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research-article  
Nitin Bhalerao, Suhas S. Joshi and N.K. Naik
J. Eng. Mater. Technol   doi: 10.1115/1.4038671
The titanium alloy (grade 5) is a two-phase material, finds significant applications in aerospace, medical, marine fields, owing to its superior characteristics like high strength-to-weight ratio, excellent corrosion resistance and good formability. Hence, the dynamic characteristics of the Ti-6Al-4V alloy are an important area to study. A compressive Split Hopkinson Pressure Bar (SHPB) was used to evaluate the dynamic properties of Ti-6Al-4V alloy under various strain rates between 997 to 1898 s-1, and at temperatures between -10°C and 320°C. It was evident that the material strength is sensitive to both strain rate and temperature; however the latter is more predominant than the former. The microstructure of the deformed samples was examined using electron back-scattered diffraction (EBSD). The microscopic observations show that the dynamic impact characteristics of the alloy are higher at higher strain-rates than at quasi-static strain rates. The SHPB tests show that the force on the transmitter bar is lower than the force on the incident bar. This indicates that the dynamic equilibrium cannot be achieved during high rate of damage evolution. Various constants in Johnson-Cook model were evaluated to validate the results. An uncertainty analysis for the experimental results has also been presented.
TOPICS: Alloys, High temperature, Temperature, Diffraction, Electrons, Weight (Mass), Pressure, Strength (Materials), Titanium alloys, Equilibrium (Physics), Aerospace industry, Corrosion resistance, Electron backscatter diffraction, Biomedicine, Uncertainty analysis, Damage
research-article  
Ladan Salari-Sharif, Tobias Schaedler and Lorenzo Valdevit
J. Eng. Mater. Technol   doi: 10.1115/1.4038672
Hybrid micro-architected materials with unique combinations of high stiffness, high damping and low density are presented. We demonstrate a scalable manufacturing process to fabricate hollow microlattices with a sandwich wall architecture comprising an elastomeric core and metallic skins. In this configuration, the metallic skins provide stiffness and strength, whereas the elastomeric core provides constrained-layer damping. This damping mechanism is effective under any strain amplitude and at any relative density, in stark contrast with the structural damping mechanism exhibited by ultralight metallic or ceramic architected materials, which requires large strain and densities lower than a fraction of a percent. We present an analytical model for stiffness and constrained-layer damping of hybrid hollow microlattices, and verify it with Finite Elements simulations and experimental measurements. Subsequently, this model is adopted in optimal design studies to identify hybrid microlattice geometries which provide ideal combinations of high stiffness and damping and low density. Finally, a previously derived analytical model for structural damping of ultralight metallic microlattices is extended to hybrid lattices, and used to show that ultralight hybrid designs are more efficient than purely metallic ones.
TOPICS: Damping, Stiffness, Density, Ceramics, Manufacturing, Simulation, Design, Engineering simulation, Finite element analysis
Review Article  
Piyas B Chowdhury and Huseyin Sehitoglu
J. Eng. Mater. Technol   doi: 10.1115/1.4038673
This article recounts recent advances on the atomistic modeling of twinning in bcc and fcc alloy. Specifically, we have reviewed: (i) the experimental evidence of twinning-dominated deformation in single- and multi-grain microstructures (ii) calculation of generalized planar fault energy landscapes, and (iii) the prediction of critical friction stresses to initiate twinning-governed plasticity (e.g. twin nucleation, twin-slip and twin-twin interactions). Possible avenues for further research are outlined.
TOPICS: Stress, Energetics, Twinning, Modeling, Nucleation (Physics), Plasticity, Deformation, Friction, Alloys
research-article  
Cinzia Menapace, Nicola Sartori, Massimo Pellizzari and Giovanni Straffelini
J. Eng. Mater. Technol   doi: 10.1115/1.4038670
The hot deformation behavior of four different steels in the as-cast condition was investigated by means of hot compression tests conducted at temperatures ranging from 1100°C up to 1200°C, and at strain rates in between 0.12 and 2.4 s-1. The primary focus of this work was to check the possibility to increase the strain rate during the rough preliminary working of the ingots, i.e., to adopt a rough rolling process in place of the more conventional rough forging. The second aim of the research was to study the influence of the different characteristics of these steels in their as-cast conditions on their hot deformation behavior. It was seen that in all deformation conditions, the stress-strain compression curves show a single peak, indicating the occurrence of dynamic recrystallization (DRX). The hot deformation behavior was studied in both the condition of dynamic recovery (DRV), modelling the stress-strain curves in the initial stage of deformation, and DRX. Data of modelling were satisfactorily employed to estimate the flow stress under different conditions of temperature and strain rate. The experimental values of the activation energy for hot deformation, QHW, were determined and correlated to the chemical composition of the steels. The critical strain for dynamic recrystallization, ?c, was determined as a function of the Zener-Hollomon parameter and correlated to the peak strain, ?p. A ratio ?c/?p in the range 0.45-0.65 was found which is in agreement with literature data.
