Current redistribution model of anomalous resistance behaviour in superconductor-topological insulator heterostructures

Anomalous resistance upturn and downturn have been observed on the topological insulator (TI) surface in superconductor-TI (NbN-Bi1.95Sb0.05Se3) heterostructures at ~ mm length scales away from the interface. Magnetotransport measurements were performed to verify that the anomaly is caused due to the superconducting transition of the NbN layer. The possibility of long range superconducting proximity effect due to the spin-polarized TI surface state was ruled out due to the observation of similar anomaly in NbN-Au and NbN-Al heterostructures. It was discovered that the unusual resistance jumps were caused due to current redistribution at the superconductor-TI interface on account of the geometry effects. Results obtained from finite element analysis using COMSOL package has validated the proposed current redistribution (CRD) model of long range resistance anomalies in superconductor-TI and superconductor-metal heterostructures.Comment: 37 pages (including references), 17 figure

Multi-Response Optimization of WEDM Process Parameters for Machining of Superelastic Nitinol Shape-Memory Alloy Using a Heat-Transfer Search Algorithm

Nitinol, a shape-memory alloy (SMA), is gaining popularity for use in various applications. Machining of these SMAs poses a challenge during conventional machining. Henceforth, in the current study, the wire-electric discharge process has been attempted to machine nickel-titanium (Ni55.8Ti) super-elastic SMA. Furthermore, to render the process viable for industry, a systematic approach comprising response surface methodology (RSM) and a heat-transfer search (HTS) algorithm has been strategized for optimization of process parameters. Pulse-on time, pulse-off time and current were considered as input process parameters, whereas material removal rate (MRR), surface roughness, and micro-hardness were considered as output responses. Residual plots were generated to check the robustness of analysis of variance (ANOVA) results and generated mathematical models. A multi-objective HTS algorithm was executed for generating 2-D and 3-D Pareto optimal points indicating the non-dominant feasible solutions. The proposed combined approach proved to be highly effective in predicting and optimizing the wire electrical discharge machining (WEDM) process parameters. Validation trials were carried out and the error between measured and predicted values was negligible. To ensure the existence of a shape-memory effect even after machining, a differential scanning calorimetry (DSC) test was carried out. The optimized parameters were found to machine the alloy appropriately with the intact shape memory effect

Shape memory effect, temperature distribution and mechanical properties of friction stir welded nitinol

Welding of shape memory alloys without deterioration of shape memory effect could vastly extend their applications. To retain shape memory behavior, a solid-state welding technique called friction stir welding was employed in this study. Austenitic NiTi alloy sheets of thickness 1.2 mm were joined at tool rotational speeds of 800,1000, and 1200 rpm. Due to dynamic recrystallization, the grain refinement has occurred in the weld region. The tensile testing has shown superelastic plateau for the welds at 800 and 1000 rpm. The phase transformation behavior of different weld regions was studied in detail using differential scanning calorimeter. A marginal drift in transformation temperatures was observed in the weld. To understand the drift in phase transformation temperatures, finite element analysis was carried out with focus on temperature distribution during welding. Finally, time-dependent shape recovery of a FSW welded joint was studied and it was found that the original position was completely recovered after 27 s at a temperature of 65 degrees C. (C) 2018 Elsevier B.V. All rights reserved

Microstructure, mechanical properties and shape memory behaviour of friction stir welded nitinol

For the first time, NiTi shape memory alloy was successfully joined by Friction Stir Welding (FSW). The weld showed significant grain refinement without formation of detrimental phases. The yield strength of the weld joint increased by 17% as compared to the base metal without substantial change in shape memory behaviour

Exploring the functional and corrosion behavior of friction stir welded NiTi shape memory alloy

The friction stir welding was proved to be a promising process to weld NiTi shape memory alloy with adequate mechanical strength and retention of shape memory effect. In this work, the tool wear during welding and the compositional change in the weld cross section has been evaluated. The tensile cyclic behavior for different strain percentages has been investigated. Interestingly, the thermomechanical behavior of the weld was studied using electrical actuation. The actuation was carried out at different current and the actuation temperatures were corroborated with phase transformation temperature range measured using differential scanning calorimetry. A maximum displacement of 17.8 mm was recorded at the actuation current of 5 A. The electrochemical corrosion testing has been performed to understand the corrosion behavior of the friction stir welded NiTi. The weld has exhibited a lower corrosion resistance than the base metal as seen from the lower breakdown potential of 250 mV and a higher current density of 1.5 x 10(-4) mA/cm(2)