Cristina Arévalo

Influence of sintering temperature on the microstructure and mechanical properties of in situ reinforced titanium composites by inductive hot pressing

This research is focused on the influence of processing temperature on titanium matrix composites reinforced through Ti, Al, and B4C reactions. In order to investigate the effect of Ti-Al based intermetallic compounds on the properties of the composites, aluminum powder was incorporated into the starting materials. In this way, in situ TixAly were expected to form as well as TiB and TiC. The specimens were fabricated by the powder metallurgy technique known as inductive hot pressing (iHP), using a temperature range between 900 °C and 1400 °C, at 40 MPa for 5 min. Raising the inductive hot pressing temperature may affect the microstructure and properties of the composites. Consequently, the variations of the reinforcing phases were investigated. X-ray diffraction, microstructural analysis, and mechanical properties (Young’s modulus and hardness) of the specimens were carried out to evaluate and determine the significant influence of the processing temperature on the behavior of the composites.

Electrophoretic Deposition of PEEK/45S5 Bioactive Glass Coating on Porous Titanium Substrate: Influence of Processing Conditions and Porosity Parameters

Commercially pure titanium (cp Ti) is typically accepted as one of the best in vitro and in vivo bone replacement biomaterial, due to its excellent balance between biomechanical and biofunctional properties. In that context, the aim of this work is to prove the hypothesis of a simultaneous solution to certain specific limitations of cpTi, which can often compromise the reliability of implants: (i) stress-shielding phenomenon, and (ii) a deficient biointerface with bone, which reduces the osseointegration. Porous samples of cp Ti, grade IV, were obtained by space-holder technique (50 vol.% NH4HCO3, 800 MPa, at 1250 oC during 2h, under high vacuum), to produce a good balance between Young ́s Modulus and yield strength. Different types of porous samples were manufactured by considering different size particles ranges of NH4HCO3: 100-200μm, 250-355μm and 355-500μm. Afterwards, they were coated with a PEEK/45S5 bioactive glass composite by electrophoretic deposition, to be finally sintered at 350oC for 1h. The coatings homogeneity, infiltration efficiency, adhesion and cracking, were studied in order to establish correlations with processing conditions (time of deposition, applied voltage, composition, concentration and stability of the colloidal suspension). Detailed structural characterization of the coatings was performed (SEM and XRD), besides the contact angle and contact profilometry testing. Additional mechanical and chemical insights were achieved by evaluating both the tribo-mechanical (instrumented microindentation and micro-scratch testing) and electrochemical behaviors (potentiodynamic polarization and in vitro corrosion tests in SBF). All these results allowed us to determine the optimal balance of properties for a porous substrate (space holder of 250-355μm) with a coating obtained for 65 V, 2 min, 6 mm (distance between electrodes), 10 g/L bioactive glass and 20 g/l PEEK. The high adhesion estimated between the bioactive/biopolymer coatings and the porous titanium substrates (excellent infiltration) suggest that this new biocomposite is a good candidate for load-bearing applications.

Nanocrystalline Al Composites from Powder Milled under Ammonia Gas Flow

The production of high hardness and thermally stable nanocrystalline aluminium composites is described. Al powder was milled at room temperature in an ammonia flow for a period of less than 5 h. NH3 dissociation during milling provokes the absorption, at a high rate, of nitrogen into aluminium, hardening it by forming a solid solution. Controlled amounts of AlN and Al5O6N are formed during the subsequent sintering of milled powders for consolidation. The pinning action of these abundant dispersoids highly restrains aluminium grain growth during heating. The mean size of the Al grains remains below 45 nm and even after the milled powder is sintered at 650°C for 1 h.

Analysis of the Influence of Starting Materials and Processing Conditions on the Properties of W/Cu Alloys

In this work, a study of the influence of the starting materials and the processing time used to develop W/Cu alloys is carried out. Regarding powder metallurgy as a promising fabrication route, the difficulties in producing W/Cu alloys motivated us to investigate the influential factors on the final properties of the most industrially demanding alloys: 85-W/15-Cu, 80-W/20-Cu, and 75-W/25-Cu alloys. Two different tungsten powders with large variation among their particle size—fine (Wf) and coarse (Wc) powders—were used for the preparation of W/Cu alloys. Three weight ratios of fine and coarse (Wf:Wc) tungsten particles were analyzed. These powders were labelled as “tungsten bimodal powders”. The powder blends were consolidated by rapid sinter pressing (RSP) at 900 °C and 150 MPa, and were thus sintered and compacted simultaneously. The elemental powders and W/Cu alloys were studied by optical microscopy (OM) and scanning electron microscopy (SEM). Thermal conductivity, hardness, and densification were measured. Results showed that the synthesis of W/Cu using bimodal tungsten powders significantly affects the final alloy properties. The higher the tungsten content, the more noticeable the effect of the bimodal powder. The best bimodal W powder was the blend with 10 wt % of fine tungsten particles (10-Wf:90-Wc). These specimens present good values of densification and hardness, and higher values of thermal conductivity than other bimodal mixtures.

Ti/B 4 C Composites Prepared by In Situ Reaction Using Inductive Hot Pressing

In the present work, in situ reinforced titanium composites (TMCs) synthesized using inductive hot pressing (iHP) are studied. The effects of B4C phases and applied processing conditions, on the microstructure and properties of TMCs, are investigated. With the addition of B4C particles, the microstructure of TMCs is refined and the strength is improved.Products of reactions which occur during the manufacturing process are analysed in detail. Microstructure observation illustrates, that B4C survives - depending on the processing conditions. The reinforcing phases are homogeneously distributed in Ti matrix. Moreover, results of densification, mechanical properties and hardness measurements help to identify the most suitable processing conditions to produce this kind of TMCs.

