MicroXRF Resolves Rhenium and Trace Contaminants in High-Performance Alloys

Table of contents

Overcoming Spectral Overlap and Trace-Element Limitations in Metallurgical Analysis

High-performance alloys depend on precise control of elemental composition.

In applications involving refractory metals and specialty alloys, even relatively small compositional differences or unexpected contaminants can matter. But identifying those elements is not always straightforward, particularly when characteristic X-ray peaks overlap or when the elements of interest occur at concentrations below the practical sensitivity of the selected analytical technique.

In this case study, energy-dispersive spectroscopy (EDS) and microXRF were compared directly on rhenium-containing alloy samples, including a small wire and a hexagonal rod.

The objective was to identify rhenium and investigate iron contamination within the material.

The comparison exposed a fundamental difference between electron- and X-ray-excited elemental analysis.

With EDS, the accessible rhenium signal overlapped with a tungsten X-ray line, preventing reliable identification. With microXRF, higher-energy excitation produced the rhenium Lα peak at approximately 8.6 keV, clearly separating the rhenium signal from the interfering tungsten line.

MicroXRF also revealed trace iron and cobalt signals that were not detected by EDS.

The study demonstrates why excitation physics, spectral interference, and detection sensitivity can be just as important as spatial resolution when selecting an analytical technique for metallurgical characterization.

The Challenge

Identifying Rhenium in a Complex Refractory-Metal Matrix

Rhenium is used in demanding metallurgical applications because of its exceptional high-temperature properties.

It is particularly valuable in high-performance alloy systems used where thermal stability and mechanical performance are critical.

But analyzing rhenium can present a practical spectroscopy problem when other heavy elements are present.

The samples investigated in this comparison included:

A small rhenium-containing wire
A hexagonal rod
Tungsten within the analytical matrix
Potential iron contamination

The key analytical questions were:

Can rhenium be identified reliably in the presence of tungsten?
Can lower-level contaminants or minor constituents also be detected?

EDS and microXRF were used to answer those questions.

The EDS Measurement

Electron Excitation Produced a Difficult Spectral Overlap

The wire sample was analyzed by EDS using an electron-beam energy of 20 keV.

Under these conditions, the rhenium signal available for identification was the Re Mα line at approximately 1.8 keV.

The problem was that this peak overlapped with a tungsten X-ray line.

That spectral interference made it impossible to distinguish the rhenium contribution reliably using the EDS spectrum alone.

For routine materials characterization, an overlapping peak may be manageable when alternative spectral lines provide sufficient information.

In this case, however, the available excitation conditions limited the usefulness of those alternatives.

The problem was therefore not simply detector energy resolution.

It was also a question of which characteristic X-ray lines could be efficiently excited and measured.

The MicroXRF Measurement

Higher-Energy X-Ray Excitation Opened a Better Analytical Line

The same sample was analyzed by microXRF using 35 keV X-ray excitation.

That higher excitation energy allowed the measurement to access the rhenium L-series fluorescence, including the Re Lα peak at approximately 8.6 keV.

At this energy, the rhenium peak was spectrally separated from the tungsten interference that complicated the EDS analysis.

The result was a clear rhenium identification.

This illustrates an important principle in X-ray spectroscopy:

Choosing a different excitation mechanism can change which elemental emission lines are available for analysis.

For heavy elements such as rhenium, tungsten, platinum, and other refractory or precious metals, access to higher-energy characteristic lines can provide a significant analytical advantage.

The Spectral Comparison

The Difference Was Visible Directly in the Overlaid Spectra

The original study overlaid spectra collected from the wire sample using both techniques:

EDS: 20 keV electron excitation
MicroXRF: 35 keV X-ray excitation

The EDS spectrum showed the difficulty associated with the low-energy rhenium signal and tungsten interference.

The microXRF spectrum, by contrast, provided access to the higher-energy rhenium Lα peak.

This direct comparison demonstrates why a technique should not be selected based solely on whether it is theoretically capable of detecting an element.

A more useful question is:

Can the technique excite and resolve an analytically useful line for that element within the actual sample matrix?

For this rhenium-containing material, microXRF provided the more useful answer.

The Results

1. MicroXRF Clearly Resolved Rhenium

The principal analytical result was successful separation of the rhenium signal from tungsten.

