A strategy for synthetic microstructure generation and crystal plasticity parameter calibration of fine-grain-structured dual-phase steel

Ultrasonic surface mechanical attrition treatment (SMAT) was employed to modify the surface microstructural layer of SLM Ti6Al4V ELI biomedical material to improve the fatigue performance. The SMAT...

/pdf/fatigue-strength-improvement-of-selective-laser-melted-4xfjzmiqqt.pdf

Fatigue strength improvement of selective laser melted Ti6Al4V using ultrasonic surface mechanical attrition

/pdf/on-the-relationship-between-microstructure-and-residual-1i4d9k2g.pdf

On the relationship between microstructure and residual stress in laser-shock-peened Ti-6Al-4V

In the investigation of fatigue properties of metals, the microstructure-based modelling has shown its powerful applicability in predicting the microcrack initiation as well as the fatigue life. However, proper treatment of the inclusions, which are the major fatigue crack trigger especially for the very high cycle fatigue regime, is still missing. It is emphasised that in addition to the geometrical representation and the basic mechanical properties assignment of the inclusions, the residual stresses developed between the steel matrix and inclusions during the cooling processes due to their distinct thermal expansion coefficients play a non-negligible role in determining the fatigue properties. Therefore, it is aimed, in this study, to propose a microstructure-based modelling approach to account for the effects of residual stresses induced by the rapid cooling process on the fatigue crack initiation behaviour of a martensitic steel, for which the majority of the fatigue crack is formed around the calcium aluminate inclusions in experiments. The entire approach is decomposed into two processes: i) simulation of the cooling process to obtain the residual stress profile around the inclusion and ii) fatigue simulation using a crystal plasticity model including the mapped residual stress profile from the previous step. It is shown that the proposed approach accurately predicts the fatigue crack initiation sites around the inclusions corresponding to the experimental findings, while the modelling approach without the residual stresses fails to predict the correct locations of the crack initiation, revealing the necessity to consider the residual stresses for the future fatigue modelling and assessment.

/pdf/microstructure-based-fatigue-modelling-with-residual-3degiw7ofw.pdf

Microstructure-based fatigue modelling with residual stresses: Prediction of the microcrack initiation around inclusions

In high-strength martensitic steels, the inclusions significantly affect the material performance especially in terms of fatigue properties. In this study, a numerical procedure to investigate the effect of the inclusions types and shapes on the residual stresses during the cooling process of the martensitic steels is applied systematically based on the scanning electronic microscopy (SEM) and energy dispersive spectrometer (EDS) results of different types of inclusions. The results show that the maximum residual stress around the interface between Mg-Al-O inclusion and the matrix is the largest, followed by TiN, Al-Ca-O-S, and MnS when the inclusions are assumed as perfect spheres for simplicity. However, these results are proved to be 28.0 to 48.0% inaccurate compared to the results considering actual shapes of inclusions. Furthermore, the convex shape of inclusion will lead to stress concentration in the matrix while the concave shape of inclusion will lead to stress concentration in the inclusion. The residual stress increases with the increase of inclusion edge angle. The increase rate is the largest for TiN inclusions on the concave angle, which leads to extreme stress concentration inside TiN inclusion.

Numerical study of the effect of inclusions on the residual stress distribution in high-strength martensitic steels during cooling

Heterogeneity of the through-thickness residual strain due to the laser shock peening (LSP) in comparison with that due to the indentation was studied in Ti-6Al-4V alloy samples. The latter is almost a quasi-static process while the former features extremely high strain-rate deformation. The synchrotron based high-energy X-ray diffraction was employed to investigate the through-thickness residual strain distribution. The studied two samples after the two processing procedures share the following features: (i) the pressure affected depths are both ~2 mm and (ii) the largest magnitudes of the compressive residual strains parallel to the surface are ~4,000 μe. However, the pit depth for the indentation sample is ~9 times larger than that for the LSP. The position featuring the largest magnitude of the compressive residual strain is in the sub-surface for the indentation while it is in the surface for the LSP. Results of the elastic-visco-plastic finite element simulation for the indentation indicate that the position featuring the maximum accumulative plastic shear as defined in this paper corresponds to the location with the largest magnitude of compressive residual strain. To validate this finding, full width at half maximum (FWHM) of the X-ray diffraction profile, which is proportional to the level of the plastic deformation, is also studied. It is found that positions with the largest FWHM indeed correspond to the largest magnitude of compressive residual strain for both tests.

The in-depth residual strain heterogeneities due to an indentation and a laser shock peening for Ti-6Al-4V titanium alloy

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The in-depth residual strain heterogeneities due to an indentation and a laser shock peening for Ti-6Al-4V titanium alloy