Jefferson Fabrício Cardoso Lins

Recrystallization behavior of a cold-rolled niobium bicrystal

A high-purity coarse-grained niobium bicrystal was 70% cold rolled in multiple passes. Deformation occurred in an inhomogeneous manner in both grains giving rise to a banded structure. In consequence, highly misoriented boundaries were developed in the microstructure in a wide range of misorientations, many reaching about 55°. These boundaries act as effective nucleation sites for recrystallization. The microstructure of both grains was investigated using electron backscatter diffraction (EBSD) in the cold-worked state in order to quantify the misorientations (ψ) associated to these bands. Upon annealing at 800 and 900 °C, the new recrystallized grains were nucleated preferentially at deformation heterogeneities and in the vicinity of the prior grain boundary in this bicrystal.

An EBSD Investigation on Deformation-Induced Shear Bands in Ti-Bearing IF-Steel under Controlled Shock-Loading Conditions

We report the results of the microstructural characterization of a Ti-bearing IF-steel deformed at high strain rates (» 6.104 s-1) in a split Hopkinson bar. The shock-loading tests were performed in hat-shaped specimens to induce the formation of adiabatic shear bands (ASB). The samples were deformed at 223 K and 298 K. High-resolution electron backscatter diffraction (EBSD) reveals the development of an ultrafine-grained structure within the ASB. A closer inspection reveals the presence of deformation twins in grains adjacent to the shear band. These twins bend towards the ASB suggesting that mechanical twinning occurs before the flow associated to shear banding. The results of microtexture have indicated the presence of a sharp <111> g-fiber texture in the ASB for both temperatures.

Sensitivity analysis of the ECAE process via 2 k experiments design

In this work the theoretical solutions based upon the upper-bound theorem recently proposed by Perez and Luri [Mech. Mater. 40 (2008) 617] for the equal channel angular extrusion process (ECAE) are analyzed by performing a 25 central composite factorial analysis. The uniaxial mechanical properties of commercial pure aluminium are considered by assuming isotropic nonlinear work-hardening combined to von Mises and Drucker isotropic yield criteria to predict the ECAE load and the effective plastic strain. From the proposed 25 factorial analysis, the main parameters affecting the ECAE pressure may be ranked as: (1) Friction factor, (2) die channels intersection angle, (3) outer and (4) inner die corners fillet radii and lastly, (5) plunger velocity. Alternatively, the effective plastic strain is mainly controlled by the die channels intersection angle and, in a less extent, by the outer and inner die corners fillet radii.

Magnetic Barkhausen Noise in quenched carburized steels

Steels with different carbon content, 0.11%C and 0.48%C were submitted to a heat treatment for carburization in the surface. The samples were analyzed after several types of heat treatment, including quenching for producing martensite. The Magnetic Barkhausen Noise (MBN) is directly related to the microstructure. Samples with lower carbon content, have ferrite, a constituent where domain walls can move freely and present higher amplitude in the envelope of MBN. It is also found that the MBN peaks are quite distinct for the samples with martensite, which have lower permeability, and the results suggest that domain rotation contributes as mechanism for reversal of magnetization in martensite. The results also indicate that MBN is very suitable for monitoring the carburizing heat treatment.

Comparison of the Magnetic Barkhausen Noise for Low Carbon Steel in Deformed and Annealed Conditions

The magnetic Barkhausen noise (MBN) in low carbon steel (0.034%C) was examined for two situations: after deformation and after recrystalization. The steel sheets previously received 50% of cold rolling and were heat treated for time periods between 15 to 540 min, originating different grain sizes. It was found that the maximum amplitude of MBN decreases with annealing time. Electron back scattered diffraction (EBSD) shows that significant grain fragmentation occurred due to plastic deformation. The results are discussed with help of the Two Gaussian model for the MBN envelope. The major contribution for MBN in the deformed sample was elimination of 90° closure domains. Displacement of 180° domain walls was limited in the deformed sample, due to the grain fragmentation.

