Combined AFM, SEM and crystal plasticity analysis of grain boundary sliding in titanium at room temperature

International audienceGrain boundary sliding (GBS) was observed at room-temperature, at a very early stage of tensile tests on Grade 2 and Grade 4 titanium specimens covered with fiducial grids. AFM images were also repeatedly captured to monitor the evolution with strain of the out-of-plane component of GBS. Sharp gradients in sliding were observed along some grain boundaries, which proved that GBS should not be considered as a rigid-body motion. Grade 4 Ti was less prone to GBS than Grade 2, which was attributed to a pinning effect of βphase particles present along the grain boundaries. Contrary to what was reported for Zinc and Magnesium alloys, GBS was not found to contribute significantly to the global plastic strain in Titanium at room temperature. Crystal viscoplasticity finite element simulations based on real microstructure were run to compute the shear stress along the grain boundaries for various strain rates. The shear stress along the boundaries that slid was found to be substantially higher than the mean shear stress over all boundaries. However, a high shear stress was not sufficient to trigger GBS, also influenced by other factors. A change in strain rate not only modified the mean shear stress on all grain boundaries, but also the relative values of the shear stress on the grain boundaries, which was attributed to the strain-rate dependence of slip activities in each grain

Characterization and analysis of deformation heterogeneities in commercial purity titanium

International audienceThe effects of solute oxygen, loading direction and strain level on the microscale plastic strain distribution in representative areas of commercially-pure titanium have been characterized by correlation of high resolution SEM images captured during in situ tensile tests. A spatial organization of highly-deformed bands was observed from the early stages of plastic flow and remained nearly unchanged as the materials were strained. The high strains close to grain boundaries were related to intense local slip activity, grain boundary sliding or kink bands formation. The plastic strain field was more homogeneous in the oxygen-rich material, which was attributed primarily to a smaller contribution of grain boundary sliding, due to the presence of hard β phase particles along the grain boundaries

Multiscale investigation of crack path and microstructural changes during liquid metal embrittlement of 304L austenitic steel in liquid sodium.

International audienceFracture micro-mechanisms and crack path of embrittled austenitic steel 304L by liquidsodium at 573K are investigated underneath the fracture surface by transmission electronmicroscopy down to the nanoscale. Automated crystal orientation and phase mappinganalyses show that abundant martensitic transformation (et947;et61664;et945;-) as well as mechanicaltwinning occur during the deformation of austenite and play a major role in the fractureprocess. A correlation between the fracture surface features and the underlyingmicrostructural interfaces is evidenced, strengthening the conclusion that liquid metalembrittlement by sodium of austenitic steels is basically an interfacial cracking phenomenon

Cold-rolling effects on the microstructure properties of 316L stainless steel parts produced by Laser Powder Bed Fusion (LPBF)

International audienceLaser Powder Bed Fusion (LPBF) technology provides new opportunities to enhance some piece-producing processes in the industry. Moreover, LPBF microstructures can heavily differ from microstructures usually obtained through traditional processes, especially 316L LPBF ones which combine both strength and ductility at room temperature. However, 316L LPBF microstructure evolution upon cold-rolling has not yet been extensively studied. In the present study, the effect of cold-rolling was investigated on the distinct 316L as-built microstructures showing different characteristics regarding mean grain size and texture. At first, LPBF 316L has sufficient ductility to be cold-rolled without any intermediate heat-treatments. Differences between initial microstructures affect the extent of grain refine-ment and texture decay. Indeed, the mean grain size and texture remain stable until 20% thickness reduction for one of the studied microstructure while they strongly decrease for the other one. Both microstructures show mechanical twinning after being cold-rolled with a twinned surface ratio exceeding 30%. Mechanical twins can also cross molten pool boundaries

Liquid metal embrittlement and deformation induced martensite: The case of 316 L austenitic steel LME by liquid eutectic gallium-indium

International audienceThe liquid metal embrittlement sensitivity of a low nickel 316 L austenitic steel has been studied with eutectic gallium indium alloy. The embrittlement case is firmly established with this steel and studied by X-Ray fractography and EBSD. Brittle to ductile recoveries were observed via a dedicated parametric study (varying cross-head speed and temperature). The return to ductility with temperature is strongly correlated with a significant reduction in plastically induced α’ phase change suggesting it is a requirement for LME. This implies a strategy focused on limiting deformation induced martensite to design LME resistant austenitic steels at low temperatur