Texture, microstructure and formability were studied in Direct Chill Cast (DC) and Strip Cast (SC) aluminum alloys with regard to crystallographic anisotropy, the Portevin-Le Chatelier effect and aging softening behavior. It was found that material properties change greatly with manufacturing processes (DC vs. SC) and chemical composition (3xxx vs. 5 xxx alloys). DC cast hot band materials are usually fully recrystallized and have strong softening textures while SC hot band materials have a rolling structure with strong deformation textures. Softening textures cause 90 earing while deformation textures result in 45 earing after deep drawing. During cold rolling, 90 earing in DC cast hot band materials decreases and eventually changes to 45 earing after certain degrees of cold reduction. Correspondingly, the intensity of the softening texture components in DC cast hot band materials decreases while the intensity of deformation texture components increases with increasing degrees of cold reduction. These two kinds of textures interact and attempt to balance each other during cold rolling which produces resultant earing. However, this is not true for SC hot band materials since it's hard to obtain strong softening textures and thus 90 earing in these materials. 5 xxx Al-Mg alloys are more difficult to work than 3 xxx aluminum alloys. Elevated temperature annealing which greatly reduces the strength (hardness) improves significantly the workability of Al-Mg alloys. On the other hand, the Portevin-Le Chatelier effect and aging softening behavior are stronger in Al-Mg alloys than in 3xxx aluminum alloys and both increase with increasing cold reduction and with increasing Mg content. An apparent tensile anisotropy exists in as received SC hot band materials. The tensile yield strength (YS) is smaller in the QD (45 to the rolling direction), and larger in the RD (rolling direction) and the TD (transverse direction). There is no obvious difference in YS between these RD and TD directions. The average stress drop of serrations in the PLC effect, D s , is strongest in the TD, smallest in the RD with QD in between but closer to TD. However, no tensile anisotropy was observed in a fully recrystallized DC hot band or in solution treated SC hot band materials. It was found that a rolling structure favors mechanical anisotropy while a recrystallized structure prevents it. The tensile anisotropy is due to anisotropic distributions of microstructures, i.e., dislocations, precipitates and solute atoms. A random microstructure is associated with material that shows little or no mechanical anisotropy. An elongated or preferably orientated microstructure is associated with material with high mechanical anisotropy. Recovery thermal treatments at sufficiently high temperatures so that dislocation annihilation and microstructure rearrangement occurs when applied to the final gauge material also lowers mechanical anisotropy because of the reduction in intensity of the elongated (preferably orientated) microstructure. In addition, plastically deforming the material in a more homogenous manner (such as cross rolling as compared to straight rolling) produces a more uniform microstructure with an accompanying lower mechanical anisotropy.

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.

Magnesium alloys have a great potential in automotive industries, compared to steel and aluminium (Al), Magnesium (Mg) is much lighter and this weight reduction improves fuel efficiency and lowers green gas emission. Due to its hexagonal crystal structure, magnesium has poor ductility at room temperature. Magnesium's ductility improves significantly above about 200°C due to thermal activation of additional slip systems. This has lead to efforts to form auto-body panels with commercial AZ31 magnesium sheet at elevated temperatures. In this work, various AZ31 magnesium alloy materials were used to investigate the influence of deformation conditions on texture and microstructure. Moreover, it is to define the correlation between formability and different deformation mechanisms. / It was observed that only basal slip and twinning contributed to room temperature deformation. As deformation temperature increased, an increase in ductility in Mg contributed to dynamic recrystallization occurring readily at elevated temperatures (≥300°C). Even coarse grain material experienced significant tensile elongation due grain refinement. Depending on temperature and strain rate, different deformation mechanisms were activated and lead to different failure modes (moderate necking, cavity, strong necking). More specifically, deformation at elevated temperature in the low-strain-rate regime with stress exponent n about 2-3 and activation energy close to grain-boundary diffusion of Mg (Q = 92 kJ/mol) is characteristic of GBS. Deformation at elevated temperature in the high strain rate regime showed that the stress exponent increased to a value close to 5 and that the activation energy was consistent with the one for Mg self-diffusion (135 kJ/mol) and for diffusion of Al in Mg (143 kJ/mol). This was indicative of a dislocation creep deformation mechanism. Plus the six-fold symmetric patterns of the {1 100} and {1120} pole figures and the splitting of basal plane distribution are another indication of slip mechanism or of dislocation creep mechanism. / The optimum deformation behavior for AZ31 sheet was found to be for the material with fine grain microstructure. The highest elongation of 265% was obtained with the material having initial grain size of 8 mum. In addition, strain-rate sensitivity, which is a good indication of material's ductility, also was the highest in material with 8 mum grain size. As a common trend, the strain-rate sensitivity increased with decreasing strain rate, increasing temperature and decreasing grain size. / In terms of drawability of AZ31 sheet, the deformation controlled by GBS resulted in a fair drawability/formability property with r-value about 1 whereas a deformation mechanism controlled by dislocation creep showed a good drawability with r-value above 1.5. Due to activation of additional slip systems (non-basal <a> and <c+a>), the thinning of the sheet was prevented, in particular at deformation conditions of 450°C with 0.1s-1 where r-value was highest. This deformation condition might suggest good forming process parameters, especially for deep drawing, for the commercial AZ31 sheet under investigation. A preliminary study of Forming Limit Diagram for AZ31 sheet was performed by the Limit Dome Height test method at 300°C. The FLD0 of AZ31 was found to be 67%; the part depth of biaxial forming was 1.86 in; and the maximum LDH varied from 2.4 to 2.6 in.

