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Severe plastic deformation (SPD) has been used over the past few decades for producing submicron grain (SMG) structures in range of metals and alloys. Equal channel angular pressing (ECAP) is a useful process for producing these types of structures whereby the material is deformed to very high plastic strains by passing a billet several times through the ECAP die. This process has an added advantage maintaining the initial dimensions of the billet. SMG materials produced by ECAP and related routes are useful as they usually exhibit excellent properties including high strength and hardness, and excellent superplastic formability: these and other properties make SMG materials useful for industrial and aerospace applications. In this thesis, a binary aluminium alloy containing a very low concentration of scandium (0.1 wt. %) Sc alloy was investigated and compared with higher Sc-containing alloys. The material was deformed by ECAP in the solution treated condition to an equivalent von Mises strain of 9.2 then pre-aged at 250 0C to generate a submicron grained material containing a relatively uniform dispersion of nanosized Al3Sc dispersiods. The thermal stability of this pre-aged microstructure was investigated by annealing at temperatures up to 450 0C resulted in continuous grain coarsening by the process of continuous recrystallization whereby the initial microstructure evolves gradually with no marked change in the grain size distribution, texture and grain boundary character. However, extended annealing (> 1h) at 4500 C resulted in discontinuous grain coarsening (often termed recrystallization) whereby a few grains grow rapidly to eventually produce a coarse-grained final microstructure. Throughout annealing, there was a good correlation between the dispersion parameter, (f/d) where f and d is the volume fraction and the mean diameter of Al3Sc particles in the alloy, respectively, and both the mean grain size (D ) and D /D max where max D is the maximum grain diameter observed in the microstructure. The grain structure was found to undergo moderate coarsening at the high f/d-values but converted to a coarsegrained structure for f/d ~<0.5/μm, and this change occurred when the mean grain diameter was ~ 3-4μm. Hence, the critical value of the dispersion parameter for the transition from continuous to discontinuous coarsening falls between the theoretical value for submicron grain size alloys (f/d ~ 1.5/μm) and the value found for conventionally-deformed alloys (f/d ~ 0.1/μm). This behaviour is the result of the alloy no longer being ultra-fine grained at the onset of discontinuous coarsening.

While inelastic mechanical behaviour of crystalline materials is well-understood in terms of lattice defects, bulk metallic glasses (BMGs) pose significant challenges in this respect due to their disordered structure. They can be produced by rapid cooling from the liquid state (among other technique) and, thus can be frozen as vitreous solids. Due to the absence of a long-range order in atomic structure and a lack of defects such as dislocations, BMGs generally show unique mechanical properties such as high strength and elastic limit, as well as good fracture toughness and corrosion resistance. Typically, inorganic glasses are brittle at room temperature, showing a smooth fracture surface as a results of mode-I brittle fracture. At small scale, it was well documented that inelastic deformation of bulk metallic glasses is localised in thin shear bands. So, in order to understand deformation mechanisms of BMGs comprehensively, it is necessary to investigate formation of shear bands and related deformation process. In this thesis, a history of development of BMGs is presented, followed by a review of fundamental mechanisms of their deformation.

