| Zinc and zinc alloys are widely used in galvanizing,casting,aerospace,automobile and other industries due to their good casting and machinability.However,due to their poor mechanical properties and corrosion resistance,their service life is seriously deteriorated during industrial applications.In recent years,high current pulse electron beam?HCPEB?,as one of the high energy beam surface modification technologies,has been developed rapidly.Non-equilibrium solidification has taken place when the surface of the materials is irradiated by the pulsed electron beams,and the surface is heated and cooled rapidly.This leads to the formation of the supersaturated solid solution,nanocrystalline and ultrafine second phase particles and thereby improves the surface properties of the materials.In this paper,pure Zn and Zn-10wt%Cu alloys were treated by pulsed electron beam surface modification at different pulse numbers.The surface states,such as microstructures,phase components,chemical composition of the pure Zn and Zn-10wt%Cu alloy samples were characterized by using X-ray diffraction?XRD?,optical microscopy?OM?,scanning electron microscopy?SEM?,energy dispersive spectroscopy?EDS?and transmission electron microscopy?TEM?techniques.Surface and cross-section microhardness tests were performed,and electrochemical tests were done in a 3.5wt%Na Cl aqueous solution to characterize changes in corrosion resistance.The main conclusions are summarized as followed:?1?XRD test results show that the intensity of the diffraction peak of the pure zinc sample on the?002?crystal plane gradually increases with the increase of the pulse number.The diffraction peak is widened and the angle slightly shifted.The interplanar spacing has changed,and the lattice constants of a and c show the same variation trend-firstly decreasing and then increasing slightly.The intensity of the?-Cu Zn5 phase diffraction peak on the surface of Zn-10wt%Cu alloy gradually increased with the increase of the pulse number.The?-Zn?101?diffraction peak of the samples is widened,and both a and c decreased to different degrees.These analyses demonstrate that the grains of the treated layers of both pure Zn and Zn-10wt%Cu alloy samples are refined.?2?Both the pure Zn and Zn-10wt%Cu alloy samples show wavy and volcanic crater-shape morphologies after HCPEB treatments.With the increase of the pulse number,the density of the craters on the surface of the pure Zn samples gradually decrease and the treated surfaces become smoother,and the cell crystals fuse with each other,leading to the formation of a long strip-shape,cell crystalline and the number of the small droplets erupted on the surface gradually decreases.The Zn-10wt%Cu alloy samples show the opposite morphological characteristics when compared with that of the pure Zn samples.As the number of pulses increasing,the density of volcanic craters in the treated surface of the Zn-10wt%Cu alloy increases,and the size of the craters decreases.The number and the size of the small droplets increase as the pulse numbers increase.This is mainly due to fatc that the content of?-Cu Zn5phase particles in the treated surface increases with the increasing pulse number,which can easily become a crater eruption site during pulsed electron beam processing.?3?The microhardness of both pure Zn and Zn-10wt%Cu alloy samples has been increased after HCPEB treatments.For pure Zn samples,the surface hardness of the sample increased from 22.8HV for the untreated sample to 61.7HV for the 15 pulsed sample?increased by nearly 2.7 times?.For Zn-10wt%Cu alloys,the hardness of the untreated,5 pulsed and 15 pulsed samples are 56.7HV,164HV and 165.9HV,respectively.The microhardness of both 5 pulsed and 15 pulsed samples are increased by nearly three times.For both pure Zn and Zn-10wt%Cu alloy samples,the cross-section microhardness shows the same variation trends—both reach the highest hardness at a certain depth from the surface,and then gradually decrease with the increase of the measured depth until it is consistent with the hardness of the substrate.The reason for the increase in hardness is mainly due to the grain refinement and thermal stress wave strengthening induced by the pulsed electron beam in the treated surfaces.?4?According to the polarization and electrochemical impedance spectroscopy tests in3.5wt%Na Cl aqueous solution,the corrosion resistance of both pure Zn and Zn-10wt%Cu alloys samples were improved after HCPEB treatments.For pure Zn sample,the 15 pulsed sample presents the best corrosion resistance.The corrosion potential increases from-1.237V to-1.021V,and the corrosion current decreases to 1.021×10-7A·cm-2,which is about an order of magnitude lower than that of the untreated sample(2.509×10-6A·cm-2).For Zn-10wt%Cu alloys samples,it can be seen from the potentio-dynamic polarization curves that all the samples show a passivation behavior during the electrochemical testing.The 5 pulsed sample presents the best corrosion resistance with the lowest icorr=3.06×10-6A?cm-2?nearly 1/50 of that of the untreated sample?and ip=2.807×10-4A?cm-2,as well as the highest Ecorr=-1.219V and Eb=-0.964V.The above results are mainly related to the surface state and the distribution and quantity of the second phase particles in the surface layer.After 5 pulse treatments,a large number of corrosion-sensitive locations such as craters appeared on the surface of pure Zn.After 15 pulses treatments,the surface becomes smoother.For Zn-10wt%Cu alloy samples,however,after 15 pulses treatments,the density of craters and droplets increases,which induce local corrosion and deteriorate the corrosion resistance. |
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