Determination of the maximum surface chromia thickness for the nondestructive identification of internal alumina scales on a heat-resistant alloy using cathodoluminescence

doi:10.21203/rs.3.rs-2492489/v1

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Determination of the maximum surface chromia thickness for the nondestructive identification of internal alumina scales on a heat-resistant alloy using cathodoluminescence

https://doi.org/10.21203/rs.3.rs-2492489/v1

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published 18 Dec, 2023

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To develop a nondestructive analytical method for identifying internal α-Al₂O₃ scales formed in heat-resistant alloys, the maximum thickness of surface Cr₂O₃ scales for identifying internal α-Al₂O₃ scales via cathodoluminescence (CL) spectroscopy was estimated using CL-peak intensity at 695 nm for Cr₂O₃ film-deposited sapphires and α-Al₂O₃ scales formed on the surface of Fe-Cr-Al alloys. The estimated maximum Cr₂O₃ thickness was validated by measuring the CL peak intensities of Ni-25Cr-5Al alloys heated at 1000°C under oxygen. Therefore, internal α-Al₂O₃ scales can be identified nondestructively by acquiring the surface CL spectra up to ~ 3-µm surface Cr₂O₃ thickness.

Cathodoluminescence

Internal alumina scale

Nickel-based alloy

Chromia scale

High temperature corrosion

The performance of heat-resistant alloys, which are materials designed to withstand extreme temperatures and adverse working conditions, relies on the formation of a continuous, adhesive, slowly growing oxide scales such as chromia (Cr₂O₃), α-alumina (α-Al₂O₃), and silica [1–3]. In general, heat-resistant alloys comprise iron, nickel, and/or cobalt as base elements and chromium, aluminum, and/or silicon as additive elements to induce the formation of these oxide scales. These alloys are used in jet engines, power plant gas turbines, petrochemical plant refineries, incinerators, and boilers [1, 3–7]. Among the protective oxide scales, α-Al₂O₃ is the most stable and efficient oxide against high-temperature oxidation and corrosion [5–9]. Thus, α-Al₂O₃-forming alloys are used for applications under harsh conditions such as temperatures exceeding 900°C [5–9]. Normally, the α-Al₂O₃ layer is formed on the surface of the alloys, although it can also appear beneath other oxide layers (e.g., NiO, NiAl₂O₄, and Cr₂O₃) on alloys such as γ´-Ni₃Al and Ni–Cr–Al [3, 10]. Such internal α-Al₂O₃ layers can also inhibit high-temperature oxidation and corrosion. Thus, identifying the internal α-Al₂O₃ layer is crucial for evaluating the performance of these alloys as heat-resistant materials.

Scanning electron microscopy (SEM) combined with energy- or wavelength-dispersive X-ray spectrometry (EDX or WDX) is conventionally used to observe the cross section of the internal α-Al₂O₃ layer; however, this analytical method is destructive. Therefore, a nondestructive analytical method is desirable for a simplified preparation process and reduced analysis time. Although photostimulated luminescence spectroscopy can be used to identify the internal α-Al₂O₃ layer nondestructively [11–14], it is not a widely used technique because it is less versatile than SEM–EDX or SEM–WDX.

As an alternative method to identify and determine the thickness of α-Al₂O₃ and SiO₂ scales on heat-resistant alloys, the author has previously focused on SEM–cathodoluminescence (CL) analysis [15–20], which provides spectra and projection images of the light emission induced by bombarding electrons from the electron gun of the SEM device on a material. Compared with photostimulated luminescence spectroscopy, the SEM–CL system is simple and versatile because it can be assembled by attaching an off-axis parabolic mirror, an optical fiber, and a portable spectrometer to a conventional SEM instrument or an electron probe microanalyzer [21], which allows conducting an elemental analysis together with the CL analysis. The author has previously revealed that SEM–CL analysis can be used to identify an internal α-Al₂O₃ layer beneath surface Cr₂O₃ scales on an Ni–Cr–Al alloy (Inconel 601 alloy) by observing a CL peak at 695 nm [19], which originates from the substitution of chromium(III) ions (Cr³⁺) for aluminum(III) ions (Al³⁺) in the α-Al₂O₃ scales [22–24]. Although the CL peak intensity at 695 nm was expected to decrease with increasing the thickness of the surface Cr₂O₃ scale layer, no effect of the latter on the CL peak intensity at 695 nm was observed in a thickness range of 0.74–1.25 µm owing to the rough surface of the Cr₂O₃ scales [19]. To realize the widespread application of the SEM–CL technique for identifying internal α-Al₂O₃ scales on heat-resistant alloys, determining the maximum surface Cr₂O₃ scale thickness that allows measuring the CL peak at 695 nm is a critical task.

