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M. Tech. Metallurgical Engineering / Titanium and its alloys exhibit poor tribological characteristics. The poor resistance to sliding wear of Ti6Al4V alloy makes it susceptible to severe wear at the surface during sliding contact. This could cause galling and seizing during sliding contact. Ti6Al4V alloy also have poor corrosion resistance under critical conditions. Some problems with Ti6Al4V MMCs produced by laser cladding technique in most cases is poor bonding as a result of wetting properties between the ceramic and metal powders for reinforcement. Occurrence of porosity is another factor which can reduce the mechanical properties of MMCs. Occurrence of agglomerates is also a concern due to poor mixing of reinforcement powders. This project is aimed at investigating the effect of laser cladding of titanium alloy substrate with zirconium (Zr), titanium carbide (TiC), titanium (Ti) reinforcement additions. The effect of combination of these powders using various fractions and variable cladding parameters on the substrate will be investigated.

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The increasing use of alpha/beta Ti-Al alloys in critical aircraft gas turbine engine and airframe applications necessitates the further development of physically-based constitutive models that account for their complex microdeformation mechanisms. Alpha/beta Ti-Al alloys are dual-phase in nature consisting of a mixture of hcp (alpha) and bcc (beta) crystal structures, which through variation in alloying elements and/or processing techniques can be produced in a wide range of microstructural compositions and morphologies. A constitutive model for these materials should address the various sources of material anisotropy and heterogeneity at both the micro and macroscales. The main sources of anisotropy in these materials are the low symmetry of the hcp phase, the texture, the relative strengths of different slip systems, non-planar dislocation core structures, phase distributions, and dislocation substructure evolution. The focus of this work is the development of a 3-D crystal plasticity model for duplex Ti-6Al-4V (Ti-64), an (alpha+beta) alloy. The model is used to study the process of attachment fatigue. Attachment fatigue is a boundary layer phenomenon in which most of the plastic deformation and damage accumulation occurs at depths on the order of tens of microns and encompasses regions of only a few grains into the depth of the material. The use of computational micromechanics-based crystal plasticity models to study attachment fatigue is a relatively new approach. This approach has the potential to offer additional insight to classical homogeneous plasticity models, since the length scales over which relative slip and crack initiation occur during this process is on the order of microstructural dimensions. Emphasis is placed on understanding the effects that texture, slip strength anisotropy, and phase distribution have on the surface and subsurface deformation fields during attachment fatigue. The deformation fields are quantified in terms of cumulative effective plastic strain distributions, plastic strain maps, and plastic strain-based critical plane multiaxial fatigue parameters.

Titanium alloys are employed in many advanced engineering applications due to their exceptional properties, i.e., a high strength-to-weight ratio, corrosion resistance, and high temperature strength. The performance of titanium alloys is known to be strongly affected by its inherent microstructure, which forms as a result of its thermo-mechanical processing. These microstructures produce compromise relationships between beneficial and detrimental effects on the alloy's performance. To study these structure-property relationships, two distinct crystal plasticity algorithms have been calibrated to data acquired from cyclic deformation experiments performed on three different Ti microstructures: (1) Ti-6Al-4V beta-annealed , (2) Ti-18 solution-treated, age-hardened (STA), and (3) Ti-18 beta-annealed, slow-cooled, age-hardened (BASCA). The calibrated models have been utilized to simulate fatigue loading of variant microstructures to investigate the influence of mean grain size, crystallographic texture, and phase volume fraction. The driving force for fatigue crack nucleation and propagation is quantified through the calculation of relevant fatigue indicator parameters (FIPs) and radial correlation functions are employed to study the correlation between favorably oriented slip systems and the extreme value FIP locations. The computed results are utilized to observe fatigue performance trends associated with changes to key microstructural attributes.

Metal matrix composites can produce mechanical and physical properties better than those of the monolithic metal. Titanium alloys are widely used matrix materials as they can offer outstanding specific strength, corrosion resistance and other advantages over its competitors, such as aluminium, magnesium and stainless steel. In past decades, titanium matrix composites served in broad areas, including aerospace, military, automobile and biomedical industries. In this project, a revised powder metallurgy method, which contains cold isostatic pressing and hot isostatic pressing, was adopted to refine the microstructure of monolithic titanium. It was also used to manufacture titanium matrix composites. TiH2 powder was selected as the starting material to form Ti matrix and the reinforcements were sub-micron and nano-metric TiB particles. Mechanical properties and microstructure of commercial titanium composites exhaust valves from Toyota Motor Corporation have been studied as the reference of properties of titanium composites manufactured in this project. It has been shown that tensile strength and hardness of exhaust valves increase about 30% than those of similar matrix titanium alloys. Examination on powder starting materials of this project was also carried out, especially the dehydrogenation process shown in the DSC result. Mechanical properties and microstructures of titanium matrix composites samples in this project, as related to the process parameter, have also been investigated. The density of these samples reached 96% of theoretical one but cracks were found through out the samples after sintering. Fast heating rates during the processing was suspected to have caused the crack formation, since the hydrogen release was too fast during dehydrogenation. Hardness testing of sintered samples was carried out and the value was comparable and even better than that of commercial exhaust valves and titanium composites in literature. Microstructure study shows that the size of reinforcements increased and the size of grains decreased as the increasing amount of TiB reinforcements. And this condition also resulted in the increasing amount of the acicular alpha structure.

