A FIRST-PRINCIPLES STUDY OF STACKING FAULTS AND LONG PERIODIC STACKING ORDER STRUCTURES IN MG AND MG ALLOYS

Open Access
Author:
Wang, Yi
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 10, 2013
Committee Members:
  • Zi Kui Liu, Dissertation Advisor
  • Zi Kui Liu, Committee Chair
  • Long Qing Chen, Committee Member
  • Jorge Osvaldo Sofo, Committee Member
  • Suzanne E Mohney, Committee Member
  • Suveen N Mathaudhu, Special Member
Keywords:
  • Stacking fault
  • Long periodic stacking order (LPSO)
  • Deformation electron density
  • Atomic array/cluster
Abstract:
Since the main challenges in developing Mg alloys are to increase their strength, ductility, as well as stability at high temperatures, it is crucial to understand the fundamental properties which affect their mechanical properties. Formation of stacking faults is fundamental in deformation of materials with the hcp structure such as Mg and Ti alloys, affecting core structures and the mobility of dislocations, twinnability and ductility, and creep rate. Moreover, long periodic stacking order (LPSO) structures, such as 6H, 10H, 14H, 18R and 24R, play significant roles in enhancing the mechanical properties of Mg alloys and have been largely investigated separately. In the present work, contributions of stacking faults, LPSOs and alloying elements to the formation energy, elastic, electronic and phonon properties of Mg and Mg alloys are investigated through the first-principles calculations. In pure Mg, the connections among stacking faults and LPSOs are discussed. Three typical basal-plane stacking faults, i.e. growth fault (I1), deformation fault (I2) and extrinsic fault (EF), are investigated, showing that the stacking fault energy increases in the order of I1 < I2 < EF. Moreover, through the electron localization morphology, electronic structures of these three stacking faults are revealed in terms of deformation electron density (Δρ) and electron localization function (ELF). These results yield a quantitative description of charge transfer between atoms in and out of the stacking faults. We also obtain a brief physical correlation between stacking fault energy and the difference of Δρ and ELF between the fault planes and the non-fault planes. Furthermore, through detailed investigations of deformation electron density, we show that the electron structures of 10H, 14H, 18R and 24R LPSO structures in Mg originate from those of deformation stacking faults in Mg, and their formation energies can be scaled with respect to formation energy and the number of layers of deformation stacking faults, while the electron structure and formation energy of the 6H LPSO structure are between those of deformation and growth stacking faults. The simulated images of high resolution transmission electron microscopy compare well with experimental observed ones. In the end, effects of fault layers in SFs and LPSOs on the local phonon density of states and vibrational entropy are discussed together with their specific electronic structures. In the binary Mg-X alloys, contributions of 17 alloying elements to the energy and the bond structure of growth, deformation and extrinsic faults are investigated. In view of electron localization morphology, the bonding structure of Mg around the fault plane can be recognized as the HCP-FCC transformations in short-range. Together with the specific electron structure of each alloying element, it has been confirmed that bond strength of the fault planes are strengthened by FCC-Al and HCP-Zn since tetrahedrons around alloying elements have more electron density. Taking Gd and Y as examples, their interactions with faults layers of 6H and 10H LPSO are presented in view of excess energy and deformation electron density. It has been determined that (i) with the addition of Gd and Y, the excess energy of 6H and 10H can be decreased significantly, indicating that the formability of 6H and 10H LPSO will be increased in Mg-10RE (wt %) alloys; (ii) Gd and Y prefer to occupy the position in fault layers of Mg-10RE with 6H and 10H LPSOs; (iii) since the excess energy will be close to and smaller than that of the pure, the atomic array model can be used in Mg-10Y with 6H and 10H LPSOs, while the atomic cluster model can be used in Mg-10Gd with 6H and 10H LPSO and (iv) the bond strength of the basal plane characterized by Δρ is strengthened around the RE effect zone, while that of prismatic and pyramidal planes will be weakened caused by the electron redistributions effected by the contributions of RE and LPSOs. In the ternary Mg-TM-RE alloys, contributions of alloying elements and fault layers to the energy, electronic structure and elastic properties of 6H and 10H LPSOs are discussed through our proposed atomic array/cluster model. In the view of excess energy, the energetic favorable configurations of the 6H LPSO in Mg98Zn2, Mg98Y2 and Mg97Zn1Y2 (at %) have been estimated via first-principles calculations. Through the formation of an atomic array of Y forming with Zn occupying its 1st nearest neighbor, the ductility of Mg97Zn1Y2 will be increased and the ratio of B/G is increased from 1.95 of Mg to 2.09. According to the energetic favorable structure of Mg-10Gd with 6H and 10H LPSOs, effect of TM elements, such as Zn and Zr, on their formability in Mg-10Gd-TM alloys has been estimated. With the addition of Zn and Zr, the excess energy of the 6H and 10H LPSOs can be decreased significantly when forming the cluster between the TM and Gd, indicating the formability of 6H and 10H will be increased in Mg-Gd-Zr and Mg-Gd-Zn alloys. Thus, with the addition of TMs into Mg alloys with LPSOs, the excess energy will be reduced to make the structure more stable than Mg alloy without TM. In view of the deformation electron density and electron localization function, the strengthen mechanism of alloying elements in the Mg alloy is that the basal plane of Mg is strengthened due to the formation of stronger chemical bond between the atomic array and Mg matrix. With the addition of TMs into Mg alloys with LPSOs, the excess energy will be reduced to make the structure more stable than that of without the TM in the Mg alloy. This work enables quantitative investigations of effects of alloying elements on the properties of Mg alloys. The understanding of stacking faults and LPSO structures in Mg enables future quantitative investigations of effects of alloying elements on properties of LPSO structures and Mg alloys.