Role of oxidation on LME of T91 steel studied by small punch test
Introduction
Heavy liquid metals such as the lead bismuth eutectic (LBE) alloy are used or are envisioned to be used for heat transfer applications. Steels such as the austenitic AISI 316L or the martensitic T91 type, having good chemical compatibility with LBE, are the structural materials of choice for this medium. Liquid metal embrittlement (LME) is the transition from a ductile fracture mode to a brittle fracture one induced by a liquid metal under plastic deformation. LME of steels has therefore important technological implications. At the same time, this phenomenon is still poorly understood and the field did not grow in importance as much as stress-corrosion cracking. Most notable is the fact that very few detailed studies have been undertaken which make use of modern surface analysis in spite of the fact that it is well suited for such a phenomenon. This is quite surprising because LME is a surface induced effect controlled by the surface chemistry. This paper is a step forward to investigate the use of surface analysis techniques that could be useful in order to reach a better understanding of this phenomenon.
The experimental requirements to observe LME are twofold: wetting by the liquid metal, which is presently understood as intimate contact i.e. oxygen free interface, and the occurrence of plastic deformation. It has been shown that the T91 martensitic steel is subjected to LME by liquid PbBi alloys when the protective native oxide is removed in a tensile test [1]. In this work, mechanical testing was performed using a cylindrical tensile geometry. Samples were prepared with an oxide free interface between a deposit of PbBi and the steel. It can be obtained by ion beam etching of the native oxide followed by a PVD deposit of a PbBi coating under ultra high vacuum (UHV). The advantage of this technique is that the solid coating prevents re-oxidation during handling between the UHV chamber and the mechanical testing setup. Here, we present an improvement of the technique which uses the small punch test (SPT) coupled with Auger electron spectroscopy (AES) or X-ray photo-spectroscopy (XPS). SPT is a simple mechanical test that allows the investigation of crack initiation in a bi-axial stress state. One of the advantages of this test is that crack initiation proceeds near the surface making it quite sensible to any surface effect. Therefore, a change induced by the surface chemistry in the crack initiation process should be detected by this technique. Another advantage is that the required sample size is small. This testing geometry has been adapted recently to work with liquid metals by using flat discs of 8.9 mm diameter and of 500 μm thickness [2]. It opens up new possibilities for detailed investigation of the LME crack initiation mechanism because this is a perfectly suited size for applying surface analysis techniques such as Auger/XPS spectroscopy. Indeed, it has become customary for the last 30 years in corrosion science to study the oxide structure and evolution by Auger/XPS techniques [3], [4]. Working under ultra high vacuum (UHV) also allows preparing surfaces with specific oxidation states. One can then test its impact on the crack initiation after a deposit of the liquid metal is performed in UHV. The crack initiation process under any liquid metal can then be studied for example as a function of the oxide layer thickness or composition.
In this work, the influence of the oxide on the protectiveness of the oxide layer has been investigated in the first oxidation stages using the SPT techniques described above. The importance of the surface state on the brittle or ductile crack initiation will be illustrated with the T91 martensitic steel in contact with PbBi. The goal is to have a better understanding of what constitutes a protective oxide ‘vis-à-vis’ LME.
Section snippets
Experimentals
Flat discs of 8.9 mm diameter and 500 μm thickness of T91 martensitic steel were machined from the as received alloy. The composition of the alloy is given in Table 1. The alloy is in the standard metallurgical state (austenitized at 1050 °C for 7 h, water quenched followed by a tempering heat treatment at 750 °C). The surface of the disc to be prepared under UHV was diamond polished down to 1 μm. The samples were taken to an Auger/XPS setup equipped with an ion gun and an Auger/XPS analysis setup.
Analysis of the re-oxidized surface
The four conditions investigated in this work are given in Table 2.