To this aim, in this study, the maximum Cr₂O₃ scale thickness was estimated on the basis of the CL peak intensity at 695 nm for two kinds of samples, i.e., sapphire (α- Al₂O₃) substrates on which a Cr₂O₃ film with various thicknesses was deposited via sputtering to obtain smooth surfaces and Fe–Cr–Al alloys forming α- Al₂O₃ scales on the surface. Then, the validity of the estimated maximum Cr₂O₃ scale thickness was investigated by acquiring the surface CL spectra of heat-treated Ni–Cr–Al alloys forming both surface Cr₂O₃ scales and internal α- Al₂O₃ scales.

Cr₂O₃ film–deposited sapphires substrates and heat-treated Ni–25 mass% Cr–5 mass% Al (Ni–25Cr–5Al) alloys were selected as samples containing internal α-Al₂O₃ layers. The Cr₂O₃ film was deposited on the sapphire substrates via sputtering using a radio frequency magnetron sputtering gun (SPG-001, Pascal Co., Ltd., Osaka, Japan) at an input power of 60 W in a vacuum chamber pumped using turbomolecular and rotary pumps to a base pressure of 3 ×10^− 5 Pa. Cr₂O₃ disc (purity: 99.9%) with a diameter of 25.4 mm was used as the sputtering target. During the sputtering deposition, argon gas (purity: 99.9999%) with a flow rate of 20 mL min^− 1 was introduced into the chamber using a mass flow controller, and the gate value was positioned to obtain a chamber pressure of 1 Pa. The thickness of the Cr₂O₃ film was monitored during the sputter deposition using a quartz crystal microbalance monitor (STM-2, INFICON Co. Ltd., Bad Ragaz, Switzerland) [25].

The Ni–25Cr-5Al alloy was prepared by melting 70 mass% Ni powder (purity: 99%, Wako Pure Chemical Industries, Ltd., Osaka, Japan), 25 mass% Cr powder (purity: 99.9%, Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan), and 5 mass% Al powder (purity: 99.99%, Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan) in an Al₂O₃ crucible. The powder mixtures were heated at 1560 °C for 30 min before being cooled to room temperature at a rate of 5 °C min^− 1 by passing 96 vol% argon and 4 vol% hydrogen at 200 mL min^− 1. The Ni–25Cr–5Al alloy was annealed at 1100°C and 0.1 Pa for 12 h, and the annealed alloy was then cut into cubic slices of approximately 5 mm. The exposed surfaces of the Ni–25Cr–5Al slices were polished using 600-, 1200-, and 2400-grit abrasive papers and finished using a 1 µm diamond slurry. The polished slices were heated at 1000°C while passing oxygen at 200 mL min^− 1 to create surface Cr₂O₃ and internal Al₂O₃ layers.

The CL spectra of the surfaces of the Cr₂O₃ film–deposited sapphires and the heat-treated Ni–25Cr–5Al alloys were acquired using the custom SEM–CL system previously described by the author [15, 16, 18, 21, 26–31]. In brief, the CL spectra were acquired using an optical spectrometer (QE65Pro, Ocean Optics Inc., Largo, Florida, USA) by connecting an optical fiber with a plano-convex lens at its tip opposite to that connected to the optical spectrometer. The tip of the optical fiber with the plano-convex lens was attached to the SEM instrument (Mighty-8DXL, TECHNEX, Tokyo, Japan). The light emitted from the samples was collimated using a gold-coated off-axis parabolic mirror with a 0.5 mm hole at the center. The collimated light was then collected by a plano-convex lens attached to the tip of the optical fiber. The acceleration voltage was set to 17 kV.

Observations and elemental analyses of the sample surface and cross section were performed using an SEM instrument (TM3030 Plus, Hitachi High-Technologies Co., Tokyo, Japan) equipped with a silicon drift EDX detector (Quantax70, Bruker Corp., Billerica, Massachusetts, USA). Prior to the observation of the cross section of the alloy, one side of the alloy sample was polished using a 2400-grit abrasive paper.

