Thesis (M.S.)--Ohio State University, 2009. / Title from first page of PDF file. Includes vita. Includes bibliographical references (p. 108-109).

Titanium alloys are widely used for aerospace and biomaterial applications since their high specific strength, and high corrosion resistivity. Besides these properties, titanium is an excellent biocompatible material widely used for internal body implants. Because the products have complex geometries in both applications, Additive Manufacturing (AM) methods have been recently applied for production. AM methods can process a direct 3-D shape of the final product, decrease total production time and cost. However, high residual stress of the final product limits the application of AM components, especially the ones that are exposed to cyclic loading. In the present study, the initial microstructures and impact toughness of Ti6Al4V processed by EBM and CMT, and CP:Ti processed by SLM were experimented. In addition to initial microstructure and impact toughness, their response to different heat treatments were examined. Gleeble® 3500 was used for rapid heat treatment process. The change of mechanical properties due to different heat treatments were monitored with impact tests. Phase transformation kinetics of CP:Ti and Ti6Al4V were investigated with a Differential Scanning Calorimeter at slow heating and cooling rates. Microstructure examination was done with a scanning electron microscope. EBSD data was used to analyze the microstructure behavior. It is observed that toughness of the samples that are produced by powder-based AM methods were improved. Overall, residual stress, strain values, and grain orientation are the key elements that affected impact toughness AM produced components.

Thesis (M.App.Sc.) - Carleton University, 2006. / Includes bibliographical references (p. 110-113). Also available in electronic format on the Internet.

The properties of titanium alloys are based on alloy compositions and microstructures that consist of mixtures of the two allotropic modifications of titanium, the low temperature α (hcp) and the high temperature β (bcc) phases. This thesis deals with the hot working behaviour of three commercial titanium alloy compositions designated IMI834, Ti17 and Ti5553 with a focus and detailed analysis of the Ti5553 alloy. These alloys represent the differing uses of titanium alloys in the aerospace industry. IMI834 is a near α alloy used in high temperature creep resistant applications as compressor discs and blades in aeroengines. Ti17 is a high strength alloy α+β used at intermediate temperatures in fan and compressor discs of aeroengines, while Ti5553 is a high strength-high toughness metastable β alloy used in the undercarriages of aircraft. The three alloys have widely differing β transus temperatures (related to α phase stability) and compositions. Titanium alloys are vacuum arc melted and thermomechanically processed. This process involves ingot breakdown in β (bcc) phase, and subsequent thermomechanical processing in two-phase α+β (hcp+bcc) region at temperatures that typically involve volume fractions of α in lath or plate form ranging from 15% to about 30%. The thermomechanical processing breaks down lath α to spheroidal particles, a process known as globularisation. Chapter I of this thesis reviews the current understanding of the hot working of titanium alloys and microstructure evolution during the hot working process. Chapter II summarises the main experimental techniques used: the hot compression test, and subsequent microstructure and microtexture analysis by scanning electron microscopy and related electron back scattered diffraction techniques (EBSD), transmission electron microscopy and related precession electron diffraction techniques (PED) for orientation imaging. The starting structure in the α+β domain of hot work is generally not a random distribution of the 12 variant Burgers Orientation Relationship (BOR) between the α and β phases, (11̅0)β || (0001)α and <111>β || <112̅0>α . A variety of morphologies and distributions ranging from the typical colony structures of near α and α+β alloys to the fine distributions of variants arranged in a triangular fashion are observed with specific growth directions and habit planes. Chapter III describes a quantitative evaluation of α distribution that are typical of some of the starting structures for the hot working conditions used in this thesis, specifically in the Ti5553 alloy. For this purpose, a Matlab based script has been developed to measure the spatially correlated misorientation distribution. It was found that experimental spatially correlated misorientation distribution varies significantly from a random frequency for both pair and triplet wise distribution of α laths. The analysis of these structures by established techniques of analysis of self-accommodated structures based on strain energy minimisation shows that the observed variant distribution arise from the residual strain energy accommodation of the semi-coherent α plates. The hot working process has been examined through hot compression tests of the 3 alloys at strain rates ranging from 10-3 s-1 to 10 s-1 over a temperature range designed to maintain constant volume fractions of the α and β phases during deformation ranging from about 30% α to a fully β structure. Since extensive prior work has been carried on the processing of titanium alloys, Chapter IV focuses on a comparative study of hot deformation behaviour of the three alloys with an emphasis on isolating microstructural and other effects. The macroscopic flow behaviour has been analysed in terms of conventional rate equations relating stress, strain, strain rate and temperature. The three alloys show very similar features in their stress-strain behaviour. β phase deformation exhibits yield points whose magnitude varies with strain rate and temperature. The flow stress curves are typical of materials undergoing dynamic recovery and recrystallization processes. The stress-strain behaviour in the α+β domain of hot work exhibits significant flow softening in the early stages of deformation with a subsequent approach to steady-state behaviour at true strain of about 0.5. Activation energy analysis of the steady state condition suggests that the rate controlling mechanism is related to recovery in the β phase in both α+β and β processing. Zener-Hollomon plots of the flow stress in the three alloys indicate that their flow stress can be normalized to a temperature-compensated strain rate and they differ only in the slopes of the plots that are related to the stress exponent. Empirical constitutive models were developed for a predictive understanding of the flow stress as a function of strain, strain rate and temperature using conventional rate equations for the flow stress Chapter V and VI examine the evolution of microstructure and microtexture in detail during hot deformation and subsequent heat treatment in Ti5553. A combination of EBSD (micron and submicron scale) and PED (nano meter scale) is used in orientation imaging to examine the globularisation process of the α phase and the recovery and recrystallization in the β phase in both supertransus and subtransus hot compression. The understanding of these processes is enhanced by tracking the same starting β grain through the deformation process. The effect of strain, strain rate and temperature on the evolution of subgrains in α and its fragmentation into spheroidal α is quantified. In the absence of shear bands, the globularisation process is seen to evolve from a strain driven Raleigh instability of the plate α, by subgrain formation in α and β phases. The related microtexture evolution is analysed. The analysis of recovery and microtexture evolution in the β phase described here has not been attempted earlier in the literature. The overall evolution of structure and texture is seen to result from the complex interplay between recovery and recrystallisation in the α and β phases in substranus deformation. While the Burgers orientation relationship between α and β is lost in the early stages of deformation, it appears to be restored at large strains as a consequence of ‘epitaxial’ recrystallisation processes that seem to result from the discontinuous nucleation of recrystallization of either phase at interphase interfaces in the Burgers orientation. The effect of substranus deformation on β texture following supertransus post deformation heat treatment is also examined and compared with β textures resulting from alternative strain paths such as friction stir processing. Finally Chapter VII summarises these results and the new insights into the evolution of structure and microtexture during hot deformation of titanium alloys and suggests directions for future work.

