Dissolution of hard-alpha inclusions in liquid titanium alloys
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INTRODUCTION
THE principal quality problem in the use of titanium and its alloys has long been the persistent occurrence of an inclusion known as ‘‘hard alpha.’’ This feature, seen as a small volume in the alloy, appears to be alpha stabilized (independent of heat treatment used) and contains enough nitrogen to render the volume extremely hard and brittle. The typical inclusion does contain nitrogen-stabilized alpha phase, but it may also contain nitrogen-stabilized compounds such as TiN. A typical example is shown in Figure 1; the dimensions depend on the mechanical working sequence of the sample, but diameters as small as 1 mm have been shown to cause early fatigue initiation in component applications. Although very intensive programs have been carried out by the industry to reduce the incidence of this inclusion in finished parts, it remains a troublesome problem to both the alloy and CPTi users. There are many potential sources of nitrogen contamination in the production route, as collated by Shamblen,[1] but the ultimate nitrogen source is invariably air reaction with hot or liquid titanium. The most serious sources appear to be connected with air leakage into titanium reduction and melting equipment, and also nitrogen pickup by the molten magnesium used in the Kroll process. Secondary reactions, such as burning during scrap or sponge handling, burning of ingot surfaces during furnace stripping, and burning during welding or machining of ingots and electrodes, have also been identified as hard-alpha sources. The present detected incidence rate in an aeroengine alloy is less than one J.P. BELLOT, Assistant Professor, and D. ABLITZER, Professor, are with the Laboratoire de Science et Ge´nie des Mate´riaux Me´talliques, Ecole des Mines de Nancy, 54042 Nancy, Cedex, France. B. FOSTER, Engineer, and A. MITCHELL, Professor, are with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada. S. HANS, Engineer, formerly with the Department of Metals and Materials Engineering, University of British Columbia, is with the Aubert et Duval Company, Les Ancizes, France. E. HESS, Engineer, formerly with Laboratoire de Science et Ge´nie des Mate´riaux Me´talliques, Ecole des Mines de Nancy, is with the IRSID Company, Maizie`re-les-Metz, France. Manuscript submitted October 22, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
inclusion/500,000 kg of alloy; but even this low rate of incidence is still a cause for concern. Attempts to remove or modify the hard-alpha inclusion by diffusion homogenization have failed due to the long times required at temperatures where inert atmosphere containment would be necessary. Similarly, inclusion removal by nondestructive examination at the billet or product stage has proved very difficult because of the background noise generated by the alloy alpha/beta structures in relatively heavy sections. For these reasons, it is of interest to understand the possibilities of the removal of hard-alpha inclusions through dissolution in the liquid s
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