MICROSTRUCTURAL EVOLUTION AND MECHANICAL PROPERTIES OF Zn-Ti ALLOYS FOR BIODEGRADABLE STENT APPLICATIONS

Stents made of biodegradable metallic materials are increasingly gaining interest within the biomaterials field because of their superior mechanical properties and biodegradation rates as compared to polymeric materials. Zinc and its alloys have been developed and investigated as possible candidates for biodegradable stent applications in the last five years. This study intended to formulate and characterize a new series of Zn-Ti alloys, with titanium additions of less than 1-3 wt%, with the primary objective to develop and select an alloy that meets benchmark values of mechanical properties for biodegradable stents. A series of Zn-Ti alloys was formulated through vacuum induction melting. The experimental approach was to analyze the effect of Ti alloying element addition on mechanical properties of zinc. The structure, mechanical properties and fractography of the as-cast alloys were investigated. It was found that the grain size was reduced from above 600 µm to ~23 µm with the Ti content increasing from 0.01 wt% to 0.3 wt%. The amount of the intermetallic phase increased from 0.3 wt% to 2.5 wt% with Ti content. The results identify the formation of a eutectic phase of zinc with intermetallics at the primary grain boundaries. Zn16Ti was identified as the intermetallic phase formed in the as-cast Zn-Ti alloys. With increasing Ti content from 0.01 wt% to 1 wt%, the ultimate tensile strength and yield strength of the as cast Zn-Ti alloys increased from 101 and 64 MPa to 177 and 122 MPa, respectively. It is proposed that the strength of as-cast Zn-Ti alloys increases with the Ti content increasing from 0.01 wt% to 0.3 wt% due to grain refinement from a small percentage of titanium. The amount of the intermetallic phase increased with the Ti content increasing from 0.3 wt% to 2.5 wt%. It is proposed that the hardness and strength of the as-cast Zn-Ti alloys increased with the Ti content increasing from 0.3 to 2.5 wt% due to the increased formation of Zn-Ti intermetallic phases. The low elongation of the as-cast Zn-0.3 wt% Ti (3%), Zn-0.5 wt% Ti (4%), and Zn-1 wt% Ti alloys (2%) was also attributed to the increasing content of Zn-Ti intermetallic phases. Based on the results of the structure and mechanical properties of as-cast Zn-Ti alloys, the most promising as-cast candidates were processed through hot extrusion. This phase study was focused on the structure-property relationships before and after hot extrusion. The as-extruded Zn-0.01 wt% Ti had the highest average ultimate tensile strength and yield strength of 269 and 177 MPa, respectively. It is proposed that a significant increase in the ultimate tensile strength and yield strength in Zn-0.01 wt% Ti alloy after hot extrusion is due to grain refinement and formation of precipitates. The as-extruded Zn-0.1 wt% Ti and Zn-0.3 wt% Ti alloys exhibited high ductility, with the elongation to failure of about 44% and 30%, respectively. It is proposed that the as-extruded Zn-0.1 wt% Ti alloy exhibited high ductility due to the grain refinement and grain shape adjustment after hot extrusion. The high elongation of the as-extruded Zn-0.1 wt% Ti and Zn-0.3 wt% Ti alloys is consistent with the microstructural observations of ductile fracture. The as-extruded Zn-0.1 wt% Ti alloy had the best combination of tensile mechanical properties (UTS=207 MPa, YS=163 MPa, and Elongation=44%), which nearly meet the mechanical requirements for stent application

Knowledge Distillation and Training Balance for Heterogeneous Decentralized Multi-Modal Learning over Wireless Networks

Decentralized learning is widely employed for collaboratively training models using distributed data over wireless networks. Existing decentralized learning methods primarily focus on training single-modal networks. For the decentralized multi-modal learning (DMML), the modality heterogeneity and the non-independent and non-identically distributed (non-IID) data across devices make it difficult for the training model to capture the correlated features across different modalities. Moreover, modality competition can result in training imbalance among different modalities, which can significantly impact the performance of DMML. To improve the training performance in the presence of non-IID data and modality heterogeneity, we propose a novel DMML with knowledge distillation (DMML-KD) framework, which decomposes the extracted feature into the modality-common and the modality-specific components. In the proposed DMML-KD, a generator is applied to learn the global conditional distribution of the modality-common features, thereby guiding the modality-common features of different devices towards the same distribution. Meanwhile, we propose to decrease the number of local iterations for the modalities with fast training speed in DMML-KD to address the imbalanced training. We design a balance metric based on the parameter variation to evaluate the training speed of different modalities in DMML-KD. Using this metric, we optimize the number of local iterations for different modalities on each device under the constraint of remaining energy on devices. Experimental results demonstrate that the proposed DMML-KD with training balance can effectively improve the training performance of DMML.Comment: submitted to IEEE Trans. on Mobile Computin

