Temperature dependent atomic-scale modeling of interfaces in cemented carbides

Material properties can now be calculated directly from first principles using density functional theory (DFT) which has a great predictive power and can, in cases that are difficult to approach experimentally, provide crucial insights on the atomic and electronic level. Such cases include the thermodynamics of interfaces and surfaces which are crucial factors for the structure and macroscopic properties of many materials, where cemented carbides are one example.Cemented carbides, or hardmetals, are composite materials manufactured by means of powder metallurgy, where carbide and binder metal powders are mixed, pressed, and sintered into a dense material. In this way the material gets a unique combination of hardness from the carbide and toughness from the binder. Cemented carbide is, therefore, an excellent choice of material in application where high hardness, wear-resistance, and toughness are crucial.In this thesis bulk, interface, and surface thermodynamics in cemented carbides are studied using DFT, but also using other atomistic descriptions derived from DFT including analytical bond order potential (ABOP), cluster expansions (CE) and force constant (FC) models. Further, free energies are calculated using methods such as thermodynamic and temperature integration from both molecular dynamics (MD) and Monte Carlo (MC) simulations, quasi harmonic approximation (QHA), effective harmonic models (EHM) from ab-initio molecular dynamics (AIMD), surface stress for liquid surface free energy and calculation of work of adhesion from separation and joining simulations.Wetting of WC surfaces and WC/WC grain boundaries is investigated in WC-Co and WC-Ni cemented carbides at elevated temperatures and it is concluded that, at liquid sintering temperatures, wetting of WC surfaces is only partial in C-rich materials while perfect in W-rich materials. Further, WC/WC grain boundaries are predicted to be stable also at liquid phase sintering temperatures. WC/WC grain boundary sliding is shown to be facilitated by infiltration of binder phase of only a few atomic layers proportion. Moreover, the hexagonal and cubic WC phases are investigated at high temperatures and a phase diagram is generated. Finally, the formation of thin cubic carbide films (complexions) in WC/Co phase boundaries is studied in both undoped and Ti-doped cemented carbides. These films are predicted at liquid phase sintering temperatures in both cases and also at solid state sintering temperatures in the Ti-doped case. In Ti-doped cemented carbides, the Ti atoms are found to mostly segregate to the second layer of the thin film and leave an essentially pure W layer towards Co

Problem reports and team maturity in agile automotive software development

Background: Volvo Cars is pioneering an agile transformation on a large scale in the automotive industry. Social psychological aspects of automotive software development are an under-researched area in general. Few studies on team maturity or group dynamics can be found specifically in the automotive software engineering domain. Objective: This study is intended as an initial step to fill that gap by investigating the connection between issues and problem reports and team maturity. Method: We conducted a quantitative study with 84 participants from 14 teams and qualitatively validated the result with the Release Train Engineer having an overview of all the participating teams. Results: We find that the more mature a team is, the faster they seem to resolve issues as provided through external feedback, at least in the two initial team maturity stages. Conclusion: This study suggests that working on team dynamics might increase productivity in modern automotive software development departments, but this needs further investigation.Comment: 5 page

Complexions and grain growth retardation: First-principles modeling of phase boundaries in WC-Co cemented carbides at elevated temperatures

WC-Co cemented carbides combine superb hardness with high toughness making them ideal for usage in metal machining and in wear resistant tools. Controlling the WC grain size is important during sintering as grain size plays a crucial role for the mechanical properties of the material. Experimental studies have observed different growth rates and grain morphologies in W-rich and C-rich materials, but the mechanism behind this has not been clarified. Here, we consider the possibility of an interface-stabilized state, a complexion, at the WC/Co phase boundary in cemented carbides, namely thin WC films with cubic structure. An interfacial phase diagram is derived using ab-initio calculations and first-principles modeling. Cluster expansions are employed to model carbon vacancies and Monte Carlo simulations to sample the configurational entropy. Force-constant fitting is used to extract the harmonic free energy for ground-state structures and the effects from anharmonicity and electronic excitations are effectively incorporated from a companion study on WC bulk phases. We predict the stabilization of thin cubic WC films at liquid phase sintering temperatures but only at W-rich conditions. This is consistent with experimental findings where thin films with cubic stacking have been observed predominantly in W-rich materials. We use this knowledge to suggest an explanation for the observed different growth rates and grain morphologies in W-rich and C-rich cemented carbides

