Mechanical Properties of Transition Metal Alloys from First-PrinciplesTheory

Abstract: The aim of the thesis is to investigate the alloying and temperature effects on the mechanical properties of body-centered cubic (bcc) random alloys. We employ the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The second-order elastic constants reflect the mechanical properties of materials in the small deformation region, where the stress-strain relations are linear. Beyond the small elastic region, the mechanical properties of defect-free solids are described by the so called ideal strength. These two sets of physical quantities are the major topic of my investigations.In part one (papers I and II), the elastic constants and the ideal tensile strengths (ITS) are investigated as a function of Cr and Ti for the bccV-based random solid solution. We find that alloys along the equi-composition region exhibit the largest shear modulus and Young’s modulus, which is a resultof the opposite alloying effects obtained for the two cubic shear elastic constants C? and C44. The classical Labusch-Nabarro solid-solution hardening (SSH) model extended to ternary alloys predicts a larger hardening effect in V-Ti than in V-Cr alloy. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, we show that the present theoretical results can account for the observed variations of DBTT with composition. Under uniaxial [001] tensile loading, the ITS of V is 12.4 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ITSs are 36.4 and 52.0 GPa for V in the [111] and [110] directions, respectively. For the V-based alloys, Cr increases and Ti decreases the ITS in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ITS values as that of pure V. We show that the ITS correlates with the fcc-bcc structural energy difference and explain the alloying effects on the ITS based on electronic band structure theory.In part two (paper III), the alloying effect on the ITS of four bcc refractory HEAs based on Zr, V, Ti, Nb, and Hf is studied. Starting from ZrNbHf, we find that the ITS decreases with equimolar Ti addition. On the other hand, if both Ti and V are added to ZrNbHf, the ITS is enhanced by about 42%. An even more captivating effect is the ITS increase by about 170%, if Ti and V are substituted for Hf. We explain the alloying effect on the ITS based on the d-band filling. We explore an intrinsic brittle-to-ductile transition, which arises due to an alloying-driven change of the failure mode under uniaxial tension. Our results indicate that intrinsically ductile HEAs with high intrinsic strength can be achieved by controlling the proportion of group four elements to group five elements.In part three (papers IV and V), the ITS of bcc ferromagnetic Fe-based random alloys is calculated as a function of compositions. The ITS of Fe is calculated to be 12.6 GPa under [001] direction tension, which is in good agreement with the available theoretical data. For the Fe-based alloys, we predict that V, Cr, and Co increase the ITS, while Al and Ni decrease it. Manganese yields a weak non-monotonic alloying behavior. We show that the previously established ideal tensile strengths model based on structural energy differences for the nonmagnetic V-based alloys is of limited use in the case of Fe-bases alloys, which is attributed to the effect of magnetism. We find that upon tension all investigated solutes strongly alter the magnetic response of the Fe host from the unsaturated towards a stronger ferromagnetic behavior.In part four (paper VI), the temperature effect on the ITS of bcc Fe and Fe0.9Co0.1alloy is studied. We find that the ITS of Fe is only slightly temperature dependent below?500K but exhibits large thermal gradients at higher temperatures. Thermal expansion and electronic excitations have an overall moderate effect, but magnetic disorder reduces the ITS with a pronounced 90% loss in strength in the temperature interval?500 - 920K. Such a dramatic temperature effect far below the magnetic transition temperature has not been observed for other micro-mechanical properties of Fe. We demonstrate that the strongly reduced Curie temperature of the distorted Fe lattices compared to that of bcc Fe is primarily responsible for the onset of the drop of the intrinsic strength. Alloying additions, which have the capability to partially restore the magnetic order in the strained Fe lattice, push the critical temperature for the strength-softening scenario towards the magnetic transition temperature of the undeformed lattice. This can result in a surprisingly large alloying-driven strengthening effect at high temperature as illustrated in our work in the case of Fe-Co alloy