| Al ion implantation and HIPIB irradiation into AZ31 magnesium alloy are carried out in a MEVVA 80-10 ion implantation system and in TEMP-6 type HIPIB apparatus,respectively. The changes in composition,structure and properties of ion beams modified AZ31 magnesium alloy are revealed and the combined improvement in wear and corrosion resistance of AZ31 magnesium alloy is achieved.The modification mechanism in the wear and corrosion resistance of Al ion implanted and H1PIB irradiated AZ31 magnesium alloy are also illustrated in terms of the changes in composition,structure and properties of modified surface.The Al ion implantation into AZ31 magnesium alloys is carried out at an implantation dose of 2×1016,6×1016 and 1×1017 ions/cm2 at room temperature and at 300℃.The concentration depth profiles of implanted Al in AZ31 magnesium alloys with an implantation dose of 1×1017 ions/cm2 at room temperature and at 300℃C were a Gaussian-type distribution in the depth up to about 840 and 1200 nm with the maximum Al concentration up to about 10 and 8 at.%respectively,measured by using Rutherford backscattering spectrometry(RBS). The microstructure,which is mainly composed ofα-Mg phase,and small amount of intermetallicβ-Mg17Al12 and MgO phase is also observed on the original and implanted samples by X-ray diffraction(XRD).The new diffraction peak ofβphase is observed on the Al ion implanted AZ31 magnesium alloys with the higher implantation dose of 1×1017 ions/cm2 at room temperature,whereas the new diffraction peak ofβphase is observed on the Al ion implanted AZ31 magnesium alloys with the lower implantation dose of 2×1016 ions/cm2 at 300℃.The combined improvement in wear and corrosion resistance is achieved for the Al ion implanted AZ31 magnesium alloys at room temperature and at 300℃using a reciprocating tribometer in a ball-on-flat configuration where the applied load is 5 and 20 N at a sliding speed of 2 mm/s and potentiodynamic polarization technology in the 0.01 mol/1 NaCl solution with a pH value of 12,respectively.For the Al ion implanted sample with a dose of 2×1016 ions/cm2 at room temperature,the wear volume is reduced by 20%respectively,compared with that of the original sample under a load of 5 N for a sliding time of 10 min,and the wear volume is reduced by 30-40%under a load of 20 N for a sliding time of 20 min.For the Al ion implanted sample with a dose of 6×1016 ions/cm2 at a temperature of 300℃,the wear volume is reduced by 1.5-20%under a load of 20 N for a sliding time of 20 min.Under a load of 5 N,the original sample exhibited a typical abrasive wear mechanism,characterized by microcutting and plastic deformation,and Al ion implantation did not change the wear mechanism of magnesium alloy.Under a load of 20 N for a sliding time of 20 min,the transitions in wear mechanism from severe metallic wear to oxidational wear for the Al ion implanted samples at room temperature and at 300℃were observed.The potentiodynamic polarization curves show that the corrosion resistance of Al ion implanted samples with an. implantation dose of 2×1016,6×1016 and 1×1017 ions/cm2 at room temperature and at 300℃are superior to that of the original one in the 0.01 mol/l NaCl solution with a pH value of 12. The sample with an implantation dose of 6×1016 ions/cm2 has the superior corrosion resistance,and the corrosion potential and pitting corrosion breakdown potential are 400 and 800 mV higher than those of the original sample,respectively.For the sample implanted with an implantation dose of 1×1017 ions/cm2 at room temperature and at 300℃,corrosion potential is higher 180 and 280 mV,and the pitting corrosion breakdown potential is higher 150 and 220 mV than those of the original sample,in the 0.08 mol/l NaCl solution with a pH value of 12.It is believed that the combined improvement in wear and corrosion resistance for Al ion implanted samples at room temperature and at 300℃is attributed to the solid solution enhancing effects and the second phaseβenhancing effects induced by Al enrichment.HIPIB irradiation into AZ31 magnesium alloy at ion current densities of 100-300 A/cm2 with shot number of 1-10 led to a typical surface morphology with crater formation and waviness features in local region due to a selective ablation and disturbance of the molten surface.Compared with the original sample,the microstructure of remelted layer and heat-affected layer are significantly refined with increasing the ion current density and the shot number.HIPIB irradiation results in the significant chemical homogeneity, microstructural refinement and the homogenous redistribution of the second phase ofβon the irradiated surface due to the repetitive rapid melting and resolidification of surface irradiated by HIPIB.The combined improvement in wear and corrosion resistance is achieved for HIPIB irradiated AZ31 magnesium alloys using a tribometer in a ball-on-disk configuration where the applied load is 0.1-0.5 N at a sliding speed of 0.08 m/s and potentiodynamic polarization technology in the 0.01-0.5 mol/1 NaC1 solution with a pH value of 12,respectively.The irradiated sample at 300 A/cm2 with 10 shots exhibited the superior wear resistance,the friction coefficient decreased from 0.378 for original sample to 0.338,and the wear volume is reduced about 75%compared with the original one.The friction coefficient for the original and irradiated samples at 300 A/cm2 with 10 shots decreases from 0.364 and 0.334 at a load of 0.2 N to 0.361 and 0.313 at a load of 0.5 N,and the wear volume is reduced about 40%and 15%compared with the original one,respectively.The wear mechanism for HIPIB irradiated samples is still typical of abrasive wear,however,the degree of abrasive wear is significantly reduced,and the improved wear resistance is characterized by reduced localized plastic deformation and shallower grooves,and the oxidational wear is observed on the local region of wear track.The apparent improvement in corrosion resistance is achieved for all the irradiated samples in 0.01 mol/l NaCl solution with a pH value of 12.The corrosion potential and pitting corrosion breakdown potential for the samples irradiated at 100 A/cm2 with 5 shots are 560 and 630 mV higher than those of the original sample,respectively.Typical corroded surface morphologies for the original and irradiated samples revealed that the original sample suffered from a severe pitting corrosion attack,leading to the formation of voluminous corrosion products that crack severely during corrosion process.In contrast to the original sample,less pitting corrosion attack is observed for the irradiated samples,indicating the enhanced protective properties of surface formed on the irradiated samples due to remelted effect induced by HIPIB irradiation.Corrosion resistance for both the original and irradiated samples decreased with increasing the Cl-concentration in NaCI solution,however,improved corrosion resistance of irradiated sample is still obtained in NaCl solution containing higher Cl- concentration.The chemical homogeneity and microstructural refinement of the irradiated surface induced by HIPIB irradiation are the main reasons for the combined improvement in wear and corrosion resistance of AZ31 magnesium alloys.
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