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Review Article

J. Eng. Mater. Technol. 2018;140(2):020801-020801-19. doi:10.1115/1.4038673.

This paper recounts recent advances on the atomistic modeling of twinning in body-centered cubic (bcc) and face-centered cubic (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 (GPFE) 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.

Commentary by Dr. Valentin Fuster

Research Papers

J. Eng. Mater. Technol. 2017;140(2):021001-021001-18. doi:10.1115/1.4037658.

Micro-computed tomography (CT) was used as a tool to investigate the deformation behavior of particulate-filled composite materials. Three different shapes of glass fillers (spherical, flake, and fiber) and filler mass fractions (5%, 10%, and 15%) were introduced to the epoxy resin. Rockwell hardness H scale indentation test was used to deform the composite material. The composite materials were scanned before and after the indentation test by using micro-CT. Displacement field for each filler type and mass fraction were measured through correlation of before and after scan data. The effects of filler type and mass fraction on the internal displacement field were investigated. It was also demonstrated that micro-CT can be used as a tool to create realistic representative volume elements (RVEs) for particulate-filled composite materials instead of randomly distributed particles within the matrix material.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(2):021002-021002-11. doi:10.1115/1.4037659.

The authors simulated the industrially used continuous annealing conditions to process dual phase (DP) steels by using a custom designed annealing simulator. Sixty-seven percentage of cold rolled steel sheets was subjected to different processing routes, including the conventional continuous annealing line (CAL), intercritical annealing (ICA), and thermal cycling (TC), to investigate the effect of change in volume fraction, shape, and spatial distribution of martensite on tensile deformation characteristics of DP steels. Annealing parameters were derived using commercial software, including thermo-calc, jmat-pro, and dictra. Through selection of appropriate process parameters, the authors found out possibilities of significantly altering the volume fraction, morphology, and grain size distribution of martensite phase. These constituent variations showed a strong influence on tensile properties of DP steels. It was observed that TC route modified the martensite morphology from the typical lath type to in-grain globular/oblong type and significantly reduced the martensite grain size. This route improved the strength–ductility combination from 590 MPa–33% (obtained through CAL route) to 660 MPa–30%. Finally, the underlying mechanisms of crack initiation/void formation, etc., in different DP microstructures were discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(2):021003-021003-8. doi:10.1115/1.4037660.

Grain size control of any engineering metal is very important in the hot upsetting process. Generally, the grain size directly controls the mechanical properties and performance of the material. Al–B4C composite finds extensive applications in nuclear industries, defense, and electronic industries. Therefore, the aim of this work is to study the dynamic recrystallization (DRX) behavior of Al–4 wt % B4C composite during the hot upsetting test. Experimental work was performed on sintered Al–4 wt % B4C preforms at various initial relative density (IRD) values of 80%, 85%, and 90%, and over the temperature range of 300–500 °C and strain rates range of 0.1–0.3 s−1. The DRXed grain size of Al–4 wt % B4C preforms for IRDes, and temperatures and strain rates were evaluated by using an optical microscope. The activation energy (Q) and Zener–Hollomon parameter of sintered Al–4 wt % B4C preforms were calculated for various deformation conditions and IRDes. The mathematical models of DRX were developed as a function of Zener–Hollomon parameter for various IRDes to predict the DRXed grain size. It was found that the DRXed grain size decreases with increasing Zener–Hollomon parameter. Verification tests were done between the measured and predicted DRXed grain size for various IRDes, and absolute and mean absolute error was found to be 9.92% and 8.58%, respectively.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(2):021004-021004-15. doi:10.1115/1.4038029.