3.1. Cr₂O₃ film–deposited sapphire

The maximum thickness of a surface Cr₂O₃ scale layer that allows detecting internal α-Al₂O₃ scales on heat-resistant alloys can be estimated by measuring the CL peak intensity at 695 nm, which originates from α-Al₂O₃, for heat-resistant alloys forming surface Cr₂O₃ scales with various thicknesses and an internal α-Al₂O₃ scale layer with a constant thickness or with a thickness beyond the information depth of the present CL experiment (several micrometers [32]). However, preparing such scales is difficult because both surface Cr₂O₃ and internal α-Al₂O₃ scales grow simultaneously on heat-resistant alloys during heat treatment. Thus, in this study, the CL peak intensity at 695 nm was measured for sapphire substrates on which Cr₂O₃ films with various thicknesses were deposited. The thickness of the sapphire substrate (500 µm) is far beyond the information depth of CL.

Smooth films were deposited on the sapphire substrates via sputtering, as shown in Figs. 1(a–c). An EDX analysis confirmed that the films were composed of Cr₂O₃. At Cr₂O₃ film thicknesses below 2.6 µm, the CL peak at 695 nm was detected, and its intensity decreased as the Cr₂O₃ film thickness increased (Figs. 1(d–f)). Figure 2 shows the Cr₂O₃ film thickness dependence on the normalized intensities of the CL peak at 695 nm, which were obtained by dividing the peak intensity of the Cr₂O₃ film-deposited sapphire substrate (I) by the reference peak intensity of a sapphire substrate without Cr₂O₃ film (I₀). The normalized intensities were in good agreement with those calculated using a reported value for the absorption coefficient of Cr₂O₃ at a wavelength of 700 nm (1.26×10⁴ cm^− 1 [33]) and a Cr₂O₃ film thickness below 0.61 µm, which also supports that the deposited films were Cr₂O₃. As the Cr₂O₃ film thickness increased above 0.61 µm, the normalized intensities negatively deviated from the calculated values. This negative deviation can be attributed to the decrease in the number of electrons bombarding the sapphire substrate. As shown in Fig. 3, the interaction volume between the bombarded electrons and a sample is pear-like in shape (a teardrop) shape [32, 34]. The interaction volume in the sapphire substrate would be much larger than in the Cr₂O₃ film with a thickness less than 0.61 µm (Fig. 3(a)) and would gradually decrease as the Cr₂O₃ film thickness increasing above 0.61 µm (Fig. 3(b)). The penetration depth of the incident electrons (R_e) can be estimated as follows [32]:

$${R}_{e}=\frac{0.0276 A}{\rho { Z}^{0.889}}{E}_{b}^{1.67}$$

where E_b is the energy of the incident electron (keV), A is the atomic weight (g mol^− 1), ρ is the density of the sample (g cm^− 3), and Z is the atomic number. Thus, the penetration depth of the incident electrons for a Cr₂O₃ film with a theoretical density of 5.21 g cm^− 3 was determined to be 1.9 µm at an accelerating voltage of 17 kV. However, the CL peak at 695 nm was detected even for a Cr₂O₃ film thickness of 2.55 µm (Fig. 2) because the electrons and holes produced by the bombardment of incident electrons diffused deeper into the Cr₂O₃ film than the incident electrons.

Next, the maximum Cr₂O₃ thickness for detecting the peak at 695 nm originating from the sapphire substrate was determined using the results depicted in Fig. 2. In this study, a detectable analyte signal intensity (I_D) was defined as being at least three times the average peak-to-peak noise level. Under the present experimental condition, the detectable analyte signal intensity was 0.36 because the peak-to-peak noise level was determined to be 0.12 according to Fig. 1(f). The average reference intensity of the peak at 695 nm (I₀) was 49000 for the sapphire substrate without the Cr₂O₃ film (Fig. 4(a)); thus, the detectable normalized intensity (I_D/I₀) was 7.4×10^− 6 for the Cr₂O₃ film-deposited sapphires. By extrapolating the normalized intensity near the detectable analyte signal in Fig. 2, the maximum Cr₂O₃ thickness for detecting a signal from the sapphire substrate was estimated to be 2.6 µm, which might be an appropriate value because no signal from the sapphire substrate was detected at a Cr₂O₃ film thickness of 3.25 µm.

