3)/Al joints.In order to utilize above research results to broaden the research significance and the scope of materials,methods to obtain composites with high-temperature thermal stability were further explored.By adding reduced graphene oxide(rGO)into Al2O3/Al composites compacted from ultrafine Al powders,it was found that rGO could stabilize am-Al2O3 and prevent it from being transformed into y-Al2O3 during hot pressing.Via the direct strengthening effect of rGO and its stabilizing effect on am-Al2O3,a small amount of rGO could realize significant strength improvement.When 0.305 vol.%rGO was added,the strength at room temperature and 350℃ could be increased by 65%and 121%,respectively.In the study of preparing high-temperature composites with high thermal stability by high-temperature treatment of ultrafine Al powders,it was found that high-temperature pretreatment of Al powders could introduce nano-sized particles into the Al matrix.The content of Al2O3 increased with the increase of the treatment temperature.When the treatment temperature rose to 600℃,nitridation of Al could further occurred,and the(AlN+γ-Al2O3)particles at the grain boundaries were almost continuously distributed.The strength of the material was higher than that of am-Al2O3/Al at room and high temperatures.After being annealed at 580℃ for 8 h,the(AlN+γ-A12O3)/Al did not exhibit any high-temperature strength degradation.Therefore,compared with am-Al2O3/Al,the(AlN+γ-Al2O3)/Al exhibited obvious performance advantages. |
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