Microstructure And Properties Of Surface Alloying Of Al With Pb And Sn By High Current Pulsed Electron Beam

In this paper,the surface alloying of pre-coated lead?Pb?and tin?Sn?film on aluminum?Al?substrate was carried out by the"HOPE-?"type high-current pulsed electron beam?HCPEB?,respectively.The Pb/Al system was irradiated with 25 and 35pulses,and the irradiation of Sn/Al system was 15,25 and 35 pulses.The phases,morphologies,roughness and microstructure evolution on the irradiated surface of specimens were characterized in detail by X-ray diffraction?XRD?,scanning electron microscope?SEM?,3D laser scanning microscope?3D LSM?and transmission electron microscope?TEM?.The surface microhardness of the samples before and after the alloying treatment was measured by using a Vickers durometer.In addition,the coefficient of friction and wear rate of samples before and after HCPEB irradiation were tested.Meanwhile,the relationship between the microstructure and performance of the alloying layer was investigated to reveal the mechanism behind it.After surface alloying treatments by HCPEB,the results of XRD analysis show that no new phase was formed on the irradiated surface of Pb/Al or Sn/Al system.However,a minor shift of Al diffraction peaks to lower angle was observed,which reveals the possibility of the dissolution of Pb and Sn atoms into Al substrate to form solid solution.The further microstructure analysis results show that the craters were formed on the surface,which has been considered as a typical feature of HCPEB irradiation.With the increasing number of pulses,the craters were gradually eliminated.Simutaneously,the surface roughness was also reduced.After 35-pulsed irradiation,the thickness of alloying layer of Pb/Al and Sn/Al system were about 15?m and 10?m,respectively.The TEM observations of Pb/Al system indicate that the size of Pb particles was gradually decreased when the pulse number was increased.The formation of the ultrafine Al grains also occurred.The uniform distribution of Pb nano-particles in the Al substrate was obtained after irradiation with 35 pulses.The excellent interfacial bonding was displayed in the Pb/Al interface.Meanwhile,some Pb particles exhibited a cube-cube orientation relationship with the surrounding Al substrate,i.e.,[011]_Al//[011]_Pb;?1^—1^—1?_Al//?1^—1^—1?_Pb;?2^—00?_Al//?2^—00?_Pb.Besides,some high-density structure defects such as dislocation walls and dislocation loops were induced by HCPEB irradiation,which supplied a large number of fast diffusion paths for Pb atoms and facilitated the formation of Al?Pb?solid solution.The TEM observations of Sn/Al system reveal that the elongated Sn particles distributed in the Al substrate were detected after 15-pulsed irradiation.The size of Sn particles was obviously reduced with the increment of pulses.After irradiation with 35 pulses,the homogeneous distribution of Sn particles with merely about 9 nm in size was obtained in the alloying layer.The Sn nano-particles exhibited hexagon and globular morphologies.In the meantime,the mutual orientation relationship between some Sn particles and the Al substrate was observed,i.e.,[321]_Al//[010]_Sn;?11^—1^—?_Al//?200?_Sn;?1^—33^—?_Al//?101?_Sn.Additionally,various structure defects such as voids,dislocation cells,dislocation walls and high-density dislocation lines were also formed on the irradiated surface,which servered as numerous fast diffusion paths for Sn atoms to be beneficial for the formation of Al?Sn?solid solution.The microhardness of the surface layer of Pb/Al and Sn/Al system was greatly improved compared to the Al substrate,which is mainly due to the combined effect of fine grain,dispersion,dislocation and solid solute strengthening.In addition,the slightly decreased microhardness of Sn/Al system after 15-pulsed irradiation is mainly owing to the soften effect caused by the nonuniform distribution of elongated Sn particles in the alloying layer.After the surface alloying of Al with Pb and Sn by HCPEB,the wear rate and the coefficient of friction of the irradiated samples were significantly decreased compared to the initial sample.Besides,with the increasing number of pulses,the wear property was drasitically enhanced,which is attributed to the combined effect of the formed Pb or Sn lubricious tribolayer,the improved microhardness and the decreased surface roughness.After 35-pulsed irradiation,the alloying layer of Sn/Al system exhibited better performance compared to Pb/Al system.