The maximum thickness of the surface Cr₂O₃ scale layer for detecting internal α-Al₂O₃ scales on heat-resistant alloys was estimated using Fig. 2 by comparing the CL peak intensity at 695 nm for the sapphire substrate without the Cr₂O₃ film with that for the α-Al₂O₃ scales formed on the surface of heat-resistant alloys (Fe–25 mass% Al (Fe–25Al) and Fe–10 mass% Cr–15 mass% Al (Fe–10Cr-15Al)) that were previously prepared to determine the thickness of surface α-Al₂O₃ scales using CL analysis [15]. Figures 4(b) and (c) show the CL spectra of α-Al₂O₃ scales formed on the surface of Fe–25Al and Fe–10Cr-15Al, respectively. The thicknesses of the α-Al₂O₃ scale layer on the Fe–25Al and Fe–10Cr–15Al alloys were 0.59 and 0.98 µm, respectively. Although the thickness of the α-Al₂O₃ layer was much larger for the sapphire substrate (500 µm) than the for α-Al₂O₃ scales formed on the Fe–25Al and Fe–10Cr–15Al alloys, the intensity of the CL peak at 695 nm was lower for the sapphire substrate owing to its extremely low chromium content. The chromium content in α-Al₂O₃ scales on heat-resistant alloys is normally 1 at% [20], whereas that in the sapphire substrate is at the ppm level or below. The chromium present in the α-Al₂O₃ scales on the Fe–25Al alloy originated from impurities in the alloy. The detectable normalized intensities (I_D/I₀) for an internal α-Al₂O₃ scale layer under surface Cr₂O₃ scales were calculated to be 3.7×10^− 6 and 1.5×10^− 6 for internal α-Al₂O₃ scale thicknesses of 0.59 and 0.98 µm, respectively, according to the CL peak intensities at 695 nm (Figs. 4(b) and (c)). Thus, the maximum Cr₂O₃ thicknesses for detecting an internal α-Al₂O₃ scales were estimated to be 2.9 and 3.2 µm for internal α-Al₂O₃ scale thicknesses of 0.59 and 0.98 µm, respectively, using the results of Fig. 2. These values for the maximum Cr₂O₃ thickness can be applied to any heat-resistant alloys that form an internal α-Al₂O₃ scales under surface Cr₂O₃ scales because the CL peak intensity at 695 nm only depends on the thickness of the α-Al₂O₃ layer, regardless of the base alloys [15, 17]. Importantly, because the CL peak intensity at 695 nm increases linearly with the thickness of the α-Al₂O₃ scale layer on heat-resistant alloys [15, 17], the maximum surface Cr₂O₃ scale thickness for detecting internal α-Al₂O₃ scales with various thicknesses can be estimated from the CL peak intensity at 695 nm for α-Al₂O₃ scale thicknesses of 0.59 and 0.98 µm. These values of the maximum surface Cr₂O₃ scale thickness are compatible to the reported thickness of surface oxide scales that allows the nondestructive identification of internal α-Al₂O₃ scales formed on a Co-based γ/γ’ alloy via photostimulated luminescence spectroscopy [14].

3.2 Internal α-Al₂O₃ scale on Ni–Cr–Al alloy

The maximum surface Cr₂O₃ scale thickness for detecting internal α-Al₂O₃ scale on heat-resistant alloys have determined using Cr₂O₃ film-deposited sapphires and α-Al₂O₃ scales formed on the surface of Fe–Cr–Al alloys. Then, the validity of the maximum surface Cr₂O₃ scale thickness was investigated using a heat-resistance alloy that forms surface Cr₂O₃ scales and internal α-Al₂O₃ scales. When an Ni–25Cr–5Al alloy was heated at 1000°C, the formation of a continuous α-Al₂O₃ scale layer between the surface Cr₂O₃ scales and the alloy surface was confirmed via SEM observation of the cross section of the alloy and the corresponding EDX elemental mappings (Fig. 5), which revealed the present of Cr and O in the surface layer and Al and O only detected between the surface layer and the alloy primarily consisting of Cr and Ni. Surface Cr₂O₃ scales and internal α-Al₂O₃ scales with different thicknesses were obtained by changing the holding time at 1000°C. The results are summarized in Table 1. When the thickness of the Cr₂O₃ layer were 1.77 (sample A) and 2.45 µm (sample B), the CL peak at 695 nm originating from internal α-Al₂O₃ scales with thicknesses of 0.63 (sample A) and 0.91 µm (sample B) was detected (Figs. 6(d) and (e)). In constant, no peak at 695 nm was detected for surface Cr₂O₃ and the internal α-Al₂O₃ scale thicknesses of 4.60 and 0.85 µm, respectively (sample C) (Fig. 6(f)). These results are consistent with the maximum surface Cr₂O₃ scale thickness determined using the CL spectra of Cr₂O₃ film-deposited sapphires and α-Al₂O₃ scales formed on the surface of Fe–Cr–Al alloys, i.e., the CL peak at 695 nm was detected at surface Cr₂O₃ scale thicknesses below 2.9 and 3.2 µm for internal α-Al₂O₃ scale thicknesses of 0.59 and 0.98 µm, respectively.

