| In this paper,high current pulsed electron beam(HCPEB)was used to study the surface alloying of some Al-based binary immiscible materials.Based on first principles,the feasibility of surface alloying immiscible elements of pure Al was theoretically calculated and analyzed.X-ray diffraction(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were used to analyze the surface microstructure.In particular,the composition distribution in the grain boundary region was systematically studied by TEM under HAADF-STEM mode.Besides,the hardness of the samples was measured by the micro-Vickers hardness tester.With the combination of the results of theoretical calculation and experiment,the internal relationship between microstructure and surface properties was established,the alloying and modification mechanism of the immiscible Al-based system under the action of HCPEB were analyzed and summarized.First principles were used to simulate the solution strengthening of Pb,Sn,W,Cr and Mo in Al matrix.The calculation results show that the insolvable solutes in Al lattices contribute to form the solid solution with thermodynamic stability,which results in obvious distortion of Al lattice,and thus enhances the surface hardness of Al matrix due to solid solution strengthening effect.The results show that the strengthening effect of the alloying elements on the surface of Al matrix in descending order is W>Mo>Cr>Sn>Pb.For 35-pulsed Pb/Al,Sn/Al,W/Al,Cr/Al and Mo/Al samples,the solid solubility of Pb,Sn,W,Cr and Mo atom in Al lattice is greatly improved.At the same time,a large number of relatively small Pb,Sn,W,Cr and Mo particles are uniformly distributed on the surface of the matrix.After electron beam irradiation,the surface hardness of the samples increased significantly due to the effect of solution strengthening,dislocation strengthening,dispersion strengthening and fine grain strengthening.The surface strengthening effect of Al-based binary immiscible materials irradiated by HCPEB technology is in the order of W>Cr>Mo>Sn>Pb,in which the strengthening effect of Mo and Cr elements deviates from the first-principle's calculation.This is because the solid solubility of Cr/Al system after HCPEB irradiation is much higher than that of Mo/Al.After HCPEB irradiation,a large number of eruptive craters were formed on the surface of the Sn/Al and Pb/Al samples,which were repaired in the subsequent irradiated process.At the initial stage of HCPEB irradiation,the alloying particles on the surface of the sample fell off,melted or even evaporated to form larger Sn and Pb phases on the surface of the matrix.With the increase of irradiation times,the alloying phases on the surface of the sample were dissolved into the matrix to form the alloying layer.After 35-pulsed irradiation,the thickness of alloy layer of Sn/Al and Pb/Al samples is about 12 ?m and 15 ?m,respectively.The microstructure of alloy layer is mainly composed of alloyed element particles(i.e.,Pb particles or Sn particles),nano Al grains and various types of defect structures(mainly dislocation,dislocation wall,hole and subcrystal structure).In addition,the HAADF results show that the defect structure(such as grain boundary and phase boundary)formed in the alloy layer can provide a "shelter" for the alloying atoms.The different cross-section morphology of X/Al film-based samples after HCPEB irradiation was related to the melting point of alloying elements.When the melting point of alloying elements is higher than the melting point of Al,a "double layer" structure was formed on the surface of the samples,which was consisted of pure Al remelting layer and alloying element layer.When the melting point of alloying element is lower than that of Al,the layer formed on the sample surface would be composed of rich alloying element layer and diffusion layer.In addition,the diffusion layer contained a large number of defects structure(such as dislocations or grain boundaries)and fine particles precipitated at these defect structures. |
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