Assessment of the effect of cooling rate on dendrite coherency point and hot tearing susceptibility of AZ magnesium alloys using thermal analysis

Dendrite coherency point (DCP) is an important parameter for examining the solidification structure and castability of alloys. In this research, the DCP of AZ magnesium alloys (AZ31, AZ61 and AZ91) is measured in the range of 0.22 °Cs^?1 to 8.13 °Cs^?1 cooling rates using the two-thermocouple thermal analysis technique. The results show that when cooling rate increased, temperature interval of coherency (T_{N –} T_DCP) and coherency time (t_DCP) are decreased; and it can postpone dendrite coherency. Also, by increasing the cooling rate, solid fraction at dendrite coherency increases initially and then decreases at higher cooling rates. To estimate the hot tearing susceptibility, Clyne and Davies’ criterion is used. Hot tearing susceptibility calculations exhibit initially reduce by increasing the cooling rate and then it increases at higher cooling rates. These results were explained based on the solidification principles.     

Ballistic-Failure Mechanisms in Gas Metal Arc Welds of Mil A46100 Armor-Grade Steel: A Computational Investigation

In our recent work, a multi-physics computational model for the conventional gas metal arc welding (GMAW) joining process was introduced. The model is of a modular type and comprises five modules, each designed to handle a specific aspect of the GMAW process, i.e.: (i) electro-dynamics of the welding-gun; (ii) radiation-/convection-controlled heat transfer from the electric-arc to the workpiece and mass transfer from the filler-metal consumable electrode to the weld; (iii) prediction of the temporal evolution and the spatial distribution of thermal and mechanical fields within the weld region during the GMAW joining process; (iv) the resulting temporal evolution and spatial distribution of the material microstructure throughout the weld region; and (v) spatial distribution of the as-welded material mechanical properties. In the present work, the GMAW process model has been upgraded with respect to its predictive capabilities regarding the spatial distribution of the mechanical properties controlling the ballistic-limit (i.e., penetration-resistance) of the weld. The model is upgraded through the introduction of the sixth module in the present work in recognition of the fact that in thick steel GMAW weldments, the overall ballistic performance of the armor may become controlled by the (often inferior) ballistic limits of its weld (fusion and heat-affected) zones. To demonstrate the utility of the upgraded GMAW process model, it is next applied to the case of butt-welding of a prototypical high-hardness armor-grade martensitic steel, MIL A46100. The model predictions concerning the spatial distribution of the material microstructure and ballistic-limit-controlling mechanical properties within the MIL A46100 butt-weld are found to be consistent with prior observations and general expectations.     

A micromechanical analysis of the coupled thermomechanical superelastic response of textured and untextured polycrystalline NiTi shape memory alloys

In this paper a micromechanical model that incorporates single crystal constitutive relationships is used for studying the pseudoelastic response of polycrystalline shape memory alloys (SMAs). In the micromechanical framework, the stress-free transformation strains of the possible martensite twinned structures, correspondence variant pairs (CVPs), obtained from the crystallographic data of NiTi are used, and the overall transformation strain is obtained by defining a set of martensitic volume fractions corresponding to active CVPs during phase transformation. The local form of the first law of thermodynamics is used and the energy balance relation for the polycrystalline SMAs is obtained. Generalized coupled thermomechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. A three-dimensional finite element framework is used and different polycrystalline samples are modeled based on Voronoi tessellations. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples, the effects of texture and the tension–compression asymmetry in polycrystalline SMAs are studied. The interaction between the stress state (tensile or compressive), the number of grains and the texture on the mechanical response of polycrystalline SMAs is studied. It is found that the number of grains (or size) affects both the stress–strain response and the phase transformation propagation in the material. In addition to tensile and compressive loadings, textured and untextured NiTi micropillars with different sizes are also studied in bending. The coupled thermomechanical framework is used for analyzing the effect of loading rate and the phase transformation latent heat on the response of both textured and untextured samples. It is shown that the temperature changes due to the heat generation during phase transformation can affect the propagation of martensite in samples subjected to high strain rates.     

An efficient threshold voltage generation for SAR ADCs

In this paper, a new architecture for successive-approximation register (SAR) analog-to-digital converters (ADCs) is presented. In the proposed scheme, the threshold voltage for each comparison is divided into two parts. This results in appreciably less switching energy and less total capacitance without a substantial increase in digital complexity compared to the conventional SAR ADC. Analytical calculations and circuit level simulation results in the context of a 10-bit 100 kS/s ADC are provided to verify the usefulness of the proposed SAR ADC scheme revealing 87 % less switching power and 40 % less total capacitance in comparison with the conventional SAR ADC.     

