Linking atomistic, kinetic Monte Carlo and crystal plasticity simulations of single-crystal tungsten strength

Understanding and improving the mechanical properties of tungsten is a critical task for the materials fusion energy program. The plastic behavior in body-centered cubic (bcc) metals like tungsten is governed primarily by screw dislocations on the atomic scale and by ensembles and interactions of dislocations at larger scales. Modeling this behavior requires the application of methods capable of resolving each relevant scale. At the small scale, atomistic methods are used to study single dislocation properties, while at the coarse-scale, continuum models are used to cover the interactions between dislocations. In this work we present a multiscale model that comprises atomistic, kinetic Monte Carlo (kMC) and continuum-level crystal plasticity (CP) calculations. The function relating dislocation velocity to applied stress and temperature is obtained from the kMC model and it is used as the main source of constitutive information into a dislocation-based CP framework. The complete model is used to perform material point simulations of single-crystal tungsten strength. We explore the entire crystallographic orientation space of the standard triangle. Non-Schmid effects are inlcuded in the model by considering the twinning-antitwinning (T/AT) asymmetry in the kMC calculations. We consider the importance of ?111?{110} and 111 {112} slip systems in the homologous temperature range from 0.08Tm to 0.33Tm, where Tm =3680 K is the melting point in tungsten.</p

Effects of iron-rich intermetallics and grain structure on semisolid tensile properties of Al-Cu 206 cast alloys near solidus temperature

The effects of iron-rich intermetallics and grain size on the semisolid tensile properties of Al-Cu 206 cast alloys near the solidus were evaluated in relation to the mush microstructure. Analyses of the stress–displacement curves showed that the damage expanded faster in the mush structure dominated by plate-like β-Fe compared to the mush structure dominated by Chinese script-like α-Fe. While there was no evidence of void formation on the β-Fe intermetallics, they blocked the interdendritic liquid channels and thus hindered liquid flow and feeding during semisolid deformation. In contrast, the interdendritic liquid flows more freely within the mush structure containing α-Fe. The tensile properties of the alloy containing α-Fe are generally higher than those containing β-Fe over the crucial liquid fraction range of ~0.6 to 2.8 pct, indicating that the latter alloy may be more susceptible to stress-related casting defects such as hot tearing. A comparison of the semisolid tensile properties of the alloy containing α-Fe with different grain sizes showed that the maximum stress and elongation of the alloy with finer grains were moderately higher for the liquid fractions of ~2.2 to 3.6 pct. The application of semisolid tensile properties for the evaluation of the hot tearing susceptibility of experimental alloys is discussed

Performance of the Antiretroviral Treatment Program in Ethiopia, 2005-2015: strengths and weaknesses toward ending AIDS

Ethiopia is one of the countries which has scaled up antiretroviral treatment (ART) over the past decade. This study reviews the performance of the ART program in Ethiopia during the past decade, and identifies successes and weaknesses toward ending AIDS in the country.A review and synthesis of data was conducted using multiple data sources: reports from all health facilities in Ethiopia to the Federal Ministry of Health, HIV/AIDS estimates and projections, and retrospective cohort and cross-sectional studies conducted between 2005/6 and 2014/15.The ART program has been successful over several critical areas: (1) ART coverage improved from 4% to 54%; (2) the median CD4 count/mm at the time of ART initiation increased from 125 in 2005/6 to 231 in 2012/13; (3) retention in care after 12 months on ART has increased from 82% to 92%. In spite of these successes, important challenges also remain: (1) ART coverage is not equitable: among regions (5.6%-93%), between children (25%) and adults (60%), and between female (54%) and male patients (69%); (2) retention in care is variable among regions (83%-94%); and, (3) the shift to second-line ART is slow and low (0·58%).The findings suggest that the ART program should sustain the successes and reflect on the shortcomings toward the goal of ending AIDS. It is important to capitalize on and calibrate the interventions and approaches utilized to scale up ART in the past. Analysis of the treatment cascade, in order to pinpoint the gaps and identify appropriate solutions, is commendable in this regard