Study On Effects Of Minor Oxygen Element And Microstructure On Corrosion Resistance Of Zr-Based Bulk Metallic Glasses

Zr-based bulk metallic glasses（BMGs）are promising candidates for practical application due to their good glass forming ability and combined properties.Corrosion resistance is one of the key properties that has to be considered for practical application.Oxygen impurity and structural relaxation,which can probably deteriorate corrosion resistance,are the two major concerns,while rejuvenation could be an effective method for improving properties,when practically applying Zr-based BMGs.So far,no reports are available for the effects of oxygen impurity on corrosion of Zr-based BMGs and the effects of structural relaxation and rejuvenation are complex with unclarified mechanisms.Therefore,it is practically meaningful for clarifying the effects,as well as the underlying mechanisms,of oxygen impurity,structural relaxation and rejuvenation on corrosion resistance of Zr-based BMGs.Meanwhile,such the three factors can significantly affect the microstructure of metallic glasses,such as free volume,local ordering and atomic clusters.In this situation,they can be regarded as research model for correlating the microstructure and corrosion resistance of metallic glasses,which is theoretically meaningful for advancing the understanding of corrosion mechanisms of metallic glasses.For the practical application of Zr-based BMGs,this work aims at clarifying the effects of oxygen impurity,structural relaxation and rejuvenation on corrosion resistance,evaluating the corrosion resistance of Zr-based BMGs containing oxygen impurity in various corrosive media,correlating the microstructure and pitting corrosion,and unmasking the micro-structural factors dominating pitting corrosion,by thoroughly investigating the corrosion behaviors of oxygen-containing,structurally relaxed and rejuvenated Zr-based BMGs.The main findings are summarized as follows:As increasing the content of oxygen impurity,the pitting corrosion resistance of Zr₆₁Ti₂Cu₂₅Al₁₂ BMGs in 3.5 wt.%NaCl solution increases first and then decreases,with the highest resistance appearing at 1200 ppm.The improvement of pitting corrosion resistance can be attributed to the oxygen-induced annihilation of free volume.When the oxygen content further rises to 1800 ppm,the formation of crystals can deteriorate the pitting corrosion resistance.The Zr₆₁Ti₂Cu₂₅Al₁₂ BMGs containing various content of oxygen impurity exhibit high resistance to the corrosion of 0.5 M H₂SO₄ and 0.5 M NaOH solutions,as long as the oxygen content does not exceed 1200 ppm.It demonstrates that the oxygen-containing Zr₆₁Ti₂Cu₂₅Al₁₂ BMGs are suitable for application in various common corrosive media.The structural relaxation has dual effects on the pitting corrosion resistance of Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMGs with fully amorphous structure in 3.5 wt.%NaCl solution.When the relaxation time is shorter than 4 h,the pit mainly initiates at the region with more free volume,corresponding to the structural heterogeneity.The relaxation-induced annihilation of free volume can inhibit the pit initiation and hence improve the pitting corrosion resistance.In contrast,further prolonging the relaxation time to 9 h,the chemical heterogeneity is generated with obvious local element composition fluctuation.Such the chemical heterogeneity can induce new pit initiation,facilitating pitting corrosion.When Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ BMGs are rejuvenated by enthalpy relaxation,the free volume is increased with slightly enhanced local ordering.The rejuvenated samples exhibit similar pitting corrosion resistance to the relaxed ones.It is because only part of the free volume is recovered and the chemical heterogeneity generated in relaxed samples is mainly attained during the rejuvenation process.The similar chemical heterogeneity can definitely lead to similar pitting corrosion resistance between the rejuvenated and relaxed samples.This work proposes a mechanism model correlating the microstructure and pitting corrosion resistance of Zr-based BMGs,based on present findings and those reported in the literature.It speculates that the dominating pit initiation mechanism can be changed depending on the structural（free volume）and chemical（local element composition fluctuation）heterogeneities,considering that pit initiation is sensitive to microstructure.This mechanism model can be used to reasonably explain the effects of oxygen impurity,structural relaxation and rejuvenation on pitting corrosion resistance of Zr-based BMGs.