EDS relied on the lower-energy Re Mα peak near 1.8 keV, where spectral overlap complicated identification.

MicroXRF excited the Re Lα peak near 8.6 keV, providing a clearly resolved rhenium signal.

For refractory-metal and high-performance-alloy analysis, this capability can be critical when overlapping low-energy peaks make identification ambiguous.

2. Trace Iron Was Detected by MicroXRF

The study also sought evidence of iron contamination.

MicroXRF resolved trace iron that was not detected in the corresponding EDS measurement.

For specialty metals, the ability to identify low-level contamination can be valuable in:

Incoming-material inspection
Process monitoring
Failure analysis
Alloy verification
Contamination investigations
Quality assurance

The importance of a trace constituent depends on the alloy and application, but the case study demonstrates that differences in analytical sensitivity can determine whether the signal is observed at all.

3. Cobalt Was Also Revealed

In addition to iron, microXRF detected a trace cobalt signal that EDS did not resolve in the comparison.

This is particularly useful when the analytical objective extends beyond verifying the principal alloying elements.

Unexpected minor or trace constituents can provide clues about:

Raw-material purity
Cross-contamination
Manufacturing history
Processing equipment
Alloy segregation
Material provenance

An analytical method capable of revealing those lower-level signals can therefore provide information beyond basic alloy identification.

Why Heavy-Element Analysis Benefits From Higher-Energy Excitation

Not All Characteristic X-Ray Lines Are Equally Useful

Each element can produce several families of characteristic X-ray lines.

For heavy elements, the lower-energy M-series lines may occur in crowded regions of the spectrum where overlaps are more common.

Higher-energy L-series lines may provide cleaner separation, but those lines require sufficient excitation energy.

This is where the difference between the two techniques became important in this study.

EDS

Electron excitation at 20 keV provided access to the rhenium M-series signal but did not provide an analytically useful separation from tungsten.

MicroXRF

X-ray excitation at 35 keV generated the higher-energy rhenium L-series fluorescence needed for a clear identification.

This same principle can become relevant when analyzing other high-atomic-number materials.

Quantitative Analysis

Moving From Identification to Concentration

Once an element has been reliably identified, the next question may be:

How much is present?

The source study describes a standards-based microXRF quantification approach using calibration standards of known composition.

A calibration curve can be established by measuring standards containing known concentrations of the target element.

The unknown sample is then measured under controlled conditions and compared with that calibration relationship.

For rhenium and other challenging alloy constituents, this creates a path from:

Element identification → calibrated measurement → concentration

The quality of the quantitative result depends on factors including:

Calibration standards
Sample matrix
Surface condition
Geometry
Excitation conditions
Counting statistics
Calibration methodology

The important point from this case is that reliable quantification first requires a spectral signal that can be distinguished from interfering peaks.

Sample Geometry Matters

Wire and Rod Samples Do Not Always Fit an Ideal SEM Workflow

The samples used in the study included a small wire and a hexagonal rod.

These geometries illustrate another practical difference between metallurgical analysis methods.

EDS measurements are commonly performed on specimens prepared for stable electron-beam analysis, often favoring relatively flat, polished surfaces and vacuum-compatible geometries.

MicroXRF can accommodate a wider range of shapes and sizes with less extensive preparation.

Potential metallurgical specimens can include:

Wires
Rods
Coupons
Fasteners
Machined components
Coated parts
Fracture specimens
Irregularly shaped material

This can make microXRF useful for rapid screening before deciding whether cutting, mounting, polishing, or other destructive preparation is justified.

Non-Destructive Analysis

Preserve the Sample for Additional Investigation

MicroXRF uses X-rays to excite the material and does not require material removal during the measurement.

For valuable or limited specimens, this allows the original component to remain available for:

Additional XRF measurements
SEM investigation
Mechanical testing
Metallography
Failure analysis
Archiving

This can be particularly important for specialty metals, aerospace components, prototype materials, and failure-analysis specimens where only one sample may be available.

A Better Metallurgical Workflow

Use Each Technique Where It Is Strongest

The case study should not be interpreted as showing that microXRF universally replaces SEM-EDS.

SEM-EDS remains highly valuable for:

High-resolution imaging
Microstructural analysis
Small inclusions
Grain-boundary characterization
Localized chemistry
Correlation between morphology and composition

MicroXRF provides different advantages:

Higher-energy excitation
Trace-element sensitivity
Large-area mapping
Broad elemental screening
Minimal sample preparation
Flexible sample geometries

A practical metallurgical workflow can therefore combine them.

