Final Report Summary - ALPAM (Atomic-Level Physics of Advanced Materials)
In recent years it has become clear that in the search for advanced and high-performance materials, computational simulations based on atomic-level theories are going to play an increasingly important role. To ensure a predictive power, these simulations have to be based on an accurate representation of the atomic-level physics, which can only be achieved by first-principles quantum mechanical methods. In the overwhelming majority of cases, these methods deliver accurate data related to the atomic, electronic, chemical and magnetic structures of materials. Furthermore, the first-principles methods give a unique opportunity to study the atomic-level structures and phenomena, which are beyond present day experimental abilities.
The nine milestones formulated within the present project (see the project description) cover a large number of materials science problems ranging from ideal bulk systems to various defect structures, and focuses on the interplay between basic thermodynamic/mechanical properties and magnetism. The systems revolve around different alloys and compounds, with special emphasis on Fe alloys and steels. By working on these questions, we aim to shed light on the atomic-scale properties and processes behind the observed macroscopic properties and at the same time deliver comprehensive first-principles data for multi-scale modelling. As a result of the proposed study, detailed information on the composition–structure–property relations, defect interaction parameters and atomistic mechanisms of processes in the alloy phases has been obtained.
As main methodological achievements, we mention the newly developed quasi-non-uniform density functional scheme and the atomic-level approach to thermodynamic and kinetic properties of alloys with non-trivial magnetic degrees of freedom. Most recently, we put forward a transparent atomic-level theory of plasticity for face centered cubic metals and alloys. Our tools have been applied to the investigation of order-disorder phase transition in important class of magnetic alloys, including interstitial alloys. We have addressed the thermodynamics of different point and planar defects (vacancies, interfaces, and stacking faults) defects in Fe alloys, and revealed a series of anomalously magnetic effects. In particular, the interfacial energies in the nucleation and growth or spinodal decomposition mechanisms in Fe-Cr (ferrite) alloys have been shown to exhibit a marked magnetic state dependence. In Fe-Cr-Ni (austenite) alloys, the stacking fault energies, controlling the plastic deformation mechanisms, follow strongly nonlinear composition dependence, which can also be ascribed to the magnetic contribution to the defect energy.
Accurate knowledge of the elastic properties is indispensable in many practical applications, including the phenomenological modeling of the strengthening mechanisms. We have investigated the elastic and structural properties of pure Fe and chemically disordered Fe-based alloys as a function of temperature and composition. The calculated peculiar trends of the single-crystal and polycrystalline elastic parameters have been found to originate from the complex magnetic interactions in Fe.
Most of the scientific problems addressed in the present project are closely related to some important industrial problems. For instance, the effect of non-equilibrium segregation is believed to be mainly responsible for the Cr depletion at grain boundaries in austenitic stainless steel under irradiation, leading to radiation-induced stress-corrosion cracking which is often involved in accidents at nuclear power stations. This kind of information is needed for the development of alloys with improved characteristics and can be used by modern methods of materials design and optimization.
As a bonus, our deep insight about the electronic properties of Fe has made it possible to perform first class interdisciplinary work in geology. Extending our ongoing works on alloys to extreme conditions, we have studied the change of the miscibility barrier between iron and different alloying elements under high-pressure and high-temperature. Using ab initio lattice dynamical calculations that go beyond the quasi-harmonic approximation, we have shown that at high-pressure and high-temperature the bcc phase of Fe is dynamically stable. Furthermore, we have demonstrated that the properties of Fe-rich bcc alloys encompassing a small amount of light metals are consistent with those obtained from seismology, indicating that the bcc-structured Fe alloys are possible models for the Earth’s inner core.
The nine milestones formulated within the present project (see the project description) cover a large number of materials science problems ranging from ideal bulk systems to various defect structures, and focuses on the interplay between basic thermodynamic/mechanical properties and magnetism. The systems revolve around different alloys and compounds, with special emphasis on Fe alloys and steels. By working on these questions, we aim to shed light on the atomic-scale properties and processes behind the observed macroscopic properties and at the same time deliver comprehensive first-principles data for multi-scale modelling. As a result of the proposed study, detailed information on the composition–structure–property relations, defect interaction parameters and atomistic mechanisms of processes in the alloy phases has been obtained.
As main methodological achievements, we mention the newly developed quasi-non-uniform density functional scheme and the atomic-level approach to thermodynamic and kinetic properties of alloys with non-trivial magnetic degrees of freedom. Most recently, we put forward a transparent atomic-level theory of plasticity for face centered cubic metals and alloys. Our tools have been applied to the investigation of order-disorder phase transition in important class of magnetic alloys, including interstitial alloys. We have addressed the thermodynamics of different point and planar defects (vacancies, interfaces, and stacking faults) defects in Fe alloys, and revealed a series of anomalously magnetic effects. In particular, the interfacial energies in the nucleation and growth or spinodal decomposition mechanisms in Fe-Cr (ferrite) alloys have been shown to exhibit a marked magnetic state dependence. In Fe-Cr-Ni (austenite) alloys, the stacking fault energies, controlling the plastic deformation mechanisms, follow strongly nonlinear composition dependence, which can also be ascribed to the magnetic contribution to the defect energy.
Accurate knowledge of the elastic properties is indispensable in many practical applications, including the phenomenological modeling of the strengthening mechanisms. We have investigated the elastic and structural properties of pure Fe and chemically disordered Fe-based alloys as a function of temperature and composition. The calculated peculiar trends of the single-crystal and polycrystalline elastic parameters have been found to originate from the complex magnetic interactions in Fe.
Most of the scientific problems addressed in the present project are closely related to some important industrial problems. For instance, the effect of non-equilibrium segregation is believed to be mainly responsible for the Cr depletion at grain boundaries in austenitic stainless steel under irradiation, leading to radiation-induced stress-corrosion cracking which is often involved in accidents at nuclear power stations. This kind of information is needed for the development of alloys with improved characteristics and can be used by modern methods of materials design and optimization.
As a bonus, our deep insight about the electronic properties of Fe has made it possible to perform first class interdisciplinary work in geology. Extending our ongoing works on alloys to extreme conditions, we have studied the change of the miscibility barrier between iron and different alloying elements under high-pressure and high-temperature. Using ab initio lattice dynamical calculations that go beyond the quasi-harmonic approximation, we have shown that at high-pressure and high-temperature the bcc phase of Fe is dynamically stable. Furthermore, we have demonstrated that the properties of Fe-rich bcc alloys encompassing a small amount of light metals are consistent with those obtained from seismology, indicating that the bcc-structured Fe alloys are possible models for the Earth’s inner core.