Model based optimization criteria for the generation of deep compressive residual stress fields in high elastic limit metallic alloys by ns-laser shock processing

Laser Shock Processing (LSP) is based on the application of a high intensity pulsed Laser beam (IN1 GW/cm2; τb50 ns) on a metallic target forcing a sudden vaporization of its surface into a high temperature and density plasma that immediately develops inducing a shock wave propagating into the material. The main acknowledged advantages of LSP consist on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behavior, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Due to these specific advantages, Laser Shock Processing is considered as a competitive alternative technology to classical treatments for improving fatigue, corrosion cracking and wear resistance of metallic materials, and is being developed as a practical process amenable to production technology. In this paper, a model based systematization of process optimization criteria and a practical assessment on the real possibilities of the technique is presented along with practical results at laboratory scale on the application of LSP to characteristic high elastic limit metallic alloys, showing the induced residual stresses fields and the corresponding results on mechanical properties improvement induced by the treatment. The homogeneity of the residual stress fields distribution following the laser treatment spatial density will be specially analyzed

Laser shock peening without absorbent coating (LSPwC) effect on 3D surface topography and mechanical properties of 6082-T651 Al alloy

The influence of nanosecond laser pulses applied by laser shock peening without absorbent coating (LSPwC) with a Q-switched Nd:YAG laser operating at a wavelength of λ = 1064 nm on 6082-T651 Al alloy has been investigated. The first portion of the present study assesses laser shock peening effect at two pulse densities on three-dimensional (3D) surface topography characteristics. In the second part of the study, the peening effect on surface texture orientation and micro-structure modification, i.e. the effect of surface craters due to plasma and shock waves, were investigated in both longitudinal (L) and transverse (T) directions of the laser-beam movement. In the final portion of the study, the changes of mechanical properties were evaluated with a residual stress profile and Vickers micro-hardness through depth variation in the near surface layer, whereas factorial design with a response surface methodology (RSM) was applied. The surface topographic and micro-structural effect of laser shock peening were characterised with optical microscopy, InfiniteFocus® microscopy and scanning electron microscopy (SEM). Residual stress evaluation based on a hole-drilling integral method confirmed higher compression at the near surface layer (33 μm) in the transverse direction (σmin) of laser-beam movement, i.e. − 407 ± 81 MPa and − 346 ± 124 MPa, after 900 and 2500 pulses/cm2, respectively. Moreover, RSM analysis of micro-hardness through depth distribution confirmed an increase at both pulse densities, whereas LSPwC-generated shock waves showed the impact effect of up to 800 μm below the surface. Furthermore, ANOVA results confirmed the insignificant influence of LSPwC treatment direction on micro-hardness distribution indicating essentially homogeneous conditions, in both L and T directions

NEUTRON-FLUX MEASUREMENTS IN A CONCENTRIC-CYLINDER FUEL ELEMENT

Neutron-flux measurements in a concentric-cylinder fuel element were made in a gas-cooled in-pile loop operated adjacent to the core of the BRR. The fuel element comprised four concentric fuel cylinders. Each fuel annulus (outside diameters- 1.248 1.018, 0.810 and 0.590 in.) consisted of a 0.031-in.- thick core of UO/sub 2/ dispersed in type 347 stainless steel and clad on each side with 0.007 in. of typee 318 stainless steel. The element was 24 in. long and the total uranium-235 content was approximately 192 g. Radial, vertical, and peripheral flux distributions were studied. The vertical flux profile was cosine- shaped with a peak-to-average ratio of 1.26. The peripheral variation around the loop wall could also be fitted to a cosine curve (with a peak-to-average ratio of 1.10). The average radial flux depression from the outer fuel cylinder to the center of the element was a factor of 2.14. Power generation in the element calculated from flux measurements agreed to within 10% with the power generated by measuring gas now rate and temperarure rise across the fuel element. The ratio of peak-to-average power density was found to be 1.75. (auth

Properties of MoNxOy thin films as a function of N/O ratio

The main purpose of this work consists on the preparation of single layered molybdenum oxynitride, MoNxOy. The films were deposited on steel substrates by dc reactive magnetron sputtering. The depositions were carried out from a pure Mo target varying the flow rate of reactive gases, which allowed tune the crystallographic structure between insulating oxides and metallic nitrides and consequently electronic, mechanical and optical properties of the material. X-ray diffraction (XRD) results revealed the occurrence of molybdenum nitride for the films with low oxygen fraction: face-centred cubic phases (gama-Mo2N) for low nitrogen flow rate or cubic MoNx and hexagonal phase (delta-MoN) for high nitrogen flow rate. The increase of oxygen content induces an amorphization of the nitride phases and appearance of MoO3 phases. The increase of the oxygen fraction in the films induces also a high decrease in films hardness. Residual stresses revealed to be of compressive type, in the range of very few tenths of GPa to 2 GPa. All these results have been analysed and will be presented as a function of the deposition parameters, the chemical composition and the structure of the films.Fundação para a Ciência e a Tecnologia (FCT) – Pograma Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/38086/2001.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Control of Urea Synthesis in Fetal Rat Liver Slices

Urea synthesis in fetal rats was studied using a liver slice system with ammonium chloride as nitrogen source. In term fetuses, the rate of urea formation increases with ammonium chloride concentration up to 100 μ&lt;i&gt;M&lt;/i&gt; and is enhanced by the addition of ornithine. The developmental pattern for urea synthesis is very similar to that found in the enzyme activity studies: urea production increases slightly during fetal life and then rises rapidly at birth. It is found that fetal liver has an absolute requirement for glucocorticosteroids to develop a normal urea synthesis. The accordance with argininosuccinate synthetase activity measured in liver homogenate is discussed.</jats:p

Glucocorticoid-Dependent Induction of the mRNA Coding for Argininosuccinate Lyase in Cultured Fetal Rat Hepatocytes

Dexamethasone increased both argininosuccinate lyase (ASL) activity and specific mRNA level in cultured fetal hepatocytes. Addition of various inhibitors of RNA synthesis showed that the increase in ASL mRNA may be related to an enhancement of ASL gene transcription, but not to a specific messenger stabilization. An apparent half-life of about 12 h for ASL mRNA was found in both untreated and dexamethasone-treated hepatocytes. About 30 h were necessary to observe the maximal effect of dexamethasone, and, in addition, both puromycin and cycloheximide (two inhibitors of protein synthesis) blocked the inducing effect of the steroid. These results suggested the involvement of intermediary protein(s) in the mechanism of induction of ASL mRNA by glucocorticoids.</jats:p