Papers by Author: R. Caram

Abstract: The high cost of processing of Ti alloys is an important limitation to their utilization in a number of applications. An approach to overcome such difficulties would be their processing by precision casting. The purpose of this work is to evaluate the centrifugal casting process of Ti-Cu and Ti-Nb alloys, analysing the influence of alloying element type and content in the obtained microstructure. Samples with different compositions were prepared in arc furnace with non-consumable tungsten electrode and water-cooled copper hearth under high purity argon atmosphere and cast in a copper mould. Microstructures were analysed by SEM, characterized by EDS and X-ray diffraction. Vickers hardness measurements helped the analysis of phases.

Abstract: This work investigates the viability of production of thixotropic semisolid Ti-Cu alloys by controlled partial melting. Samples were submitted to different holding times at temperatures above solidus, from different initial microstructure conditions. In some cases, deformation by rolling before heating to semisolid was imposed. It is investigated the influence of deformation ratio and holding time in the obtained structure. Results show that thixotropic semisolid can be easily produced; pre-deformation leads to fine, globular primary phase in the liquid for short holding times. Therefore, thixoforming of Ti alloys is feasible, and can bring a whole new concept on forming complex shapes of these alloys, at low energy consumption, and new possibilities in engineering components with special properties.

Abstract: Titanium alloys form the most versatile class of metallic materials used as biomaterials. Among them it is foreseen that the  type titanium alloy will be a prominent one for orthopedic applications. Aim of the present work was to prepare and characterize a  type titanium alloy containing 35 wt.% Nb. Samples were cooled from the  phase temperatures at different rates. This work includes the effects of heat treatment on the microstructure and hardness, tensile and fatigue properties in air at room temperature. The results showed that microstructure of slow cooled samples are formed by precipitates of  and  phases in a  matrix. After rapid cooling, the microstructure consists of  phase and ” martensite. Mechanical testing showed that the elastic modulus and Vickers hardness of slow cooled samples were significantly higher than that obtained by rapid cooling. On the other hand, it was observed that slow cooled samples showed higher tensile strength and lower ductility. The rapid cooled sample showed fatigue resistance higher than that of slow cooled samples.

Abstract: The structural materials phase transformations and failure mechanisms have been under scrutiny for many years. However, the advent of new and more powerful techniques is always making possible to address unsolved problems. Nowadays, the implementation of sophisticated in-situ electron microscopy tests is providing new insights in several fields of chemistry, physics, and materials science by allowing direct observation of a wide variety of phenomena at submicron and even atomic scale. These experiments may involve controlled temperature and atmospheres, mechanical loading, magnetic and/or electric field among other conditions that are imposed to the sample while its response or evolution is studied. An in-situ high temperature deformation experiment was developed and adapted within the vacuum chamber of a scanning electron microscope (SEM). This setup was used to study the grain boundary sliding (GBS) mechanism and its effect on the high temperature cracking phenomenon known as ductility-dip cracking (DDC). The Ni-base filler metals AWS A5.14, ERNiCrFe-7 and ERNiCr-3, which correspond to alloys 690 and 600, respectively, were studied within the temperature range between 700 and 1000 °C. Analysis of the recorded digital videos that registered the high temperature deformation made possible differentiating and quantifying, with submicron resolution, two different components of GBS. The designated pure-GBS and deformation-GBS components were described and quantified. In addition, the GBS relationship with the material high temperature ductility and the DDC failure mechanism was established.

Abstract: Titanium alloys comprise a versatile class of biomedical materials. Among them, β- titanium alloy is one of the most promising metallic materials for biomaterial applications thanks to their high mechanical strength, good corrosion resistance and excellent biocompatibility. This study purported to analyze the phase stability in Ti-Nb alloys, evaluating the influence of the Nb content on the microstructure obtained under different heat treatment conditions. To this end, Ti-Nb alloys containing 5 to 35% (wt.%) of Nb were prepared and evaluated. The samples were arc melted and characterized using optical microscopy, transmission electron microscopy and X-ray diffraction. Young’s modulus was evaluated primarily by acoustic techniques.