VACUUM BRAZING OF DIAMOND TO TUNGSTEN CARBIDE

Diamond tools are increasingly gaining importance as cutting and drilling materials for a wide variety of industrial applications. Polycrystalline diamond (PCD) is the main ultrahard material commercially used in the oil and gas drilling industry. In this study, a reactive brazing process was developed to join polycrystalline diamond (PCD) to WC-13 wt% Co, to form the cutter for fixed-cutter drill bit applications. Most nonmetals including polycrystalline diamond are not wet by and cannot easily be joined with conventional brazing alloys due to their chemical stability. The experimental approach was first to analyze the effect of adding an active metal (Ti, Zr, or V) to copper, silver, or a silver-copper eutectic alloy on the wettability of diamond and WC-Co substrates. Sessile drop tests were utilized to compare wettability between the liquid braze alloy and the substrate. The addition of Ti, Zr, and V decreased the apparent contact angle, which improved both the wetting and bonding behavior between braze alloy and diamond substrate. For all three alloy systems evaluated, all three base alloys (Cu, Ag, and Ag-Cu) with active metal additions (Ti, Zr, or V) exhibited good wettability on diamond and WC-Co substrates. Microstructural analysis of the diamond and WC-Co sessile drop samples was performed via scanning electron microscopy (SEM) to characterize the interfacial layers formed. Two different types of reactions were observed between the braze alloys and the WC-Co substrates: reduction and dissolution reactions. For the diamond sessile drop samples, only intermetallic solidification products were observed at the interface for the Ag-Cu eutectic based alloys with additions of 2 and 5 wt% Ti. SEM/EDS analysis revealed that the chemical changes at the interface between the braze alloy and diamond substrate were in agreement with the intermetallic solidification products predicted from the phase diagrams. Based on the Gibbs energies of formation for carbides, it is predicted that the formation of TiC is thermodynamically favored at the interface. However, no TiC reaction product was identified within the resolution of SEM/EDS analysis possibly because the TiC reaction layer is too thin. Based on the results of the wetting studies, an effort was made to optimize the shear strength of diamond brazed to WC-Co. This phase study was focused on the relationship between the braze alloy composition, the braze layer thickness, the brazing thermal cycle, the braze microstructures and the resulting joint mechanical properties. The average shear strength for Ag-2 wt% Ti alloy was approximately constant in the braze thickness range of 0.1 to 0.2 mm. It was observed that the brazed samples failed in the silver braze layer. More visible cracking and larger cracks were observed on the surface region of diamond substrates of the joint thickness of 0.2 mm for the Ag-Cu-2 wt% Ti alloys. It is possible that thermal stresses generated from coefficient of thermal expansion (CTE) mismatch resulted in the formation of interfacial cracks. The Ag-Cu eutectic alloy with addition of a 2 wt% Ti has the highest average shear strength of 95 MPa when the hold time is 30 minutes and the cooling rate is 5 °C/min

The possible role of ribosomal protein S6 kinase 4 in the senescence of endothelial progenitor cells in diabetes mellitus

<p>Abstract</p> <p>Background</p> <p>The decrease and dysfunction of endothelial progenitor cells (EPCs) has been assumed as an important cause/consequence of diabetes mellitus (DM) and its complications, in which the senescence of EPCs induced by hyperglycemia may play an immensurable role. However, the mechanisms of EPCs senescence has not been fully investigated. Recently, ribosomal protein S6 kinase 4 (RSK4), a member of serine/threomine (Ser/Thr) kinase family and p53-related gene, is reported to regulate the replicative and stress-induced senescence of different cells.</p> <p>Presentation of the hypothesis</p> <p>These above lead to consideration of an evidence-based hypothesis that RSK4 may serve as a mediator of EPCs senescence in DM.</p> <p>Testing the hypothesis</p> <p>EPCs of healthy subjects and DM patients are isolated from peripheral blood and incubated with high glucose (HG). Then, the EPCs senescence would be detected by senescence associated β-galactosides (SA-β-gal) staining. Meanwhile, the RSK4 expression is assessed by RT-PCR and western blot. Moreover, overexpressing or RNA interfering of RSK4 in EPCs to investigate the relationship between RSK4 expression and the senescence of EPCs are necessary to substantiate this hypothesis. Also, studies on possible upstream and downstream factors of RSK4 would be explored to reveal the RSK4-mediated senescence pathway in EPCs.</p> <p>Implications of the hypothesis</p> <p>If proved, this hypothesis will provide another mediator of EPCs senescence, and may establish a novel pathogenesis for DM and further benefit to the management of DM.</p

Continental drift, plateau uplift, and the evolutions of monsoon and arid regions in Asia, Africa, and Australia during the Cenozoic

Monsoon and arid regions in the Asia-Africa-Australia (A-A-A) realm occupy more than 60% of the total area of these continents. Geological evidence showed that significant changes occurred to the A-A-A environments of the monsoon and arid regions, the land-ocean configuration in the Eastern Hemisphere, and the topography of the Tibetan Plateau (TP) in the Cenozoic. Motivated by this background, numerical experiments for 5 typical geological periods during the Cenozoic were conducted using a coupled ocean-atmosphere general circulation model to systemically explore the formations and evolutionary histories of the Cenozoic A-A-A monsoon and arid regions under the influences of continental drift and plateau uplift. Results of the numerical experiments indicate that the timings and causes of the formations of monsoon and arid regions in the A-A-A realm were very different. The northern and southern African monsoons existed during the mid-Paleocene, while the South Asian monsoon appeared in the Eocene after the Indian Subcontinent moved into the tropical Northern Hemisphere. In contrast, the East Asian monsoon and northern Australian monsoon were established much later in the Miocene. The establishment of the tropical monsoons in northern and southern Africa, South Asia, and Australia were determined by both the continental drift and seasonal migration of the Inter-Tropical Convergence Zone (ITCZ), while the position and height of the TP were the key factor for the establishment of the East Asian monsoon. The presence of the subtropical arid regions in northern and southern Africa, Asia, and Australia depended on the positions of the continents and the control of the planetary scale subtropical high pressure zones, while the arid regions in the Arabian Peninsula and West Asia were closely related to the retreat of the Paratethys Sea. The formation of the mid-latitude arid region in the Asian interior, on the other hand, was the consequence of the uplift of the TP. These results from this study provide insight to the important roles played by the earth’s tectonic boundary conditions in the formations and evolutions of regional climates during geological times