The mechanical properties of pure Aluminium, Al-0.2%Cu and Al-0.4%Cu at large strains were studied by channel-die compression at three different temperatures: 77K, 200K and 300K. The results were interpreted in terms of work hardening rate versus stress (0/r) diagrams. The evolution of the structure was studied on a range of scales from macroscopic to microscopic, by optical study of slip lines, X-ray diffraction for texture measurements, Electron Back-Scattering Kikuchi Patterns for local texture measurements, and by Transmission Electron Microscopy for microstructural information. Intense shear banding was observed at large strains in all alloys at all temperatures. The texture evolution was shown to be consistent with this change in deformation mode. At low temperatures, stage HI of deformation was shown to be represented by a straight line in the 6lr diagram. Increasing the temperature lead to a dramatic decrease in work hardening rate and to an increasing concavity of the 0lr plots. The addition of solutes to pure Aluminium was shown to result in an increase of the work hardening rate, which could be represented by a simple translation of the 0/r plots on the stress axis. At large strains, all three materials experienced a stage (stage IV) of constant work hardening at low rate. The stage IV work hardening rate decreased with increasing temperature, and was not influenced by solute content. The stage Ill-Stage IV transition was very sharp at 77K and smoother at higher testing temperatures. Phenomenological models were developed for the prediction of the influence of temperature and solute content on work hardening. Moderate strains were modelled taking into account the evolution of the dislocation density into two different populations during the deformation. The influence of solutes on work hardening was modelled by considering how segregation of solute atoms at the dislocation cores influences dynamic recovery. Stage IV work hardening was considered to arise from the accumulation of dislocation debris resulting from the dynamic recovery events. / Thesis / Master of Engineering (ME)

<p>Tantalum, niobium, stainless steels, and nickel are corrosion-resistant metals that have become critical in many industrial sectors. Due to the demanding environments and temperatures in which they operate, few materials can serve as substitutes. The advantages of these materials are offset by the difficulties in their machining. Belonging to a group of metals and alloys often referred to as ‘gummy’, their poor machinability or gumminess is manifest as thick chip formation, large cutting forces, and poor finish on cut surface. Hence, machining costs can be prohibitive, and applications limited. The gumminess has been attributed broadly to their high strain-hardening capacity.</p> <p>To examine why these metals are difficult to machine, we used direct <i>in situ</i> observations of the cutting process with a high-speed imaging system, complemented by force measurements. The observations showed that chip formation occurred by repeated large-amplitude folding of the material – sinuous flow – with vortex-like components and extensive redundant deformation. The folding was particularly severe in Ta and Nb. Although Ta and Nb displayed a higher rate of fold nucleation than the Ni and stainless steel, the flow dynamics underlying chip formation across the metals was the same – sinuous flow nucleated by a plastic (buckling-type) flow instability on the workpiece surface just ahead of the advancing tool. The large strains and energy dissipation associated with sinuous flow is the reason for the poor machinability of these metals. </p> <p>Prior work with Cu and Al has shown that sinuous flow can be disrupted and replaced by an energetically more favorable (segmented) flow mode, characterized by quasi-periodic fracture, when suitable chemical media are applied to the initial workpiece surface – a mechanochemical effect. The segmented flow is beneficial for machining processes since it involves much smaller forces and plastic strains. It has been hypothesized that the chemical media influence the flow through their adsorption onto the workpiece surface, thereby altering the surface energy and/or surface stress, and effecting a local embrittlement (ductile-to-brittle transition). </p> <p>We demonstrate similar media (mechanochemical) effects and segmented flow development in cutting of the corrosion-resistant metals, with significant benefits for their machining. These benefits include > 35 percent reduction in the cutting force/energy, and an order of magnitude improvement in cut surface quality (finish, tears and residual strain). Importantly, the experiments with the corrosion-resistant metals provide strong evidence that it is indeed adsorption – not corrosion, as in case of hydrogen embrittlement – that underpins the mechanochemical effect. The experiments used chemical agents well-known for their strong adsorption to metal surfaces, namely green corrosion inhibitors (e.g., plant extracts, propolis) and other natural organic molecules (e.g., dyes, antibacterial drugs, cow’s milk). Lastly, the suitability and application of the mechanochemical effect at industrial cutting speeds is explored in turning experiments with these corrosion-resistant metals. Collectively, our observations, measurements, and analysis show that the gumminess of metals in cutting is due to sinuous flow; the gumminess can be eliminated by use of chemical media; and adsorption is the key to engendering the mechanochemical effect. Implications of the results for industrial processes ranging from machining to particle comminution, and for sustainable manufacturing are discussed.</p> <br>