P. Villechaise

On slip band features and crack initiation in fatigued 316L austenitic stainless steel: Part 1: Analysis by electron back-scattered diffraction and atomic force microscopy

Electron back-scattered diffraction (EBSD) and atomic force microscopy (AFM) have been used to study surface slip features on 316L austenitic stainless steel polycrystals tested in the low cycle fatigue range. EBSD investigations allow activated slip planes to be identified for each grain and the local inclination of these slip planes to the surface to be calculated. AFM allows the height of steps induced at the surface along slip bands to be measured and the local morphology of extrusions to be characterized at a nanometer scale. In this study, both techniques are used on the same surface in order to combine crystallographic and topographic information. Based on the results, a schematic model of the slip band emergence is proposed.

Very high temperature creep behavior of a single crystal Ni-based superalloy under complex thermal cycling conditions

Very high temperature thermal cycling has been performed on the single crystal superalloy MC2 to evaluate the effect of periodic overheating on creep behavior. The experiments consist of alternately performing 1 min dwell time at 1100°C and 1150°C for every 15 min during creep test at 1050°C/120 MPa. Both thermal cycling and prior γ′-rafting appear to be deleterious to the cyclic creep properties. The observed non-isothermal creep behavior is correlated with γ′-dissolution/coalescence processes, especially during overheatings where γ′ micro-rafts seem to play a significant role.

Slip line analysis around nanoindentation imprints in Ti3SnC2: a new insight into plasticity of MAX-phase materials

The plasticity of Ti3SnC2, a recently synthesized MAX phase, was investigated. Localized deformation was induced by nanoindentation in a polycrystalline sample, and the resulting surface topography was observed by atomic force microscopy (AFM). For several grains, buckling around the indent was observed, in agreement with the kink band deformation process often reported for MAX-phase materials. For other grains, slip lines have been revealed by AFM. The corresponding slip systems have been identified through the determination of the local crystalline orientation by electron backscatter diffraction. First- and second-order pyramidal slip systems are shown to be active for some grain orientations, as well as dislocation interactions and cross slip from one system to the other.

Cyclic behaviour and surface slip features in 〈111〉 oriented copper single crystals

Abstract New information about cyclic behaviour and surface deformation features have been obtained on 〈111〉 oriented copper crystals cycled in air and in vacuum at different stages of fatigue life. In this way a secondary hardening stage has been observed and different types of slip events have been distinguished; in particular, evidence of {100} glide has been obtained which had never been observed before at room temperature in cyclically deformed copper crystals.

Microstructural features controlling the variability in low-cycle fatigue properties of alloy Inconel 718DA at intermediate temperature

The low-cycle fatigue behavior of two direct-aged versions of the nickel-based superalloy Inconel 718 (IN718DA) was examined in the low-strain amplitude regime at intermediate temperature. High variability in fatigue life was observed, and abnormally short lifetimes were systematically observed to be due to crack initiation at (sub)-surface non-metallic inclusions. However, crack initiation within (sub)-surface non-metallic inclusions did not necessarily lead to short fatigue life. The macro- to micro-mechanical mechanisms of deformation and damage have been examined by means of detailed microstructural characterization, tensile and fatigue mechanical tests, and in situ tensile testing. The initial stages of crack micro-propagation from cracked non-metallic particles into the surrounding metallic matrix occupies a large fraction of the fatigue life and requires extensive local plastic straining in the matrix adjacent to the cracked inclusions. Differences in microstructure that influence local plastic straining, i.e., the δ-phase content and the grain size, coupled with the presence of non-metallic inclusions at the high end of the size distribution contribute strongly to the fatigue life variability.

Determination of young's modulus by a resonant technique applied to two dynamically ion mixed thin films

Abstract A resonant frequency technique developed to determine the elastic constants and the internal friction of isotropic and anisotropic bulk materials from 300 K to 1500 K was extended to the determination of Youngs moduli of thin coatings. This paper presents the measurement technique and the associated calculations. Results for very thin films(approximately 2 μm) of SiC and NiTi obtained by dynamic ion mixing (DIM) are presented. This technique, which involves ion sputtering evaporation combined with a high energy ion implantation, gives well adherent and homogeneous coatings. It is shown that such films have very low Youngs modulus compared with the classically prepared crystalline materials. It is assumed that theselow values have their origin in the amorphous structure of the DIM films.

Microscopic Strain and Crystal Rotation Measurement within Metallurgical Grains

This work describes an experimental procedure to measure the progressive strain localization and crystal lattice rotation within metallurgical grains. A digital image correlation software was implemented and associated with mechanical tests carried out inside a scanning electron microscope on specimens exhibiting nanometric grainy patterns. Cross-correlation analyzes between electron backscattering diffraction maps were also developed to quantify the corresponding local crystal rotation relative to the original structure. The microscale strain and rotation fields on the surface of a tensile-loaded specimen made of austenitic stainless steel 316L are presented as an illustration. Their direct spatial correlation between strain heterogeneities and the progressive activation of slip systems is put into evidence and discussed.

Improvement of the fatigue resistance of materials with thin coatings developed from dynamic ion mixing

Abstract A dynamic ion mixing (DIM) technique, which involves a physical vapor deposition (PVD) combined with a simultaneous ion implantation method, has been applied to elaborate adherent NiTi and SiC amorphous thin coatings. Two different metallic materials, a Ti—6Al—4V titanium alloy and a 316L type austenitic stainless steel were used as substrates. The characteristics of such deposits were established by performing scratch-tests and micro hardness measurements, and by measuring their Youngs moduli. This process leads to a significant improvement of the fatigue life for both materials, at room temperature, in the low cycle fatigue range. Scanning electron microscopy observations revealed that thin coatings constitute a barrier at the surface which modifies the near surface deformation mode impeding the formation of extrusions—intrusions pairs and strain localization within intense slip bands. Consequently, crack initiation can be considerably delayed or even suppressed. Several factors controlling beneficial effects of DIM treatments on fatigue resistance are pointed out. These factors concern the nature and properties of the coatings and the cyclic deformation processes in the substrates.

Physically-Based Simulations of the Cyclic Behavior of FCC Polycrystals

Two homogeneization approaches are used in order predict the cyclic elastic-plastic behaviour of 316L(N) polycrystals, either

Influence of thin coatings deposited by a dynamic ion mixing technique on the fatigue life of TITANIUM ALLOYS

The technique of dynamic ion mixing involving a physical vapour deposition method and a simultaneous ion implantation has been used in order to improve the fatigue resistance of two titanium alloys. This process allows the deposition of adherent NiTi and SiC amorphous coatings of the order of 1 μm thick. Both treated substrates have been tested at room temperature in the low cycle fatigue range, revealing significant fatigue life improvement. NiTi and SiC films modify the surface deformation mechanisms of fatigued materials and largely suppress or delay crack initiation. These effects depend, however, on the nature of the film, the microstructure of the substrate and the stress amplitude applied during the fatigue tests. To explain the fatigue results, the mechanical properties of these thin films have been characterized by different techniques: scratch-tests, micro-Vickers indentations, Youngs modulus measurements by a resonant frequency method and “fracture stress” determination by in situ tensile tests. The results have shown that their mechanical properties are very different to those of the corresponding classically deposited solid materials and are influenced by the film thickness. The results are discussed according to the mechanical properties of the coatings and the substrate deformation and damage modes associated with their microstructure.