A.B. Lopes

Morphological field emission-SEM study of the effect of six phosphoric acid etching agents on human dentin

OBJECTIVES This study evaluated the effects of six phosphoric acid-etching agents on dentin, the independent variables being two acid concentrations (10% and 32%-37%) and three thickener conditions (no thickener, silica, and polymer). The tested hypothesis was that the use of different etchants with similar concentrations of phosphoric acid would result in similar depths of dentin demineralization. METHODS Thirty dentin disks were obtained from extracted human teeth by microtome sectioning. The dentin surfaces were etched with one of the etching agents, fixed, dehydrated and dried. The specimens were observed using a FE-SEM. The mean deepest demineralization of intertubular dentin was measured from fracture surfaces of the disks. These values were analyzed by ANOVA and Duncans Test. The morphological appearance of the dentin surfaces was compared using the following observation criteria: 1) Presence of a cuff of peritubular dentin; 2) Relative thickness of the layer containing residual collagen or smear layer particles; and 3) Formation of a submicron hiatus at the bottom of the exposed collagen network. The pH of each of the etching agents was measured. A correlation analysis was made of the pH vs. the depth of dentin demineralization. RESULTS Silica-thickened etchants did not demineralize dentin as deeply as did polymer-thickened etchants and unthickened etchants. High magnifications revealed three distinct zones within the demineralized dentin layer; an upper porous zone of residual smear layer or denatured collagen and residual silica particles (in groups etched with silica-thickened etchants), an intermediate area with randomly oriented collagen fibers, and a lower zone with submicron hiatus, few collagen fibers, and scattered hydroxyapatite inclusions. This hiatus was observable in all the specimens etched with the polymer-thickened etchants, in 90% of the specimens etched with the unthickened phosphoric acid liquids, and in 60% of the specimens etched with the silica-thickened gels. SIGNIFICANCE The results obtained suggest that similar concentrations of phosphoric acid etchants containing distinct thickeners result in different demineralization depths as well as different morphology of etched dentin.

Plastic flow for non-monotonic loading conditions of an aluminum alloy sheet sample

Abstract Non-linear deformation paths obtained using uniaxial tension followed by simple shear tests were performed for a 1050-O aluminum alloy sheet sample in different specimen orientations with respect to the material symmetry axes. In order to eliminate the time influence, the time interval between the first and second loading steps was kept constant for all the tests. Monotonic uniaxial tension tests interrupted during loading were used to assess the recovery that takes place during this time. In order to eliminate the influence of the initial plastic anisotropy and to compare the results as if the material hardening was isotropic, the flow stress was represented as a function of the plastic work. The behavior of the material after reloading was analyzed in terms of dislocation microstructure and crystallographic texture evolutions. For more quantitative assessment, the full constraints [Int. J. Plasticity 13 (1997) 75] and visco-plastic self-consistent [Acta Metall. Mater. 41 (1993) 2611] polycrystal models were used to simulate the material behavior in the non-linear deformation paths. Based on experimental and simulation results, the relative contributions of the crystallographic texture and dislocation microstructure evolution to the anisotropic hardening behavior of the material were discussed.

Retrogression and re-ageing of 7075 aluminium alloy: microstructural characterization

Abstract Industrial Summary: The 7075 aluminium alloy presents a low stress corrosion cracking strength when aged to achieve maximum mechanical strength, T6 temper; high stress corrosion cracking strength is attained with overageing, T7 temper; but with loss of mechanical strength. Retrogression and re-ageing treatments improves the stress corrosion behaviour of the alloy whilst maintaining the mechanical resistance of the T6 temper. The microstructures produced by the retrogression and re-ageing treatments were characterized in this study by transmission electron microscopy, electron diffraction and differential scanning calorimetry. The precipitation is extremely fine and distributed homogeneously inside the grains, being slightly denser and more stable than that resulting from the T6 temper; whilst the grain boundary precipitation is quite different from that resulting from T6 treatment, the particles being coarser, and much closer to the precipitation resulting from T7 temper. The retrogression temperature is the main property controlling factor; a higher retrogression temperature, increasing the dissolution degree, promotes the formation of more stable precipitates on re-ageing.

Effect of texture and microstructure on strain hardening anisotropy for aluminum deformed in uniaxial tension and simple shear

Abstract Uniaxial and simple shear stress–strain curves were obtained for a 1050-O aluminum alloy sheet sample in different specimen orientations with respect to the material symmetry axes. For uniaxial tension, a strong anisotropy of strain hardening was observed leading to about 30% difference in uniform tensile elongation between the extreme conditions. For simple shear, the hardening was also significantly different. These results were rationalized with an analysis that accounts for dislocation substructure observations, crystallographic texture measurements and polycrystal modeling of texture-induced strength evolution.