The development of a high speed pin-on-disc wear testing rig has been undertaken and it has enabled the wear behaviour of zirconia sliding against silicon carbide to be examined. Sliding wear experiments were conducted for zirconia pins and silicon carbide discs under both water-lubricated and unlubricated conditions. Under water-lubricated conditions, tests at a relative sliding speed of 6 m s-1 showed that wear was geometry dependent and the exact alignment of the pin was crucial. At this speed it was possible, with pins polished in-situ on the rig, to achieve hydrodynamic lubrication (HDL) which resulted in low wear. Increasing the nominal contact pressure to 70 MPa resulted in the breakdown of the lubrication and led to high wear coefficients (e.g. 1.1 x 10-6 mm3 N-1 m-1). Under unlubricated conditions, the zirconia pin wore faster than with water lubrication present, the wear rate increasing with speed and nominal contact pressure in the range 1-6 m s-1 and 2-14 MPa. Wear coefficients ranged from 1.4 x 10-6 to 5.1 x 10 -5 mm3 N-1 m-1. The wear tests were followed by examination of the worn surfaces, using a variety of techniques including reflected light and scanning electron microscopy, in order to elucidate likely wear mechanisms. These techniques revealed that there was some degree of commonality between water-lubricated and unlubricated tests, suggesting a universal mechanism which operates over a broad spectrum of testing conditions. The electron microscopy study, combined with observations in the literature, led to the development of a physical model for the wear mechanism, including surface modification and material removal. During the initial stages of wear, and under mild testing conditions, grooves were formed on the surface by plastic deformation due to counterface asperities and trapped debris. The surface was further smeared and deformed as the test proceeded. A deformed surface layer built up which caused intergranular cracks parallel to the surface, at a depth of approximately 3 m. Wear occurred when these cracks linked up with the surface, a process which may be helped by the formation of a network of cracks parallel and normal to the sliding direction. The mechanism of formation of these crack networks remains controversial but is thought to involve thermal shock. Once material is removed from the surface, it is either thrown out of the contact or trapped in it causing further damage.

Oxide dispersion strengthened (ODS) alloys are known for their desirable mechanical properties and unique microstructures. These alloys are characterized by an even dispersion of oxide phase throughout a metallic matrix, and exhibit high strength and enhanced creep properties at elevated temperatures. This makes them ideal candidate materials for use in many structural applications, such as coal-fired power plants or in next generation nuclear reactors. Currently most often produced by mechanical alloying, a powder metallurgy based process that utilizes high energy ball milling, these alloys are difficult and costly to produce. One proposed method for forming ODS alloys without high-energy ball milling is to internally oxidize a bulk alloy before subjecting it to severe plastic deformation to induce an even oxide distribution. This work examines such a processing scheme with a focus on the internal oxidation behavior. Internal oxidation has been shown to occur orders of magnitude faster than expected in multi-phase alloys where a highly reactive oxidizable solute has negligible solubility and diffusivity in other, more-noble, phases. Commonly referred to as in situ oxidation, this accelerated oxidation process has potential for use in a processing scheme for ODS alloys. While in situ oxidation has been observed in many different alloy systems, a comprehensive study of alloy composition and microstructure has not been performed to describe the unusual oxidation rates. This work used Fe-Y binary alloys as model system to study effects of composition and microstructure. These alloys have been shown to exhibit in situ oxidation, and additionally, Y is typically introduced during mechanical alloying to form Y-rich oxides in Fe-based ODS alloys. Alloys with Y content between 1.5 and 15 wt% were prepared using a laboratory scale arc-melting furnace. These alloys were two phase mixtures of Fe and Fe17Y2. First, samples were oxidized between 600 and 800 °C for 2 to 72 hours, using a Rhines pack to maintain low oxygen partial pressures so that in situ oxidation could occur. Oxidation rates were accelerated when compared to traditional theory, and were not well described by a single parabolic rate constant throughout the duration of the experiment. While results agreed with Wagner theory that increased Y content should lead to decreased oxidation rates, this was attributed to a depletion of oxygen supply from the Rhines pack over time. Samples were also subjected to plastic deformation to observe how changes in microstructure influenced kinetics. Connectivity of the oxidizable phase was found to be critical to promoting the fastest rates of oxidation. Oxidation studies where then carried out using thermogravimetric analysis. A gaseous mixture of Ar-H2 was passed through a dew point control unit to vary oxidant partial pressure between 10-25 and 10-20 atm. Flow rate of the gas parallel to the sample surface was also altered. Canonical correlation analysis was then used to analyze and simplify the relationships between input and output variables. This analysis pointed to the importance of quantifying the relationship between the size of formed oxides and changes in oxidation kinetics over time. Where sustained parabolic kinetics were observed, oxides were small throughout the depth of internal oxidation. The effects of oxide size on penetration depth were then numerically modeled and incorporated into existing oxidation theory to show that the observed kinetics could be qualitatively described. After oxidation experiments, severe plastic deformation was applied to both oxidized and unoxidized microstructures using equal channel angular pressing. By manipulating pressing temperature and the number of passes, microstructures were altered to varying degrees of success. No oxide refinement was observed, but increasing temperatures and number of passes allowed for even dispersion of both oxides and Fe17Y2 intermetallic.

