In situ electrochemical nanoindentation of FeAl (100) single crystal: Hydrogen effect on dislocation nucleation
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The hydrogen effect on dislocation nucleation in FeAl single crystal with (100) surface orientation has been examined with the aid of a specifically designed nanoindentation setup for in situ electrochemical experiments. The effect of the electrochemical potential on the indent load–displacement curve, especially the unstable elastic-plastic transition (pop-in), was studied in detail. The observations showed a reduction in the pop-in load for both samples due to in situ hydrogen charging, which is reproducibly observed within sequential hydrogen charging and discharging. Clear evidence is provided that hydrogen atoms facilitate homogeneous dislocation nucleation.
I. INTRODUCTION
Iron aluminides are of considerable interest for low- to moderate-temperature structural applications in which low cost, low density, and good corrosion or oxidation resistance are required. Such applications include heat exchangers, piping for chemical process industries, furnace heating elements, and automotive exhaust systems and valves.1 In spite of all these inherent advantages, the ordered FeAl alloys exhibit poor ductility because of their susceptibility to hydrogen embrittlement (HE).1–5 Hydrogen embrittlement is known to be a complex phenomenon in metals and alloys.6–8 One of the predominant factors causing hydrogen embrittlement is the interaction of hydrogen with dislocations, which has been studied by many authors.9–13 The underlying mechanisms suggested for hydrogen embrittlement in iron aluminides are mainly based on two experimental approaches: (i) Conventional mechanical testing in hydrogen or water vapor containing atmospheres where environmental embrittlement in the aluminides results in brittle cleavage fracture. This suggests a reduction of cohesive strength across cleavage planes (i.e., {100} planes) which was interpreted as hydrogen-enhanced decohesion (HEDE). (ii) In situ transmission electron microscopy (TEM) studies of microcrack propagation in an environment cell where, upon introduction of hydrogen gas into the cell, the dislocation velocity and the crack growth rate increased significantly.14 The influence of hydrogen on dislocation activities and enhanced crack-tip plasticity was interpreted as hydrogen-enhanced local plasticity (HELP). a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0084 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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Although the qualitative effects of hydrogen on dislocations were observed from conventional mechanical experiments, the results are difficult to interpret quantitatively. This is mainly because of difficulties in exploring the details of plastic deformation locally. On the other hand, in situ environmental TEM experiments on thin films are also difficult to interpret, since the electron beam produces high hydrogen fugacities and local heating.15 Another limiting factor is that the hydrogen fugacity can only be varied over a small range.16 The instrumented nanoindentation t
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