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J. Eng. Mater. Technol. 2018;140(4):041001-041001-9. doi:10.1115/1.4039690.

The aim of this paper is to analyze the macroscopic behavior of an aluminum alloy after severe plastic deformations (SPD). Samples of 6061 aluminum alloy are processed at room temperature by two techniques of SPD: equal channel angular pressing (ECAP) under quasi-static loading and impact under dynamic loading, using Taylor's test setup. In addition to the mechanical properties, the microstructure evolution of the material is investigated. Half of the samples are aged at 400 °C for 2 h, to remove internal stress in a commercial alloy in order to increase workability of the material. The evolution of the properties and the material behavior after 2, 4, 6, and 8 passes of the 120 deg ECAP process is investigated.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(4):041002-041002-6. doi:10.1115/1.4039791.

The grain refinement of Mg–Al alloy AZ91 via carbon inoculation, including the significant role of Mn in advanced nucleation, was analyzed, and the corresponding mechanical properties and aging behavior were investigated. To this end, various amounts of C were added into the liquid at the desired temperatures. Al8Mn5 particles, which are suitable nucleation sites for α-Mg, were identified as the primary grain refiners. In situ particle formation, along with appropriate wetting and a suitable orientation relationship (OR), facilitated the grain refinement mechanism. Al4C3 particles contributed to heterogeneous nucleation by providing suitable Al8Mn5 nucleation sites. Mn removal resulted in poor grain refinement in the Mg–Al alloy. The Hall–Petch relationship, high-temperature tensile behavior, and aging mechanism of the samples refined by 1 wt % C addition (as the best grain refiner) are discussed and compared with industrial practice.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(4):041004-041004-11. doi:10.1115/1.4040003.

We present a phenomenological three-dimensional (3D) nonlinear viscoelastic constitutive model for time-dependent analysis. Based on Schapery's single integral constitutive law, a solution procedure has been provided to solve nonlinear viscoelastic behavior. This procedure is applicable to 3D problems and uses time- and stress-dependent material properties to characterize the nonlinear behavior of material. The equations describing material behavior are chosen based on the measured material properties in a short test time frame. This estimation process uses the Prony series material parameters, and the constitutive relations are based on the nonseparable form of equations. Material properties are then modified to include the long-term response of material. The presented model is suitable for the development of a unified computer code that can handle both linear and nonlinear viscoelastic material behavior. The proposed viscoelastic model is implemented in a user-defined material algorithm in abaqus (UMAT), and the model validity is assessed by comparison with experimental observations on polyethylene for three uniaxial loading cases, namely short-term loading, long-term loading, and step loading. A part of the experimental results have been conducted by (Liu, 2007, “Material Modelling for Structural Analysis of Polyethylene,” M.Sc. thesis, University of Waterloo, Waterloo, ON Canada), while the rest are provided by an industrial partner. The research shows that the proposed finite element model can reproduce the experimental strain–time curves accurately and concludes that with proper material properties to reflect the deformation involved in the mechanical tests, the deformation behavior observed experimentally can be accurately predicted using the finite element simulation.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(4):041005-041005-11. doi:10.1115/1.4040099.

2219Al and 2219Al + 0.1 wt % Ag alloys were processed by casting route. The hot compression tests were carried out at constant true strain rates and temperatures in the range of 10−3 to 101 s−1 and 300–500 °C, respectively. Flow stress of the alloy decreases with the addition of silver. The flow stress of both alloys increases with the decrease in deformation temperature and the increase in strain rates. Constitutive models correlating the peak flow stress with deformation temperature and strain rates for the two alloys were developed using hyperbolic–sine relationship. The activation energy for hot deformation of 2219 Al alloy decreases with the addition of silver. Comparison of the predicted and experimental values of peak flow stress reveals that 92% of the data could be predicted within a deviation error of ±10% indicating good predictive capability for the developed constitutive relationships.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(4):041006-041006-8. doi:10.1115/1.4040004.

Shot peening is a cold working process, which is used to enhance the properties of materials, especially the fatigue life as it induces large compressive residual stresses in the subsurface of materials. In this paper, the effect of the shot peening process on the topography of the shot peened surface and the distribution of the residual stresses in the subsurface of the material was systematically investigated. A technique to estimate the shot peening coverage was employed using a finite element model which was further developed using experimental results for increased accuracy. The comparison between the numerical and experimental studies gives a good agreement of the distribution of the residual stresses in the subsurface of the shot peened material. The shot peening pressure and media size are two main factors affecting on the presence of compressive residual stresses in the subsurface of the material.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(4):041007-041007-8. doi:10.1115/1.4040005.

Tungsten inert gas arc (TIG) process was employed to remelt Fe-based coating deposited by plasma spraying. Subsequently, the microstructure, interface, and the wear resistance of the coatings before and after remelting were studied. The results showed that the lamellar structure, pores, and inclusions of Fe-based coating were eliminated and the porosity significantly decreased from 4% to 0.4%. The as-sprayed coating contained microcrystalline region, nanocrystalline region, and transition region, while single crystal region and rod-shaped (Fe,Cr)23C6 were observed in the remelted coating. There was no element diffusion and dissolution phenomenon at the interface; thus, the bonding form between the as-sprayed coating and substrate mainly was mechanical bonding. On the contrary, the diffusion transfer belt (DTB) emerged at the interface of the remelted coating and substrate, the remelted coating was bonded with the substrate metallurgically. Additionally, the average microhardness and elastic modulus of the remelted coating increased by 33.4% and 53.2%, respectively, compared with the as-sprayed coating. During wear process, the as-sprayed coating exhibited obvious brittle fracture characteristics, while the remelted coating appeared typical plastic deformation characteristics and its weight loss reduced by 39.5%. Therefore, TIG remelting process significantly improved the microstructure, mechanical properties, and wear resistance of Fe-based coating.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2018;140(4):041008-041008-11. doi:10.1115/1.4040223.

Next-generation, reusable hypersonic aircraft will be subjected to extreme environments that produce complex fatigue loads at high temperatures, reminiscent of the life-limiting thermal and mechanical loads present in large gas-powered land-based turbines. In both of these applications, there is a need for greater fidelity in the constitutive material models employed in finite element simulations, resulting in the transition to nonlinear formulations. One such formulation is the nonlinear kinematic hardening (NLKH) model, which is a plasticity model quickly gaining popularity in the industrial sector, and can be found in commercial finite element software. The drawback to using models like the NLKH model is that the parameterization can be difficult, and the numerical fitting techniques commonly used for such tasks may result in constants devoid of physical meaning. This study presents a simple method to derive these constants by extrapolation of a reduced-order model, where the cyclic Ramberg–Osgood (CRO) formulation is used to obtain the parameters of a three-part NLKH model. This fitting scheme is used with basic literature-based data to fully characterize a constitutive model for Inconel 617 at temperatures between 20 °C and 1000 °C. This model is validated for low-cycle fatigue (LCF), creep-fatigue (CF), thermomechanical fatigue (TMF), and combined thermomechanical-high-cycle fatigue (HCF) using a mix of literature data and original data produced at the Air Force Research Laboratory (AFRL).

Commentary by Dr. Valentin Fuster

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