Linking Surface Precipitation in Fe-Au Alloys to Its Self-healing Potential During Creep Loading
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While the field of (ferrous) metallurgy has a history of many centuries, the very nature of the materials and their many transformation modes leads to a continuous stream of discoveries even until today. In recent years, two new (intentional) discoveries were made due to dedicated research: (i) transient solid-state precipitation of new phases on the surface of binary and ternary alloys[1] and (ii) the self-healing of creep damage in steels and related alloys[2–4] due to the autonomous filling of creep loading induced cavities leading to a significant life extension. Until now, the two phenomena were studied in isolation. The current work aims to bridge the two phenomena by linking the surface precipitation of a Fe-Au alloy to its capability to heal creep damage.
W.W. SUN and C.R. HUTCHINSON are with the Department of Materials Science and Engineering, Monash University, Clayton 3800, VIC, Australia. Contact e-mail: [email protected] H. FANG is with the Fundamental Aspects of Materials and Energy group, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands and also with the Novel Aerospace Materials group, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS, Delft, The Netherlands. N.H. VAN DIJK is with the Fundamental Aspects of Materials and Energy group, Faculty of Applied Sciences, Delft University of Technology. S. VAN DER ZWAAG is with the Novel Aerospace Materials group, Faculty of Aerospace Engineering, Delft University of Technology. Manuscript submitted December 7, 2016. Article published online February 27, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
Traditional bulk (internal) precipitation reactions that lead to a significant increase in strength are well known in both non-ferrous and ferrous alloys and are used to design new alloys with improved mechanical properties. However, only recently it has been discovered that there are alloys where the annealing conditions may be tuned to generate precipitates with a high areal surface density (1010-1011 m2) on the external surface of the sample (surface precipitation) at elevated temperatures.[1] The observed surface precipitation shows a surprising periodicity that is not directly related to the microstructure. Surface precipitation (which may be enhanced by concurrent surface segregation) occurs in competition with traditional bulk precipitation and the subsurface volume acts as a huge reservoir of solute atoms for the surface precipitation reaction. For a system to be capable of surface precipitation, there must be a lower energy barrier for precipitation on the surface than for internal (bulk) precipitation. Precipitates exhibiting a significant lattice misfit with the bulk matrix may be good candidates. These studies on precipitate formation on free surfaces by segregation from the bulk show some similarities (but also significant differences) with island formation and self-organization of atoms individually deposited on free surfaces during processes s
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