Adding programmable function to elastic metamaterials makes them versatile and intelligent. The objective of this study is to design and demonstrate thermomechanically tunable metamaterials with a compliant porous structure (CPS) and to analyze their thermomechanical behaviors. CPS, the unit cell of the metamaterial, is composed of rectangular holes, slits, and bimaterial hinges. By decomposing kinematic rotation of a linked arm and elastic deformation of a bimaterial hinge, a thermomechanical constitutive model of CPS is constructed, and the constitutive model is extended to a three-dimensional (3D) polyhedron structure for securing isotropic thermal properties. Temperature-dependent properties of base materials are implemented to the analytical model. The analytical model is verified with finite element (FE) based numerical simulations. A controllable range of temperature and strain is identified that is associated with a thermal deformation of the bimaterial hinge and contact on the slit surfaces of CPS. We also investigate the effect of geometry of CPS on the thermal expansion and effective stiffness of the metamaterial. The metamaterial with CPS has multiple transformation modes in response to temperature while keeping the same mechanical properties at room temperature, such as effective moduli and Poisson’s ratios. This work will pave the road toward the design of programmable metamaterials with both mechanically and thermally tunable capability, providing unique thermomechanical properties with a programmable function.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2017;140(2):021005-021005-9. doi:10.1115/1.4038393.

In this work, two advanced high-strength steels (AHSS) have been developed by designing alloy systems with suitable alloying elements, Mn, Si, Al, and Cr, and postforming 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 first and the current third 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 third generation of AHSSs. The thermomechanical process conducted on the alloy containing Cr has developed steel with tensile strength up to 1790 MPa.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(2):021006-021006-11. 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), modeling the stress–strain curves in the initial stage of deformation, and DRX. Data of modeling 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; a power law curve was found to describe the relation of QHW and the total amount of substitutional elements of the steels. The critical strain for DRX, εc, was determined as a function of the Zener–Hollomon parameter and correlated to the peak strain, εp. A ratio εcp in the range 0.45–0.65 was found, which is in agreement with literature data. All this information is crucial for a correct design of the rough deformation process of the produced ingots.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(2):021007-021007-9. 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. Two TMP schemes were employed using four cycles of low-strain compression (ε = 0.15) and annealing, and two cycles of medium-strain compression (ε = 0.3) and annealing. 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 Σ3n boundaries (n = 1, 2, and 3) with increasing cycles, producing maximum fraction of 68%. For medium-strain scheme, a drop in the fraction of Σ3n boundaries occurred in cycle 2. Reducing compression temperature lowered the fraction of Σ3n boundaries for low-strain scheme, while it enhanced the formation of Σ3n boundaries for medium-strain scheme. Annealing textures showed that 〈101〉 compression fiber was strongly retained for samples processed by small-strain scheme, while weakening of 〈101〉 fiber accompanied by the formation of 〈111〉 recrystallization fiber occurred for the 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 the formation of Σ3n boundaries. Yet, SIBM is thought to strongly promote regeneration of Σ3n boundaries at higher TMP cycles. This is consistent with the development of microstructure and texture using small-strain scheme.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(2):021008-021008-9. doi:10.1115/1.4038780.

The widespread use of copper in power and data cabling for aircraft, ships, and ground vehicles imposes significant mass penalties and limits cable ampacity. Experimental research has suggested that iodine-doped carbon nanotubes (CNTs) can serve as energy efficient replacements for copper in mass sensitive cabling applications. The high computational costs of ab initio modeling have limited complimentary modeling research on the development of high specific conductance materials. In recent research, the authors have applied two modeling assumptions, single zeta basis sets and approximate geometric models of the CNT junction structures, to allow an order of magnitude increase in the atom count used to model iodine-doped CNT conductors. This permits the ab initio study of dopant concentration and dopant distribution effects, and the development of a fully quantum based nanowire model which may be compared directly with the results of macroscale experiments. The accuracy of the modeling assumptions is supported by comparisons of ballistic conductance calculations with known quantum solutions and by comparison of the nanowire performance predictions with published experimental data. The validated formulation offers important insights on dopant distribution effects and conduction mechanisms not amenable to direct experimental measurement.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(2):021009-021009-10. doi:10.1115/1.4038671.

The titanium alloy (grade 5) is a two-phase material, which 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 and 1898s−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 (JC) model were evaluated to validate the results. An uncertainty analysis for the experimental results has also been presented.

Commentary by Dr. Valentin Fuster

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