Phase field simulation of dendritic microstructure in additively manufactured titanium alloy

Additive manufacturing (AM) processes for metals, such as selective laser sintering and electron beam melting, involve rapid solidification process. The microstructure of the fabricated material and its properties strongly depend on the solidification. Therefore, in order to control and optimize the AM process, it is important to understand the microstructure evolution. In this work, using Ti-6Al-4V as a model system, the phase field method is applied to simulate the microstructure evolution in additively manufactured metals. First, the fundamental governing equations are presented. Then the effects of various processing related parameters, including local temperature gradient, scan speed and cooling rate, on dendrites’ morphology and growth velocity are studied. The simulated results show that the dendritic arms grow along the direction of the heat flow. Higher temperature gradient, scan speed and cooling rate will result in small dendritic arm spacing and higher growth velocity. The simulated dendritic morphology and arm spacings are in good agreement with experimental data and theoretical predictions

Motivating a Symmetric Copula ‘Be’ in Attitude Reports

Natural languages generally use such patterns as ‘A is B’ or ‘A and B are the same’ to mean an identity relation. However, it remains unclear what the cognitive mechanism actually is in using these identity statements. In this study, I embed the identity statements in attitude reports and investigate possible and impossible readings for such attitude reports as ‘John thinks A is B’, ‘John thinks A and B are the same’, etc. Intriguingly, the study reveals that felicitous ‘de dicto’ identity reports have no corresponding ‘de re’ reports. To account for this effect, I propose that the identity relation between A and B as encoded in natural languages means the contextually salient properties coerced from the expression A hold in a certain world (e.g., in the belief worlds of an attitude holder) for the individual named B and vice versa. The current analysis also suggests that natural language users can have access to some expressions in two different ways simultaneously: both as descriptions to describe certain objects and as variable names to refer to certain objects

Breaking de Morgan's law in counterfactual antecedents

The main goal of this paper is to investigate the relation between the meaning of a sentence and its truth conditions. We report on a comprehension experiment on counterfactual conditionals, based on a context in which a light is controlled by two switches. Our main finding is that the truth-conditionally equivalent clauses (i) "switch A or switch B is down" and (ii) "switch A and switch B are not both up" make different semantic contributions when embedded in a conditional antecedent. Assuming compositionality, this means that (i) and (ii) differ in meaning, which implies that the meaning of a sentential clause cannot be identified with its truth conditions. We show that our data have a clear explanation in inquisitive semantics: in a conditional antecedent, (i) introduces two distinct assumptions, while (ii) introduces only one. Independently of the complications stemming from disjunctive antecedents, our results also challenge analyses of counterfactuals in terms of minimal change from the actual state of affairs: we show that such analyses cannot account for our findings, regardless of what changes are considered minimal

The presupposition of even

I present a new observation with regard to the felicity of using even: There is no apparent focus/QUD congruence for even-sentences. For example,  Even Mary came cannot be used to answer a question like who came or who was unlikely to come. Instead, the felicitous use of Even Mary came is to address issues like how successful the exhibition was, how enthusiastic people were, how urgent the matter was, etc. Thus I propose that the use of even is QUD-sensitive, always with regard to a contextually salient degree question. Even brings a degree-based presupposition of additivity, not an entity-based one (see also Greenberg 2018 for a similar view).  An even-sentence presupposes that its prejacent is associated with a degree value, a benchmark value higher than the usual contextual threshold, resolving a degree question with an increasingly positive answer. E.g., under a relevant scenario about how popular a certain talk was, Even Mary came is roughly interpreted as (the talk was so popular) that Mary came. Under the current analysis,  the entity-based additivity and likelihood-based scalarity of even, which are considered presuppositions under the traditional view, are now considered implicatures.