First-principles modeling of complexions at the phase boundaries in Ti-doped WC-Co cemented carbides at finite temperatures

WC-Co cemented carbides have a unique combination of high hardness and good toughness, making them ideal as tool materials in applications such as metal machining or rock drilling. Dopants are commonly added to retard grain growth and thereby creating a harder material. Thin films with cubic structure have been observed experimentally at phase boundaries between hexagonal WC and fcc Co-rich binder when doping with, e.g., Ti, V, or Cr. These films are generally considered to play a crucial role in the grain growth inhibition effect. Therefore, the thermodynamics of these thin cubic films is important to understand. Here, we construct, using ab initio calculations and modeling, an interfacial phase diagram for thin cubic films in Ti-doped WC-Co. We consider C↔vacancy and W↔Ti substitutions by constructing alloy cluster expansions and use Monte Carlo simulations to calculate the configurational free energy. Furthermore, force-constant fitting is used to extract the harmonic free energy for the ground-state structures. Additionally, we use information from thermodynamic databases to couple our atomic-scale calculations to overall compositions of typical WC-Co materials. We predict that Ti segregates to WC/Co phase boundaries to form thin cubic films of two metallic layer thickness, both at solid-state and liquid-phase sintering temperatures. Furthermore, we predict that these films are stable also for low doping concentrations when no Ti-containing carbide phase precipitates in the material. We show that Ti essentially only segregates to the inner layer of the thin cubic film leaving an almost pure W layer towards Co, an ordering which has been observed in recent experimental high-resolution transmission electron microscopy studies

Cost-effective management of a eutrophicated sea in the presence of uncertain technological development and climate change

We analyse effects of uncertain climate change and technological development on cost-effective abatement of nitrogen and phosphorus for a eutrophied sea. A dynamic model is developed which accounts for differences in the sea’s adjustment to the loads of the two nutrients, uncertainty in climate change effects with probabilistic constraints on nutrient pool targets, and uncertain technological development in a mean-variance framework. The analytical results show that introduction of uncertainty increases abatement costs but that the effect on marginal abatement cost differ for the two types of uncertainty. Marginal abatement cost is increased by technological uncertainty but decreased by the reduction in the risk discount of climate change uncertainties. It is also shown that abatement along the optimal time path is delayed by the introduction of technological uncertainty, but made earlier when considering climate change uncertainty. An application to the eutrophied Baltic Sea indicates that climate change and technological development can reduce total abatement cost by 1/3, but also increase it considerably when uncertainty is included

Modeling of vibrational and configurational degrees of freedom in hexagonal and cubic tungsten carbide at high temperatures

Transition metal carbide is a class of materials characterized by high hardness, high melting points, and low chemical reactivity. It is widely used in industrial applications involving exposure to elevated temperatures, aggressive media, and heavy loads, and is thus of technological and industrial importance. In this paper the high-temperature thermodynamic properties of tungsten carbide, WC, is studied. At most temperatures below melting, WC assumes a hexagonal structure with essentially no vacancies (δ-WC). Only at very high temperatures (around 3000 K), WC crystallizes in the cubic rocksalt structure (γ-WC), which is more common for the transition metal carbides and in the case for WC can contain up to 40% carbon vacancies. At lower temperatures, γ-WC can, however, form as thin interfacial structures or nanoparticles. Hence, the thermodynamic properties of both γ-WC and δ-WC are of relevance. Here, we conduct a first-principles density-functional theory based computational investigation of the γ-WC and δ-WC phases, which requires modeling of high carbon vacancy concentrations at high temperatures. The configurational degrees of freedom are modeled with an alloy cluster expansion and sampled through Monte Carlo simulations. To account for the dynamic instability of the cubic γ-WC phase at low temperatures, the vibrational degrees of freedom are treated using effective harmonic models constructed from ab initio molecular dynamics simulations. Finally, we obtain a part of the W-C phase diagram in reasonably quantitative agreement with experimental data

Denying bogus skepticism in climate change and tourism research

This final response to the two climate change denial papers by Shani and Arad further highlights the inaccuracies, misinformation and errors in their commentaries. The obfuscation of scientific research and the consensus on anthropogenic climate change may have significant long-term negative consequences for better understanding the implications of climate change and climate policy for tourism and create confusion and delay in developing and implementing tourism sector responses

The Time and space for Earthly reckoning is here and now

The Time and space for Earthly reckoning is here and no