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Combined with scanning electron microscopy, transmission electron microscope, X-ray diffraction and electron backscatter diffraction, the effects of three treatment modes consisting of as-deposited, solid solution and solution aging state on mechanical structure-properties of a nickel-based superalloy fabricated by selective laser melting were studied. Grains have preferred orientation, i.e. strong texture along {100}. The phase transformation processes during post-heat treatments include dissolution of Laves phase, precipitation of strengthening phases γ′, γ′′ and δ, and size reduction of δ phase. Especially for solution aging treatment, the refined grains, high dislocation density and added high-angle grain boundaries cause a significant increase in strength and a decrease in ductility, which is consistent with the results of the tensile tests. The interior cracking behavior related to the micro-voids and microcracks formed by dislocation pile-ups and uncoordinated deformation around harder δ phase in a vacuum environment becomes a typical ductile failure mode. [Display omitted] • A microstructure evolution process with precipitation and reduction of δ phrase is proposed. • Mechanical properties are affected by grain size, dislocation density and HAGBs. • Interior ductile failure mechanism related to micro-voids and microcracks is elucidated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fatigue lives with interior failure are longer at 750 °C and shorter at 1000 °C. • Interior crystallographic plane cracking behavior belong to Mode I & Ⅱ cracking. • Climbing, bypassing and shearing mechanisms are remarkable at elevated temperature. • Threshold value and transition crack size are lower at 1000 °C. • A crystallographic version of FIP controlled crack initiation life is proposed. Asymmetric load tests with stress ratio of 0.1 at 25 °C, 750 °C and 1000 °C were performed to investigate the high-cycle and very-high-cycle fatigue properties of Inconel-713C Ni-based superalloy. Results show that the life at 750 °C is longer than that at 25 °C, but shortens as temperature keeps rising to 1000 °C. Relevant crack initiation and growth belong to the mixed Modes I & Ⅱ cracking along the maximum shear stress. Moreover, due to the effects of temperature and vacuum environment, threshold values for small and long cracks, and the transition crack size decreases as the temperature increases. Moreover, further analysis of dislocation structures such as tangled dislocations, dislocation loops and finer stacking faults shows that the deformation mechanism with faceting cracking at 750 °C should be the interaction of bypassing and shearing mechanisms, especially under stress concentration effect of crack. Based on these, a crystallographic version of fatigue index parameter model for crack nucleation life is proposed and the predicted results are within a factor of two with respect to experimental data. The present model can be applied to predict the fatigue life based on the microstructure attributes and load conditions at different temperatures, which can effectively improve operational safety and reliability of structural components in reality. [ABSTRACT FROM AUTHOR]

The axial fatigue tests at elevated (650°C) temperature with stress ratios of 0.1 and −1 were conducted under the loading cycle range of 104–108 cycles to investigate interior cracking mechanism at elevated temperature of one selective laser melted GH4169 superalloy. Results showed that the interior fatigue crack nucleation was the main failure mode with both stress ratios at 650°C in the long‐life regime. The interior crack nucleation region consisting of many facets was formed under shear stress and strain according to the 3D morphology restructure of the fracture surface. Combined with the evaluation of the threshold stress intensity factor, the transition lengths from a small crack to a long crack for interior failure at both stress ratios were evaluated. Based on the above analysis, the microstructure‐related interior failure mechanism was summarized. Finally, crack nucleation and propagation lives were predicted to obtain the total fatigue life, which showed that the prediction and experimental data scattered between the factor‐of‐three boundaries. [ABSTRACT FROM AUTHOR]

• The interior fatigue failure with longer life is greatly promoted at 650 °C. • The interior cracks are mainly nucleated from the facets associated with grain structure. • The interior crack nucleation and growth mechanisms at T = 25 °C & 650 °C are elucidated. • A microstructure-based crack nucleation life prediction model is proposed. Axial loading tests with stress ratios of −1 and 0.1 at 25 °C and 650 °C were performed to investigate the effect of elevated temperature on high-cycle and very-high-cycle fatigue properties of a Ni-based superalloy manufactured by selective laser melting. Interior fatigue failure with longer life is greatly promoted at elevated temperature, which is induced from the crystallographic facets associated with grain structure. By means of electron-backscattered diffraction and stress intensity factor evaluation, the interior failure mechanisms are elucidated. Based on the proposed microstructure-based crack nucleation life model, a good agreement between the predicted and experimental results is obtained. [ABSTRACT FROM AUTHOR]

The structure-property of a Ni-based superalloy fabricated by selective laser melting (SLM) was investigated under very high cycle fatigue (VHCF), and the subsurface cracking with fisheye pattern becomes a remarkable failure mode. Due to the presence of subsurface failure, the descending S–N curve property is represented. Based on 2D & 3D microscopic observation, a rough region with facets and inclusions, named as "multi-inclusion assisted facetted cracking zone (MAFCZ)", is present within the fisheye. By combining electron backscatter diffraction analysis, the subsurface failure is related to grain size & orientation, microtexture and crystal defects. The crack is nucleated from the large grain slip with the highest Schmid factor under the assistance of the inclusions. The rough surface morphology with crack deflection reveals ultra-slow crack growth behavior within MAFCZ. Combined with failure modeling and fracture mechanics analysis, the subsurface failure mechanisms of SLM Ni-based superalloy under the VHCF are elucidated. Image, graphical abstract [ABSTRACT FROM AUTHOR]