The microstructure and recrystallization of flow-formed oxide-dispersion-strengthened ferritic alloy: Part II. Recrystal
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I. INTRODUCTION
THE potential for an attractive combination of hightemperature creep and gas oxidation resistance makes Febased oxide dispersion strengthened (ODS) alloys produced by mechanical alloying one of the few metallic systems with potential for application in service at temperatures above 1000 ⬚C.[1] Such alloys may be suitable for components in gas turbine chambers and diesel engines and for components in advanced energy conversion systems. One application of current interest is the potential use of Fe-based ODS alloy tubing in high-temperature heat exchangers in advanced indirect combined cycle gas turbine systems, where planned metal operating temperatures may extend up to 1150 ⬚C.[2] Current commercial Fe-based ODS alloy tubes exhibit creep performance under internally pressurized loading, which falls substantially short of anticipated heat exchanger creep duty requirements. Principally, this is due to the axially aligned, high aspect ratio, coarse-grained microstructure, which forms in these tubes, which confers excellent axial but poor hoop creep performance, the latter being the direction of maximum principal stress under internal pressurization. For improved creep resistance in these alloys, previous studies have shown the importance of developing high grain aspect ratio (GAR) structures aligned with the direction of principal creep stress.[3,4] Ideally, therefore, to develop the maximum creep strength in ODS alloy tubing subject to internal pressurization, coarse, high GAR microstructures more closely aligned with the hoop direction need to be developed. However, current ODS alloy tube manufacturing routes are dominated by processes involving substantial elements of axial deformation. This results in axial alignment of the stringers of coarse Al2O3 and other primary particles (Ti(CN)), which Y.L. CHEN, formerly Research Associate, Department of Engineering, University of Liverpool, Liverpool, L69 3GH England, is Experimental Officer, MicroStructural Studies Unit (MSSU), School of Engineering, The University of Surrey, Guildford, GU 2 7XH England. Contact e-mail: [email protected] A.R. JONES, Senior Lecturer, and R.C. POND, Professor, are with the Department of Engineering, The University of Liverpool. U. MILLER, Head of Business Segment, is with Compound Materials, Plansee GmbH, D-86983 Lechbruck, Germany. Manuscript submitted September 18, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
tend to dominate grain boundary alignment (and aspect ratio) within the coarse grain structures, which develop during the final “secondary recrystallization” of these materials. It has proven very difficult to change grain morphology in these microstructures. Even zone annealing along a direction normal to the extrusion direction cannot change the principal direction of grain growth away from the working direction.[5] Encouraged by some preliminary work by Jaeger on Febased ODS alloys,[6] efforts have been made within an ongoing European BRITE-EuRAM project to produce PM2000 Fe-based ODS alloy tubes with a hi
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