Reduction of porosity in oxide dispersion-strengthened alloys produced by powder metallurgy
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INTRODUCTION
OXIDE dispersion–strengthened (ODS) alloys produced by powder metallurgy techniques such as mechanical alloying (MA) are important materials for high-temperature applications.[1,2] The two main groups of commercial ODS materials are the Fe-based and Ni-based alloys. In these alloys, an oxide dispersion generated by the addition of Yttria during milling contributes to high-temperature creep and stress-rupture performance by the inhibition of dislocation motion.[3] High-temperature strength can be further improved by the formation of a coarse-grained microstructure with a high grain-aspect ratio (GAR) in the loading direction.[3] This high-aspect-ratio grain structure is produced by a process usually referred to as secondary recrystallization. Unfortunately, porosity may also be developed during such high-temperature recrystallization annealing. Such porosity can degrade the performance of these materials by reducing the load-bearing capacity and aiding the linkup of creep cracks to cause premature failure.[4,5] Substantial investigations have been carried out to understand the origins of porosity.[5–10] A recent review on this subject was presented by Jones and Jaeger.[6] Porosity in Fe-based ODS alloys is generally believed to be thermally induced.[6] The principal results from the literature can be summarized as follows. (1) The occurrence of porosity in the alloys during heat treatment is not an intrinsic phenomenon of the alloy systems and is only observed in alloys produced by powder metallurgy methods.[7] (2) There is no evidence of gross porosity developing in alloys in the primary recrystallized, fine-grained condition. Secondary recrystallization had been regarded as responsible for the development of porosity, but was later ruled out.[6,7,11] (3) Porosity generally arises when the alloys are annealed at temperatures above 1150 ⬚C.[6,7] (4) Porosity is strongly dependent on sample size. Thin samples exhibit substantially less porosity than thicker Y.L. CHEN, Research Associate, and A.R. JONES, Senior Lecturer, are with the Division of Materials Science and Engineering, Department of Engineering, The University of Liverpool, Liverpool L69 3GH, England. Manuscript submitted October 18, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
samples. Moreover, after a relatively short high-temperature exposure, the porosity tends to concentrate in the middle third of annealed samples.[6,10] (5) A higher nitrogen and/or oxygen content appears to favor pore formation.[7] (6) The volume fraction of porosity in each Fe-based ODS alloy is different, with a noticeable trend toward decreasing level in the following sequence: MA956, ODM331, ODM751, and PM2000.[6,10] These results represent a summary of the mainly experimental phenomena. However, the principal reason for porosity formation has been unclear until now, there being no widely accepted single explanation. For example, based mainly on results from some Fe-based ODS model alloys, Heine et al.[9] suggested that the partial pressure of hydrogen liberated
