Origin of porosity in oxide-dispersion-strengthened alloys produced by mechanical alloying
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INTRODUCTION
OXIDE-DISPERSION-STRENGTHENED (ODS) alloys produced by mechanical alloying (MA) are materials with important engineering potential because of the combination of excellent high-temperature creep strength and oxidation resistance. However, porosity is often found in ODS alloys after secondary recrystallization annealing. This is likely to be detrimental to their mechanical strength. For example, porosity may degrade the performance of these materials by reducing load bearing capacity and aiding the link-up of creep cracks to cause premature failure. Figure 1 is an optical micrograph from a crept ferritic ODS alloy sample showing that the crack is indeed formed by porosity link-up. Earlier studies have been undertaken to understand the occurrence of porosity in ODS alloys.[1–7] In particular, Chen and Jones have demonstrated that the rapid kinetics of secondary recrystallization may leave no time for gas trapped in the material to dissipate to free surfaces via rapid grain boundary diffusion and porosity is formed as a result. Based on this understanding, a double annealing procedure has been proposed to reduce porosity in ODS alloys.[1] Essentially, an additional anneal is applied at a temperature below that for secondary recrystallization, in order to facilitate gas diffusion through the grain boundary network in the finegrained condition. The double annealing method has been demonstrated to work well on a ferritic ODS alloy.[1] It has also been suggested that porosity observed in ODS alloys is no different from pores in other sintered products as far as its mechanism of formation is concerned.[1] In fact, pores are a generic feature of all sintered components and pressure sintering has been proved to be an effective way to reduce them. A density of more than 99 pct of the theoretical density can be obtained by hot isostatic pressing (“hipping”) even for advanced ceramics, which are difficult to process because of their high hardness and high melting temperature.[8,9] It has long been believed that full density could be obtained by pressure sintering of metallic products.[9] However, in contrast to this general understanding, it has 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. Contact e-mail: [email protected] U. MILLER, Project Manager, is with Plansee GmbH/Lechbruck, D-86983 Lechbruck, Germany. Manuscript submitted August 7, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
been observed that the volume fraction of porosity formed in ODS alloys can reach 3 to 7 pct after secondary recrystallization, even though most of these alloys are consolidated by either hipping or hot extrusion; the latter could also be classified as a pressure sintering process.[10] Furthermore, it was also found that even double annealing can only reduce porosity, but not eliminate it completely.[1] Hence, further work is needed to determine more about the factors lea
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