Austempered Ductile Iron (ADI) excels through strength, wear resistance and
toughness - characteristics that make ADI the material of choice for use in
combustion engines and gear box components. This means that safety aspects are
also involved in addition to purely functional aspects. For this reason, changes
in the ADI production process need to be monitored with respect to the material's
characteristics and must be optimized systematically. For the micro- and nanoscopic
analysis of the structure and precipitations, scientists typically use both
light and electron microscopes. To date, however, there has been no possibility
of relocating regions of interest without doubt when transferring the sample
from the light to the electron microscope or vice versa. "Shuttle &
Find" - the interface for correlative microscopy in materials analysis
offers an easy-to-use solution, allowing seamless integration of these two complementary
technologies for the first time.
EN-GJS-1200-2 according to DIN EN 1564. Austempered Ductile Iron (ADI) is a
family of iron-based materials, which among different processes can be obtained
by heat treatment (so-called austempering) of nodular cast iron. ADI is
well-known for its excellent strength, wear resistance and toughness. The
ultimate tensile strength of min. 1200 MPa and elongation at fracture of min. 2%
results in a cost-effective solution providing comparable performance to high
strength aluminum alloys (per unit weight) or even steels.
The current investigations were done on a ZEISS Axio Imager.M1 light microscope
with motorized stage, SUPRA™ 40VP Field Emission-SEM (FE-SEM) equipped with
an AsB® detector, SUPRA™ 55VP FE-SEM with an AsB® and Bruker Quantax
200 EDS detectors.
In general, in order to enable the correlative workflow, the following points
should be fulfilled:
- Light microscope equipped with digital camera and motorized stage of Axio
Observer and SteREO Discovery families with corresponding mounting frames,
- Scanning electron microscope of EVO®,
SIGMA, SUPRA™, ULTRA and MERLIN® families
with an SEM adapter for correlative microscopy,
- Correlative holder,
- Software compatibility requirements.
Supporting the tribological characterization the main investigation task was
an exact description, including elemental analysis, of the materials
microstructure. This was, however, only possible with the help of correlative
microscopy, because the (hard) precipitates allowed a systematic investigation
in the scanning electron microscope (SEM) after being examined in the light
microscope only in case of a precise relocation. In addition, the task became
more complicated by the fact that it was only possible to detect the
precipitates in the SEM with the backscatter electron detector (e.g. AsB®).
Fig. 1. Light microscope image of ADI sample with a
magnification of approx. 400:1. One can see the ROI with the precipitates.
Fig. 2. SE image of the same area as in Fig. 1. The
problem of relocating the same area with SE becomes evident.
The identification of the chemical composition of precipitates is possible
only by EDS. Thus, the exact relocation of the region of interest (ROI) - in
order to avoid time consuming searches - is of great importance. "Shuttle
& Find" - correlative microscopy solution from Carl Zeiss
fulfills exactly these application requirements and allows micron-precise relocation
of the ROI on metallographic samples even at high magnifications in both light
and electron microscopes, making subsequent EDS in a SEM a matter of routine
Fig. 3. BSE image of the same area as in Fig. 1.The
microstructure is clearly visible in comparison to the SE image.
Fig. 4. SE image of the same area as in Fig. 1 overlapped
with Mo, Fe and P mappings obtained with EDS.
The matrix basically consists of bainite (in fact ausferrite), graphite nodules
and of individual areas of retained austenite. The precipitates were found in
these areas. After the relocation of the precipitates in SEM with the help of
"Shuttle & Find", the area was scanned with EDS for Mo, Fe, P
and C. The spatial resolution of EDS in this experiment is limited in order
to provide information about the exact position of the neighboring elements.
Nevertheless, the EDS mappings showed that Mo, P and C are found in the same
area, verifying the existence of iron phosphides and molybdenum carbides respectively.
The wear resistance increases through such hard precipitates whereas the toughness
Note: The Mo (La) line (2.29 keV) and S (Ka) line (2.31 keV) interfere. An
acceleration voltage of 30 kV and a long analysis time are required in order to
Source: "Microstructural Investigation of Austempered Ductile
Iron (ADI) with "Shuttle & Find" Interface for Correlative Microscopy
in Materials Analysis" by Carl Zeiss
For more information on this source, please visit Carl