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Research Papers

Wang Y. Y., Sun X., Wang Y. D., et al. Effects of Mn Content on the Deformation Behavior of Fe–Mn–Al–C TWIP Steels—A Computational Study J. Eng. Mater. Technol. 137(2), 021001 (2015) (9 pages);   Paper No: MATS-14-1171;   doi:10.1115/1.4029041

This paper presents a double-slip/double-twin polycrystal plasticity model using finite element solution to investigate the kinetics of deformation twinning of medium manganese (Mn) twinning-induced plasticity (TWIP) steels. Empirical equations are employed to estimate the stacking fault energy (SFE) of TWIP steels and the critical resolved shear stress (CRSS) for dislocation slip and deformation twinning, respectively. The results suggest that the evolution of twinning in Fe–xMn–1.4Al–0.6 C (x = 11.5, 13.5, 15.5, 17.5, and 19.5 mass%) TWIP steels, and its relation to the Mn content, can explain the effect of Mn on mechanical properties. By comparing the double-slip/double-twin model to a double-slip model, the predicted results essentially reveal that the interaction behavior between dislocation slip and deformation twinning can lead to an additional work hardening. Also, numerical simulations are carried out to study the influence of boundary conditions on deformation behavior and twin formation. The nucleation and growth of twinning are found to depend on internal properties (e.g., mismatch orientation of grains and stress redistribution) as well as on external constraints (e.g., the applied boundary conditions) of the material.

Moraes J. F. C., Jordon J. B., Bammann D. J. Finite Element Analysis of Self-Pierce Riveting in Magnesium Alloys Sheets J. Eng. Mater. Technol. 137(2), 021002 (2015) (9 pages);   Paper No: MATS-14-1016;   doi:10.1115/1.4029032

Conventional fusion joining methods, such as resistance spot welding (RSW), have been demonstrated to be ineffective for magnesium alloys. However, self-pierce riveting (SPR) has recently been shown as an attractive joining technique for lightweight metals, including magnesium alloys. While the SPR joining process has been experimentally established on magnesium alloys through trial and error, this joining process is not fully developed. As such, in this work, we explore simulation techniques for modeling the SPR process that could be used to optimize this joining method for magnesium alloys. Due to the process conditions needed to rivet the magnesium sheets, high strain rates and adiabatic heat generation are developed that require a robust material model. Thus, we employ an internal state variable (ISV) plasticity material model that captures strain-rate and temperature dependent deformation. In addition, we explore various damage modeling techniques needed to capture the piercing process observed in the joining of magnesium alloys. The simulations were performed using a two-dimensional axisymmetric model with various element deletion criterions resulting in good agreement with experimental data. The simulations results of this study show that the ISV material model is ideally suited for capturing the complex physics of the plasticity and damage observed in the SPR of magnesium alloys.

Paunović Perica, Grozdanov Anita, Češnovar Andrej, et al. Characterization of Nanoscaled TiO2 Produced by Simplified Sol–Gel Method Using Organometallic Precursor J. Eng. Mater. Technol. 137(2), 021003 (2015) (7 pages);   Paper No: MATS-13-1184;   doi:10.1115/1.4029112

This work is concerned with development of sol–gel method for preparation of nanoscaled TiO2 using organometallic precursor—titanium tetraisopropoxide (TTIP) and determination of the present crystalline phases depending on the temperature of further thermal treatment. The characteristic processes and transformations during the thermal treatment were determined by means of thermal gravimetric analysis and/or differential thermal analysis (TGA/DTA) method. The crystalline structure and size of the TiO2 crystallites were analyzed by means of Raman spectroscopy and X-ray powder diffraction (XRPD) method. At 250 °C, cryptocrystalline structure was detected, where amorphous TiO2 is accompanied with crystalline anatase. The anatase crystallite phase is stable up to 650 °C, whereas at higher temperature rutile transformation begins. It was observed that at 800 °C, almost the whole TiO2 is transformed to rutile phase. According to XRPD analysis, the increase of the temperature influences on the increase of the size of the crystalline particles ranging from 6 nm at 250 °C to less than 100 nm at 800 °C. The size and shape of the TiO2 crystalline particles were observed by transmission electron microscopy (TEM). The shape of the studied samples changes from nanospheres (250, 380, and 550 °C) to nanorods (650 and 800 °C). Morphology of the formed TiO2 aggregates was observed by scanning electron microscopy (SEM).

Senthil Saravanan M. S., Kumaresh Babu S. P. Synthesis, Characterization, and ECAP Consolidation of Carbon Nanotube Reinforced AA 4032 Nanocrystalline Composites Produced by High Energy Ball Milling J. Eng. Mater. Technol. 137(2), 021004 (2015) (9 pages);   Paper No: MATS-14-1096;   doi:10.1115/1.4029196

In the present work, multiwalled carbon nanotubes (MWNTs) were synthesized by electric arc discharge method in open air atmosphere. The synthesized nanotubes were subjected to multistep purification followed by characterization using Raman spectroscopy and transmission electron microscopy (TEM). These carbon nanotubes (CNTs) have inner and outer diameters of the order of 3.5 nm and 16 nm with an aspect ratio of 63. AA 4032 nanocomposites reinforced with MWNTs were produced by high energy ball milling using elemental powder mixtures. X-ray diffraction (XRD) and scanning electron microscope (SEM) studies showed different phases of composite with and without CNTs. The crystallite size and lattice strain were calculated using an anisotropic model of Williamson–Hall peak broadening analysis, which showed in decreased crystallite size with increasing milling time. TEM studies reveal that the MWNTs were uniformly distributed in the matrix. Thermal stability of the nanocrystalline powders was studied using a differential thermal analyzer (DTA). The mechanically alloyed powders were consolidated using a novel method called equal channel angular pressing (ECAP) at room temperature. The consolidated samples were sintered at 480 °C in argon atmosphere for 90 min. ECAP method was investigated as an alternative to conventionally sintered powder composites. CNT addition has shown significant improvement in the hardness of the system, even though the observed density is relatively low compared with a base alloy. Thus, the results show that ECAP enables sufficient shear deformation results in good metallurgical bonds between particles at lower compaction pressures. Hence, it is proven that ECAP can be effectively used as one of the consolidation technique especially for powders that are difficult to consolidate by other means.

Technical Brief

Ostanin Igor, Wang Yuezhou, Ni Yuxiang, et al. Mechanics of Nanocrystalline Particles With the Distinct Element Method J. Eng. Mater. Technol. 137(2), 024501 (2015) (5 pages);   Paper No: MATS-14-1168;   doi:10.1115/1.4029249

In geomechanics and civil engineering, the distinct element method (DEM) is employed in a top-down manner to simulate problems involving mechanics of granular media. Because this particle-based method is well adapted to discontinuities, we propose here to adapt DEM at the mesoscale in order to simulate the mechanics of nanocrystalline structures. The modeling concept is based on the representation of crystalline nanograins as mesoscopic distinct elements. The elasticity, plasticity, and fracture processes occurring at the interfaces are captured with contact models of interaction between elements. Simulations that rely on the fitting of the peak stress, strain, and failure mode on the experimental testing of Au and CdS hollow nanocrystalline particles illustrate the promising potential of mesoscopic DEM for bridging the atomistic-scale simulations with experimental testing data.

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