Abstract: To be updated

Abstract: The scatter of the fatigue life of additive manufacturing specimens is quite large because of the defects dispersed in material. Due to the various defects and the large life scatter of additive manufacturing material, the relationship between life and defect characterization parameters such as defect type, defect location, and defect size are complex. This paper analyses the effect of defects on fatigue life from the perspective of statistical sense. The statistical results based on both the minimum life of the samples under the same stress level and the half of the samples with the fatigue life less than the median show that the defect size has a great influence on the fatigue life. However, the "effective area", i.e., the projection area of a defect on the plane vertical the loading direction, is not an ideal parameter to characterize the defect. In other word, the sample with the maximum defect in term of the effective area is not necessarily the shortest life sample. Fatigue life is more sensitive to the defects on surface or near-surface than those inside a specimen. There is little difference in the hazards of the two types of defects to fatigue performance, porosity defect induced failure is slightly higher than non-fusion defect induced failure.

Keywords: Additive manufacturing material, defects, defect size, defect location, fatigue life.

Abstract: This study presents a novel method to determine the stress tensor for grain groups with preferred texture orientations and the Critical Resolved Shear Stresses (CRSSs) required to activate slip systems, applied to investigate the elastic-plastic behavior of textured duplex steel [1] and two-phase brass. The approach relies on in situ neutron diffraction measurements of lattice strains in ferritic and austenitic grains for steel, and in α- and β-phase grains for brass, during tensile testing. These measurements enabled direct experimental determination of the evolution of both the stress tensor and the Resolved Shear Stress (RSS) for groups of grains with similar orientations.
For the first time, CRSS values for slip systems in both phases of duplex steel and two-phase brass were directly obtained from experimental data. A key advantage of this methodology is that both grain stress tensors and CRSSs are determined for representative polycrystalline volumes without relying on elastic-plastic models.
Results show that heat treatment significantly hardens the ferritic phase in duplex steel, resulting in a markedly higher CRSS compared to the austenitic phase, strongly influencing the overall yield stress. In contrast, the CRSS values of α- and β-phase grains in two-phase brass are approximately equal, leading to more uniform mechanical behaviour.
Finally, the experimental data were compared with predictions from a multi-scale Elastic-Plastic Self-Consistent (EPSC) model using the experimentally obtained CRSSs as input. Direct determination of CRSS values reduces input parameters in multiscale models and allows verification of theoretical calculations.

Reference: [1] A. Baczmański, et al., International Journal of Mechanical Sciences 283 (2024) 109745.

Keywords: Elastic-plastic deformation, neutron diffraction, crystallographic slip, multiscale model
Acknowledgements: This work was financed by a grant from the National Science Centre, Poland (NCN), No. UMO-2023/49/B/ST11/00774.

Abstract: Crystallographic texture evolution during symmetric and asymmetric rolling of aluminum and copper was investigated experimentally and through modelling using the Finite Element Method (FEM) combined with crystal plasticity (CP) approaches. Rolling asymmetry was introduced by varying the ratio of the roll diameters, with the asymmetry coefficient defined as A=R₂ / R₁. A second type of asymmetry was generated by adjusting the material insertion angle α, defined as the angle between the incoming strip and the horizontal plane. Consequently, both flat and tilted entry configurations were examined. The variation of crystallographic texture across the thickness of the rolled bars was measured using X-ray diffraction and predicted using FEM–CP simulations. In parallel, the microstructure was characterized by Electron Backscatter Diffraction (EBSD). The dominant effect observed during asymmetric rolling is the homogenization of texture across the sample thickness, arising from the presence of strong shear stresses and strain components within the material. The extent of this homogenization depends on the selected rolling geometry parameters, namely A and α. Changes in texture distribution significantly influence the mechanical response of the material, particularly its plastic ductility and drawability. These effects manifest as an increased maximum strain at fracture and a reduction in planar plastic anisotropy. The developed computational software enables optimization of the rolling geometry parameters. Based on the modelling results, the most favorable rolling configurations have been identified and recommended for technological implementation.

Keywords: Rolling geometry, crystallographic texture, aluminium, X-ray diffraction, mechanical properties, numerical modeling

Abstract: The effect of long-term ageing on degradation of tensile properties and mechanism of a directionally-solidification superalloy DZ409 were studied, to simulate servicing of heavy industrial gas turbine blades during the long-term aged at 900℃ and 980℃. The results show that the spherical secondary γ' phase dissolves rapidly, the cubic γ' phase becomes smooth, and some γ' phases merge at 900℃ and 980℃. γ' phase coarsened continuously during long-term ageing, and the coarsening rate at 980℃ was much higher than that at 900℃. The MC carbides gradually transform into M₂₃C₆ and γ' phases. At 900℃ to 1000h and 980℃ to 100h, M₂₃C₆ began to precipitate at the MC/γ' interface. When (γ+γ') eutectic structure coexists with MC, M₂₃C₆ particles are gradually precipitated. No TCP phase was found after aged at 900℃/20000h and 980℃/3000h. Both the yield and tensile strength of the alloy at RT, 650℃ and 900℃ can be divided into three stages after aged at 900℃. The first stage (0~100/200h) has a rapid decline in strength, mainly due to the rapid dissolution of the spherical secondary γ' phase. In the second stage (100/200~2000h), the decline rate of the strength continued to slow down, mainly due to the coarsening of the γ' phase and the decrease of the cubic degree of the γ' phase, which led to the decrease of the mismatch of γ/γ', and the decomposition of MC carbide began. In the third stage (2500~8000h), the strength tends to be stable, which is because of the negative effect of continuous coarsening and merging of γ' phase on the deformation resistance of the alloy. The positive effect of M₂₃C₆ carbide formation on the pinning of dislocation and the inhibition of grain boundary slip makes the tensile properties of the alloy stable. Compared with the long-term aged at 900℃, the damage regularity of microstructure evolution and tensile properties of the alloy after 980℃ ageing is similar, but the change rate is faster.

Keywords: Long-term ageing, property degradation, directionally solidified superalloy DZ409, microstructure evolution.