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Major Melt—Crucible Systems
Published in Nagaiyar Krishnamurthy, Metal–Crucible Interactions, 2023
Although titanium is cast under vacuum and in a mould made of an appropriate non-reactive material that current technology can provide, it is not yet possible to avoid contamination of the casting surface by oxygen, carbonaceous material or first-coat oxides from the mould surface picked up by the molten metal at the time of pouring. The resulting impurity-laden layer, known as alphacase, is very hard and can range in thickness from zero upto 0.6 mm. The alpha case (α-case) is the subsurface layer that forms on a titanium casting in the area that is in contact with the mould. It is a thin, impurity-laden layer that develops due to mass transfer of elements such as oxygen, carbon, nitrogen, aluminium or silicon from the ceramic mould into the alloy as it solidifies (Saha and Jacob 1986). There is always a concentration gradient of elements across the thickness of the α-case, which has a break at the point where this layer ends. The α-case significantly decreases ductility, fracture toughness and fatigue life of the casting. There are various methods for removing it, including grinding, laser irradiation, water jetting, cathodic de-oxygenation and chemical milling in mixed acids. Yet, the better option would be to prevent or limit it rather than treat it. The best case is no case (layer). A mould material resulting in a thinner α-case is better than one that gives rise to a thicker α-case.
Towards a better preservation of current and future outdoor architectural heritage; maximum suppression of discolouration in anodized and non-anodized titanium sheets
Published in Environmental Technology Reviews, 2020
Maryam Mokhtarifar, MariaPia Pedeferri, Maria Vittoria Diamanti
Alpha-case is a heat treatment quality issue which can occur during the manufacturing of Ti sheets, particularly in ‘Kroll process’ [43]. Along with the formation of a thin layer, an inward diffusion of oxygen into the bulk metal will contribute to the total oxidation of titanium [44]. While the first layer enhances the corrosion immunity of metal, the latter layer, which is indeed formed at elevated temperature (mainly above 480°C due to interaction of liquid titanium with alpha-stabilizers such as oxygen, nitrogen and hydrogen and downgrades its protective nature and leads to exhibiting different structural properties including embrittlement, increased hardness, and reduction in fatigue life [43]. Such layer is highly probable at initiating and growth of the microcracks and voids which can cause mechanical damage. Overall, thermal, chemical diffusion and mechanical processes are involved in characterizing the response of such layer induction and formation (Figure 3). The scale of the alpha-case boundary region, titanium grain structure and overall structure are depicted by Figure 4 [45].
CIP-FAST: assessing the production of complex geometry titanium components from powders by combining Cold Isostatic Pressing (CIP) and Field Assisted Sintering Technology (FAST)
Published in Powder Metallurgy, 2023
S. J. Graham, Y. Azakli, J. Withey, M. Jackson
Figure 8(e) shows how the amount of β and secondary α is reduced towards the edge of the part, which is consistent with α stabilisation from oxygen diffusion. Some microcracks are also visible at the edge, possibly formed during metallographic preparation, indicating that this region is relatively brittle. This is similar to the effect of alpha case, which is often observed in titanium alloy components after processing and is often removed by chemical milling before using fatigue-sensitive applications [28]. Unfortunately, attempts at X-EDS analysis to measure oxygen content across this region were unsuccessful due to difficulties in analysing light elements with this technique.