STUDY ON THE STRUCTURAL, THERMAL AND MECHANICAL PROPERTIES OF LICOO2 USING FIRST PRINCIPLES METHOD

Thin film rechargeable battery has become a research hotspot because of its small size and high energy density. Lithium cobalt oxide as a typical cathode material in classical lithium ion batteries is also widely used in thin film rechargeable batteries. In this work, structural, mechanical and thermal properties of LiCoO2 were systematically investigated using first principles. Lattice constants, band gap and density of states were studied using Castep and VASP package, respectively. Elastic constants by applying various hydrostatic pressures between 0 to 40 GPa were computed. Specific heat and Debye temperature at low temperature were also discussed in this work. Other property including thermal conductivity was obtained using the imposed-flux method. The results show good agreement with experimental data and computational results in literature

Numerical Simulation of Li Diffusion in 3D Polycrystalline LiCoO2

poster abstractLiCoO2 is a commonly used cathode material of Li-ion rechargeable batteries. In battery applications, crystal anisotropy and grain boundaries have large influence on ion diffusion properties. To improve the battery performance, a thorough understanding of the diffusion process of Li ions is significant. In this study, 3D microstructures of various grain sizes were generated using phase field models. The apparent Li diffusion coefficient was obtained using finite element method. The relationship between the apparent Li diffusion coefficient, the grain boundary diffusivity, spatial distribution of grain orientations, and the grain size was discussed

Ab initio study of anisotropic mechanical properties of LiCoO2 during lithium intercalation and deintercalation process

The mechanical properties of LixCoO2 under various Li concentrations and associated anisotropy have been systematically studied using the first principles method. During the lithium intercalation process, the Young's modulus,bulk modulus,shear modulus, and ultimate strength increase with increasing lithium concentration. Strong anisotropy of mechanical properties between a-axis and c-axis in LixCoO2 is identified at low lithium concentrations, and the anisotropy decreases with increasing lithium concentration. The observed lithium concentration dependence and anisotropy are explained by analyzing the charge transfer using Bader charge analysis, bond order analysis, and bond strength by investigating partial density of states and charge density difference. With the decrease of Li concentration, the charge depletion in the bonding regions increases, indicating a weaker Co-O bond strength. Additionally, the Young's modulus,bulk modulus,shear modulus, and toughness are obtained by simulating ab initio tensile tests. From the simulated stress-strain curves, LixCoO2 shows the highest toughness, which is in contraction with Pugh criterion prediction based on elastic properties only

Phase Field Simulation of Dendritic Solidification of Ti-6Al-4V During Additive Manufacturing Process

In this study, the phase field method is applied to simulate the phase transformation of Ti-6Al-4V from liquid phase to solid phase during solidification. The simulated results show the dendritic arms grow along the direction of the heat flow. Droplets are found formed inside the dendrites. Solute enriches in the liquid near the dendritic tips and between the dendritic arms. The effects of various processing parameters, including local temperature gradient, scan speed, and cooling rate, on dendrites morphology and growth velocity are studied. The results show that the higher temperature gradient, scan speed, and cooling rate will result in smaller dendritic arm spacing and higher growth velocity. The simulated dendritic morphology and arm spacings are in good agreement with experimental data and theoretical predictions

Finite element study of Li diffusion and stress in LiNi0.33Mn0.33Co0.33O2 cathode using microstructures reconstructed by synchrotron X-ray tomography

poster abstractLiNi0.33Mn0.33Co0.33O2 is a good substitute for LiCoO2 because of its good thermal stability and high energy density. In this study, the diffusion and stress in LiNi0.33Mn0.33Co0.33O2 cathode with realistic three-dimensional (3D) microstructures have been studied systematically. Synchrotron Xray tomography was used to obtain the 3D reconstructions of porous LiNi0.33Mn0.33Co0.33O2 microstructures. Li concentration distributions under various C-rates were investigated. The obtained charge/discharge curves under various C-rates were compared with the results from Newman’s model. The stress generation in the cathode was computed coupled with the diffusion. The hydrostatic stress, shear stress and von Mises stress in the particles were analyzed. The results show that the von Mises stress in particle boundaries is higher than the stress inside the particle due to the Li concentration gradient during discharge, which is consistent with the literature. Additionally, the von Mises stress near the particle contact region is much higher than other areas

Abrasive Resistant Coatings—A Review

Abrasive resistant coatings have been widely used to reduce or eliminate wear, extending the lifetime of products. Abrasive resistant coatings can also be used in certain environments unsuitable for lubrications. Moreover, abrasive resistant coatings have been employed to strengthen mechanical properties, such as hardness and toughness. Given recently rapid development in abrasive resistant coatings, this paper provides a review of major types of abrasive coatings, their wearing mechanisms, preparation methods, and properties