Modeling of precipitation hardening in Mg-based alloys

This work deals with the development of Mg-based alloys with enhanced properties at elevated temperatures. This is achieved by precipitation of binary phases such as MgZn₂ and Mg₂Sn during the aging of these alloys. The aim of the present work is to develop and calibrate a model for precipitation hardening in Mg-based alloys, as different types of precipitates form simultaneously. The modified Langer-Schwartz approach, while taking into account nucleation, growth and coarsening of the new phase precipitations, was used for the analysis of precipitates’ evolution and precipitation hardening during aging of Mg-based alloys. Two strengthening mechanisms associated with particle-dislocation interaction (shearing and bypassing) were considered to be operating simultaneously due to particle size-distribution. Parameters of the model, R _N i and k _{σ i} , were found by fitting of calculated densities and average sizes of precipitates with ones estimated from experiments. The effective diffusion coefficients of phase formation processes, which determine the strengthening kinetics, were estimated from the hardness maximum positions on the aging curves.     

A review on the synthesis of in situ aluminum based composites by thermal,mechanical and mechanical–thermal activation of chemical reactions

The aim of the present paper is to review the recent progress in the synthesis of in situ particle reinforced aluminum composites using thermal, mechanical and combined mechanical-thermal activation of aluminothermic reduction reactions. The combination of combustion synthesis (CS) and mechanosynthesis (MS) is the most recent development in the processing of advanced materials like micro and nano aluminum based composites. The combined mechanical thermal synthesis (MTS) has widened the possibilities for both CS and MS. MTS holds great potential for commercial viability and offers exciting processing route for the synthesis of advanced materials. Enhanced reaction kinetics and extended concentration limits in MTS are demonstrated by illustrating the synthesis of aluminum based nanocomposite involving Al–CeO₂.     

Preparation and antibacterial activity of novel fluoroalkyl end-capped oligomers/silica nanocomposites-encapsulated low molecular biocides

Fluoroalkyl end-capped N-(1,1-dimethyl-3-oxobutyl)acrylamide oligomer [R_F-(DOBAA) _n -R_F] reacted with tetraethoxysilane (TEOS) and silica nanoparticles in the presence of low-molecular weight biocides such as hibitane, hinokitiol, and hinokioil under alkaline conditions to afford R_F-(DOBAA) _n -R_F/silica nanocomposites-encapsulated these biocides in excellent to moderate isolated yields. Fluoroalkyl end-capped N,N-dimethylacrylamide oligomer [R_F-(DMAA) _n -R_F] and acrylic acid oligomer [R_F-(ACA) _n -R_F]/silica nanocomposites-encapsulated hibitane were obtained under similar conditions. Dynamic light scattering measurements showed that the size of these fluorinated nanocomposites-encapsulated biocides thus obtained is nanometer size-controlled. Additionally, these fluorinated nanocomposites were shown to have a good dispersibility and stability in methanol and water. Of particular interest, these fluorinated nanocomposites-encapsulated biocides were found to have a good antibacterial activity against Staphylococcus aureus, and these nanocomposites were applied to the surface modification of traditional organic polymers such as poly(methyl methacrylate).     

Prediction of ultrasonic wave velocity in particleboard and fiberboard

Ultrasonic wave velocities were determined at parallel and perpendicular to manufacturing direction and at the interval angles of 15° in clockwise and counterclockwise directions of particleboard and fiberboard. The experimental results were compared with the predicted values using some empirical formulae such as Hankinson and Jacoby equations. The results showed that the ultrasonic wave velocity were the highest in parallel direction in particleboard and fiberboard and decreases with increase of angle and the lowest values occurred in perpendicular direction. The predicted ultrasonic velocity using Hankinson and Jacoby equations are in close agreement with the measured values. Relationship between ultrasonic wave velocities and particles and fibers angle could be successfully presented by cubic and quadratic regression equations as well.     

Dissolution processes,hydrolysis and condensation reactions during geopolymer synthesis: Part I—Low Si/Al ratio systems

Initial geopolymeric reaction processes governing dissolution of solid aluminosilicate particles in alkali solutions have been investigated using conventional experimental techniques, and the data analysed by speciation predictions of the partial charge model (PCM). For metakaolin powders activated with 5.0 M NaOH, solid-state nuclear magnetic resonance (NMR) spectra disclose the existence of monomeric [Al(OH)₄]⁻ species after two hours of dissolution, consistent with PCM predictions. However, no equivalent monomeric silicate species were observed for 5.0–10.0 M NaOH activator solutions characteristic of systems with nominal Si/Al ≤ 1. The apparent absence of monomeric silicate species suggest rapid condensation of silicate units with [Al(OH)₄]⁻ to generate aluminosilicate species, as indicated by the evolution of the shoulder at around −87 ppm in the ²⁹Si NMR spectra. Of the two possible stable silicate species [SiO₂(OH)₂]²⁻ and [SiO(OH)₃]⁻, the latter appears most likely to condense with [Al(OH)₄]⁻ to produce aluminosilicate oligomers, from which larger oligomers subsequently form through further condensation with [Al(OH)₄]⁻ leading to a gradual build up of aluminosilicate networks and a lowering of system alkalinity. This dissolution and hydrolysis sequence at the early stages of synthesis suggests a reaction path wholly consistent with predictions of the partial charge model.