Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

This method can help answer key questions on the evolution of crystallization in iron-based metallic glasses with respect to varying time and/or temperature. The main advantage of this procedure is the use of two complementary nuclear-based analytical methods that can follow structural and magnetic transformations through hyperfine interactions. Mossbauer spectroscopy performed ex situ describes how the structural arrangement and magnetic microstructure appears after annealing at certain temperature and time, so it reflects a static situation.

The method of nuclear forward scattering of synchrotron radiation provides data which are recorded in situ during dynamically changing temperature and thus inspects transient states. Demonstrating the procedure will be Dr.Irena Janatova, a junior research fellow from our laboratory. First cast a previously-prepared melt upon the rotating quenching wheel of an apparatus for planar flow casting under ambient conditions.

Check the amorphous nature of the produced ribbons by performing X-ray diffraction in Bragg-Brentano geometry. Use a copper anode and record the diffraction pattern from 20 to 100 degrees of 2-theta with an angular step of 0.05 degrees and acquisition of 20 seconds for one point. Next prepare small pieces of the produced ribbons with a total mass of about three to five milligrams and place them into a DSC instrument.

Perform the DSC experiment with a temperature ramp of 10 Kelvin per minute at a temperature range of 50 to 700 degrees Celsius under argon atmosphere. Determine the temperature of the onset of crystallization, which is taken at the kink of the most pronounced peak on the DSC curve. Choose five temperatures of annealing that cover both the pre-crystallization and crystallization regions on the DSC for further ex situ annealing.

Following this, prepare five groups of seven-centimeter long pieces of the as-quenched ribbon. For ex situ annealing, set the destination temperature on a furnace and wait for 15 minutes for its stabilization. Following this, insert pieces of the ribbon into the evacuated and thermally-stabilized zone by opening a seven to 10-millimeter gap between the two blocks and sliding the ribbons directly into the center of the heated zone.

Close the gap immediately so that the temperature of the sample achieves the furnace temperature in less than five seconds within a 0.1 Kelvin difference. After annealing, remove the heated ribbons and place them on a cold substrate inside the vacuum system to ensure fast cooling of the samples to room temperature. Prepare six to eight pieces of one-centimeter long ribbons for each sample for Mossbauer spectrometry experiments.

Assemble the ribbons side-by-side using adhesive tape over the ends of the ribbons. Attach them to an aluminum holder to form a compact area of about one by one centimeters squared. Note the different appearance of both sides of the ribbons.

Be sure that all ribbons are positioned on the aluminum holder with the same side upwards. Next insert the aluminum holder with the sample into the detector of the instrument. Prior to the measurement, thoroughly wash the inner detector volume with a stream of the detection gas for 10 to 15 minutes to expel all residual air.

After washing, adjust the gas flow through the detector by a needle valve to three milliliters per minute. Next connect a high voltage to the detector. Record the CEMS and CXMS Mossbauer spectra using a constant acceleration spectrometer equipped with a Cobalt 57 radioactive source embedded in a rhodium matrix.

Operate the spectrometer with a gas detector at room temperature according to the manual. For NFS experiments, place an approximately six-millimeter long ribbon of the investigated metallic glass in a vacuum furnace. Following this, record the NFS time-domain patterns during continuous heating of the sample to a temperature of up to 700 degrees Celsius with a ramp of 10 degrees per minute, using one-minute time intervals for acquisition of experimental data during the entire in situ annealing process.

Finally, evaluate the NFS experimental data using suitable software. Mossbauer spectrometry allows direct identification of the type of structural arrangement, specifically crystalline versus amorphous as shown here. CEMS spectra taken from the air and wheel sides of the ribbons are displayed here.

They show increasing contribution of the crystalline components after ex situ annealing. Comparison of the amounts of crystallites from CEMS and CXMS techniques is shown here. The crystallization starts after annealing at 410 degrees Celsius.

NFS patterns that contain in time domain the same information on hyperfine interactions as Mossbauer spectra in energy domain but are recorded over a shorter time are shown here. The contour plot of NFS time domain patterns recorded during in situ heating of as-quenched ribbons is shown here. It exhibits a magnetic transition at the Curie temperature and the onset of crystallization.

Examples of NFS patterns are shown here. The evolution of the total relative amount of nanocrystals with temperature is shown here. The onset of crystallization is marked with TX1.

Total number of counts of the individual NFS time domain patterns with respect to the temperature of the in situ NFS experiment show three well distinguished regions that evolve during dynamically changing temperature. Mossbauer spectrometry unveils fine details of structural and/or magnetic transformations induced by heat treatment. However, the time needed for recording one spectrum can extend over several hours, which limits this technique to ex situ experiments.

Using ex situ Mossbauer experiments, we can investigate the local structure and magnetic arrangement in a material under static conditions. These can be inspected before and/or after temperature treatment. Following the procedure of nuclear forward scattering, rapid data acquisition is ensured by extremely brilliant synchrotron radiation.

Thus, in situ experiments can be performed during temperature treatment and immediate states of a material can be inspected under dynamic conditions. After watching this video, you should have a good understanding of benefits of the in situ nuclear forward scattering technique of synchrotron radiation. It is complementary with ex situ Mossbauer spectroscopy from the viewpoint of the obtained results and conditions under which they are achieved.

These techniques pave the way for researchers in other fields that are associated with structural and/or magnetic transitions, especially when existence of intermediate states is foreseen.