Enhanced Oxidation Resistance of Ultrafine-Grain Microstructure AlCoCrFeNi High Entropy Alloy.

This work investigates the effect of ultrafine-grain microstructure on the oxidation behavior of AlCoCrFeNi high entropy alloy (HEA). The ultrafine-grain microstructure is obtained using stationary friction processing performed at two different rotational speeds, 400 and 1800 rpm, for 5 min duration. Processed samples demonstrate high depth of refinement (DOR) and ultrafine grain size (0.43-1 µm) at high rotational speeds along with significant phase transformations from BCC/B2 to FCC microstructure. Further, surface free energy of the ultrafine-grain microstructure is enhanced up to 35%. Oxidation kinetics of the ultrafine-grained sample is decelerated up to 12-48% in a temperature range of 850-1050 °C for a duration of 100 h. Chromia and alumina were the predominant oxides formed in almost all the samples oxidized at elevated temperature. In addition, spinel Co(Cr,Fe)2O4/Fe(Co,Cr)2O4 formation is also detected in the unprocessed oxidized samples. Processed samples rich in grain boundaries (GBs) promote internal oxidation to form Al-rich inner oxides. The enhanced oxidation resistance of the processed samples is attributed to the microstructural refinement and homogenization resulting in the formation of protective chromia followed by Al-rich inner oxides.

Bioinspired micro/nano structured aluminum with multifaceted applications.

Inspired by many biological systems such as lotus leaves, insect wings and rose petals, great attention has been devoted to the study and fabrication of artificial superhydrophobic surfaces with multiple functionalities. In the present study, a simple and ecological synthesis route has been employed for large scale fabrication of self-assembled, sustainable nanostructures on unprocessed and micro imprinted aluminum surfaces named 'Nano' and 'Hierarchy'. The processed samples show extreme wettability ranging from superhydrophilicity to superhydrophobicity depending on post-processing conditions. The densely packed ellipsoidal nanostructures exhibited superhydrophobicity with excellent water, bacterial and dust repellency when modified by low surface energy material 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FOTES), characterized by a static contact angle of 163 ± 1° and contact angle hysteresis (CAH) ~3°. These coated surfaces show significant corrosion resistance with current density of 6 nA/cm2 which is 40 times lower than unprocessed counterpart and retain chemical stability after prolonged immersion in corrosive media. These surfaces show excellent self-cleaning ability with significantly low water consumption (< 0.1 µl/mm2-mg) and prevent biofouling which ensures its applicability in biological environment and marine components. The nanostructured superhydrophilic aluminum shows maximum antibacterial activity due to disruption of cell membrane. This work can offer a simple strategy to large scale fabrication of multifunctional biomimetic metallic surfaces.

Enhanced antibacterial properties and the cellular response of stainless steel through friction stir processing.

Biofilm related bacterial infection is one of the primary causes of implant failure. Limiting bacterial adhesion and colonization of pathogenic bacteria is a challenging task in health care. Here, a highly simplistic processing technique for imparting antibacterial properties on a biomedical grade stainless steel is demonstrated. Low-temperature high strain-rate deformation achieved using submerged friction stir processing resulted in a nearly single phase ultra-fine grain structure. The processed stainless steel demonstrated improved antibacterial properties for both Gram-positive and Gram-negative bacteria, significantly impeding biofilm formation during the in vitro study. Also, the processed stainless steel showed better compatibility with human fibroblasts manifested through apparent cell spreading and proliferation. The substantial antibacterial properties of the processed steel are explained in terms of the favorable electronic characteristics of the metal-oxide and by using classical Derjaguin-Landau-Verwey-Overbeek (DLVO) and the extended DLVO (XDLVO) approach at the cell-substrate interface.