Step 1: Screen the Material With MicroXRF

Use microXRF to identify:

Principal alloying elements
Trace contaminants
Unexpected constituents
Elemental segregation
Spatial variation

For heavy-element alloys, select excitation conditions capable of accessing useful higher-energy emission lines.

Step 2: Map the Elemental Distribution

If compositional heterogeneity is suspected, collect elemental maps.

These can reveal:

Segregation
Inclusions
Contamination
Coating variation
Diffusion zones
Weld or interface chemistry

This creates a chemical overview of the specimen.

Step 3: Identify Regions of Interest

Use spectra and elemental maps to locate regions that differ from the surrounding material.

These regions can then be targeted for more detailed analysis.

Step 4: Move to SEM-EDS When Higher Spatial Resolution Is Needed

If the analytical question requires grain-scale imaging, microstructural detail, or investigation of very small inclusions, transfer selected regions to SEM-EDS.

This turns the workflow from:

search first, characterize later

into:

chemically locate first, then characterize.

Applications in Metallurgy

The analytical advantages demonstrated by the rhenium alloy comparison extend to several metallurgical workflows.

High-Temperature and Refractory Alloys

Elements such as:

can occur in alloy systems designed for demanding thermal environments.

High-energy X-ray excitation can provide useful access to characteristic lines for these heavy elements.

Superalloys

Nickel- and cobalt-based superalloys contain complex mixtures of major, minor, and trace elements.

MicroXRF can support investigations of:

Alloy composition
Segregation
Trace contamination
Coating chemistry
Diffusion regions
Failure-analysis specimens

Corrosion-Resistant Alloys

Stainless steels and other corrosion-resistant materials rely on controlled elemental composition.

MicroXRF can help compare alloy chemistry and identify compositional variations or unexpected constituents.

Quality Control of Specialty Metals

Manufacturers can use microXRF to screen:

Incoming raw materials
Wires
Rods
Finished components
Specialty alloy products

The ability to analyze unusual sample shapes with minimal preparation can simplify inspection workflows.

Coatings and Platings

MicroXRF can also evaluate metallic coatings and plated structures.

Depending on the application and calibration method, analysis may address:

Elemental composition
Coating uniformity
Layer distribution
Thickness

Materials such as gold, platinum, and other high-value coatings are particularly well suited to XRF-based characterization.

The Value

Resolving What EDS Could Not

The rhenium alloy comparison demonstrates several practical advantages of microXRF for metallurgical analysis.

Clear Rhenium Identification

MicroXRF accessed the Re Lα peak at approximately 8.6 keV, avoiding the tungsten interference that complicated the Re Mα signal measured by EDS.

Detection of Trace Constituents

Iron and cobalt signals were resolved by microXRF but were not detected in the EDS comparison.

Standards-Based Quantification

Once the target element was spectrally resolved, calibration standards could be used to support quantitative analysis.

Flexible Sample Geometry

Wire and hex-rod specimens could be examined without requiring the extensive preparation associated with many microscopy workflows.

Non-Destructive Measurement

The specimens remained available for further analysis.

Together, these advantages demonstrate why the excitation method itself should be considered when selecting an elemental-analysis technique for complex alloys.

The Takeaway

The Best Elemental Technique Depends on the Spectral Problem

EDS and microXRF both use characteristic X-rays to identify elements, but they do not generate those X-rays in the same way.

That difference can fundamentally change the analysis.

In this rhenium alloy case study, EDS at 20 keV relied on a low-energy rhenium Mα signal near 1.8 keV that overlapped with tungsten.

MicroXRF analysis at 35 keV excited the higher-energy Re Lα signal near 8.6 keV, allowing rhenium to be clearly resolved.

The microXRF spectrum also revealed trace iron and cobalt signals not detected by EDS.

The result illustrates an important principle for metallurgical characterization:

When spectral overlap or trace-element sensitivity limits one technique, changing the excitation method may reveal information that was previously inaccessible.

For high-performance alloys, refractory metals, quality control, contamination analysis, and failure investigations, microXRF provides a complementary elemental-analysis capability that can extend well beyond conventional EDS screening.