Composition-dependent effects of oxygen on atomic structure and mechanical properties of metallic glasses

Although minor alloying in metallic glasses (MGs) has been extensively investigated, the effect of O doping is still a debatable topic. In the present study, the atomic-level structures and mechanical properties of Zr-based MGs doped with different O contents have been analyzed using ab initio molecular dynamics simulations. It is revealed that O atoms prefer to bond to Zr atoms due to their low mixing enthalpy, and that O atoms degrade the properties of Zr-lean MGs but hardly affect the properties of Zr-rich MGs, with results suggesting a compositional dependence of O doping. For Zr-lean MGs, the fraction of full icosahedra, size of the medium-range-order clusters, Young's modulus and shear modulus decrease sharply with O content, while accompanied by a sharp increase of the non-Frank-Kasper polyhedra, and the ratio of bulk modulus to shear modulus and Poisson's ratio, indicating decreased strength and improved plasticity. For Zr-rich MGs, however, the above-mentioned structural and mechanical features experience little change or only change slightly after O doping, showing low oxygen sensitivity. It is shown that the high Zr content weakens the effect of Zr-O bonding to some extent. The present study not only sheds light on the atomic-level structures of O-doped MGs, which may provide guidelines for designing MGs with low-grade materials, but also helps to explain the previous conflicting results based on the composition-dependence effect.

[Transapical transcatheter aortic valve replacement for high risk elderly patients with predominant aortic incompetence]

OBJECTIVES

To introduce the application of the J-Valve™ system in elderly patients with predominant aortic incompetence without significant valve calcification, and to evaluate its feasibility.

METHODS

From April 2014 to July 2015, 33 cases of transapical implantation of J-Valve™ were performed in Department of Cardiac Surgery, Zhongshan Hospital, Fudan University. Sixteen of these patients were diagnosed as predominant aortic incompetence without significant valve calcification. There were 11 male and 5 female patients aged from 61 to 84 years, with a mean age of (76±6) years. All patients had symptoms of left ventricular dysfunction for at least 3 months. They were considered to be prohibitive for surgical valve replacement (logistic European system for cardiac operative risk evaluation: 22.2% to 44.4%, mean 27%±6% after evaluation by an interdisciplinary heart team. The J-Valve™ system was applied in transapical transcatheter aortic valve replacement for patients. The multi-slice CT was performed before discharge. Clinical evaluation including patients' history, symptoms and New York Heart Association classification and echocardiogram evaluation were performed before discharge, 1(st) month, 3(th) month and 12(th) month after the operation respectively.

RESULTS

Implantations were successful in all patients. One patient died from moderate paravalvular leak which led to multi-organ failure during the hospital stay. The mean time of postoperative hospital stay of the other 15 patients was (6.1±1.3) days. The 15 patients were followed by 174 to 410 days, with a median time of 188 days. Only two patients had trivial prosthetic valve incompetence, the other 13 patients had no prosthetic valve incompetence; two patients had no paravavular leak and the other 13 patients had paravavular leak of no more than moderate grade. There were no major complication or mortality during the follow-up.

CONCLUSIONS

The transapical implantation of the J-Valve™ system in high risk elderly patients with predominant aortic incompetence is feasible.The early postoperative outcome is satisfactory.