EBSD Texture Analysis of NdFeB Magnets

The crystallographical texture is relevant information for NdFeB magnets, since the maximum energy product is directly related to orientation of the crystals. EBSD (Electron Backscattered Diffraction) is a very suitable tool for preferred orientation measurement of NdFeB magnets. The advantages of EBSD against X-ray Diffraction (XRD) pole figures for texture determination are discussed. EBSD identifies misaligned grains, and this is not feasible with XRD pole figures. EBSD is also helpful on the identification of oxides.

Evaluation of Bonding Resistance after Surface Treatment of Two Glass-Ceramics Used in Dentistry

The goal of this study was to evaluate the bonding resistance of two dental ceramics. The samples, one of each type of ceramics, were split into 4 groups (n=4) separated from those from control group, the others were conditioned with 5% hydrofluoric acid, according to the times of 20, 40 and 60 seconds. Cylindrical pins of resin cement were applied in each sample. After a shearing test, the obtained results showed that experimental groups has given higher bonding strength values than the control groups for both ceramics. For the e.max ceramic, the highest values were observed with timings of 20 and 40 seconds and for the Suprinity ceramic the conditioning timings were insignificant. The authors concluded that the surface treatment with hydrofluoric acid improved the bonding strength for both ceramics; and the e.max ceramic showed higher bonding values than the Suprinity.

Evaluation of Load Parameters and Hold Time of a Vickers Ultra-Micro Hardness Tester for Measure of Hardness and Modulus of Elasticity of Dental Composites

The aim of this study was to standardize the parameters for testing the hardness and elastic modulus for dental composites. It used the equipment Dynamic Ultra Micro Hardness Tester DUH-211 / DUH-211S (Shimadzu) varying the load and the hold time. two composites were used with various inorganic particles that were embedded and sanded for the assay. one Vickers indenter was used being held nine rows with ten penetrations. For each composite loads were used (20, 50 and 100gf) and times (zero, five and ten seconds). The results showed that the effect of the load and the hold time had influence on the modulus and hardness values of the studied composites. The authors concluded that loads and hold larger team showed greater stability of hardness values and modulus of elasticity for the studied composites and these values for both hybrid composites as for nanoparticulate were dependent on load and time.

Analysis of the Effect of Cyclic Fatigue on the Flexural Strength of a Ceramoceramic System Used in Dental Prostheses

The aim of this study was to analyze the flexural strength of ceramics based on yttria-stabilized zirconia (YTZP) , used in the manufacture of dental prostheses infrastructure before and after aging with cyclic fatigue in moisture. The samples were made by pre-sintered blocks of ZirkonTransluzent (Zirkonzahn GMHB) from YTZP ceramics with and without feldspathic ceramic coating, divided into groups: G1 = YTZP control; G2 = YTZP aged; G3 = YTZP + feldspathic; G4 = YTZP + aged feldspathic. The samples were submitted to a three-point bending test with a speed of 0.5 mm / min. The mean values of the flexural strength values were G1 = 645 MPa (DP ± 124), G2 = 681 MPa (DP ± 129), G3 = 904Mpa (DP ± 157) and G4 = 954Mpa (DP ± 243). The values of groups G1 and G2; G3 and G4 were statistically similar, depending on the presence or absence of coating. Cyclic stresses did not affect the flexural strength of the material

ESTUDO DA INFLUÊNCIA DO BORO NO AÇO 10B45 (FIO MÁQUINA E ARAME TREFILADO) ANALISANDO PROPRIEDADES MECÂNICAS E MICROESTRUTURAS

O presente trabalho avaliou as influencias da adicao de diversos teores de boro nas caracteristicas mecânicas e microestruturais do fio-maquina e arame trefilado no aco 10B45. Foi verificado que a adicao de boro reduz a resistencia do material, atuando sobre os mecanismos de envelhecimento, pois associa-se ao nitrogenio formando o nitreto de boro (BN) no contorno de grao. Assim, quanto maior a adicao de boro, menor resistencia a trefilacao, o que pode ser comprovado pelo ensaio de torcao nos arames. Ainda foi possivel avaliar o modo de fratura relacionado com a quantidade de boro adicionada. Nesse ponto, nao foi possivel atribuir maior ductilidade a amostra com maior teor de boro.