X-rays reveal secrets of metallic glass

PETRA watches hidden nanocrystals grow

Thanks to the superior X-ray vision of DESY's light source PETRA III, scientists have watched nanocrystals grow live inside a metallic glass. The method allows to follow the growth of metallic crystals from sizes as small as two nanometres (billionth of a metre) in real-time and may lead to an optimisation of these high tech materials. "The technique can also be applied to other materials and may be used to follow crystallisation processes during chemical reactions in real-time," says DESY scientist Jozef Bednarcik, lead author of a scientific paper appearing on the front cover of the journal Physical Chemistry Chemical Physics today.

X-ray diffraction pattern of a sample in nanocrystalline state, recorded at PETRA III beamline P07. The bright sharp rings correspond to the so-called Bragg reflections of Fe3Si nanocrystals whereas much broader and less intense rings seen in the centre of the pattern correspond to the amorphous matrix. Credit: Jozef Bednarcik et al/DESY

Metallic glasses are advanced magnetic materials that find broad application in sensors, transformers and other fields of electronics and electrical engineering. For instance, many anti-theft tags in shops contain strips of metallic glass. They can be detected in an external magnetic field at the shop exit and trigger the alarm. Different from what their name might suggest, metallic glasses are not optically transparent. The name glass refers to their amorphous internal structure that is as disordered as in conventional glass. Usually, metals have a highly ordered internal structure.

Metallic glasses are produced as thin films of molten alloys that are sprayed onto a fast rotating copper cylinder. Because of the high thermal conductivity of copper, the alloy cools so rapidly that it simply has no time to build an ordered internal structure. In a second step, this amorphous precursor is heated again just below its primary crystallisation point. The heat makes magnetic nanocrystals grow inside the amorphous glass. "It's like a pudding with raisins inside," explains Bednarcik.

Under the right conditions these nanocrystals all have approximately the same diameter of 10 to 20 nanometres and are evenly distributed throughout the amorphous matrix. The nanocrystals interact magnetically with each other on distances larger than their diameter and significantly determine the magnetic properties of the material, which becomes easy to magnetise. Depending on the size and number of the 'raisins', the magnetic properties can be fine-tuned.

Bednarcik's team investigated an alloy of iron, copper, niobium, molybdenum, silicon and boron (Fe72.5Cu1Nb2Mo2Si15.5B7) that is commercially available. The researchers heated raw stripes of this metallic glass to 500 and 520 degrees Celsius, respectively. The samples were kept at this temperature for at least 100 minutes while shooting PETRA's bright X-ray beam at them, taking a snapshot every twelve seconds. "We could see crystals grow live already at sizes of two or three nanometres," says Bednarcik.

This sort of investigation can elucidate the crystal growth in-situ and in real-time, leading to a better understanding of the process. With this knowledge, manufacturers of various nanocrystalline substances can optimise their production processes as well as the properties of their materials. But without the high energy and the bright intensity of PETRA's X-rays the scientists would not be able to see the full picture right down to nanometre resolution.

Apart from the crystal growth, the X-rays also revealed a change of the magnetic properties of the metallic glass during heating well before crystallisation sets in. It was known that the material turns from ferromagnetic to paramagnetic at some point before finally becoming ferromagnetic again after the build-up of the nanometre sized iron silicon (Fe3Si) crystals. The X-ray view could detect this transition in terms of an accelerated thermal expansion rate.

As long as the amorphous material is ferromagnetic, the magnetic force counteracts the thermal expansion during heating. But at the Curie temperature thermal energy prevails and the magnetic interactions among the iron atoms fade away. The amorphous material becomes paramagnetic, and the magnetic interaction forces no longer work against the increasing of interatomic distances. From here on, the glass expands faster, which can be seen in the X-ray diffraction image. "This way, a non-magnetic method can detect a magnetic transition in real-time," Bednarcik points out.

Jozef Bednarcik, Stefan Michalik, Vladimir Kolesar, Uta Rütt and Hermann Franz: "In situ XRD studies of nanocrystallization of Fe-based metallic glass: a comparative study by reciprocal and direct space methods"; Phys. Chem. Chem. Phys., 2013, 15, 8470-8479; DOI: 10.1039/C3CP44445G