This article deals with the effect of periodically acting liquid droplets on the polished surfaces of AISI 316L stainless steel and Ti6Al4V titanium alloy. These materials were exposed to a pulsating ...water jet produced using an ultrasonic sonotrode with an oscillation frequency of 21 kHz placed in a pressure chamber. The only variable in the experiments was the time for which the materials were exposed to water droplets, i.e., the number of impingements; the other parameters were kept constant. We chose a low number of impingements to study the incubation stages of the deformation caused by the pulsating water jet. The surfaces of the specimens were studied using (1) confocal microscopy for characterizing the surface profile induced by the water jet, (2) scanning electron microscopy for detailed surface observation, and (3) transmission electron microscopy for detecting the changes in the near-surface microstructure. The surface described by the height of the primary profile of the surface increased with the number of impingements, and was substantially more intense in the austenitic steel than in the Ti alloy. Irregular surface depressions, slip lines, and short cracks were observed in the Ti alloy, whereas pronounced straight slip bands formed in the austenitic steel. The dislocation density near the surface was measured quantitatively, reaching high values of the order of 1014 m−2 in the austenitic steel and even higher values (up to 3 × 1015 m−2) in the Ti alloy. The origins of the mentioned surface features differed in the two materials: an intense dislocation slip on parallel slip planes for the Ti alloy and mechanical twinning combined with dislocation slip for the austenitic steel.
The objective of the study is to investigate the erosion effects of periodic water clusters interacting with flat AISI 304 surface to evaluate erosion regimes, surface morphology and variation in the ...subsurface microhardness below the disintegrated grooves. Stairs trajectory within the standoff distance 5–101 mm with step height 2 mm and length 20 mm was used. The pulsating water jet was changed by varying the supply pressure of the liquid from 40 MPa–100 MPa, at a traverse speed of 5 mm/s which is equivalent to constant impingement distribution of 4000 impingements per mm. Morphology of the PWJ defines different erosion regimes based on specific value of supply pressure and standoff distance. Increasing of supply pressure shifts prevalence of both impact pressure and stagnation pressure at higher standoff distances leading to the delay in formation of incubation, acceleration, culmination, depletion, and termination regime. Impinged surface features such as micro-pits, craters, upheaved surface and isolated voids near the periphery of the trace were observed along with PWJ footprint. The values of the micro-hardness showed strengthening of material subsurface at selected standoff distances. At certain standoff distance effect of supply pressure is inverse to micro-hardness values. Determination of studied technological parameters can be used for effective surface peening operation at v > 5 mm and surface activation of the metals. Results opens a new avenue for determination of erosion resistance of material.
•Erosion effects of pulsating water jet as function of standoff distance and flow rate was investigated.•Linear offset of erosion stages and erosion intervals due to interaction of water flow rate and standoff distance was found.•Different erosion stages were observed due to non-uniform plastic deformation with the variation of standoff distance.
Austenitic heat resistant Sanicro 25 steel developed for high temperature applications in power generation industry has been subjected to strain controlled low cycle fatigue tests at ambient and at ...elevated temperature in a wide interval of strain amplitudes. Fatigue hardening/softening curves, cyclic stress–strain curves and fatigue life curves were evaluated at room temperature and at 700°C. The internal dislocation structures of the material at room and at elevated temperature were studied using transmission electron microscopy. High resolution surface observations and FIB cuts revealed early damage at room temperature in the form of persistent slip bands and at elevated temperature as oxidized grain boundary cracks. Dislocation arrangement study and surface observations were used to identify the cyclic slip localization and to discuss the fatigue softening/hardening behavior and the temperature dependence of the fatigue life.