Neutron diffraction is a widely used technique for measuring internal stresses inside polycrystalline materials. By examining the diffraction patterns collected during in situ uniaxial deformation, the lattice strains along various crystallographic directions can be calculated. These lattice strains give insight into the active deformation mechanisms active within the material during plastic deformation. This is most commonly done by fitting model results to the experimentally measured lattice strains through an iterative process of refining the model parameters. A numerical optimization technique was successfully applied to the problem of refining the input parameters of an elastoplastic self-consistent (EPSC) model. The results were found to be comparable to those obtained by a past researcher manually refining the model parameters and subjectively judging the fit to the experimental data. The numerical optimization method was able to reach an acceptable result much faster than is possible by a human being (days as opposed to weeks or months), meaning that it has the potential to reduce the turn-around time from data collection to interpretation/publication significantly. At the same time, common experimental techniques for conducting diffraction experiments during uniaxial deformation tests were examined. It is common to use an interrupted loading scheme where the sample is brought to a certain loading condition and then held steady while the neutron data is collected, a processes that often takes several minutes. This interrupted loading may be done such that the sample is held at constant stress, strain, or simply by having the load frame stay in a constant position. Each of these different loading modes results in a particular type of relaxation within the sample as it is being held, so a series of experiments were conducted to investigate any impact these different relaxation types may have on the measured values of the lattice strains. Overall it was found that both qualitative and quantitative differences in the recorded data can arise as a result of the different loading modes, and that such differences tend to manifest themselves at or near the point at which the material begins to yield macroscopically. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-03-05 19:37:19.995

Many metals used in modern engineering exhibit anisotropy. A common assumption when modelling anisotropic metals is that the level of anisotropy is fixed throughout the calculation. As it is well understood that processes such as cold rolling, forging or shock loading change the level of anisotropy, it is clear that this assumption is not accurate when dealing with large deformations. The aim of this project was to develop a tool capable to predict large deformations of a single crystal or crystalline aggregate of a metal of interest and able to trace an evolution of anisotropy within the material. The outcome of this project is a verified computational tool capable of predicting large deformations in metals. This computational tool is built on the Crystal Plasticity Finite Element Method (CPFEM). The CPFEM in this project is an implementation of an existing constitutive model, based on the crystal plasticity theory (the single crystal strength model), into the framework of the FEA software DYNA3D® . Accuracy of the new tool was validated for a large deformation of a single crystal of an annealed OFHC copper at room temperature. The implementation was also tested for a large deformation of a polycrystalline aggregate comprised of 512 crystals of an annealed anisotropic OFHC copper in a uniaxial compression and tension test. Here sufficient agreement with the experimental data was not achieved and further investigation was proposed in order to find out the cause of the discrepancy. Moreover, the behaviour of anisotropic metals during a large deformation was modelled and it was demonstrated that this tool is able to trace the evolution of anisotropy. The main benefit of having this computational tool lies in virtual material testing. This testing has the advantage over experiments in time and cost expenses. This tool and its future improvements, which were proposed, will allow studying evolution of anisotropy in FCC and BCC materials during dynamic finite deformations, which can lead to current material models improvement.

