Strengthening at High Temperatures in an Iron-Aluminium Alloy by the Precipitation of Stable and Coherent Intermetallic
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0980-II01-04
Strengthening at High Temperatures in an Iron-Aluminium Alloy by the Precipitation of Stable and Coherent Intermetallic Particles David G. Morris, Maria A. Muñoz-Morris, and Luis M. Requejo Physical Metallurgy, CENIM, CSIC, Avenida Gregorio del Amo 8, Madrid, E-28040, Spain
ABSTRACT Despite decades of intensive research iron aluminides remain characterised by relatively poor ductility at room temperature and low strength at high temperatures, especially under slow strain rate or creep conditions. A variety of strengthening particles has been tested for improving high temperature strength, but each has serious limitations: typical carbide precipitates are unable to resist dissolution or coarsening at high temperatures; as-solidified iron aluminides with sufficient amounts of transition elements such as Nb or Mo show heavy solidification segregation and are embrittled by a network of Laves phase; mechanical milling with stable oxides appears an excessively expensive processing route. A new iron-aluminium alloy has been developed with Zr and Cr additions that forms fine coherent precipitates even after extended annealing at temperatures as high as 900ºC. These precipitates have a complex Fe3Zr structure and form in a cube-on-cube orientation relationship in the bcc matrix. The low solubility and diffusivity of the solute, as well as the low energy, near-coherent interface ensures excellent stability of these intermetallic precipitates. Interesting strengthening is possible for this material under the relevant high temperature creep conditions.
INTRODUCTION There has been a large amount of research effort devoted over decades to the improvement of high temperature and creep strength of iron aluminides [1-3]. While high strength at room temperature and up to about 500ºC is relatively easily achieved, it has proved much more difficult to retain good strength at the 700-900ºC temperature range of most relevance [1-4]. Solute additions, for example of Mo or Nb [3], can lead to some strength increase at the high temperatures, but fine precipitate or dispersoid particles appear to be the best strengtheners [4-17]. All alloys containing these strengthening particles suffer from some problems, however, such as: rapid particle coarsening above about 600ºC for carbide precipitates [3,5,6]; good structural stability but requiring complex processing by rapid solidification or mechanical alloying for boride and oxide dispersoids [7-9]; solidification segregation and tendency to form brittle networks in cast Laves-forming alloys [10-17]. A recent study [18] has shown that good high temperature strength could be retained in a cast iron aluminide alloy strengthened by Fe-Zr intermetallic precipitates. The present study examines in more detail these precipitate particles and compares their structure, composition and stability with strengthening particles reported in other, similar alloys.
EXPERIMENTAL DETAILS The alloy examined here had a nominal composition of Fe-20%Al-5%Cr-0.5%Zr (atomic percent throughout this paper
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