Papers by Keyword: Microstructural Development

Abstract: Metal matrix composites (MMCs) have received considerable attention due to their low density with good elastic modulus and high strength to weight ratio. Discontinuous reinforced Ti matrix composites have been found as a promising material for applications in various fields, such as aerospace, automotive, biomedical and advanced military applications, because of their low cost, improved performance and ease of fabrication. Among the discontinuous ceramic reinforcements, TiC is identified as a very suitable reinforcement for the Ti system because of its excellent properties and high compatibility with Ti matrices. In this study, investigations have been conducted on the influence of volumetric percentage of TiC (10%) on microstructural development of TiC reinforced titanium beta matrix composite prepared by the blended elemental method from hydrided powders using ex situ processing route. Samples were produced by mixing of elemental hydrided powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering (900°C- 1500°C), in high vacuum. Sintered samples were characterized for phase composition, microstructure, microhardness and mechanical properties by X-ray diffraction, scanning electron microscopy, Vickers indentation, respectively. Density was measured by Archimedes method. The experiment results revealed that TiC content has significant influence on the microstructure and improving the hardness values of Ti-35Nb-TiC composites. A homogeneous distribution of TiC particles was observed, with a reduced presence of agglomerates and macroporosities. There was an increase of 28.5 % in the hardness of the composites with the addiction of TiC, which indicates the possibility of using components manufactured using this technique, for example, in aircraft landing gears that are subject to high mechanical stress and orthopedic implants.

Abstract: The characteristics of β-phase metastable Ti alloys make them an attractive choice for advanced engineering applications in demanding conditions. Ti-35Nb alloy has high strength-to-weight ratios, deep hardenability and high biocompatibility exhibiting high potential for use in niche applications for aircraft structures, orthopedic implants, and orthodontic devices. The difficulty of producing complex shapes of these alloys by conventional methods for reasonable costs makes Metal Injection Moulding (MIM) attractive. Sintering behavior, microstructure and mechanical properties of a Ti–35Nb alloy processed by MIM technology from hydrided powders were investigated in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and thermal and microhardness analysis. Samples with relative density up to 93% have been produced using a feedstock based on wax-polymer binder. The microstructural evolution observed during sintering from 900 °C up to 1500 °C indicates a combination of densification and optimized microstructure reached because of the complete dissolution of the β stabilizer (Nb) in the titanium matrix. The injection and sintering parameters provided a homogeneous microstructure with some TiC precipitates at grain boundaries and relative high porosity. Higher sintering temperatures or longer holding times can lead to intensive grain growth.

Abstract: A three-dimensional cellular automata (CA) model is developed for the kinetic and microstructural modelling of the relevant metallurgical mechanisms occurring in the annealing stage of low–alloy steels: recrystallisation, pearlite–to–austenite transformation and ferrite–to–austenite transformation on heating and austenite–to–ferrite transformation on cooling. In this model the austenite–to–ferrite transformation is described by a mixed–mode approach, which implies that the transformation kinetics is controlled by both the interface mobility and the diffusivity of the partitioning elements. This approach also allows incorporation of the ferrite nucleation occurring on structural defects. The developed CA algorithm, in which the transformation rules for the grain boundary and interface cells are controlled by the growth kinetics of the forming phase, allows three-dimensional systems to be treated within relatively short simulation times. The simulated microstructure reproduces quite well the microstructure observed in experimental samples. A good agreement is obtained between the experimental and simulated ferrite recrystallisation and ferrite and austenite transformation kinetics. The present approach also models the development of the carbon concentration profile in the austenite, which is, for instance, essential for subsequent martensite formation.

Abstract: Duplex stainless steels (DSS) are alloys made of ferrite and austenite, with a proportion of each phase around 50%. Their main advantage in comparison with other austenitic and ferritic stainless steels is the attractive combination of high strength and corrosion resistance together with good formability and weldability. Unfortunately, DSS often present a poor hot workability. This phenomenon can stem from different factors associated to the balance of the phases, the nature of the interface, the distribution, size and shape of the second phase, and possibly also from difference in rheology between ferrite and austenite. In order to determine the specific influence of phase morphology on the hot-workability of DSS, two austenite morphologies (E: Equiaxed and W: Widmanstätten) with very similar phase ratio have been generated using appropriate heat treatments. It was checked that the latter treatments generate stable microstructures so that subsequent hot mechanical tests are performed on the microstructures of interest. One microstructure consists of a ferritic matrix with austenitic equiaxed islands while the other microstructure is composed of a ferritic matrix with Widmanstätten austenite. The latter morphology corresponds to the morphology observed in as-cast slabs.

Abstract: Ti-35Nb-7Zr-5Ta alloy is considered an attractive material for implants manufacture due to an excellent combination of properties, including high mechanical and corrosion resistance, beyond the lowest elastic modulus among the titanium alloys. The alloy processing by powder metallurgy (P/M) eases the obtainment of parts with near-net shape forming and low production costs. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering between 800-1600 °C, in vacuum. The isochronal sintering demonstrated to be efficient for the study of the microstructural evolution. The samples presented high densification and adequate microstructure. The results show that a beta-homogeneous microstructure is obtained in the whole sample extension when sintered at high temperatures beyond that P/M technology allows an effective porosity control.

Abstract: The primary factors that effect solid state diffusion during solidification are described and binary solute redistribution equations that permit estimation of the significance of solid state diffusion are discussed. Model calculations suggest that solid state diffusion of substitutional alloying elements in FCC alloys is insignificant under most processing conditions, while that of interstitial alloying elements is likely to be complete. Experimental data that supports these results are presented. Several cases that highlight the practical importance of microsegregation on performance of engineering alloys are described as well as methods for avoiding or minimizing microsegregation for improved properties. A solute redistribution model for handling the limiting cases of solute diffusion in ternary alloys is presented and model calculations are reviewed to reveal the strong influence diffusion can have on the solidification path and resultant microstructure.

Abstract: Al-rich sludge produced from industrial anodising and surface treatment processes had been tested in the fabrication of mullite-based materials, by using unidirectional dry pressing as shaping technique. Mixture with common natural silica and magnesium-containing materials, like ball clay, kaolin and diatomite were prepared in order to achieve interesting final fired refractory and/or electrical insulating materials. Microstructural changes upon sintering and crystalline phase evolution are detailed and their relationship with the electrical behaviour is also studied, by the use of impedance spectroscopy. Finally, different models were used to fit the experimental electrical resistivity values and to estimate the percolation limit of the bi-phase material.