Mechanical Properties of Transition Metal Alloys from First-Principles Theory

Detta är en avhandling från KTH : KTH Royal Institute of Technology

Sammanfattning: The aim of the thesis is to investigate the alloying effect on the mechanical properties of random alloys using the all-electron exact muffin-tin orbitals methodin combination with the coherent-potential approximation. The second-order elastic constants describe the mechanical properties of materials in the small deformation region, where the stress-strain relations arelinear. Beyond the small elastic region, the mechanical properties of dislocation-free solids are described by the ideal strength.The elastic constants and ideal tensile strengths have been investigated as a function of Cr and Ti for the body centered cubic V-based random solidsolution. Alloys along the equi-composition region are found to exhibit the largest shear and Young’s modulus as a result of the opposite alloying effectsobtained for the two cubic shear elastic constants C' and C44.Classical solid-solution hardening (SSH) model predicts larger hardening effect in V-Ti thanin V-Cr alloy. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, itis shown that the present theoretical results can account for the variations of DBTT with composition. Under uniaxial [001] tensile loading, the ideal tensilestrength of V is 12.4 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ideal tensile strength are 36.4 and 52.0 GPa for V inthe [111] and [110] directions, respectively. For the V-based alloys, Cr increases and Ti decreases the ideal tensile strength in all principal directions. Addingthe same concentration of Cr and Ti to V leads to ternary alloys with similar ideal tensile strength values as that of pure V. The alloying effects on the idealtensile strength are explained using the electronic band structure.The ideal tensile strengths of bcc ferromagnetic Fe-based random alloys have been calculated as a function of compositions. The ideal tensile strength of Fe in the [001] direction is calculated to be 12. 6GPa,in agreement with the available data. For the Fe-based alloys, we predict that V, Cr, and Co increase the ideal tensile strength, while Al and Ni decrease it. Manganese yields a weak non-monotonous alloying behavior. We show that the limited use of the previouslyestablished ideal tensile strengths model based on structural energy differences in the case of Fe-bases alloys is attributed to the effect of magnetism. We find that upon tension all the investigated solutes strongly alter the magneticresponse of the Fe host from the unsaturated towards a stronger ferromagnetic behavior.