Autonomous Repair Mechanism of Creep Damage in Fe-Au and Fe-Au-B-N Alloys
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NTRODUCTION
CORROSION-RESISTANT steels serve as key alloys in multiple high-temperature applications found in for example power plant components. When these steel components are operated at high temperatures for long times, they can show premature creep failure, arising from the formation, growth, and coalescence of creep cavities located at grain boundaries into macroscopic cracks.[1,2] Extensive research has focussed on identifying the metallurgical processes that help preventing the formation of this pore-related high-temperature creep damage.[3–5] A viable alternative to these established approaches to avoid premature creep damage can be achieved by incorporation self-healing capabilities in these steels. As recently demonstrated, self-healing of damage can significantly enhance the component lifetime for a wide range of materials, including metals.[6–9] For austenitic stainless steels, Shinya and coworkers have recently S. ZHANG, Ph.D. Student, E. BRU¨CK, Professor, and N.H. VAN DIJK, Researcher, are with the Fundamental Aspects of Materials and Energy, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands. Contact e-mail: [email protected] C. KWAKERNAAK, Research Assistant, and W.G. SLOOF, Researcher, are with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. F.D. TICHELAAR, Researcher, is with the Kavli Institute of Nanoscience, National Centre for HREM, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands. M. KUZMINA, Ph.D. Student, M. HERBIG, Researcher, and D. RAABE, Professor, are with the Max-Planck-Institut fu¨r Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Du¨sseldorf, Germany. S. VAN DER ZWAAG, Professor, is with the Novel Aerospace Materials group, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands. Manuscript submitted April 10, 2015. Article published online October 6, 2015 5656—VOLUME 46A, DECEMBER 2015
proposed that creep damage can be healed when the alloys were compositionally modified with additions of Cu and additions of both B and N.[10–13] The resulting enhanced creep resistance was proposed to be attributed to dynamic precipitation of either Cu or BN at the crack surface. In our previous work, copper precipitation was indeed observed at open-volume defects in solutionized Fe-Cu and Fe-Cu-B-N model alloys.[14–16] However, Cu precipitation at deformation-induced defects was found to be relatively weak as spherical Cu precipitates are simultaneously formed inside the matrix. In order to preserve the driving force for segregation from the matrix, but at the same time to suppress undesired precipitation inside the matrix, we substituted Au for Cu as the healing element. Earlier studies[17,18] confirmed that Au atoms in Fe-Au alloys show a pronounced preference to segregate to dislocations. In our earlier investigations on thermal aging of pre-deformed high-purity Fe-Au and Fe-Au-
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