Dehydration-induced damage and deformation in gypsum and implications for subduction zone processes

Experimental heating tests were performed on Volterra gypsum to study the micromechanical consequences of the dehydration reaction. The experimental conditions were drained at 5 MPa fluid pressure and confining pressures ranging from 15 to 55 MPa. One test was performed with a constantly applied differential stress of 30 MPa. The reaction is marked by (1) a porosity increase and homogeneous compaction, (2) a swarm of acoustic emissions, (3) a large decrease in P and S wave velocities, and (4) a decrease in V P/V S ratio. Wave velocity data are interpreted in terms of crack density and pore aspect ratio, which, modeling pores as spheroids, is estimated at around 0.05 (crack-like spheroid). Complementary tests performed in an environmental scanning electron microscope indicate that cracks first form inside the gypsum grains and are oriented preferentially along the crystal structure of gypsum. Most of the visible porosity appears at later stages when grains shrink and grain boundaries open. Extrapolation of our data to serpentinites in subduction zones suggest that the signature of dehydrating rocks in seismic tomography could be a low apparent Poisson's ratio, although this interpretation may be masked by anisotropy development due to preexisting crystal preferred orientation and/or deformation-induced cracking. The large compaction and the absence of strain localization in the deformation test suggests that dehydrating rocks maybe seen as soft inclusions and could thus induce ruptures in the surrounding, nonreacting rocks

Characterization of oxide scales formed on Alloy 82 in nominal PWR primary water at 340 degrees C and in hydrogenated steam at 400 degrees C

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SCC crack initiation of alloy 82 in hydrogenated steam at 400 degrees c

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SCC Crack initiation in nickel based alloy welds in hydrogenated steam at 400°c

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A synchrotron transmission X-ray microscopy study on precipitate evolution during solid-state thermal cycling of a stainless steel

International audienceDuring additive manufacturing of stainless steels, sub-micron sized oxide (i.e., MnSiO3 , SiO2 , and CrMn2O4) and non-oxide (i.e., sulfide, in particular MnS, and possibly carbides, phosphides and nitrides) precipitates form during solidification. But do they evolve during the subsequent solid-state thermal cycling (SSTC) until the end of the printing process? A recent study on subjecting thin-film lamellae extracted from an additively manufactured stainless steel to heating-cooling treatments inside a transmission electron microscope (TEM) confirmed that precipitate composition can indeed evolve during SSTC. However, that study could not provide any conclusive evidence on precipitate volume fraction, density, and size evolution. In this work, we have quantified these changes using a novel experimental procedure combining (i) micropillar extraction from an additively manufactured stainless steel, (ii) subjecting them to different SSTC (including annealing) inside a TEM, (iii) performing synchrotron transmission X-ray microscopy to identify precipitates, and (iv) using a machine learning model to segment precipitates and quantify precipitate volume fraction, density, and size. Comparing these quantities before and after each SSTC/annealing sequence reveals that new oxides nucleated during rapid SSTC with high maximum temperature. However, during slow SSTC with high maximum temperature and annealing, precipitates dissolve because of oxygen evaporation during SSTC inside the TEM. A new empirical relationship correlating precipitate sizes and cooling rates is proposed. It is in good agreement with data collected from conventional casting, directed energy deposition, and powder bed fusion processes

Micromechanical tensile test investigation to identify elastic and toughness properties of thin nitride compound layers

International audienceThe measurement of thin-layer mechanical properties, like compounds generated by thermo-chemical nitriding process, is a key issue to optimize and predict the endurance of surface treatment against contact fatigue and wear. An original FIB micro-tensile test strategy involving plain and micro-notched tensile specimens is proposed. These specimens were FIB machined in a thin ε(50%)- γ’(50%) compound layer resulting from a common low pressure gaseous nitriding process (Allnit ©) and tested using a dedicated micro-testing system. Combined with DIC analysis, such micro-tensile test strategy allows extracting both the elastic modulus and the Poisson's ratio. Additionally, testing micro-notched specimens underlines the necessity to include FE simulations in order to take into account the radius of the micro-notch tip as well as the surface roughness induced by the FIB machining process. The investigation of this ε - γ’ compound layer suggests a Young's modulus E = 200 GPa, a Poisson's ratio ν =0.31 and a rather low fracture toughness KIC = 0.55 MPa√m . This methodology involving FIB machining, DIC analysis, test procedures and FE post processing simulations is fully detailed and the given results are discussed regarding literature

Parameter identification of crystallographic constitutive law using local strain fields

Communication to : 2004 SEM international congress, Costa Mesa (USA), 7-10 juin 2004SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : 22419, issue : a.2004 n.114 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Coupling and scale effects: two main issues to begin to understand intergranular stress corrosion cracking in nickel bases alloy.

International audienceIntergranular stress corrosion cracking (IGSCC) of Nickel based alloys in PWR environment is a coupled phenomenon that involves several factors related to the material (crystallographic orientations and misorientations of the grains, nature in term of lattice structure coincidence factor and orientation of the grain boundaries versus the maximum stress applied, the environment (temperature, pH of the solution, partial pressure of O$2$) and the mechanical loading (effect of the strain path, residual stresses, strain heterogeneities). In this study, we propose a methodology based on a local approach that combined EBSD mapping to characterize the microstructures, DIC technique to determine strain heterogeneity localizations and finite element simulations to estimate local stress states that allow us to evaluate a stress corrosion cracking criterion. We will discuss how this criterion could be related with the depth of the penetration oxide, what modify the intrinsic cohesive energy of the grain boundaries. So, discussion should be open concerning the use of cohesive zone models, with evolution of the open stress regarding the evolution of the nature of the species (oxide or metal) that define the grain boundaries

Acqueous alteration probed by mutli-assembly and 3D heterogeneity in small Ryugu fragments

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Acqueous alteration probed by mutli-assembly and 3D heterogeneity in small Ryugu fragments

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