Study of Titanium Metal Matrix Composites Reinforced by Boron Carbides and Amorphous Boron Particles Produced via Direct Hot Pressing

Nowadays, the demands for materials with high strength based on a titanium matrix are increasing. The manufacturing of titanium composites through low cost and near-net-shape techniques is a challenge for the industry. There are different processing routes to meet these requirements of the market. As it is well known fast powder metallurgical densification techniques could satisfy these needs. In the present work, several titanium metal matrix composites (TiMMCs) have been fabricated by using a fast hot consolidation technique, namely direct hot pressing (dHP) in order to reduce the manufacturing time. Through a pressure assisted sintering with direct heating of a pressing die the consolidated composites can be formed directly from powders in a short period of time (15 min). The matrix materials were selected from two titanium grade 1 powders and as reinforcement materials boron carbide and boron amorphous particles were employed. Varying the reinforcement’s content in addition to their particle size, their influence on the composites behaviour was expected. Furthermore in this research work, the mechanical and microstructural characterisation of the specimens was carried out in order to identify the best combination of process parameters, material reinforcement and matrix powders.Nowadays, the demands for materials with high strength based on a titanium matrix are increasing. The manufacturing of titanium composites through low cost and near-net-shape techniques is a challenge for the industry. There are different processing routes to meet these requirements of the market. As it is well known fast powder metallurgical densification techniques could satisfy these needs. In the present work, several titanium metal matrix composites (TiMMCs) have been fabricated by using a fast hot consolidation technique, namely direct hot pressing (dHP) in order to reduce the manufacturing time. Through a pressure assisted sintering with direct heating of a pressing die the consolidated composites can be formed directly from powders in a short period of time (15 min). The matrix materials were selected from two titanium grade 1 powders and as reinforcement materials boron carbide and boron amorphous particles were employed. Varying the reinforcement’s content in addition to their particle size, their influence on the composites behaviour was expected. Furthermore in this research work, the mechanical and microstructural characterisation of the specimens was carried out in order to identify the best combination of process parameters, material reinforcement and matrix powders.

Quasicontinuum Method at Finite Temperature Applied to the Study of Nanovoids Evolution in Fcc Crystals

Breaking tensile test of ductile materials starts with the formation, in the test material central area, of a choking followed by the nucleation of several cavities at nanoscopic scale. Nanovoids growth and coalescence give rise to a crack which propagates towards the surface in the perpendicular direction to the applied charge. This work is focused in the study of the evolution of these nanovoids for face centered cubic (fcc) crystals. The Quasicontinuum (QC) method at finite temperature has been performed to carry out such an analysis.

Defect diffusion in hcp Zirconium: A kinetic Monte Carlo approach

We have studied diffusion of defects produced during irradiation in hcp zirconium through a kinetic Monte Carlo model. The input data for these simulations is based on molecular dynamics calculations and from experiments whenever available. The initial cascade damage produced by recoils of 25 keV energy from molecular dynamics simulations has been followed for times of hours at a fixed temperature of 600K. We have calculated the number of freely migrating defects, the recombination ratio between vacancies and interstitials, the defects surviving in the bulk as well as the average cluster size for these remaining defects.

Powder-materials impact on nanoparticle-reinforced Ti-6Al-4V matrix composites produced via inductive hot pressing

This paper addresses the optimal manufacture of titanium alloy composites via inductive hot pressing, and studies the influence of nanosized reinforcement and matrix powders on the final properties of the titanium-based composites. Before hot consolidation, two types of Ti-6Al-4V powders were mixed with various percentages of nano-diamonds. After inductive hot pressing at 900°C and 950°C, characterisation of the specimens took place, which revealed significant differences in the final properties of the composites produced. Further to the well-known effect of how oxygen content in the initial powder considerably affects the properties of the specimens, the influence of the nano-diamonds in the microstructure is also evaluated.

Analysis of the Microstructure and Mechanical Properties of Titanium-Based Composites Reinforced by Secondary Phases and B4C Particles Produced via Direct Hot Pressing

In the last decade, titanium metal matrix composites (TMCs) have received considerable attention thanks to their interesting properties as a consequence of the clear interface between the matrix and the reinforcing phases formed. In this work, TMCs with 30 vol % of B4C are consolidated by hot pressing. This technique is a powder metallurgy rapid process. Incorporation of the intermetallic to the matrix, 20 vol % (Ti-Al), is also evaluated. Here, the reinforcing phases formed by the reaction between the titanium matrix and the ceramic particles, as well as the intermetallic addition, promote substantial variations to the microstructure and to the properties of the fabricated composites. The influences of the starting materials and the consolidation temperature (900 °C and 1000 °C) are investigated. By X-ray diffraction, scanning and transmission electron microscopy analysis, the in-situ-formed phases in the matrix and the residual ceramic particles were studied. Furthermore, mechanical properties are studied through tensile and bending tests in addition to other properties, such as Young’s modulus, hardness, and densification of the composites. The results show the significant effect of temperature on the microstructure and on the mechanical properties from the same starting powder. Moreover, the Ti-Al addition causes variation in the interface between the reinforcement and the matrix, thereby affecting the behaviour of the TMCs produced at the same temperature.