A.L. Rivas

Physical and mechanical properties evaluation of Acropora palmata coralline species for bone substitution applications

The search for ideal materials for bone substitution has been a challenge for many decades. Numerous natural and synthetic materials have been studied. For this application, exoskeletons of coral have been considered a good alternative given its tendency to resorption, biocompatibility and similarity to the mineral bone phase. Very few studies of these materials consider a detailed analysis of the structure–property relationship. The purpose of this work was to carry out the microstructural characterization of a coralline species named Acropora palmata and the determination of the mechanical and physico-chemical properties. Measurements of hardness, compressive strength, bulk density and apparent porosity were performed. From these results it was determined that this marine coral species could be an alternative xenograft due to its mechanical properties and osteoconductive nature.

Microstructural Evolution during Induction Hardening Heat Treatment of a Vanadium Microalloyed Steel

The microstructural evolution during induction heat treatment applied to a forged 0.1 wt. % V microalloyed steel has been investigated. The study examined both as-forged and forged plus normalized starting microstructures. The core of the as-forged steel was comprised of ferrite and pearlite. The prior austenite grain size was about 150 μm and only a faint indication of banding was observed. After normalizing the ferrite/pearlite microstructure was refined. The prior austenite grain size was reduced to 10 μm and banding became more evident. During induction heating, the banded appearance persisted in all but the extreme outer portion of the surface layer that underwent austenitization. Banding was more apparent in the as-forged steel after induction hardening. The austenite average grain sizes developed in both the as-forged and forged and normalized steels after induction hardening were less than 20 μm. High levels of compressive residual stresses were measured in the hardened outer surface layer using an x-ray diffraction technique. Residual stress relaxation due to the application of cycling loading will be discussed.

Hydrogen damage of a welded API 5L X52 steel

The main objective of the present investigation was to study the susceptibility to hydrogen damage on a type API 5L X52 steel welded by electrical resistance. Several techniques, such as hydrogen permeation and cathodic charging, were used. The metallic material was characterized using a scanning electron microscope and transmission electron microscope. The base metal microstructure was very similar to that of the one corresponding to the welded area. This microstructure mainly comprised ferrite and perlite, differing only in the grain size. Therefore, the susceptibility to hydrogen damage was similar in both cases. It is worth mentioning that the welded area has a very small dimension. Indeed, the fusion zone is only 5 mm wide while the heat affected zone is 1 mm wide. The hydrogen damage observed was mainly in the form of blisters, which were associated with the presence of aluminium-rich inclusions. Also, partial dissolution of inclusions and some matrix attack adjacent to the inclusions were noticed.

Phase Transformation of Amorphous Rice Husk Silica

This work aims at assessing the crystallization process of amorphous silica from rice husk (RH) by in situ X-ray diffraction, upon heating up to 1450 °C in a high-temperature chamber. The study includes microstructural characterization by transmission electron microscopy. The work was conducted on rice husk ash (RHA), obtained by combustion of RH at 650 °C with and without leaching in 1 N HCl solution, obtaining samples with variable impurity content. The results showed that crystallization of RHA amorphous silica upon continuous heating occurred at higher temperatures compared to the isothermal one, considering the sample purity. The type of polymorph silica identified in the samples during continuous heating consisted of cristobalite or a combination of cristobalite and tridymite depending on the impurity levels in the samples. The study revealed silica crystallites with spherical shape nucleated by a dominant homogeneous mechanism. The size of crystallites ranges from around 2 nm to 20 nm. Larger particles nucleated on impurities and constituted by combinations of P, Ca, K, Mg, and Na were also observed.

Study of austempering kinetics of high silicon bainitic cast steels

ABSTRACT This study investigates the effect of chemical composition and microsegregation on the bainitic transformation of high Si cast steels at two different austempering temperatures. The advance of the bainite precipitation as a function of time and temperature was monitored through measurements of the amount of retained austenite by X-ray diffraction, the evolution of hardness and microstructural characterization. The results showed that the bainitic transformation starts preferentially at zones with lower contents of alloying elements but later covers the entire matrix. The austempering heat treatments resulted in an apparently homogeneous bainitic microstructure throughout the entire matrix after a short period of time. The results encourage the continuation of the development of high Si steels suitable to produce carbide-free austempered cast parts.

Characterization of Rice Husk and the Crystallization Process of Amorphous Silica from Rice Husk Ash

Rice husk (RH) is a rich silica by-product of rice production with great potential for industrial applications. After combustion, a rice husk ash rich in silica is obtained, with some amount of other inorganic components constituted mainly of alkaline elements. The study was conducted to characterize both the rice husk and rice husk ash (RHA) in order to evaluate their constitution and the form in which mentioned impurities are present. The amorphous silica crystallized when heated under certain conditions. In this way, the examination of the crystallization process of rice husk ash with different impurity levels was also carried out by varying the temperature, time and heating conditions. The study reveals a preferential distribution of silica at the cell tip of the outer RH surface cells. The impurities form compounds with combinations of different elements such as Ca and Mg, P and K, and are mainly localized below the inner surface in the form of spherical and cuboidal shapes, as well as irregular morphologies. . I. INTRODUCTION

Microstructural Evolution of a Commercial 0.04%C Steel During Batch Annealing

The effect of heating rate and annealing time on the microstructure and texture of a commercial 0.04 %C steel, cold rolled up to 80 %, is studied. Samples have been isothermally annealed at various heating rates (12 °C/h, 20 °C/h, 40 °C/h and 650 °C/min) and then soaked at 700 °C for 15 hours. The microstructural evolution of the samples during the heating process and hold period has been followed by optical microscopy, scanning and transmission electron microscopy. The electron back-scattered diffraction technique is used to reveal the texture of the samples. Tensile tests and hardness measurements are correlated with the microstructural features. Results show that (a) recrystallization occurs between 600 and 650 °C; (b) a “pancake” structure develops during recrystallization at low heating rates without appreciable grain growth; (c) samples heated at 650 °C/min exhibit an equiaxed grain structure and significant grain growth; (d) only at low heating rates the material develops a strong {111} recrystallization texture, in ccordance with the high plastic anisotropy found by mechanical testing.

The influence of fabrication parameters on the selective properties of aluminum-copper alloys

Abstract The aluminum-copper eutectic alloy presents a fine microstructure which could used to produce textured surfaces through a selective chemical attack method. In this work results are presented on the behaviour of the above alloy as a selective absorber for variations of fabrication parameters; composition, solidification rate, chemical attack and thermal cycling. The results show that the figure of merit α/ϵ has a maximum value of 2.5 with an absorptance α of 0.95, the best results were obtained for exact eutectic composition with solidification rate higher than 3 × 10 −2 cm/s and with a chemical attack time greater than 90 s. Thermal cycling between 20°C and 200°C causes superficial degradation but the optical properties are not seriously affected.