Continuum understanding of twin formation near grain boundaries of FCC metals with low stacking fault energy

Deformation twinning from grain boundaries is often observed in face-centered cubic metals with low stacking fault energy. One of the possible factors that contribute to twinning origination from grain boundaries is the intergranular interactions during deformation. Nonetheless, the influence of mechanical interaction among grains on twin evolution has not been fully understood. In spite of extensive experimental and modeling efforts on correlating microstructural features with their twinning behavior, a clear relation among the large aggregate of grains is still lacking. In this work, we characterize the micromechanics of grain-to-grain interactions that contribute to twin evolution by investigating the mechanical twins near grain boundaries using a full-field crystal plasticity simulation of a twinning-induced plasticity steel deformed in uniaxial tension at room temperature. Microstructures are first observed through electron backscatter diffraction technique to obtain data to reconstruct a statistically equivalent microstructure through synthetic microstructure building. Grain-to-grain micromechanical response is analyzed to assess the collective twinning behavior of the microstructural volume element under tensile deformation. Examination of the simulated results reveal that grain interactions are capable of changing the local mechanical behavior near grain boundaries by transferring strain across grain boundary or localizing strain near grain boundary.116Ysciescopu

판상금속재료의 파괴인성과 신장플랜지성간의 상관관계

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Suppressed Plastic Anisotropy via Sigma-Phase Precipitation in CoCrFeMnNi High-Entropy Alloys

The effect of sigma-phase precipitation on plastic anisotropy of the equiatomic CoCrFeMnNi high-entropy alloy was investigated. Annealing at 700 °C after cold-rolling leads to the formation of the Cr-rich σ phase with a fraction of 2.7%. It is experimentally found that the planar anisotropy (∆r = −0.16) of the CoCrFeMnNi alloy annealed at 700 °C is two times lower than that of the alloy annealed at 800 °C (∆r = −0.35). This observation was further supported by measuring the earing profile of cup specimens after the deep drawing process. The plastic strain ratio, normal anisotropy, and planar anisotropy were also predicted using the visco-plastic self-consistent model. The results indirectly indicated that the reduction of plastic anisotropy in alloy annealed at 700 °C can be attributed to the formation of the σ phase

Numerical analysis on the formation of P-orientation near coarse precipitates in FCC crystals during recrystallization

Particles or precipitates in physical metallurgy are important microstructural components which control mechanical properties and phase transformation behaviors such as the recrystallization, grain growth and anisotropy of metallic materials. In particular, particle stimulated nucleation followed by an oriented growth of P-oriented grains is a commonly accepted premise, and many industrial grade cold rolled Al alloys exhibit strong P-texture after annealing. Accordingly, substantial efforts have been invested in texture and engineering studies to explore exactly how particle stimulated nucleation leads to the formation of P-oriented grains after annealing in cold-rolled Al alloys. Despite extensive experimental observations on the formation of P-oriented grains, the theoretical grounds for the formation of P-oriented grains is not yet fully established. In this work, we employ crystal plasticity theory and strain energy release maximization theory to uncover the nucleation mechanism of P-orientations near a coarse precipitate of a plane strain compressed Al alloy. The strain energy release maximization theory used in this work demonstrates that the nucleation of P-orientation is primarily possible due to stable P-orientations and near P-orientations formed during plane strain compression, which act as nuclei that recrystallize into P-orientations. ? 2017 Acta Materialia Inc.113sciescopu

Modeling the recrystallization texture for a non-grain oriented electrical steel using the strain energy release maximization theory

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On the phase transformation and dynamic stress-strain partitioning of ferrous medium-entropy alloy using integrated analysis

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Effect of grain size on stretch-flangeability of twinning-induced plasticity steels

The effect of grain size on stretch-flangeability was investigated to determine its influence on stretch-flangeability of high strength steels. To avoid other effects of microstructure, single-phase twinning-induced plasticity (TWIP) steels were selected for the investigation. To control the grain size of two types of TWIP steels, 1) the initial specimen was annealed at 1100 degrees C to increase its grain size, or 2) subjected to high-pressure torsion then annealed at 650 degrees C to reduce the grain size. The microstructural features were analyzed using the electron backscatter diffraction. The stretch-flangeability of TWIP steels with various grain sizes was evaluated using a hole-expansion test. It was found that the hole-expansion ratio follows the Hall-Petch correlation as does fracture toughness. To improve the stretch-flangeability of high strength steels, microstructural features should be designed to increase their fracture toughness.11Nsciescopu

Factors governing hole expansion ratio of steel sheets with smooth sheared edge

Stretch-flangeability measured using hole expansion test (HET) represents the ability of a material to form into a complex shaped component. Despite its importance in automotive applications of advanced high strength steels, stretch-flangeability is a less known sheet metal forming property. In this paper, we investigate the factors governing hole expansion ratio (HER) by means of tensile test and HET. We correlate a wide range of tensile properties with HERs of steel sheet specimens because the stress state in the hole edge region during the HET is almost the same as that of the uniaxial tensile test. In order to evaluate an intrinsic HER of steel sheet specimens, the initial hole of the HET specimen is produced using a milling process after punching, which can remove accumulated shearing damage and micro-void in the hole edge region that is present when using the standard HER evaluation method. It was found that the intrinsic HER of steel sheet specimens was proportional to the strain rate sensitivity exponent and post uniform elongation.1175sciescopuskc

Small-Scale System for Evaluation of Stretch-Flangeability with Excellent Reliability

We propose a system for evaluating the stretch-flangeability of small-scale specimens based on the hole-expansion ratio (HER). The system has no size effect and shows excellent reproducibility, reliability, and economic efficiency. To verify the reliability and reproducibility of the proposed hole-expansion testing (HET) method, the deformation behavior of the conventional standard stretch-flangeability evaluation method was compared with the proposed method using finite-element method simulations. The distribution of shearing defects in the hole-edge region of the specimen, which has a significant influence on the HER, was investigated using scanning electron microscopy. The stretch-flangeability of several kinds of advanced high-strength steel determined using the conventional standard method was compared with that using the proposed small-scale HET method. It was verified that the deformation behavior, morphology and distribution of shearing defects, and stretch-flangeability results for the specimens were the same for the conventional standard method and the proposed small-scale stretch-flangeability evaluation system.111sciescopu

Development of Methodology with Excellent Reproducibility for Evaluating Stretch-Flangeability Using a Sheared-Edge Tensile Test

Stretch-flangeability is an important formability factor for advanced high-strength steels (AHSS) when manufacturing automotive parts. However, the reproducibility of the hole expansion test (HET), a standard testing method established by the international organization for standardization, is quite poor compared with other mechanical testing methods. In this study, we propose a new method, the sheared-edge tensile test (SETT), for evaluating stretch-flangeability with excellent reproducibility. SETT takes into account the deformation behavior during the HET and the critical extrinsic factors influencing the hole expansion ratio (HER). The correlation between the sheared-edge surface characteristics, the tensile properties of the SETT specimen, and stretch-flangeability were investigated to gain in-depth understanding of the proposed evaluation method. Although the correlation between HER and the post-uniform elongation of the SETT specimen with an HER > 80% was ambiguous, the SETT results exhibited good reproducibility, and the stretch-flangeability indicated by HER tended to increase linearly with increases in the post-uniform elongation of the SETT specimen. Furthermore, the roughness of the sheared-edge surface had a significant effect on the local deformation behavior of the materials. The proposed method requires only a small amount of specimen, as compared with the HET and can evaluate stretch-flangeability with good reproducibility.112sciescopu