Joan Josep Roa Rovira

Influence of laser cutting on the fatigue limit of two high strenght steels

Laser cutting is widely used in the metal industry, particularly when components of high strength steels sheets are produced. On the other hand, the roughness of cut - edges produced by laser differs from that obt ained by other methods, such as mechanical blank ing, and this fact influence s the fatigue performance. Moreover, thermal effects are another factor to consider. In the present investigation, specimens of two grades of high strength austenitic steels were cut by laser and tested in the high cycle fatigue regime to determine their corresponding fatigue limits. One of the steels was a metastable stainless grade (AISI 301LN) whereas the other corresponded to the TWIP family (TWIP17Mn). Load control fatigue te sting was conducted at a stress ratio R of 0.1 and using the stair - case methodology. A series of fatigue specimens were tested without polishing the cut - edges and other s eries after a careful polishing, in order to assess the influence of the cut - edges con dition. R esults indicate a significant influence of the edges roughness, more marked for the AISI 301LN than for the TWIP steel. In the latter material, the presence of nitrides induced a premature fatigue crack nucleationAbstract Laser cutting is widely used in the metal industry, particularly when components of high strength steel sheets are produced. However, the roughness of cut edges produced by laser differs from that obtained by other methods, such as mechanical blanking, and this fact influences the fatigue performance. In the present investigation, specimens of two grades of high strength austenitic steels, i. e. AISI 301LN and TWIP17Mn, were cut by laser and tested in the high cycle fatigue regime to determine their corresponding fatigue limits. A series of fatigue specimens were tested without polishing and other series after a careful polishing of the cut edges, in order to assess the influence of the cut edges condition. Results indicate a significant influence of the edge roughness, more distinctive for AISI 301LN than for TWIP steel.

Evaluation of the bulk and alpha-case layer properties in Ti-6Al-4V at micro- and nano-metric length scale

In the present study the hardness of individual alpha ( )-Ti grains in Ti-6Al-4V was measured by nanoindentation using Berkovich tip indenter. Additionally, alpha-case layer was induced by performing isothermal heat treatment at 700 °C in air for 500 hours. The average hardness of the -Ti grains found in the bulk material and in the alpha-case layer were 6.7 ± 0.7 GPa and 9.4 ± 1.4 GPa, respectively. The high hardness of the -Ti grains in the alpha-case layer is due to solid solution strengthening caused by interstitial oxygen diffusion. The thickness of the developed alpha-case layer was estimated metallographically and compared with that measured from a hardness profile performed along the layer. Moreover, electron back-scattered diffraction was used to determine the local crystallographic orientation, the texture of the alloy microstructure, as well as phase fraction changes, where the nanoindentation measurements were performed.

Magnetic irreversibility: An important amendment in the zero-field-cooling and field-cooling method

This work was supported under the Brazilian grant MCTI= CNPq Universal 14=2012 (contract number 477506=2012-7). The experimental work at the HLD was supported by Euromagnet II (contract number 228043).

Mechanical properties of Al2O3 inverse opals by means of nanoindentation

In order to understand the mechanical behaviour of Al2O3 inverse opals, nanoindentation techniques have been implemented in material layers with three different microstructures, in terms of hollow or polystyrene spheres, with Al2O3 shells of distinct wall thickness. Different indenter tip geometries as well as contact loading conditions have been used, in order to induce different stress field and fracture events to the layers. Field emission scanning electron microscopy and focused ion beam have been employed to understand accommodation of plastic deformation induced during the indentation process. Results show that materials with polystyrene spheres exhibit higher hardness and modulus under sharp indentation, and cracking resistance under spherical indentation. Furthermore, deformation is discerned to be mainly governed by the rotation of the microspheres. In the case of the inverse opals made of hollow spheres, the main deformation mechanisms activated under indentation are the rearrangement and densification of them

Chemical introduced disorder effects on the critical current density and pinning mechanisms of YBa2-XSrXCu3O7-¿

We report on isotherm (T = 77.5 K) dc magnetization hysteresis loops of a series of YBa_2-xSr_xCu₃O_7-δ (x = 0, 0.02, 0.1, 0.25, and 0.37) single crystals with the purpose of studying the influence of chemically introduced site disorder on the YBa₂Cu₃O_7-δ critical current density J_c(B, T) and normalized pinning force density f(h). The Sr ion chemical disorder is inserted at YBa₂Cu₃O_7-δ structure by lattice distortion. Transmission electronic microscopy of the doped samples shows a structure constituted by a high density of twins, which is, probably, decorated by many small precipitates. The J_c(B, T) transported by the doped samples with x ≤ 0.10 is higher than that transported by the YBa₂Cu₃O_7-δ pure sample. In contrast, the J_c(B, T) transported by the doped samples with x > 0.10 is significant and systematically lower than that transported by the undoped sample. The analysis of the f(h) plots indicates, in the light of the Dew-Hughes model, that the preponderant pinning mechanism of YBa_2-xSr_xCu₃O_7-δ single crystals with x≤0.10 is the core normal point type. However, to the samples with x > 0.10, the f(h) analyses indicate that the preponderant pinning mechanism undergoes a crossover from the core normal point type to the core normal surface type. This crossover of pinning mechanism dynamics could be appointed as the possible reason of the decrease of the J_c(B, T) observed in the samples with x > 0.10.

Comparison Between the Magnetic Irreversibility and Zero Resistance of High-Quality Melt-Processed YBaCuO Superconductors

This paper portrays a detailed study of the magnetic irreversibility limit Tirr (H) and of the zero resistance point Tc0 (H) of three different top-seeding melt-textured YBa2Cu3O7-δ superconducting samples, with well-aligned c-axis and doped with a high density of nonsuperconducting Y2Ba1Cu1O5 (Y211) pinning centers. We have performed measurements for applied magnetic fields up to 140 kOe and for the whole set of the different field-current configurations. The magnetization measurements were performed using an MPMS-XL SQUID magnetometer and a vibrating sample magnetometer, both from Quantum Design. The electric transport measurements were made using a physical properties measurement system from Quantum Design. The goal of this exhaustive study is obtaining precise data about magnetic flux mobility along the various directions in the sample for the different field-current configurations, thereby defining the nature and effects, due to the strength and anisotropy of the pinning mechanisms and disclosing the various physical mechanisms dissipating electric transport in these systems below the superconducting transition temperature. We discuss our results in terms of the anisotropic flux pinning by the Y211 grains dispersed into the superconducting matrix.