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.

No description available.

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.

The formability of Mg alloy sheet in the as-hot rolled condition depends on the microstructure developed during hot rolling. In general, the formability of Mg alloys is improved by finer grain sizes. 'Microalloying' levels of calcium (Ca), strontium (Sr), and cerium (Ce) have been found to refine the as-cast structure, but there is no information as to whether this effect will be reflected in the as-hot worked structure and formability. Thus, in this work, the effects of microalloying levels of calcium (Ca), strontium (Sr), and cerium (Ce) on the microstructures (from as-cast to as-hot rolled) and subsequent hot deformation behavior of AZ31, nominally 3% Al, 1% Zn, and 0.3%Mn, were systematically investigated. / To include the effect of solidification rate these alloys were cast in different moulds (preheated steel mould, Cu-mould, and water cooled Cu-mould). One-hit compression testing at temperatures between 250&deg;C &sim; 400 &deg;C, strain rates of 0.001, 0.01, 0.1 s-1 and strains from 0.2 up to 1.0, was performed to investigate the basic hot compression behavior, while two-hit compression testing was conducted to determine the static softening behavior. Hot rolling of the microalloyed AZ31 alloys was then carried out to study the effects of microalloying on as-hot rolled structure under two sets of rolling schedules. To investigate the formability of these microalloyed sheets, tensile tests were completed over a temperature range between ambient and 450&deg;C, at strain rates between 0.1 and 0.0003 s-1. / Results show that Ca and Sr act to refine the as cast grain size and the second phases, consistently promoting fine and uniform as-hot rolled grain structure. With regard to grain refinement, calcium has the strongest effect, whereas Ce is most effective for second phase refinement. In addition, microalloying retards grain growth during hot tensile testing. Multiple alloying presents a combined and complementary effect. / A refined and uniform grain structure combined with well dispersed and thermally stable second phases significantly improves the hot formability of AZ31 sheets by promoting dynamic recrystallization (DRX) in the matrix, resisting grain coarsening, and retarding the development of cavitation and necking. Under the superplastic condition of 450&deg;C and 0.0003 s -1, the elongation was improved by 17% with Ca only, 26% with Ca and Ce, 51% with Ca and Sr, and 59% with Ca, Sr and Ce.

[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

Magnesium and its alloys are considered as the potential biomaterials due to their biocompatibility and biodegradable characteristics but suffer from poor corrosion performance. Various surface modification techniques are employed to improve their corrosion resistance. In present case, laser surface melting was carried out on AZ31B Mg alloy with various laser energy densities using a continuous wave ytterbium laser. Effect of laser treatment on phase and microstructure evolution was evaluated by X ray diffraction and scanning electron microscopy. Multi-physics thermal model predicted time temperature evolution along the depth of the laser treatment zone. Additionally, electrochemical method and bio-immersion test were employed to evaluate the corrosion behavior in simulated body fluid medium. Microstructure revealed grain refinement and even distribution of Mg17Al12 phase along the grain boundary for laser treated samples leading to substantial enhancement in the corrosion resistance of the laser treated samples compared to the untreated alloy. The laser processed samples also possessed a superior wettability in SBF solution than the untreated sample. This was further reflected in enhanced bio-integration behavior of laser processed samples. By changing the parameters of laser processing such as power, scanning speed, and fill spacing, a controllable corrosion resistance and bioactivity/biocompatibility of the implant material was achieved.