Hydrogen trapped at intermetallic particles in aluminum alloy 6061-T6 exposed to high-pressure hydrogen gas and the reason for high resistance against hydrogen embrittlement

https://doi.org/10.1016/j.ijhydene.2017.08.035Get rights and content

Highlights

  • Hydrogen trapping in an aluminum alloy was investigated.

  • Hydrogen charging was performed with high-pressure hydrogen gas.

  • Secondary ion mass spectroscopy was employed for local hydrogen detection.

  • We succeeded in visualizing hydrogen trapped by intermetallic particles.

  • The hydrogen trapped by the intermetallic particles had high desorption energy.

Abstract

Hydrogen (1H) trapped at intermetallic particles (IPs) in an aluminum alloy, 6061-T6, was visualized with secondary ion mass spectrometry (SIMS) by precisely excluding the false signal which is caused by background hydrogen (HBG). The interference of the HBG was avoided by a unique continuous pre-sputtering (pre-digging) by a primary ion beam of SIMS into a sample in combination with silicon sputtering prior to the SIMS measurement of the sample and we succeeded in visualizing the exact signal of 1H trapped by IPs at subsurface layer of the sample charged in high-pressure hydrogen gas. The thermal desorption analysis clarified that the desorption energy (Ed) of the IPs was 200 kJ/mol or higher, which was extremely higher than Ed for lattice interstice, dislocations, and vacancies. High density hydrogen was concentratedly trapped at IPs in the subsurface layer in contact with the hydrogen gas. This nature causes an extremely low effective hydrogen diffusivity of 6061-T6 of the order of 10−14 m2/s even at 200 °C and may eventually give a high HE resistance to 6061-T6.

Introduction

The hydrogen atom easily invades into metals and interacts various lattice defects, such as dislocations, grain boundary, and secondary particles [1], [2], [3], [4], causing degradation of the mechanical properties, well-known as hydrogen embrittlement (HE) [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. According the NASA database [18], aluminum alloys, 6061-T6 and 7075-T73, are categorized into “negligibly embrittled”. However, the detailed mechanism of a high resistance to HE of such aluminum alloys has not been clarified from the viewpoint of hydrogen trapped at intermetallic particles (IPs). To elucidate a precise mechanism of HE, it is crucially important to exactly identify the hydrogen trapping sites in materials.

The secondary ion mass spectrometry (SIMS) is one of most promising techniques to visualize the local distribution of hydrogen compared to other hydrogen-analysis techniques [19], [20]. In the SIMS measurement, a sample is irradiated with a primary ion beam. Subsequently, ions emitted from the sample (secondary ion) are analyzed with a mass spectrometer to identify elemental species and to measure intensities of the secondary ions under an ultra-high vacuum of around 1 × 10−8 Pa. However, there are various challenges caused by several factors which interfere exact visualization of hydrogen (1H). The most annoying interfering factor is the background hydrogen (HBG) originated from moisture (H2O), hydrocarbon (CXHY), or organic materials (CXHYOZ) existing in the SIMS chamber or on sample surface. This is the reason why many researchers use the deuterium (2H) doping method [21] to avoid interference of HBG. In the semiconductor field, analytical techniques with the SIMS have been developed by Magee et al. [22] and the analytical conditions, such as an intensity of primary beam and a raster size (a size of scanning range of the primary ion beam), have been optimized [23], [24], [25], [26]. In contrast, no reliable method of avoiding HBG in 1H-analysis for metals has not been established. For the exact 1H-analysis, it is absolutely necessary to reduce or exclude HBG, or separate HBG from the hydrogen truly trapped in metals.

The influence of the HBG on detection of hydrogen trapped by secondary-phase particles in metals such as precipitates or inclusions must be carefully investigated in SIMS measurement, because it has been revealed that the simultaneous irradiation of both matrix and secondary-phase particle with a primary ion beam causes a difference in sputtering (digging) rate. The difference in sputtering rate results in different detected intensities of HBG between the matrix and secondary-phase particle, even if real intensities of HBG are the same between the two phases [23]. This problem on the false detection has not been solved in the existing literature [19], [27], [28].

In order to obtain the accurate SIMS signal of 1H in metals, Awane et al. developed a unique method, i.e., the highly sensitive analytical method for hydrogen in metallic materials with SIMS [29]. This method mainly consists of (I) putting a dummy pure silicon sample besides a measurement sample (Fig. 1), (II) the reduction in HBG mainly contained in the SIMS chamber wall by sputtering the dummy pure silicon sample prior to the measurement (silicon sputtering method) [29], [30], (III) detection of the exact signals of 1H by digging the sample into the region without interference of HBG (Fig. 2). The validity of this method has been verified by the hydrogen analyses achieved by the authors' method [31], [32], [33], [34].

This study presents 1H trapped at IPs in an aluminum alloy, 6061-T6, exposed to high-pressure hydrogen gas with our unique SIMS methodology. Also, the desorption energy of the IPs was identified by thermal desorption analysis (TDA) and the morphology of the IPs was observed by scanning electron microscopy in combination with energy dispersive X-ray spectroscopy (SEM-EDX).

Section snippets

Material, specimens and hydrogen-exposure condition

The material was an aluminum alloy, 6061-T6, composed of 0.66 Si, 0.03 Mn, 0.29 Cu, 0.32 Fe, 1.04 Mg, 0.18 Cr, 0.01 Zn, 0.02 Ti in mass %, and the balance Al. The sample for TDA had a diameter, 2r0 = 5 mm and thickness, z0 = 0.15 mm or 1 mm. The samples for SIMS and SEM-EDX had 2r0 = 7 mm and z0 = 7 mm. These samples were cut from the core of the same round bar with 2r0 = 20 mm. The samples for SIMS and SEM-EDX (2r0 = z0 = 7 mm) were cut in half (2r0 = 7 mm and z0 = 3.5 mm) and cross-sectional

Hydrogen spectra obtained by TDA

The TDA spectrum of the non-charged sample showed that the hydrogen desorption occurred only at temperatures of 480 °C or higher (Fig. 5A). In contrast, in the H-charged sample, a new peak associated with the peak-1 hydrogen was observed at a peak temperature (Tp) of 350 °C in addition to the hydrogen desorbed at temperatures of 480 °C or higher (Fig. 5A). The content of the peak-1 hydrogen was 40 mass ppm and it was confirmed that hydrogen uptake in the aluminum alloy actually occurred under

Conclusion

This study has made clear hydrogen (1H) trapped at intermetallic particles (IPs) in an aluminum alloy, 6061-T6, exposed to high-pressure hydrogen gas with our unique secondary ion mass spectrometry (SIMS) methodology. In addition, the desorption energy of the IPs was identified by thermal desorption analysis (TDA) and the morphology of the IPs was observed by scanning electron microscopy in combination with energy dispersive X-ray spectroscopy (SEM-EDX). Our conclusions are summarized as

Acknowledgement

The authors are grateful to Mr. Shiro Miwa (CAMECA, AMETEK) for the helpful technical supports on SIMS analyses.

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