Equal channel angular extrusion (ECAE) has attracted a substantial attention for it provides the opportunity to introduce large plastic strain into the material in the bulk form. Both die angles and processing routes have been recognized as the important parameters in applying ECAE to fabricate ultrafine-grained materials. Unfortunately, studies of different group provided inconsistent conclusions on the effectiveness of processing routes, which are believed to be due to the incomplete microstructural information obtained in each investigation. In the present work, quantitative analysis of the microstructure developed by different processing conditions were conducted using transmission electron microscopy (TEM), in which the morphology, size, and shape of subgrains as well as boundary misorientation were fully characterized. A commercial pure aluminum (AA 1050) was deformed by ECAE to strain of ~ 8 with different routes (A, Bc and C, in terms of reorientation angle 0o, 90o, and 180o respectively of the billet between two extrusion passes) and die angles. The results show that the effectiveness of high angle boundary (HAB) formation is in the sequence of route A¡ÜBc>C. However, in terms of grain refinement, the effectiveness is in the order of route Bc>A>C. In addition, route A produces subgrains with the most elongated shape, while route Bc produces subgrains with the most equiaxed shape. These results may be attributed to the different shear pattern introduced in each route. ECAE die angle determines both the strain per pass and the shear plane orientation. In route C, the shear is maintained in the same plane and the effect of strain per pass can be studied. With route C, both the 90o and 120o die produce microstructure with similar HAB proportions, but they result in different arrangement of HABs. The 120o die produces subgrains with larger size and higher aspect ratio than the 90o die does in route C. Generally speaking, for the die angle range studied, the different values of strain per pass used in ECAE mainly affect the morphology of the subgrains. On the other hand, the effect of die angle is weakened with route Bc as compared to route C, which may be attributed to the intersection of shear planes involved in route Bc.

This document proposes modifications to an established cloth simulation algorithm to allow for stretch deformation and tearing of simulated cloth in computer-generated imagery. Previous research is presented, followed by the development of a cloth simulation system with the addition of tearing behavior. Several results are given that show off individual features and behaviors that this thesis models.

Zircaloy-2 and its sister alloy, Zircaloy-4, have extensive applications in the nuclear industry as core components in heavy water reactors and fuel cladding in both heavy and light water reactors. Intergranular stresses and texture can greatly affect the mechanical performance of these components. A complete understanding of the development of intergranular constraints and texture in Zircaloy-2 will allow an improved understanding of the plastic deformation of zirconium alloys, and the prediction of in-reactor deformation of tubes made by different manufacturing routes. Neutron diffraction was used to track the development of lattice strain and peak intensity in three dimensions for various crystallographic planes in samples cut from a rolled Zircaloy-2 slab. The samples were subject to room temperature compression or tension in-situ in the neutron spectrometer in each of the three principal directions of the slab. Textures in the deformed samples were measured using neutron diffraction. Strong evidence was found for tensile twinning in tensile tests in the plate normal direction and compression tests in the transverse and rolling directions. The lattice strain development inside the newly formed twins was recorded for the first time in a Zr alloy. An elasto-plastic self-consistent model and a visco-plastic self-consistent model were used to interpret the lattice strain and texture data, respectively. Various slip and twinning modes were considered in both models. Prism <a> slip, basal <a> slip, pyramidal <c+a> slip and tensile twinning were concluded to be indispensable, while pyramidal <a> slip was unnecessary in the modeling. The critical resolved shear stresses and hardening parameters were obtained by simultaneously achieving a ‘best-fit’ with the complete experimental data set. The effects of anisotropic latent hardening due to dislocation interactions were found to be critical, and the inclusion of Lankford coefficients as modeling constraints was necessary. This research provided a comprehensive experimental data set obtained by neutron diffraction, forming a sound basis to investigate active plastic deformation mechanisms and to rigorously test plasticity models and twinning models. The research also made a substantial improvement in understanding the plastic deformation of Zircaloy-2 through polycrystalline modeling by introducing extensive data sets to constrain the modeling parameters. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2007-12-21 11:56:23.743 / NSERC, COG, OPG and Nu-Tech Precision Metals