Microstructure evolution in L1 2 hardened Co-base superalloys during creep
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Florian Pyczak,a) Jonathan Paul, Michael Oehring, and Uwe Lorenz Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Geesthacht 21502, Germany
Zekun Yao School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China (Received 28 April 2017; accepted 21 August 2017)
The plastic deformation mechanisms and the microstructure development during creep deformation of L12-hardened Co-base superalloys show a number of unique features. The preferred orientation of rafting is determined by their positive lattice mismatch. In addition, the regular interfacial dislocation networks often found in rafted specimens of other types of superalloys do not form. While the ordered c9-L12 precipitates are supposed to harden the material, they are actually found to be frequently cut by partial dislocations generating stacking faults. In this work, specimens from creep tests interrupted at different strains were investigated using transmission and scanning electron microscopy. By this, it is possible to find out which of these processes take place in which stage of creep deformation. For a better understanding of creep deformation, the balance between c9 cutting and dislocation activity within the matrix channels is of special interest.
I. INTRODUCTION
In 2006, a novel class of superalloys was discovered by Sato et al.1 by their finding of a c9-Co3(Al,W) phase with an L12 lattice structure in the ternary Co–Al–W system. For compositions exhibiting a c-Co 1 c9-Co3(Al,W) microstructure, the c9-precipitates exhibit a cubic shape and are coherently embedded in a fcc c-Co matrix. The precipitate sizes were in the range of some hundred nanometers with matrix channels of some ten nanometers in between and a c9 volume fraction of up to 70%.2,3 Depending on the W content of the alloy, the solvus temperature of the c9 phase can be higher than 1000 °C. Other alloying elements, as, for example, Ta and Ti, were also found to increase the c9 solvus temperature.2 Taking all these observations into account and keeping in mind that Ni-base superalloys, which exhibit similar microstructures and properties, are very successful hightemperature materials, the Co–Al–W system has a good potential to be an attractive material for high-temperature applications.4–8 Despite the high similarity of the c/c9 microstructures between these novel Co-base and the well-known Ni-base superalloys, the creep and high-temperature plastic deformation mechanisms seem to differ between Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: fl[email protected] DOI: 10.1557/jmr.2017.362
the two alloy classes. In the high-temperature regime at around 850–1000 °C, creep deformation in Ni-base superalloys is dominated by the dislocation movement in the c matrix channels.9,10 Depending on the lattice mismatch between the c matrix and c9 phase and the externally applied stress, one orientation of matrix channels with respect to the externally applied stress deforms first, while plastic deformation in
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