High temperature creep strength in a nanodispersion-strengthened ferritic alloy prepared by heavy plastic deformation
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High temperature creep strength in a nanodispersion-strengthened ferritic alloy prepared by heavy plastic deformation David G. Morris and Maria Antonia Muñoz-Morris Department of Physical Metallurgy, CENIM, CSIC, Avenida Gregorio del Amo 8, 28040 Madrid, Spain ABSTRACT Processes of severe plastic deformation have been investigated for a wide range of ductile alloys over the past decade, generally with an objective of refining the microstructural scale, for example the grain size, but have hardly been considered for intermetallics. This presentation discusses processing of a boride-containing Fe3Al alloy using a multidirectional, high-strain and high-temperature forging technique. Iron aluminides with relatively low Al contents can be regarded as Al-rich ferritic steels with outstanding oxidation-corrosion properties. However, as for many ferritic steels, they show poor creep resistance at temperatures above about 600ºC. The deformation processing leads to a material with large grain size and refined dispersion of thermally-stable boride particles. The particles produce a large increase in creep strength under conditions of moderate stresses and low strain rates at temperatures near 700ºC. This high-strain forging technique can be seen as an intermediate processing method between conventional wrought metallurgy and mechanical-alloying powder metallurgy, whereby an initially coarse and inhomogeneous dispersion of second phase is refined and made more homogeneous, and can be considered as a useful processing technique for a wide range of particle-containing materials. INTRODUCTION There is considerable interest in the development of iron based alloys for engineering applications where strength at high temperatures and limited creep deformation over long periods is required, for example in power generation equipment using fossil fuel and nuclear energy sources. Two routes of alloy development have received much attention: one involves increasing Cr content and adding elements such as Mo and W for the martensitic/ferritic super heat-resistant steels for super-critical and ultra-super-critical power plants where additional strengthening can come from precipitation of Laves phases as well as carbides [1-5]; the other involves the use of mechanical alloying with additions such as Cr, W, Ti and Y2O3 [6-10]. Another topic that has received considerable attention has been the development of alloy composition and processing methods for high strength, high ductility and creep resistant iron aluminides, often for application in fossil fuel based power generation. For example, mechanical alloying has been used to introduce fine yttria particles, improving ductility and creep resistance [11-14]. Processing by severe plastic deformation has received much attention for refinement of microstructure of a wide range of simple metals and alloys [15,16], and has recently been used to modify iron aluminides, both to refine grain size and to obtain a dispersion of fine strengthening particles [17-19]. Refinement of grain size to near micron
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