Modeling of Advanced High Strength Steels with the realistic microstructure–strength relationships

The objective of the work is to consider the first-order effects of the realistic microstructure morphology in the macroscale modeling of the multiphase Advanced High Strength Steels (AHSS). Instead of using constitutive equations at macroscale, the strength–microstructure relationship is studied in the forms of micromechanical and multiscale models that do not make considerable simplifications with regard to the microscale geometry and topology. The trade-off between the higher computational time and the higher accuracy has been offset with a stochastic approach in the construction of the microscale models. The multiphase composite effects of AHSS microstructure is considered in realistic microstructural models that are stochastically built from AHSS micrographs. Computational homogenization routines are used to couple micro and macroscale and resultant stress–strain relations are compared for models built with the simplified and idealized geometries of the microstructure. The results from this study show that using a realistic representation of the microstructure, either for DP or TRIP steel, could improve the accuracy of the predicted stress and strain distribution. The resultant globally averaged effective stress and strain fields from realistic microstructure model were able to accurately capture the onset of the plastic instability in the DP steel. It is shown that the macroscale mechanical behavior is directly affected by the level of complexities in the microscale models. Therefore, greater accuracy could be achieved if these stochastic realistic microstructures are used at the microscale models.     

Effects of cold work prior to aging on microstructure of AEREX™350 superalloy

The influence of different amounts of cold work on the evolution of microstructure during subsequent aging of AEREX™350 superalloy was investigated. Simple compression testing was used to impose desired plastic strain while microhardness testing, XRD, SEM and TEM techniques were used to characterize the response of the alloy following aging treatment. It was found that high levels of cold work followed by aging resulted in a drastic increase in the formation of Widmanstättan η phase and incidence of shear bands in the microstructure. Based on microstructural data, the effect of plastic strain on η phase formation and the physical origin of the localized shear bands are discussed.     

Synthesis of Macrocycles Derived from Substituted Triazines

A triazine ring derivatized with morpholine, an N-alkyl-N′-BOC-hydrazine (alkyl=isopropyl or benzyl) and the diethylacetal of glycinylpropionaldehyde undergoes spontaneous dimerization in good yields upon acid-catalyzed deprotection. The resulting 24-member macrocycles can be characterized by NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction. In the solid state, both homodimers adopt a taco-like conformation. Although each shows π–π stacking between the triazine rings, different patterns of hydrogen bonds emerge. The crystal structure of the isopropyl dimer shows that it includes two molecules of trifluoracetic acid per macrocycle. The trifluoroacetate anion charge balances the protonated triazines, which engage in bifurcated hydrogen bonds with the carbonyl acceptor of the distant glycine. This carbonyl also forms a hydrogen bond with the NH of the proximate glycine. The crystal structure of the benzyl derivative does not include trifluoracetic acid. Instead, two hydrogen bonds form, each between a glycine NH and the lone pair of the C=N nitrogen of the hydrazine group. In the solid state, both molecules present the alkyl side chains and morpholine groups in close proximity. A heterodimer is accessible in approximately statistical yields—along with both homodimers—by mixing the two protected monomers prior to subjecting them to deprotection.     

An evolutionary testbed for software technology evaluation

Empirical evidence and technology evaluation are needed to close the gap between the state of the art and the state of the practice in software engineering. However, there are difficulties associated with evaluating technologies based on empirical evidence: insufficient specification of context variables, cost of experimentation, and risks associated with trying out new technologies. In this paper, we propose the idea of an evolutionary testbed for addressing these problems. We demonstrate the utility of the testbed in empirical studies involving two different research technologies applied to the testbed, as well as the results of these studies. The work is part of NASAs High Dependability Computing Project (HDCP), in which we are evaluating a wide range of new technologies for improving the dependability of NASA mission-critical systems.     

Turning titanium metal matrix composites (Ti-MMCs) with carbide and CBN inserts

Despite excellent mechanical and physical features of titanium metal matrix composite (Ti-MMC), hard and abrasive ceramic particles within the matrix structure, as well as high price, may lead to severe difficulties on machining and machinability of Ti-MMCs. Review of literature denotes that only limited studies are available on machining Ti-MMCs with commercial cutting tools under various cutting conditions and cutting tools/inserts. Furthermore, limited studies are available on machinability attributes of Ti-MMC under various cutting conditions used. Therefore, to remedy the lack of knowledge observed, this work intends to report turning Ti-MMCs with carbide, and cubic boron nitride (CBN) inserts under various cutting conditions. The mean values of surface roughness (R_a) and directional cutting forces, as well as flank wear (V_B) were studied as the machinability attributes. The microstructural evaluations were conducted to discover the wear modes. Furthermore, the statistical tools were used to present the factors governing machining attributes studied. Adhesion, abrasion, and oxidation were observed as the principle wear modes on the flank sides of the tested inserts. According to experimental results and statistical analysis, the R_a and V_B could be controlled by cutting parameters only when CBN inserts were used. Despite the inset used, factors governing both responses were not identical. Although average cutting forces were directly affected by cutting parameters used, however, the relatively low correlation of determination (R²) of directional cutting forces can be attributed to effects of cutting speed, elevated temperature in the cutting zone as well as rapid tool wear which are all correlated to others.     