[Truncated abstract] The study of NiTi shape memory alloys, although comprehensive and diverse, still encounters numerous uncertainties and misunderstandings that often jeopardise the effective use of these alloys in various applications. One such key area is the understanding of the micromechanics and thermodynamics of the deformation mechanisms, such that their deformation behaviour can be accurately predicted and modelled. Furthermore, most research involves polycrystalline NiTi of varying compositions and processing history, both of which complicate and damage the internal structure of the matrix even before deformation. This work aims to study the micromechanisms of deformation of near-equiatomic NiTi alloys, both in polycrystalline and single crystal forms, with particular attention given to the commonly observed phenomena of Luders-like deformation behaviour and deformation induced martensite stabilisation. This work was carried out in three sections. Firstly, the tensile deformation of polycrystalline NiTi samples via martensite reorientation and stress-induced martensitic transformations was carried out. The samples were deformed to various stages of deformation and then thermally cycled to study the thermomechanical response to deformation as a means to explore the various mechanisms of deformation. Next, the deformation and post-deformation transformation behaviour of NiTi single crystals were studied to verify the effect of grain boundaries and other hypotheses raised regarding the deformation mechanisms. The single crystal samples were deformed along three low-index axial orientations. Finally, microscopic analysis was carried out on as-annealed and the deformed polycrystal and single crystal samples by means of transmission electron microscopy. The microstructural analyses accompanied the thermodynamic study and provided evidences to support various hypotheses

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.

A compositionally graded Ti-xCr (10≤x≤30 wt%) alloy has been fabricated using Laser Engineered Net Shaping (LENSTM) to study the microstructural phase evolution along a compositional gradient in both as-deposited and heat treated conditions (1000°C followed by furnace cooling or air cooling). The alloys were characterized by SEM BSE imaging, XRD, EBSD, TEM and micro-hardness measurements to determine processing-structure-property relations. For the as-deposited alloy, α-Ti, β-Ti, and TiCr2 (C15 Laves) phases exist in varying phase fractions, which were influential in determining hardness values. With the furnace cooled alloy, there was more homogeneous nucleation of α phase throughout the sample with a larger phase fraction of TiCr2 resulting in increased hardness values. When compared to the air cooled alloy, there was absence of wide scale nucleation of α phase and formation of ω phase within the β phase due to the quicker cooling from elevated temperature. At lower concentrations of Cr, the kinetics resulted in a diffusionless phase transformation of ω phase with increased hardness and a lower phase fraction of TiCr2. In contrast at higher Cr concentrations, α phase separation reaction occurs where the β phase is spinodally decomposed to Cr solute-lean β1 and solute-rich β2 resulting in reduced hardness.