Atomic-scale Investigation Of Crystalline Defects And Solute Segregation In Them In Mg-Bi And Mg-Gd Alloys

Mg alloys are the lightest metallic structural materials.They have great potential applications in automotive,3C(computer,communication and consumer electronic product),aircraft,aerospace industries due to the excellent properties such as high specific strength,high specific stiffness,superior damping performance,good thermal conductivity,etc.However,their low strength and poor ductility hinder their industrial applications.The plastic deformation process of Mg alloys involves slip and twinning.Therefore,knowledge on the structural features and formation mechanisms of crystal defects(dislocations and twins),interactions between different crystal defects,and interactions between solute atoms and crystal defects in Mg alloys provides a basis for the better understanding of deformation mechanisms of Mg alloys and,more importantly,for the rational alloy design in practice.In this thesis,the stacking faults and dislocation structures in a cold-rolled Mg–Bi alloy were systematically investigated using atomic-scale high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM).It is found that there are two types of stacking faults in the cold-rolled alloy:I₁ and I₂.Most of I₁ faults are formed by dissociation of<c+a>dislocations,and only a few have a structure similar to that of growth I₁.The I₁ faults are distributed densely inside{10(?)2}twins but sparsely in the magnesium matrix.The I₂ fault results from dissociation of a basal<a>dislocation into two Shockley partials and exists in both matrix and{10(?)2}tension twins.An unusual I₁ is also observed.The total projected Burgers vector of this I₁ fault resembles that of a 60°basal<a>dislocation.The formation of this I₁ fault is rationalized by the interaction of the bounding Frank partial of an I₁ and a basal<a>dislocation.Unexpectedly,Shockley partials(1/3<10(?)0>)are often found lying at the edge of steps within the I₁ faults.Three cases are categorized in terms of the number and the sign of the Shockley partials located in each single I₁ fault:one Shockley partial,two Shockley partials having the same sign,and two Shockley partials having opposite signs.The fault bounded by two Shockley partials is explicitly I₁ but not I₂,which is different from the dissociation of the basal<a>dislocation.Geometric analysis indicates that the formation of Shockley partials formed in the I₁ fault results from interactions between basal dislocations and the bounding Frank partial of the I₁ faul.Solute atom segregation behaviors at coherent twin boundaries(CTBs)of a Mg-Bi and a Mg-Pb alloy have also been investigated.It is found that Bi or Pb atoms that are larger than Mg unexpectedly segregate to the compression sites of{10(?)1}CTBs and do not segregate to the extension or compression site in{10 (?)2}CTBs.The HAADF-STEM image simulation suggests that the occupancy of Bi and Pb atoms that segregate into the compression sites of{10(?)1}CTBs are?20%and?40%,respectively,rather than the value of 100%that is widely accepted in the past.Apart from the strain field effect,the strong interaction between solute atoms and Mg atoms in the CTB might be another important factor in triggering solute segregation patterns in the CTBs.The effect of Gd additions on the twinning behavior in Mg were studied.It is found that,(1)in the Mg–Gd alloys,{11(?)1}and{10(?)2}twinning modes are both activated and the number density of{11(?)1}twins increases with the Gd addition in Mg.(2)First-principles calculations suggest that the formation of{11(?)1}twins in Mg–Gd alloys is related to the lowering of{11(?)1}twin boundary energy with the increasing solute concentration.This reduction is caused by relaxation of the elastic strain along the{11(?)1}twin boundary.