The origin of the extraordinary strengthening of the highly alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700 °C was investigated by the use of advanced scanning transmission ...electron microscopy (STEM). Along with substantial change of the dislocation structure, nucleation of two distinct populations of nanoparticles was revealed. Fully coherent Cu-rich nanoparticles were observed to be homogeneously dispersed with high number density along with nanometer-sized incoherent NbC carbides precipitating on dislocations during cyclic loading. Probe-corrected high-angle annular dark-field STEM imaging was used to characterize the atomic structure of nanoparticles. Compositional analysis was conducted using both electron energy loss spectroscopy and high spatial resolution energy dispersive X-ray spectroscopy. High-temperature exposure-induced precipitation of spatially dense coherent Cu-rich nanoparticles and strain-induced nucleation of incoherent NbC nanoparticles leads to retardation of dislocation movement. The pinning effects and associated obstacles to the dislocation motion prevent recovery and formation of the localized low-energy cellular structures. As a consequence, the alloy exhibits remarkable cyclic hardening at elevated temperatures.
•Erosion damage accumulation varies across the entire cross-section.•A thin hardened layer was observed under the surface.•Additional deformation is inhibited by grain boundaries.•The subsurface ...hardening is caused by shear stress wave propagation.
Erosion of solids by liquid droplets is a phenomenon which is a compromise between mechanical properties of the material and droplet hydrodynamic parameters. While a number of studies deal with the deformation of drops, the deformation evolution inside the material has not yet been revealed, mainly from the point of view of the time action of the impinging drops The mechanical response of AISI 316L was investigated under gradually increasing numbers of impingements of liquid droplets, with a droplet volume of Vd ≅ 0.9 mm3, generated by an ultrasonic pulsating water jet with the frequency f = 40 kHz from 1 to 20 s. The surface roughness and the wear rates were determined using a laser profilometer. The cross-section of the selected samples was subjected to microhardness measurement with a load of 0.150 N in a 2D grid, which included the entire perimeter of the deformed area. The minimal microhardness measurement grid under the groove had dimensions of 15 × 15 indents, equal to an area of approximately 450 × 600 µm. A maximum hardness increase was observed at the lowest measured depth of 30 µm. An increase in hardness was observed at 300 µm below the surface. The hardening in the deeper subsurface area was most likely caused by shear stress. This shows the high degree of similitude between the solid and liquid droplet impingements. The results indicate that the currently accepted theory on the development of erosion over time has shortcomings, as demonstrated in this work by the ratio between the utilised droplet diameter and the grain size of the material.
Excellent adhesion of electrospun nanofiber (NF) to textile support is crucial for a broad range of their bioapplications, e.g., wound dressing development. We compared the effect of several low- and ...atmospheric pressure plasma modifications on the adhesion between two parts of composite-polycaprolactone (PCL) nanofibrous mat (functional part) and polypropylene (PP) spunbond fabric (support). The support fabrics were modified before electrospinning by low-pressure plasma oxygen treatment or amine plasma polymer thin film or treated by atmospheric pressure plasma slit jet (PSJ) in argon or argon/nitrogen. The adhesion was evaluated by tensile test and loop test adapted for thin NF mat measurement and the trends obtained by both tests largely agreed. Although all modifications improved the adhesion significantly (at least twice for PSJ treatments), low-pressure oxygen treatment showed to be the most effective as it strengthened adhesion by a factor of six. The adhesion improvement was ascribed to the synergic effect of high treatment homogeneity with the right ratio of surface functional groups and sufficient wettability. The low-pressure modified fabric also stayed long-term hydrophilic (ten months), even though surfaces usually return to a non-wettable state (hydrophobic recovery). In contrast to XPS, highly surface-sensitive water contact angle measurement proved suitable for monitoring subtle surface changes.
This paper describes and applies a methodology to determine the elastic properties of freestanding thin membranes by means of a bulge test and a numerical approach. The numerical procedure is based ...on the combination of two standard methods i.e. finite element analysis and classical analytical solutions to calculate elastic properties of thin films. Bulge tests were conducted on silicon nitride (Si3N4) monolayer of 2 × 2 mm (square) and 3.5 × 1.5mm (rectangular) membranes with the aim to determine elastics properties (Young's modulus (E) and Poison's ratio (v)) that define the load-deflection curves of both membranes. With this purpose, an error function was constructed for each membrane which involved finite element solutions, analytical solutions and experimental measurements. Error functions were found and minimized by mapping a set of elastic parameters for the two membranes (square and rectangular). A unique solution was determined in the intersection of both linear approximations, obtaining 236 GPa for E and 0.264 for v. It is well known that in a traditional bulge test analysis only one of both biaxial modulus can be determined and not a combination of E and v. Numerical results show that calculated load-deflection curves agree well with the measurements obtained for both square and rectangular membranes experimentally. The proposed methodology is only applicable in thin films with elastic behavior, however generalization for more complicated geometries is possible.