A wideband time‐based continuous‐time sigma‐delta modulator with 2nd order noise‐coupling based on passive elements

Embedding the time encoding approach inside the loop of the sigma‐delta modulators has been shown as a promising alternative to overcome the resolution problems of analog‐to‐digital converters in low‐voltage complementary metal‐oxide semiconductor (CMOS) circuits. In this paper, a wideband noise‐transfer‐function (NTF)‐enhanced time‐based continuous‐time sigma‐delta modulator (TCSDM) with a second‐order noise‐coupling is presented. The proposed structure benefits from the combination of an asynchronous pulse width modulator as the voltage‐to‐time converter and a time‐to‐digital converter as the sampler to realize the time quantization. By using a novel implementation of the analog‐based noise‐coupling technique, the modulator's noise‐shaping order is improved by two. The concept is elaborated for an NTF‐enhanced second‐order TCSDM, and the comparative analytical calculations and behavioral simulation results are presented to verify the performance of the proposed structure. To further confirm the effectiveness of the presented structure, the circuit‐level implementation of the modulator is provided in Taiwan Semiconductor Manufacturing Company (TSMC) 90 nm CMOS technology. The simulation results show that the proposed modulator achieves a dynamic range of 84 dB over 30 MHz bandwidth while consuming less than 25 mW power from a single 1 V power supply. With the proposed time‐based noise‐coupling structure, both the order and bandwidth requirements of the loop filter are relaxed, and as a result, the analog complexity of the modulator is significantly reduced. Copyright © 2015 John Wiley & Sons, Ltd.     

Correlation between hydrogen storage properties and textures induced in magnesium through ECAP and cold rolling

It is feasible to obtain a significant enhancement of the hydrogen storage capability in magnesium by selecting an appropriate sequence of mechanical processing. The Mg metal may be produced with different textures which will then give significant differences in the absorption/desorption kinetics and in the incubation times for hydrogenation. Using processing by equal-channel angular pressing (ECAP), different textures may be produced by changing both the numbers of passes through the ECAP die and the ram speed. Significant grain refinement is easily avoided by using commercial coarse-grained magnesium as the starting material. The use of cold rolling after ECAP further increases the preferential texture for hydrogenation. The results show that the hydriding properties are enhanced with a (002) texture where the improved kinetics lie mainly in the initial stages of hydrogenation. An incubation time is associated with the presence of a (101) texture and this is probably due to the magnesium oxide stability in this direction.     

Microstructural Evolution in Hot and Cold-Rolled Ti-Nb Alloy

Phase transformations, morphology, and crystallographic texture evolution in hot and cold-rolled Ti-25.51 wt.% Nb alloys are investigated. The experimental procedure involves synthesis of the alloy by arc melting followed by cold or hot rolling with intermediate prior and postheat treatments. Composition and phase analysis of all alloys are conducted using x-ray diffraction techniques and microstructural observations are conducted using an optical microscope. These examinations reveal that the as-melted alloy possesses large millimeter size grains with no stored strain energy and a two phase β ? α′ microstructure. Direct cold rolling followed by a short homogenization leads to a β ? α′′ mixture with ω precipitates. Two hour annealing before cold rolling leads to an α′ ? α′′ mixture with a characteristic triangular martensitic microstructure evidencing the act of shear on formation of the phase. Hot rolling followed by a water quench results in a β ? α′′ mixture, while annealing prior to hot rolling transforms the arc-melted material to a α′ ? α′′ mixture. The crystallographic textures of similar microstructure mixtures in hot and cold-rolled samples are distinctively different. The analysis shows that the microstructure serves as an identifying characteristic of the processing paths and is highly dependent on the mode of processing.     

Serrated Flow Model for Metallic Glasses under Compressive Loading

A rheological model is proposed that incorporates the serrated flow nature of metallic glasses. It involves the process of the nucleation, propagation and the arrest of a shear bands in the samples subjected to compressive deformation at room temperature. Numerical resolution of the constitutive equations resulting from the model is compared with the stress-strain curve obtained from in-situ nano-compression test in SEM of Zrbased metallic glass. Parametric identification method was applied and enabled us to release the physical parameters of the model. The obtained results showed that the model is adequately valid to describe the experimental data and the almost adjustable model parameters are physically meaningful and comparable to literature.     

Extension of a Current Continuum-Level Material Model for Soil into the Low-Density Discrete-Particle Regime

In this article, an attempt is made to construct a soil-material model which can be used over a wide range of soil densities. To construct such a model, an existing purely continuum-type soil material model (used in the high-density regime), within which the granular structure of the soil is neglected, is combined with an existing discrete-type soil material model (used in the low-density regime) within which soil is treated as an assembly of interacting particles. In order to enable it to be used in conventional transient, nonlinear dynamics, and finite element analyses, the new soil material model is cast using a continuum-type framework. Thus, while in the low-density regime soil behavior is fully dominated by the discrete-type soil-material model, soil has been treated as a continuum constituent properties of which are governed by particle geometrical parameters and particle-particle interaction laws. To demonstrate the utility and fidelity of the new soil material model, a series of uniaxial strain computational tests involving rectangular, parallelepiped-shaped soil-slug normal impact onto a rigid, fixed, flat surface is carried out. While these tests are of a one-dimensional character, they are generally considered as being representative of the loading and deformation histories experienced by mine-blast-ejected soil during its impact with the target structure. The results obtained using the newly proposed soil material model, in the low-density regime, are found to be fully consistent with their discrete-particle modeling and simulation counterparts, suggesting that the new model can be used in transient nonlinear dynamics, finite element simulations involving low-density soil.