This study is concerned with the effect of particles, which are too small to produce Particle Stimulated Nucleation, on recrystallization. A single crystal model system of internally oxidised copper-aluminium was deformed by rolling. The deformed and annealed structures were investigated both electron-microscopically and optically. Measurements of the stored energy of deformation were combined with measurements of the recrystallization kinetics by the use of anisothermal differential scanning calorimetry. The effect of particles on the mechanical properties of metals and the microstructural inhomogeneities produced during deformation in the single phase are reviewed. The literature concerning recrystallization is surveyed with emphasis on the possible sites for nucleation and the effects of a second phase. Comparison of the deformed structure of copper-alumina and copper revealed that cells form at lower strains in the particle-containing material. These cells are smaller than those found in copper and show alignment with {111} planes. Dynamic recovery at higher strains leads to the formation of a banded structure from these cells. Microbands were found at all strains, they increased in frequency and decreased in width as the strain increased. Misorientation measurements using STEM microdiffraction could find no evidence of a slip homogenizing effect due to the particles, in fact larger misorientations were found across cell boundaries in particle-containing material. Addition of particles raised the stored energy and this increase was greater at higher strains. A comparison of the recrystallization kinetics of particle-hardened and single phase material found complex behaviour in the former. Increasing the strain could lead to a change from complete suppression, to retardation, to acceleration of recrystallization in two-phase alloys. A model based on the properties of microbands and particle pinning is presented to account for these effects.

No description available.

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.

The purpose of this work was to study learn about grain refinement mechanisms and texture development in cast n-type Bi90Sb10 alloy caused by severe plastic deformation. The practical objective is to produce a fine grained and textured microstructure in Bi90Sb10 alloy with enhanced thermoelectric performance and mechanical strength. In the study, twelve millimeter diameter cast bars of Bi90Sb10 alloy were encapsulated in square cross section aluminum 6061 alloy containers. The composite bars were equal channel angular (ECAE) extruded through a 90 degree angle die at high homologous temperature. Various extrusion conditions were studied including punch speed (0.1, 0.3 and 0.6 in/min), extrusion temperature (220, 235 and 250oC), number of extrusion passes (1, 2 and 4), route (A, BC and C), and exit channel area reduction ratio (half and quarter area of inlet channel). The affect of an intermediate long term heat treatment (for 100 hours at 250oC under 10-3 torr vacuum) was explored. Processed materials were characterized by optical microscopy, x-ray diffraction, energy dispersive spectroscopy, wavelength dispersive spectroscopy and scanning electron microscopy. Texture was analyzed using the {006} reflection plane to identify the orientation of the basal poles in processed materials. The cast grains were irregularly shaped, had a grain size of hundreds-of-microns to millimeters, and showed inhomogeneous chemical composition. Severe plastic deformation refines the cast grains through dynamic recrystallization and causes the development of a bimodal microstructure consisting of fine grains (5-30 micron) and coarse grains (50-300 micron). ECAE processing of homogenizied Bi-Sb alloy causes grain refinement and produces a more uniform microstructure. Texture results show that ECAE route C processing gives a similar or slightly stronger texture than ECAE route A processing. In both cases, the basal-plane poles become aligned with the shear direction. Reduction area exit channel extrusion is more effective for both grain refinement and texture enhancement than simple ECAE processing.

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

No description available.

No description available.

Antistaling properties of enzymes (xylanase, bacterial maltogenic and conventional a-amylases), CMC and vital wheat gluten on corn tortillas were evaluated during storage for up to 21 days. Effect of storage time (0-21 days) and temperature (-40, -20, 3, 10 and 21 oC) on tortilla staling was evaluated with or without additives. Addition of 275-1650 AU of ICS maltogenic amylase effectively reduced amylopectin retrogradation without reducing tortilla yields, but did not improve tortilla flexibility. The combination of 825 AU of ICS amylase (to interfere with intra-granular amylopectin re-crystallization) and 0.25% CMC (to create a more flexible inter-granular matrix than retrograded amylose) produced less stiff, equally flexible and less chewy tortillas than 0.5% CMC. Corn tortilla staling followed the basic laws that control aging in starch-based semi-crystalline systems such as starch gels, bread and other baked products. Amylopectin re-crystallization was the driving force behind the staling of corn tortillas. Increasing levels of re-crystallized amylopectin measured by DSC correlated significantly with increased tortilla stiffness and reduction in tortilla rollability, pliability and rupture distance during storage. Re-crystallization of amylopectin in fresh tortillas was not detected. It increased rapidly during the first 24 hr reaching a plateau after 7 days storage. The level of amylopectin re-crystallization on tortillas showed a bell-shaped trend along the evaluated storage temperature range with a maximum around 7 oC. However, a negative linear relationship of peak pasting viscosity with storage temperature of tortilla extracts without additives after 21 days suggests other compounds besides amylopectin affect tortilla staling. Thus, interfering with amylopectin re-crystallization is not the only way to retard staling. Further research is required to optimize the addition of maltogenic amylases in continuous processing lines that use fresh masa instead of nixtamalized corn flour, to determine how these amylases interfere with amylopectin re-crystallization and to elucidate if amylose retrogradation continues during storage and plays a role in tortilla staling.

No description available.

No description available.