The Effect of Yttrium on Wear, Corrosion and Corrosive Wear of Mg-Al Alloys

• Author / Creator

  Nouri, Meisam

• Magnesium remarkably deserves considerable interest from industry especially transportation sector because of its high strength to weight ratio. However, poor wear resistance and low corrosion resistance of magnesium are barriers to extend use of its alloys. Yttrium is one of rare-earth elements which has been demonstrated effectiveness in improving the corrosion resistance and tribological properties of some metallic alloys. In this work, the effects of yttrium on wear, corrosion and corrosive wear of as-cast Mg-3 wt% Al and AZ31 alloys were investigated. It was shown that the added yttrium increased the dry wear resistance of the alloys through the formation of a harder second phases. The corrosion resistance of the alloys was evaluated in tap water and salt solution. A small amount of added Y improved the corrosion resistance but such benefit decreased at higher yttrium contents. Corrosive wear tests were performed in the same environment for the alloys. The improvement in corrosive wear resistance mainly came from the reduction of corrosion-wear synergy caused by the added yttrium, which increased both hardness and corrosion resistance. Relevant mechanisms to express the observations and the correlation between the performance and microstructure were analyzed and discussed in details. In addition to modification of the corrosion potential of bulk magnesium through alloying elements, tailoring its surface oxide film is an alternative approach for enhanced resistance to corrosion. In this study, effects of small amounts of a yttrium on the strength of surface oxide film formed on Mg-3 wt% Al alloy and its adherence to the substrate were investigated. It is demonstrated that the yttrium addition decreased the component ratio of hydroxides to oxides, mainly Mg(OH)2 to MgO, in the surface oxide film, leading to markedly enhanced mechanical strength and resistance to scratch with stronger adherence to the substrate. However, as the nominal yttrium concentration exceeded a certain level, the beneficial effect of Y on the film was weakened. Efforts were made to clarify underlying mechanisms. Surface modification is an effective approach for increasing wear and corrosion resistance of materials without affecting their bulk properties. Among various surface modification techniques, aluminizing is an economical and effective one which has been applied to various alloys. The effectiveness of aluminizing might be further enhanced by alloying with rare earth elements such as yttrium. In this research, effects of yttrium-incorporated aluminizing on the resistance of AZ31 alloy to corrosion, wear, and corrosive wear were investigated. The study demonstrates that aluminizing can be beneficial to the resistance of the alloy to wear, corrosion and corrosive wear. The addition of yttrium increased the benefits, largely attributed to the associated passive film. Effectiveness of aluminizing can be further enhanced by surface nanocrystallization. In this study, effects of surface nanocrystallization on the effectiveness of aluminizing in enhancing the resistance of AZ31 alloy to corrosion, wear and corrosive wear were investigated. The results demonstrated that aluminizing was effective to increase the resistance of the Mg alloy to surface failures. Surface nanocrystallization expanded the improvement, largely attributed to increased surface hardness and enhanced passivation capability.

• Subjects / Keywords

  • Surface film
  • Magnesium Alloy
  • Corrosion
  • Yttrium
  • Wear
  • Corrosive wear
  • Surface treatment

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3S757203

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.