Hardening behavior and structural evolution upon strain reversal of aluminum alloys

Abstract The mechanical behavior and the dislocation structure change upon strain reversal are analyzed for 1050-O and 6022-T4 aluminum alloys. Dissolution of the dislocation structure and related transient hardening rate are observed in the AA1050-O but impeded by the numerous precipitates and the high solute content in the AA6022-T4.

Porosity control of hydroxyapatite implants

Conformation of hydroxyapatite (HAp) bodies was carried out using a multiple slip-casting technique, in order to obtain dual-layer samples with differential sized porosities. The external layer, because of its porosity, controlled by the addition of organic compounds (polyvinyl polyacrylate, PVC), will promote bone ingrowth. The internal denser layer, due to the addition of lithium phosphate (Li3PO4) as sintering additive, will give mechanical resistance to the implant. HAp aqueous suspensions were characterized by rheological measurements. Scanning electron microscopy (SEM) and intrusion mercury porosimetry (MP) were used to characterize sintered bodies. It can be concluded that it is possible to introduce gross porosity in HAp bodies by the addition of organic compounds. The results show that a compatible shrinking of the layers during the sintering process and a good frequency of pores with an appropriate size in the external layer can be achieved with the use of organic additives.

Microstructure, toughness and flexural strength of self-reinforced silicon nitride ceramics doped with yttrium oxide and ytterbium oxide.

Self‐reinforced silicon nitride ceramics with additions of either yttrium oxide or ytterbium oxide have been investigated at room temperature after various processing heat treatments. Devitrification of the intergranular phase in these materials is very sensitive to the heat treatment used during processing and does not necessarily improve their strength and toughness. Hot‐pressed ceramics without a subsequent devitrification heat treatment were the strongest. The ytterbium oxide‐doped silicon nitride ceramics were consistently tougher, but less strong, than the yttrium oxide‐doped silicon nitride ceramics. In all the ceramics examined, the fracture toughness showed evidence for R‐curve behaviour. This was most significant in pressureless sintered ytterbium oxide‐doped silicon nitride ceramics. A number of toughening mechanisms, including crack deflection, bridging, and fibre‐like grain pull‐out, were observed during microstructural analysis of the ceramics. In common with other silicon nitride‐based ceramics, thin amorphous films were found at the grain boundaries in each of the ceramics examined. Arrays of dislocations left in the elongated silicon nitride grains after processing were found to belong to the {101¯0}<0001> primary slip system.

Textural vs structural plastic instabilities in sheet metal forming

Abstract Negative work-hardening rates are precursors of plastic instabilities. The objective of the work is to determine the origin of the stress decrease in the case of prestrained sheet metals. Samples of low carbon steel and pure aluminium deformed in rolling and reloaded in simple shear exhibit softening for specific shear directions with respect to the rolling direction. Structural and textural evolutions are analysed with transmission electron microscopy and X-ray measurements, respectively. The low carbon steel shows clear evidence of intragranular structural destabilization while the Taylor factor, estimated by self-consistent calculations, remains nearly constant. It is deduced that for steel, the softening has a structural origin. By contrast the aluminium samples present identical structural changes whatever the sign of the work-hardening rate. It is shown that the latter is strongly correlated with Taylor factor evolutions. A textural instability is obtained in this case.

Accumulative Roll Bonding of Pure Copper and IF Steel

Severe plastic deformation is a new method to produce ultrafine grain materials with enhanced mechanical properties. The main objective of this work is to investigate whether accumulative roll bonding (ARB) is an effective grain refinement technique for two engineering materials of pure copper and interstitial free (IF) steel strips. Additionally, the influence of severely plastic deformation imposed by ARB on the mechanical properties of these materials with different crystallographic structure is taken into account. For this purpose, a number of ARB processes were performed at elevated temperature on the materials with 50% of plastic deformation in each rolling pass. Hardness of the samples was measured using microhardness tests. It was found that both the ultimate grain size achieved, and the degree of bonding depend on the number of rolling passes and the total plastic deformation. The rolling process was stopped in the 4th cycle for copper and the 10th cycle for IF steel, until cracking of the edges became pronounced. The effects of process temperature and wire-brushing as significant parameters in ARB process on the mechanical behaviour of the samples were evaluated.

Analysis of plastic instability in commercially pure Al alloys

Abstract The present work deals with sequences of rolling-shear experiments conducted on an Al 1050 alloy. Particular attention was paid to the transient mechanisms produced by a change of loading mode. It is shown that the flow behaviour during shearing strongly depends on the amplitude in the strain path change. More precisely, plastic instabilities take place just after yielding only for shear tests performed at some definite orientations from the rolling direction. It is shown that such a mechanical anisotropy is related to the deformation texture. The texture evolution is evaluated through self-consistent calculations. Flow softening is obtained when the Taylor factor, estimated with the model, decreases.