TOPICS: Deformation, Steel, Surface roughness, Stress, Recrystallization, Temperature, Modeling, Compression, Forging, Flow (Dynamics), Stress-strain curves
research-article  
Khaled J. Al-Fadhalah
J. Eng. Mater. Technol   doi: 10.1115/1.4038674
Repetitive thermomechanical processing (TMP) has been applied to evaluate the effect of compression strain and temperature on microstructure and texture development in an alpha-brass alloy. TMP cycles were employed using four cycles of low-strain compression (e = 0.15), and two cycles of medium-strain compression (e = 0.3). Compression tests were conducted at 25, 250 and -100°C, while annealing was made at 670°C for 10 min. Examination by electron backscattered diffraction (EBSD) indicated that the low-strain scheme was capable to increase the fraction of S3n boundaries (n = 1, 2 and 3) with increasing cycles, producing maximum fraction of 68%. For medium-strain scheme, a drop in the fraction of S3n boundaries occurred in cycle 2. Reducing compression temperature lowered the fraction of S3n boundaries for low-strain scheme, while it enhanced formation of S3n boundaries for medium-strain scheme. Annealing textures showed that <101> compression fiber was strongly maintained for samples processed by small-strain scheme, while weakening of <101> fiber accompanied by formation of <111> recrystallization fiber occurred using medium-strain scheme. The results indicate that the increase in strain energy stored during compression, via increasing strain and/or decreasing deformation temperature, is responsible to favor recrystallization twinning over strain induced grain boundary migration (SIBM). Both mechanisms are important for formation of S3n boundaries. Yet, SIBM is thought to strongly promote regeneration of S3n boundaries at higher TMP cycles. This is consistent with the development of microstructure and texture using small-strain scheme.
TOPICS: Brass (Metal), Texture (Materials), Thermomechanical treatment, Compression, Cycles, Fibers, Temperature, Annealing, Recrystallization, Deformation, Twinning, Electron backscatter diffraction, Grain boundaries, Diffraction, Electrons, Alloys
research-article  
Abdel-Wahab El-Morsy and Ahmed Farahat
J. Eng. Mater. Technol   doi: 10.1115/1.4038393
In this work, two advanced high strength steels have been developed by designing alloy systems with suitable alloying elements, Mn, Si, Al, and Cr, and post-forming heat treatment processes. Thermomechanical process of ? 90% forging reductions has been applied on the designed alloys at a temperature of 1100?C, followed by austenitizing above Ac3. Four cooling rates, air-cooling, air-cooling with tempering, oil quenching with tempering, and water quenching with tempering, have been applied on the forged samples. The results revealed that the estimated tensile properties of the ferrite/bainite microstructures of alloy A, without Cr, is situated between the bands of the 1st and the current 3rd generation AHSS, whereas the estimated properties corresponding to the ferrite/fine bainite with 8% retained austenite of alloy B, with Cr, is overlapped with the properties exhibited by the current 3rd generation of advanced high strength steels. The thermomechanical process conducted on the alloy containing Cr has developed steel with tensile strength up to 1790 MPa.
TOPICS: High strength steel, Mechanical properties, Thermomechanical treatment, Alloys, Cooling, Ferrites (Magnetic materials), Quenching (Metalworking), Tensile strength, Thermomechanics, Water, Heat treating (Metalworking), Design, Temperature, Steel, Forging
research-article  
Ahmad A Mousa, Gert Heinrich, Udo Wagenknecht and Omar Arrabeyat
J. Eng. Mater. Technol   doi: 10.1115/1.4037169
Exfoliated graphite (EXG) was prepared from commercially available natural graphite flakes (NGF), through strong acid treatment followed by thermal shock at 950 oC. The EXG sheets were characterized with respect to their thermal stability via thermo-gravimetric analysis (TGA) and Raman spectra. Their morphology and particle size were evaluated using scanning electron microscope (SEM) and particle size analyzer. The potential of EXG as reinforcement on the mechanical and thermal properties of the dynamically vulcanized polystyrene/styrene butadiene rubber (PS/SBR) composites were evaluated. The influence of EXG on the electrical properties of the composites was measured as well.
TOPICS: Composite materials, Thermal properties, Graphite, Particle size, Raman spectra, Thermal shock, Thermal stability, Electrical properties, Scanning electron microscopes, Styrene-butadiene rubber

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