Effect of fuel composition on NOx formation in high‐pressure syngas/air combustion

In this study, an experimental investigation of lean premixed syngas/air flames with H₂/CO ratio of 1.0 and equivalence ratio of 0.5 has been conducted in a high‐pressure burner facility to investigate the effects of pressure and the presence of hydrocarbons on NO_x speciation. Detailed NO_x speciation measurements in the post‐flame region were conducted for various pressures up to 1.5 MPa (15 bar) using Fourier transform infrared (FTIR) spectroscopy. When the pressure is increased, NO concentration decreases while NO₂ increases due to pressure dependence of NO to NO₂ conversion. For a given pressure, the presence of hydrocarbons in syngas leads to an increase in NO_x concentrations possibly due to prompt NO formation. Comparison of NO concentrations in presence of CH₄ at different pressures shows that the effect of CH₄ due to prompt NO formation is more dominant than the effect of pressure on NO. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3134–3140, 2018     

Electrochemical Behavior Assessment of As-Cast Mg-Y-RE-Zr Alloy in Phosphate Buffer Solutions (X Na<Subscript>3</Subscript>PO<Subscript>4</Subscript> + Y Na<Subscript>2</Subscript>HPO<Subscript>4</Subscript>) Using Electrochemical Impedance Spectroscopy and Mott–Schottky Techniques

In the present study, electrochemical behavior of as-cast Mg-Y-RE-Zr alloy (RE: rare-earth alloying elements) was investigated using electrochemical tests in phosphate buffer solutions (X Na₃PO₄?+?Y Na₂HPO₄). X-ray diffraction techniques and Scanning electron microscopy equipped with energy dispersive x-ray spectroscopy were used to investigate the microstructure and phases of the experimental alloy. Different electrochemical tests such as potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S) analysis were carried out in order to study the electrochemical behavior of the experimental alloy in phosphate buffer solutions. The PDP curves and EIS measurements indicated that the passive behavior of the as-cast Mg-Y-RE-Zr alloy in phosphate buffer solutions was weakened by an increase in the pH, which is related to formation of an imperfect and less protective passive layer on the alloy surface. The presence of the insoluble zirconium particles along with high number of intermetallic phases of RE elements mainly Mg₂₄Y₅ in the magnesium matrix can deteriorate the corrosion performance of the alloy by disrupting the protective passive layer that is formed at pH values over 11. These insoluble zirconium particles embedded in the matrix can detrimentally influence the passivation. The M–S analysis revealed that the formed passive layers on Mg-Y-RE-Zr alloy behaved as an n-type semiconductor. An increase in donor concentration accompanying solutions of higher alkalinity is thought to result in the formation of a less resistive passive layer.     

Microstructural studies and wear assessments of Ti/TiC surface composite coatings on commercial pure Ti produced by titanium cored wires and TIG process

Tungsten Inert Gas (TIG) process and titanium cored wires filled with micro size TiC particles were employed to produce surface composite coatings on commercial pure Ti substrate for wear resistance improvement. Wire drawing process was utilized to produce several cored wires from titanium strips and titanium carbide powders. Subsequently, these cored wires were melted and coated on commercial pure Ti using TIG process. This procedure was repeated at different current intensities and welding travel speeds. Composite coating tracks were found to be affected by TIG heat input. The microstructural studies using optical and scanning electron microscopy supported by X-ray diffraction showed that the surface composite coatings consisted of α′-Ti, spherical and dendritic TiC particles. Also, greater volume fractions of TiC particles in the coatings were found at lower heat input. A maximum microhardness value of about 1100 HV was measured which is more than 7 times higher than the substrate material. Pin-on-disk wear tests exhibited a better performance of the surface composite coatings than the untreated material which was attributed to the presence of TiC particles in the microstructure.     

Thermal energy storage of phase change materials in the solidification process inside the quasi-square heat exchanger: CFD simulation

The high latent heat of phase change materials (PCMs) makes them one of the most important sources of heat energy storage systems. However, due to the slow rate of heat transfer in these materials, using conductive materials such as fins and nanoparticles could improve the thermal efficiency of these energy storage systems. So in this article, cross-shaped fins and Copper(II) oxide nanoparticles with different synthesized forms and various volume fractions have been employed to increase the thermal efficiency of paraffin PCMs. In this simulation, three fin models based on the installed size, the shape of the synthesized nanoparticles in brick, cylindrical, and platelet forms, and the nanoparticle volume fraction of the Copper(II) oxide is 1%–4% are studied. Increasing the volumetric ratio of nanoparticle and shape coefficient decrease the time of solidification, while increasing the length of the cross-shaped fins raises the solidification rate and improves heat transfer. Finally, it was found that when the inner and outer walls play a role in the solidification process at the same time, the solidification rate will increase by more than 66% as more zone of the surface is exposed to cold.