Key Engineering Materials Vol. 423

Abstract: An experimental study of the variation of the mechanical properties of sheets of aluminium alloy 1050A due to heat treatment has been carried out. For this purpose, sheets and foils rolled from continuous casting strip (CCS) and from hot rolling strip (HRS) have been prepared in the laboratory. Initially, CCS and HRS had a thickness of 7,5 mm and 3,0 mm respectively. The final thickness achieved in both cases was 0,20 mm. In order to obtain hardening curves for CCS and HRS sheets and foils, specimens of different thickness were taken during the rolling process. Oven treatments were carried out for drawing softening curves of foils. Foil proceeding from CCS presents hardening due to the precipitation of Al3Fe particles at a temperature range of 160°C to 220 °C. HRS foil has a lower hardening in the same range of temperatures owing to its low supersaturation of Fe. Correlation between ultimate tensile strength Rm and Vickers hardness is studied. Micrographies of optical microscopy are offered at hard and recrystallized conditions.

Abstract: The grain size dependence of the tensile properties of a TWIP steel has been determined for a wide range of grain sizes obtained by grain growth after complete recrystallization of cold rolled material. The near-linear stress-strain behaviour typical of either TWIP steels or other materials that deform by twinning has been observed, the work hardening rate being larger for the smaller grain sizes. The Hall-Petch slope increases as a function of strain, from 350 MPa μm1/2 for the yield stress to 630 MPa μm1/2 for the maximum uniform strain in the tensile tests, ε  0.40. Profuse twinning is observed in deformed specimens by means of FIB-ISE.

Abstract: Hybrid silica-organic polymer aerogels based on tetraethoxysilane (TEOS) have been synthesized, polydimethylsiloxane (PDMS) and methyltriethoxysilane (MTES) have been used as organic phases. Synthetic wollastonite powder was added as bioactive phase. The composites were prepared by dispersing wollastonite powder in the sol with the assistance of high power ultrasounds to control the gelling time. Wet composites were dried under supercritical conditions of the solvent. The mechanical characterization was performed by uniaxial compression and by nanoindentation. Young`s modulus from uniaxial compression increase from 5 MPa to 100 MPa for increasing MTES content and also rupture modulus was enhanced from 0.85 MPa to about 50 MPa, so the incorporation of cross linkers in this kind of aerogels, was proven to enhance the mechanical resistance. However the inclusion of wollastonite powders provokes a dropping of the Young’s modulus and hardness. All the composites showed bioactivity by the formation of an apatite layer when immersed in Simulated Body Fluid (SBF).

Abstract: The mechanical properties of black thin film on glass substrates and metallic, obtained from the synthesis of organic-inorganic silica hybrids via sol-gel are described. Hybrid silica-based coatings with tetraethoxysilane (TEOS), methyltrietoxysilane (MTES), polyurethane (PU) and commercial silica colloidal solution (Ludox HS30) as precursors, were used. The procedure is divided in the following three steps: 1) obtaining homogenous and stable solutions of different organic silica-polymer composition, 2) aggregation of the black pigment with the formulate FeMnOx, 3) Coating substrates by both dip-coating and aerograph airbrush, at room temperature. The films obtained were characterized by SEM, AFM and nano-indentation. The coatings of the Ludox-MTES-PU/FeMnOx system showed the better characteristics concerning the morphology and the adhesion to the glass and metallic copper substrates

Abstract: Organic/inorganic hybrids silica aerogels were synthesized by the classical sol-gel method with application of high power ultrasounds to the liquid mixture. Precursors were tetraethoxysilane (TEOS), as inorganic phase, and polydimethyl siloxane (PDMS), as organic one. These hybrid organic-inorganic materials are known as ORMOSIL (ORganically MOdified SILicates). Monolithic aerogels were obtained by supercritical drying in ethanol. Failure tests by uniaxial compression shows an increase of the rupture modulus as well as a decrease of the Young’s modulus with the polymer content, tuning from a brittle solid to a rubbery-like one. These hybrid aerogels behave as elastomers showing a decrease in the relaxation viscoelastic modulus. Nanoindentation tests have been performed in these hybrid aerogels: load/unload cycles about 1.5 mN of maximum load have shown a decreasing value of the reduced modulus, as well as both plastic and elastic work with the organic content, while hardness remains almost constant. Elastic recovery parameter rised with the increasing organic content. Results from creep tests made with uniaxial compression configuration are discussed and compared with nanoindentation. Viscoelastic behavior of these hybrids materials can be described by a rheological model.