X-ray photoelectron spectroscopy analysis of the re-oxidized surfaces were achieved with a MAC II CAMECA spectrometer (resolution ΔE = 1.0 eV) operating in the constant analyser energy mode with a 15 eV pass energy setting the energy resolution at 1.0 eV. The Al Kα anode of the dual Al–Mg unmonochromated X-ray source (260 W, 57° mean incident angle) was employed in this study. Calibration of the analyser energy scale is performed
SPT results
The load–displacement curves of the SPT performed at 250 °C and 300 °C of the different oxidation conditions are given in Fig. 4. Taking as reference the SPT curve of the air oxidized specimen (DAO2), the load-displacement curves of the other specimen (D4O2, D0.5O2, DNO2) are systematically below. The behaviors of the DNO2 specimens are very remarkable in their lower values of the yield point values. This is indicative of a strong LME effect occurring at low strain. Interestingly, there is also a
SEM analysis
Due to the fact that the LBE layer is not very thick (of the order of a few micrometers at most), the SEM analysis were first performed on most samples with the remaining coating of PbBi and then after complete removal. This was accomplished by the selective dissolution of PbBi coating in a chemical composition composed of 1/3 ethanol, 1/3 acetic acid, 1/3 hydroxed water. This chemical solution is known not to affect fracture surfaces of steels.
Fig. 5 shows a SEM macro top view of the DNO2
Discussion: on the role of the oxide nature on LME cracking
One interesting fact is that the SPT experiment has been able to reproduce the effect of the liquid PbBi alloy in intimate contact with the T91 steel as shown previously with the tensile test [1]: combining intimate contact and plastic deformation, a brittle fracture can occur in the ductile T91 steel. The bi-axial stress state existing at the surface seems more damaging than a pure tensile component because the effect is very strong from the yield point. On the other hand, the native oxide is
Acknowledgements
The authors would like to acknowledge the financial support from the FP6 European program IP-EUROTRANS (contract N° FI6W-CT-2004-516520). Financial support from the French GDR GEDEPEON is also gratefully acknowledged.
References (8)
- et al.
Scr. Mater.
(2005) - et al.
Nucl. Eng. Des.
(2007) - et al.
Surf. Sci.
(1979) - et al.
Surf. Interf. Anal.
(1990)
Cited by (29)
Mechanical behavior in liquid lead of Al<inf>2</inf>O<inf>3</inf> coated 15-15Ti steel and an Alumina-Forming Austenitic steel designed to mitigate their corrosion
2022, Engineering Failure AnalysisCitation Excerpt :Note that there is no particular material and metal liquid properties leading to a sensitivity to LME i.e. any metal or ductile metallic alloy may be sensitive to LME by a specific liquid metal, depending on intrinsic conditions (the microstructure of the solid metal (heat treatment, grain size, cold working, precipitations …), the liquid metal (purity, physico-chemistry), the liquid metal/solid alloy interface (roughness, presence of native oxide at the surface of the solid alloy …)) or experimental conditions (the temperature, the mechanical loading (monotonic, cyclic …), the strain rate). Different studies have shown the influence of test temperature [9–14], strain rate [10,15–16], oxygen content in liquid metal [15–18], surface roughness [19], nature of oxide layer [20], microstructure state of the steel [21–23] on the LME sensitivity of steels in contact with liquid Pb or liquid Pb-Bi. The ferritic/martensitic steels, especially the T91 steel, have been for a long time considered as promising candidates for structural materials used in contact with liquid lead or liquid LBE.
Environmental degradation of structural materials in liquid lead- and lead-bismuth eutectic-cooled reactors
2022, Progress in Materials ScienceCitation Excerpt :This clarified the intrinsic sensitivity of T91 F/M steels to LME by showing that the only criterion that explains the difference in its sensitivity is linked with the quality of direct steel/HLM contact during testing. Auger et al. [465] claimed that the chemical composition of the oxide scales could also play an important role in crack initiation. Different oxides have either different mechanical properties or different HLM wettabilities, leading to different sensitivities to crack initiation [121,251,465].
Pathway to understand liquid metal embrittlement (LME) in Fe-Zn couple: From fundamentals toward application
2021, Progress in Materials ScienceLiquid metal embrittlement sensitivity of the T91 steel in lead, in bismuth and in lead-bismuth eutectic
2020, Journal of Nuclear MaterialsCorrosion of structural materials by liquid metals used in fusion, fission, and spallation
2020, Nuclear Corrosion: Research, Progress and ChallengesToF-SIMS investigation of absorption of lead and bismuth in T91 steel deformed in liquid lead bismuth eutectic
2019, Applied Surface Science