Corrosion And Wear Resistance Of Al Ion Implanted AZ31 Magnesium Alloys

Magneium alloys have low density, high strength to weight ratio, high dimensional stability, good machinability and are easily recycled. These properties make them valuable in a number of applications including automobile, aerospace and electronics. Despite the growing interests in magnesium alloys, their inferior corrosion and wear resistance is still a serious problem for widespread applications. So far, little is known about the corrosion and wear mechanisms of magnesium alloys, so it is difficult to find a way to improve their corrosion and wear resistance. Corrosion and wear are essentially surface related degradations that can be minimized by surface alloying using ion implantation. In this paper, metal element Al is implanted into AZ31 magnesium alloy. The purpose of this investigation is to study the wear and corrosion behaviors of the Al ion implanted AZ31 magnesium alloys under dry sliding condition and electrochemical polarization measurement, respectively, in order to explore the corrosion and wear mechanisms of magnesium alloys.The Al ion implantation into AZ31 magnesium alloys are carried out in a MEWA 80-10 ion implantation system with an ion implantation dose ranging from 2 ×10¹⁶ ions cm^-2 to 1 × 10¹⁷ ions cm^-2 at an ion energy of 40-50 keV at room temperature and a temperature of 300℃ The concentration depth profiles of implanted Al in AZ31 magnesium alloys with a dose of 1 ×10¹⁷ ions cm^-2 at room temperature and a temperature of 300℃ were a Gaussian-type distribution in the depth up to about 840 and 1200nm with the maximum Al concentration up to about 10 and 8 at. % respectively, measured by using Rutherford backscattering spectrometry (RBS). The microstructure, which is composed of α-Mg phase, intermetallic β-Mg₁₇Al₁₂ and MgO phase is observed on the unimplanted and implanted samples by x-ray diffraction (XRD). Another diffraction peak of β-Mg₁₇Al₁₂ phase is observed on the Al ion implanted AZ31 magnesium alloys with the higher Al ion implantation dose of 1×10¹⁷ ions cm^-2 at room temperature. The diffraction peak of β-Mg₁₇Al₁₂ phase is observed on the Al ion implanted AZ31 magnesium alloys with the lower Al ion implantation dose of 2×10¹⁶ ions cm^-2 at a temperature of 300℃.The potentiodynamic polarization curves show that the corrosion resistance of Al ion implanted samples with an implantation dose from 2×10¹⁶, 6×10¹⁶ and 1×10¹⁷ ions cm^-2 is superior to that of unimplanted sample in the 0.01 mol/1 NaCl solution with a pH value of 12.The samples with an implantation dose of 6 X1016 ions cm"2 have the higher corrosion potential, passivity properties and pitting corrosion breakdown potential compared with the samples with lower ion implantation doses. Corrosion potential has an improvement of 180 mV (SCE) and 280 mV (SCE), pitting corrosion breakdown potential has an improvement of 150 mV (SCE) and 220 mV (SCE) respectively, for the samples with a dose of 1X1017 ions cm"2 at room temperature and a temperature of 300sC in the 0.08 mol/1 NaCl solution with a pH value of 12.The increases in microhardness are observed for all the Al ion implanted samples as compared to the unimplanted sample. The microhardness reduces with the increasing ion implantation doses. The microhardness of the Al ion implanted sample with 2X 1016 ions cm"2 was achieved to HV0.05 n =900 MPa. The wear resistance of AZ31 magnesium alloys is improved by ion implantation under the investigated wear conditions. The profilometer patterns of wear track cross-section of the unimplanted sample show that the plastic deformation occurred and deep and wide grooves are observed. For the Al ion implanted sample with a dose of 2 X1016 ions cm"2 at room temperature, the wear track depth is reduced by 30 % and the wear rate is reduced by 20 % respectively, compared with that of unimplanted sample under a load of 5 N for a duration of 10 min; The wear rate is reduced by 30-40 % under a load of 20 N for a duration of 20 min. For the Al ion implanted sample with a dose of 6X1016 ions cm"2 at a temperature of 300QC, the wear rate is reduced by 15-20 % under a load of 20 N for a duration of 20 min. The early friction coefficients of Al ion implanted samples are higher than that of umimplanted sample. The SEM and EDS analysis show that the prevailing wear mechanism, abrasion, are confirmed by the appearance of the wear debris, formed during the wear tests under the applied loads of 5 N and 20 N for the unimplanted sample. The bamboo-like magnesium oxide whose oxygen content is above 30 % are observed on the wear tracks surface of the Al ion implanted samples at room temperature and a temperature of 3002C under a load of 20 N for a duration of 20 min. The formation of magnesium oxide provides lubricating action and minimizes friction and wear.The Al ion implanted sample with a dose of 2 X1016 ions cm"2 at room temperature has the best corrosion and wear resistance.