Vacuum brazing of cubic boron nitride to medium carbon steel with Zr added passive and Ti activated eutectic Ag-Cu alloys

Active element like Ti is usually added to Ag-Cu passive alloy to braze cubic boron nitride (cBN) ceramic particles with steel substrate under high vacuum. Current work focusses on another group-IV element Zr, having more negative Gibbs free energy than Ti, concerning its reaction capability with cBN. In the present study, its effectiveness as an additive to the 72Ag28Cu alloy in brazing of cBN has been investigated. Zr is added to both passive and Ti-activated Ag-Cu alloys. Interestingly, the addition of 2 wt% Zr to the composition of eutectic alloy fails to braze cBN, unlike the addition of 2 wt% Ti. The underlying science is critically investigated. A new formulation of the alloy with the composition of (72Ag28Cu)-4Zr-4Ti alloy is found to be a beneficial alternative. Wetting of this new alloy on cBN is appreciable and simultaneously an additional benefit of 50% increase in wear resistance is achieved, compared to the (72Ag28Cu)-2Ti alloy. A significantly hard new intermetallic phase, AgCu₄Zr is detected in the microstructure which contributes to the enhancement of wear resistance of the new alloy. The joint strength thus achieved in brazing cBN with medium carbon steel using (72Ag28Cu)-2Ti and (72Ag28Cu)-4Zr-4Ti alloys is also compared but found to be compromised in case of the later. The results are analyzed in the light of microstructure, alloy-cBN interfacial chemistry and interfacial fracture under load.     

Simulation of continuous stirred rotating disk-membrane module: An approach based on surface renewal theory

A rigorous surface renewal model has been developed describing the aspects of mass transfer in a rotating disk-membrane (RDM) ultrafiltration cell. The model takes into consideration of two distribution functions of random surface elements, one with respect to their point of origin and the other related to the corresponding residence time on the membrane surface. The back transport flux and the permeate flux are evaluated at the membrane surface in order to develop a surface component balance equation. The component balance equation and a flux-rejection relationship arising from irreversible thermodynamics are solved simultaneously to develop a dynamic simulation. The simulation elucidates on permeate flux, membrane surface concentration and the permeate concentration under various operating conditions of transmembrane pressure, bulk concentration, membrane and stirrer speeds. For validation of the proposed model, experiments were conducted with bovine serum albumin (BSA)/water as feed in a standard rotating disk membrane module fitted with polyethersulfone (PES) membrane of 30 kDa molecular weight cut-off (MWCO). The model predicted flux and permeate concentration was found to be in good agreement with the experimental data, and the maximum absolute deviation for both cases was found to be well within ±5%.     

Deformation mechanism of bimodal microstructure in Ti-6Al-4V alloy: The effects of intercritical annealing temperature and constituent hardness

The so-called bimodal microstructure of Ti-6Al-4V alloy,composed of primary α grains (αp) and transformed β areas (βtrans),can be regarded as a dual-phase structure to some extent,the mechanical properties of which are closely related to the sizes,volume fractions,distributions as well as nano-hardness of the two constituents.In this study,the volume fractions of primary α grains (vol.％(αp)) were systematically modified in three series of bimodal microstructures with fixed primary α grain sizes (0.8 μm,2.4 μm and 5.0μm),by changing the intercritical annealing temperature (Tint).By evaluating the tensile properties at room temperature,it was found that with increasing Tint (decreasing vol.％(αp)),the yield strength of bimodal microstructures monotonically increased,while the uniform elongation firstly increased with Tint until 910 ℃ and then drastically decreased afterwards,thereby dividing the Tint into two regions,namely region Ⅰ (830-910 ℃) and region Ⅱ (910-970 ℃).The detailed deformation behaviors within the two regions were studied and compared,from the perspectives of strain distribution analysis,slip system analysis as well as dislocation analysis.For bimodal microstructures in region Ⅰ,due to the much lower nano-hardness of βtrans than αp,there was a clear strain partitioning between the two constituents as well as a strain gradient from the αp/βtrans interface to the grain interior of αp.This activated a large number of geometrically necessary dislocations (GNDs) near the interface,mostly with components,which contributed greatly to the extraordinary work-hardening abilities of bimodal microstructures in region Ⅰ.With increasing Tint,the αp/βtrans interface length density gradually increased and so was the density of GNDs with components,which explained the continuous increase of uniform elongation with Tint in this region.For bimodal microstructures in region Ⅱ,where the nano-hardness ofβtrans and αp were comparable,neither a clear strain-partitioning tendency nor a strain gradient across the αp/βtrans interface was observed.Consequently,only statistically stored dislocations (SSDs) with component were activated inside αp.The absence of dislocations together with a decreased volume fraction of αp resulted into a dramatic loss of uniform elongation for bimodal microstructures in region Ⅱ.     

Local deformation heterogeneity in cyclically deformed interstitial free steel

Heterogeneity of cyclic plastic deformation in interstitial‐free steel has been studied through a measurement of local average misorientation using electron back‐scattering diffraction technique. It is found that with increase of strain amplitude, the local average misorientation increases linearly within ±0.15 to ±0.40% and then decreases with further increase of strain amplitude. It is proposed that due to more homogeneous deformation vis‐à‐vis less generation of geometrically necessary dislocations, misorientation development per cycle becomes almost saturated in the predominantly low cycle fatigue regime. It is also found that the local average misorientation depends upon grain size in such a manner that in small grains, the misorientation is less as compared with large grains.     

Impact of exopolysaccharides on the stability of silver nanoparticles in water

The stability of commercial silver nanoparticles (SNPs) in aquatic environment plays a significant role in its toxicity to the environment and to human health. Here, we have studied the impact of bacterial exopolysaccharides (EPS) to the stability of engineered SNPs. When nanoparticles are present in neutral water, the nanoparticles exhibited low zeta potential and are least stable. However, in the presence of EPS (10-250 mg/L), the negative surface charge of nanoparticles increased and therefore the propensity of nanoparticles to aggregate is reduced. In UV-visible spectroscopic analysis a decrease in absorbance at plasmon peak of SNPs (425 nm) was observed till the addition of 50 mg/L of EPS, beyond that a blue shift towards 417 nm was observed. The adsorption of EPS was confirmed by Fourier-transform infrared spectroscopy. The EPS adsorbed SNPs were more stable and exhibited the zeta potential of higher than −30 mV.