Creep Behavior and Damage of Ni-Base Superalloys PM 1000 and PM 3030
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INTRODUCTION
CREEP is one of the most critical damage processes for components in high-temperature applications such as blades for stationary gas or aircraft turbines. Single crystalline, c¢-strengthened nickel-base alloys are commonly used in the first few turbine stages in order to limit diffusion and grain boundary sliding as well as void formation, but some polycrystalline alloys are also still in service.[1,2] A serious drawback of the strengthening via c¢ precipitates is their thermal instability,[3,4] which makes them more suitable for the intermediate temperature range rather than for temperatures significantly beyond 1000 C. At these high temperatures, oxide dispersion strengthening (ODS) is known to provide the best resistance to creep due to the excellent thermal stability of oxide dispersoids.[5–8] Because single crystalline ODS alloys are nearly impossible to obtain via secondary recrystallization due to the difficulty to achieve grain growth and simultaneously maintain a homogeneous and fine distribution of oxide particles, coarse and elongated grain structures are commonly used instead.[1,9] High-temperature turbine applications generally involve start, acceleration, deceleration, and stop phases. Consequently, materials are not only subjected to constant high temperature and creep under stationary conditions, but to a rather large temperature range as well as varying quasi-static, fatigue, and thermomechanical loading. Therefore, a combination of solid solution, c¢ precipitation, and ODS has been realized in some ODS alloys including PM 3030. In order to M. NGANBE, Assistant Professor, is with the Department of Mechanical Engineering, University of Ottawa, Ottawa, ON, Canada K1N 6N5. Contact e-mail: [email protected] M. HEILMAIER, Professor, is with Otto-von-Guericke Universita¨t Magdeburg, Institut fu¨r Werkstoff- und Fu¨getechnik, D-39016 Magdeburg, Germany. Manuscript submitted November 12, 2008. Article published online September 30, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
explore potential improvements and cost savings, it is important to rationalize the creep behavior of such alloys from the viewpoint of the contribution of these different strengthening contributions to their deformation resistance for a variety of temperatures and loads. Furthermore, the aspect of grain morphology and size has to be tackled for applications with varying temperature. It is well known that fine grain materials benefit from additional grain boundary strengthening and therefore show superior strength at lower temperatures. However, high temperatures can cause strong creep deformation and damage in fine grain materials due to grain boundary sliding as well as void formation and growth.[10,11] Therefore, it is crucial to understand the notion of the equicohesion temperature at which the grain morphology has no impact on strength,[12] and to shed light on the dominating deformation processes above and below this equicohesion point in order to achieve improved performance through an optimal combination of strengthe
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