[SY-C6] Effect of interstitial solutes on the structure and mobility of screw dislocations in bcc metals
Invited
There is ample experimental evidence of the effects of solute interactions with dislocations, such as Cottrell atmospheres and dynamical strain ageing (DSA). In this work we investigated, using Density Functional Theory (DFT) electronic structure calculations, the interaction between interstitial solute atoms, in particular carbon, and screw dislocation cores in body centered cubic (bcc) metals, in particular iron.
First considering carbon in bcc Fe, our calculations evidence a strong attractive interaction of carbon with screw dislocation cores inducing an unexpected reconstruction towards a hard-core configuration, which is unstable in pure metals. The carbon atoms are at the center of regular trigonal prisms formed by the Fe atoms. Around this decorated core, the 4th nearest neighbor octahedral sites are found to be also highly attractive for carbon. The complex thermodynamic behavior of carbon segregation on these dislocation sites, as function of temperature and bulk carbon concentration, is modelled using mean field and Monte Carlo methods. The parameters of the Hamiltonian, fitted to DFT calculations, account for the C-C repulsions between neighboring sites. It is concluded that in the temperature range corresponding to the occurrence of DSA, the carbon atoms occupy every other prismatic sites, while the other sites are essentially empty. This configuration is then used to study the kink-pair mechanism suggested by in situ straining experiments. The results obtained for the kink-pair formation energy and single kink migration energies are consistent with the activation energy of the end of DSA regime.
The solute segregation on dislocation-core prismatic sites was investigated for other solutes in Fe and for C in other bcc metals. A behavior similar to Fe-C is exhibited for B, N and O in Fe and for C in group 6 metals, Mo and W, but with solute-solute interactions between prismatic sites which can be either attractive or repulsive. By way of contrast, the configuration of lowest energy in group 5 consists of the dislocation in its easy core and the carbon atom in a fifth nearest neighbor octahedral site. We show that this group dependence is consistent with the carbon local environment in the stable stoichiometric carbide structures, namely cubic NaCl-type for group 5 and hexagonal WC-type for group 6.
First considering carbon in bcc Fe, our calculations evidence a strong attractive interaction of carbon with screw dislocation cores inducing an unexpected reconstruction towards a hard-core configuration, which is unstable in pure metals. The carbon atoms are at the center of regular trigonal prisms formed by the Fe atoms. Around this decorated core, the 4th nearest neighbor octahedral sites are found to be also highly attractive for carbon. The complex thermodynamic behavior of carbon segregation on these dislocation sites, as function of temperature and bulk carbon concentration, is modelled using mean field and Monte Carlo methods. The parameters of the Hamiltonian, fitted to DFT calculations, account for the C-C repulsions between neighboring sites. It is concluded that in the temperature range corresponding to the occurrence of DSA, the carbon atoms occupy every other prismatic sites, while the other sites are essentially empty. This configuration is then used to study the kink-pair mechanism suggested by in situ straining experiments. The results obtained for the kink-pair formation energy and single kink migration energies are consistent with the activation energy of the end of DSA regime.
The solute segregation on dislocation-core prismatic sites was investigated for other solutes in Fe and for C in other bcc metals. A behavior similar to Fe-C is exhibited for B, N and O in Fe and for C in group 6 metals, Mo and W, but with solute-solute interactions between prismatic sites which can be either attractive or repulsive. By way of contrast, the configuration of lowest energy in group 5 consists of the dislocation in its easy core and the carbon atom in a fifth nearest neighbor octahedral site. We show that this group dependence is consistent with the carbon local environment in the stable stoichiometric carbide structures, namely cubic NaCl-type for group 5 and hexagonal WC-type for group 6.