Modification of primary carbides in hypereutectic HCCIs

• Author / Creator

  Li, Jiaqi

• High chromium cast irons (HCCIs) are widely used in oil sands, mining, and manufacturing industrial sectors due to its high resistance to wear, corrosion and corrosive wear. HCCIs are composed of hard carbides and ferrous matrix, resulting in high hardness and reasonably good toughness. In hypereutectic HCCIs, the primary M7C3 carbides have greater brittleness compared to the supporting matrix, which may result in detrimental stress at the interface, due to the lattice mismatch increasing the probability of cracking and thus deteriorating the wear resistance. Moreover, since the cracking would be zigzagged at grain boundaries, the expanding of crack would be accelerated with bulky M7C3 carbides. Therefore, modification of primary carbides can be an effective method to improve the hypereutectic HCCIs’ overall performance.
  Core-shell structured carbides have been reported to be beneficial to wear resistance of HCCIs. However, the compositional range of forming such carbides is limited. In the first part of the thesis, thermodynamics analysis was conducted to investigate the conditions for forming core-shell structured carbides and determine the probability of producing such carbides in a wider compositional range via alloying elements. With the guide of thermodynamics calculations, core-shell structured carbides were successfully formed in HCCIs with different carbon concentrations by alloying with boron, confirmed by experiments. For more information, first-principles calculations were conducted to investigate the effect of the B addition on mechanical properties of the M23C6 shell in comparison with those of non-doped M23C6. Also, the reason for B addition promoting the formation of core-shell structured carbides was demonstrated by interfacial energies of M23C6/Fe and M23C6/M7C3 interfaces with or without B, which were achieved using first-principles calculations.
  In the second part of the thesis, the effect of alloying elements W and V on mechanical properties of (Fe, Cr, X)7C3 (X=W, V) is investigated. M7C3 carbides were tailored by partially substituting the metallic elements with W or V via thermodynamics guidance. A significant correlation between Young’s modulus and electron work function of carbides was experimentally observed using multimode atomic force microscope. First-principles calculations were conducted to calculate the modulus and electron work function as a comparison to the experimental data. The atomic bond characteristics in M7C3 were investigated. The study correlates the electronic characteristics and mechanical behavior of carbides and guides in designing advanced multi-component M7C3 carbides.

• Subjects / Keywords

  • carbides
  • Cast irons

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-k7pn-mc87

• License

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