Table 1

Sample names, holding time at 1000°C, thickness of surface Cr₂O₃ and internal α-Al₂O₃ scales, CL peak intensity at 695 nm, and reference CL intensity used for calculating the normalized intensity (I/I₀) at 695 nm for the Ni–25Cr–5Al alloys.
Sample name	Holding time (h)	Thickness (µm)		Peak intensity at 695 nm (I)	Reference peak intensity (I₀)
Sample name	Holding time (h)	Cr₂O₃	α-Al₂O₃	Peak intensity at 695 nm (I)	Reference peak intensity (I₀)
A	10	1.77	0.63	140	95900
B	16	2.45	0.91	29	239000
C	25	4.60	0.85	n.d.*	–
* “n.d.” means not detected.

Subsequently, the normalized intensities of the CL peak at 695 nm were calculated for samples A and B using the intensities of 0.59 and 0.98 µm-thick α-Al₂O₃ scales formed on Fe–Cr–Al alloys (Figs. 4 (b) and (c)) as refence intensities (I₀), respectively, because the thicknesses of the α-Al₂O₃ scales on the Fe–Cr–Al alloys were close to those of the internal α-Al₂O₃ scales for samples A (0.63 µm) and B (0.91 µm). The normalized intensities for samples A and B were calculated to be 1.5×10^− 3 and 1.2×10^− 4, respectively, which were higher than those for Cr₂O₃ film-deposited sapphires with Cr₂O₃ thicknesses of 1.80 and 2.55 µm (Fig. 2) due to the presence of areas where the Cr₂O₃ thickness was lower than the average thickness because the surface Cr₂O₃ scales on samples A and B exhibited rough surfaces (Figs. 6 (f) and (g)). Given the surface roughness of samples A and B, these results suggest that the normalized intensities for internal α-Al₂O₃ scales beneath Cr₂O₃ scales on a heat-resistant alloy showed a similar behavior to those on Cr₂O₃ film-deposited sapphires. Thus, the maximum Cr₂O₃ scale thickness for detecting internal α-Al₂O₃ scales with a certain thickness on a heat-resistant alloy can be estimated on the basis of the normalized intensities for Cr₂O₃ film-deposited sapphires (Fig. 2). Therefore, acquiring CL spectra of the surface of heat-resistant materials could aid in the nondestructive identification of internal α-Al₂O₃ scales. Future work will focus on investigating the applicability of this method for identifying internal α-Al₂O₃ scales beneath other surface scales such as NiO and NiAl₂O₄ on Ni–Al alloys [3].

The maximum thickness of surface Cr₂O₃ scales for detecting internal α-Al₂O₃ scales in a heat-resistant alloy was investigated by acquiring the surface CL spectra. Sapphire on which a Cr₂O₃ film with different thicknesses was deposited was used for conducting this investigation. By investigating the Cr₂O₃ film thickness dependence on the CL peak intensity at 695 nm and acquiring the CL spectra of α-Al₂O₃ scales formed on the surface of the Fe–Cr–Al alloys, the maximum thicknesses of the surface Cr₂O₃ scales for detecting α-Al₂O₃ scales with thicknesses of 0.59 and 0.98 µm were estimated to be 2.9 and 3.2 µm, respectively. The validity of these values was confirmed by measuring the CL peak intensities at 695 nm for Ni–25Cr–5Al alloys heated at 1000°C under oxygen and via cross-sectional SEM observations. Thus, internal α-Al₂O₃ scales with a thickness of 1.0 µm beneath the surface Cr₂O₃ scales can be detected nondestructively in heat-resistant alloys by acquring the surface CL spectrum up to a surface Cr₂O₃ scale thickness of 3.2 µm.

Funding: Financial support for the present study was provided by JSPS KAKENHI Grant Number 22H01837.

Conflict of Interest: The authors declare that they have no conflict of interest.

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No competing interests reported.

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Determination of the maximum surface chromia thickness for the nondestructive identification of internal alumina scales on a heat-resistant alloy using cathodoluminescence

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Journal Publication

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Abstract

Figures

1. Introduction

2. Experimental

3. Results And Discussion

3.1. Cr₂O₃ film–deposited sapphire

3.2 Internal α-Al₂O₃ scale on Ni–Cr–Al alloy

4. Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

Determination of the maximum surface chromia thickness for the nondestructive identification of internal alumina scales on a heat-resistant alloy using cathodoluminescence

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experimental

3. Results And Discussion

3.1. Cr2O3 film–deposited sapphire

3.2 Internal α-Al2O3 scale on Ni–Cr–Al alloy

4. Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1

3.1. Cr₂O₃ film–deposited sapphire

3.2 Internal α-Al₂O₃ scale on Ni–Cr–Al alloy