Titanium alloys are important engineering alloys that are extensively used in various industries. This is due to their unique combination of mechanical and physical properties such as low density combined with high strength and toughness as well as outstanding corrosion resistance. An additional benefit associated with Ti alloys, in general, is that their properties are relatively temperature-insensitive between cryogenic temperature and ~500 °C. Amongst the Ti alloys, Ti-6Al-4V (referred as Ti64) is a widely used alloy. Conventionally cast Ti64 possesses classical Widmanstätten microstructure of (hcp) α and (bcc) β phases. However this microstructure suffers from large prior β grain size, which tends be in the order of a few mm. Such large grain sizes are associated with poor processability as well as inferior mechanical performance. The necessity to break this coarse as-cast microstructure down, through several successive thermo-mechanical processing steps, adds considerably to the cost of finished Ti alloy products, making them expensive vis-à-vis other competing alloys. The addition of small amount of B (~0.1%) to Ti64 alloys, on the other hand reduces the cast grain size from couple of mm to ~200 µm. Moreover, addition of B to Ti alloys produces the intermetallic TiB needles during solidification by an in situ chemical reaction. The overall objective of this work is to gain insights into the role of microstructural modifications, induced by B addition to Ti64, on the mechanical performance of the alloys, in particular the room temperature damage tolerance (fracture toughness and fatigue crack growth) characteristics. The key questions we seek to answer through this study are the following: (a) What role does the microstructural refinement plays on the quasistatic as well as fracture and fatigue behavior and high temperature deformability of the alloys? (c) A hierarchy of microstructural length scales exist in Ti alloys. These are the lath, colony and grain sizes. Which of these microstructural parameters control the mechanical performance of the alloy? (b) What (possibly detrimental) role, if any, do the TiB needles play in influencing the mechanical performance of Ti64 alloys? This is because TiB being much stiffer, strain incompatibility between the matrix and the TiB phase could lead to easy nucleation of cracks during cyclic loading as well as can pose problems during dynamic deformation. (d) What is the optimum amount of B that can be added to Ti64 such that the most desirable combination of properties can be achieved? Five B-modified Ti64 alloys with B content varying from 0.0 to 0.55 wt.% were utilised to answer the above questions. Marked prior β grain size reduction was noted with up to 0.1 wt.% B addition. Simultaneous refinement of α/β colony size has also been observed. The addition of B to Ti64, on the other hand increases the α lath size. The TiB needles that form in-situ during casting are arranged in a necklace like structure surrounding the grain boundaries for higher B added Ti64 alloys. An anomalous enhancement in elastic modulus, E, of the alloy with only 0.04 wt.% B to Ti64 was found. E has been found to follow the same trend of variation with B content at higher temperatures (up to 600 °C) as well. Nanoindentation experiments were conducted to evaluate the moduli of the various phases present in the microstructure and then rationalize the experimental trends within the framework of approximate models. Marginal but continuous enhancement in strength of the alloys with B addition was observed. It correlates well with the grain size refinement according to Hall-Petch relationship. Ductility on the other hand increases initially with up to 0.1 wt.% B addition followed by a reduction. While the former is due to the microstructural refinement, the latter is due to the presence of significant amount of brittle TiB phase. Room temperature fracture toughness decreases with B addition to Ti64. Such reduction in fracture toughness with the refinement of prior β grain size has been justified with Ritchie-Knott-Rice model. Contradictory roles of microstructural refinement have been observed for notched and un-notched fatigue. While reduction in length scale has a negative role in crack propagation, it enhances the fatigue strength of the alloy owing to better resistance to fatigue crack initiation. TiB needles on the other hand act as sites for crack initiation and hence limit the enhancement in fatigue strength of alloys with 0.30 and 0.55 wt.% B. An investigation of the high temperature deformability of the alloys has been performed over a wide range of temperature (within the two phase α+β regime) and strain rate windows. Results show that microstructural refinement does not alter the high temperature deformation characteristics as well as optimum processing conditions of the alloys. TiB needles, however act as sites for instability owing to differences in compressibility between the matrix and the whisker phase. In summary, this study suggests that the addition of ~0.1 wt.% B to Ti64 can lead to the elimination of certain thermo-mechanical processing steps that are otherwise necessary for breaking the as-cast structure down and hence make finished Ti components more affordable. In addition, it leads to marginal enhancement in the quasi-static properties and significant benefits in terms of high cycle fatigue performance.