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The recent acceptance and standardized certification of powder metallurgy titanium for commercial aircraft engineering applications marks a milestone in titanium powder metallurgy. However, there are still many issues to be addressed for the wider development of powder metallurgy of titanium such as in-depth identification of impurities arising from raw materials and sintering atmosphere, development of novel sintering methods and new alloy design. Since limited works currently focus on the study of residual magnesium chloride impurities originating from the mainstream Ti sponge production process — Kroll process, the starting point of this thesis is to identify the chloride impurities in titanium powders. Jigsaw-like agglomerates were observed on the particle surface of Kroll-processed hydrogenated-dehydrogenated (HDH) commercially pu...
There has been much confusion evident in the literature in terms of the influence of impurity elements on grain refinement of magnesium alloys. This thesis addresses how impurity elements such as iron, manganese, carbon and beryllium affect grain refinement in magnesium alloys. The thesis starts with an investigation into the effect of the uptake of iron on grain refinement of Mg-Zr alloys. The highly detrimental influence of the uptake of iron on grain refinement in Mg-Zr alloys has been confirmed. The gradual loss of grain refinement of Mg-Zr alloys partly arises from the consumption of Zr by the formation of Fe2Zr via the reaction between soluble Zr and Fe picked up from mild steel crucibles. (Settling of undissolved Zr particles also partly attributes to the gradual loss of grain refinement.) The morphological evolution of Zr...
Carbon inoculation has no effect on magnesium alloys that do not contain aluminium. The hypothesis proposed in a recent article [Scripta Materialia 49 (2003) 1129] that segregation of carbon plays a major role in the grain refinement of magnesium alloys by carbon inoculation is inconsistent with many of the observed facts. The Al4C3 or Al–C–O hypothesis, which is supported by experimental observations, is still the most reasonable mechanism proposed to date for the grain refinement of magnesium alloys by carbon inoculation.
The grain growth kinetics of nanocrystalline copper thin film samples was investigated. The grain size of nanocrystalline copper samples was determined from the broadening of X-ray spectra. It was found that the grain size increased linearly with isothermal annealing time within the first 10 minutes, beyond which power-law growth kinetics is applied. The activation energy for grain growth was determined by constructing an Arrhenius plot, which shows an activation energy of about 21 – 30 kJ/mol. The low activation energy is attributed to the second phase particle drag and the porosity drag, which act as the pinning force for grain growth in nanocrystalline copper.
The effect of iron on the grain refinement of high-purity Mg–3%Al and Mg–9%Al alloys has been investigated using anhydrous FeCl3 as an iron additive at 750 °C in carbon-free aluminium titanite crucibles. It was shown that grain refinement was readily achievable for both alloys. Fe- and Al-rich intermetallic particles were observed in many magnesium grains.
High purity Mg–Al type alloys have a naturally fine grain size compared to commercial purity alloys with the same basic composition. This is referred to as native grain refinement. It is shown that native grain refinement occurs only in magnesium alloys containing aluminium. The mechanism is attributed to the Al4C3 particles existing in these alloys.
A trace of beryllium can lead to dramatic grain coarsening in Mg–Al alloys at normal cooling rates. It is, however, unclear whether this effect applies to aluminium-free magnesium alloys or not. This work shows that a trace of beryllium also causes considerable grain coarsening in Mg–Zn, Mg–Ca, Mg–Ce and Mg–Nd alloys and hinders grain refinement of magnesium alloys by zirconium as well.
A study on the kinetics of grain growth of an Mg-12.1 wt%Cu alloy produced by mechanical alloying was carried out. The grain sizes of as-mechanically alloyed powder and of cold-compacted annealed powder were determined from the broadening of X-ray lines. The grain size deceases initially due to recrystallization and then increases gradually, and finally ceases to reach an ultimate value regardless of annealing time. From isothermal anneals, the grain growth kinetics can be described by Dn − D0n = ct, where n (n = 5 to 8) is a constant essentially dependent on the annealing temperature. The activation energy for grain growth Q has been determined to be 118 kJ/mol, which is longer by 26 kJ/mol than that for pure magnesium. Second-phase intermetallic particle Mg2Cu produced during ball-milling influences not only on activation energy but ...
The mechanisms for grain refinement of magnesium alloys by superheating have remained ambiguous since 1931. A model has been proposed on the basis of the recent understanding of the grain refinement of both high purity and commercial purity Mg–Al alloys. The model explains most of the experimental observations about superheating. Analysis of the grain size data obtained from different Mg–Al alloys as a function of the growth restriction factor with and without superheating provides good support to the model.
Manganese is a grain refiner for high purity Mg–3%Al, Mg–6%Al, Mg–9%Al, and commercial AZ31 (Mg–3%Al–1%Zn) alloys when introduced in the form of an Al–60%Mn master alloy splatter but the use of pure Mn flakes and ALTAB™ Mn75 tablets shows no grain refinement. Long time holding of the melt at 730 °C leads to an increase in grain size. The mechanism is attributed to the presence of an ε-AlMn phase (hexagonal close-packed) in the master alloy splatter.
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