•Bulge test apparatus•Finite element modelling of bulge test for elastic thin films•Numerical approach for the estimation of elastic properties•Effects of elastic properties in the load-deflection relation•Young's modulus and Poisson's ratio determination
The present work focuses on the low-cycle fatigue (LCF) behavior of conventionally and additively manufactured (laser powder bed fusion (LPBF) technique, Ni-based VDM Alloy 699 XA. LCF tests were ...performed on miniature cylindrical specimens in a symmetrical push-pull cycle under strain control with a constant total strain amplitude at 23°C. Cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves were obtained. Despite the present manufacturing defects associated with the LPBF technology, the fatigue lifetime of the LPBF manufactured material is higher than that of conventionally prepared material. Fatigue crack initiation sites and fatigue crack propagation areas were investigated by means of scanning electron microscopy (SEM). A quantitative fractography of fracture surfaces was introduced to provide a detailed description of fatigue crack growth from the evaluation of striation spacing. The dependency of the stress intensity factor range on macroscopic crack growth rates was displayed in the form of the Paris Law. The differences in fatigue lifetimes and fatigue crack growth rates of additively manufactured and hot rolled solution annealed alloy 699 XA plate material were discussed and correlated with respect to the microstructural specifics of both material batches.
Oxide dispersion-strengthened (ODS) materials contain homogeneous dispersions of temperature-stable nano-oxides serving as obstacles for dislocations and further pinning of grain boundaries. The ...strategy for dispersion strengthening based on complex oxides (Y-Hf, -Zr, -Ce, -La) was developed in order to refine oxide dispersion to enhance the dispersion strengthening effect. In this work, the strengthening of EUROFER steel by complex oxides based on Y and elements of the IIIB group (lanthanum, scandium) and IVB group (cerium, hafnium, zirconium) was explored. Interparticle spacing as a dispersoid characteristic appeared to be an important factor in controlling the dispersion strengthening contribution to the yield strength of ODS EUROFER steels. The dispersoid size and average grain size of ODS EUROFER steel were altered in the ranges of 5–13 nm and 0.6–1.7 µm, respectively. Using this strategy, the yield strength of the prepared alloys varied between 550 MPa and 950 MPa depending on the doping element.
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Deformation twinning in body-centered cubic (BCC) metals occurs by shearing the crystal along {112} planes parallel to 〈111〉 directions. One of these directions (twinning shear) ...produces a twin, but it is often argued that the opposite sense of shearing (antitwinning shear) does not lead to twin formation. However, recent slip trace and orientational mapping analyses made on many BCC metals after low-temperature plastic deformation show clear evidence of fine misoriented lamellae along the traces of {112} planes sheared in the antitwinning sense. To resolve this controversy, we have utilized molecular statics simulations to determine the energy barriers for uniformly shearing all transition and alkali BCC metals in the twinning and antitwinning sense. The results of these simulations show that twins in transition BCC metals of the 5th and 6th groups can be produced on both types of {112} planes, irrespective of whether the shear is applied in the twinning or the antitwinning sense. However, this is not the case for α-Fe and the BCC structures of the alkali metals Li, Na, and K, where twins are likely to occur only on {112} planes sheared in the twinning sense. Furthermore, we have used TEM diffraction pattern analyses to investigate the characters of misoriented deformation lamellae in Nb and Cr compressed at 77 K, which were found along {112} planes sheared in the antitwinning sense. We demonstrate that these regions constitute regular twins. Similar studies on α-Fe compressed at 77 K prove that twinning in this material takes place exclusively on {112} planes sheared in the twinning sense.
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