Hard and soft regions in a BMG microstructure can be explained through the use of NBED, which reveals that clusters of atoms with high MRO control the local hardness. Regions with larger MRO cluster sizes and higher volume fractions of those clusters possess decreased local hardness.Bulk metallic glasses (BMGs) have outstanding mechanical properties but because the atoms in these amorphous materials do not have long-range order their properties are difficult to understand. Now researchers from the University of New South Wales (UNSW Sydney), University of Sydney, Austrian Academy of Sciences, and University of Leoben have uncovered hierarchical structure in BMGs at the nanoscale that determines their properties [Nomoto et al., Materials Today (2020), https://doi.org/10.1016/j.mattod.2020.10.032].
“BMGs are carefully alloyed and processed to avoid crystallization,” explains Jamie J. Kruzic of UNSW Sydney, who led the study. “Our ability to control their properties precisely is limited because we have difficulties observing and quantifying the structural arrangements of the atoms and understanding how those atomic arrangements control the final properties.”
The researchers turned to nanobeam electron diffraction (NBED) in a transmission electron microscope (TEM) to look for tell-tale atomic structural features in a promising class of Zr-based BMGs. As-cast material was deformed or subjected to cryogenic thermal treatment to create hard and soft regions. Cross-sections of the BMGs were then examined to reveal the amount and size of locally ordered atomic arrangements over the scale of a few nanometers, which is known as medium range order (MRO).
The analysis revealed that the size and volume fraction of MRO regions change with deformation or thermal treatment and, more importantly, larger MRO cluster sizes and higher volume fractions are associated with decreased local hardness.
“Our findings represent the first detailed experimental characterization of the hierarchical structure of BMGs,” says Kruzic. “We have connected the nanoscale structure to the microscale structure by revealing how local microscale hardness heterogeneities arise from differences in the MRO cluster size and volume fraction.”
The findings hold true for BMGs of different compositions, as well as after deformation or cryogenic thermal cycling. The ordering of atoms on a local scale within BMGs appears to be responsible for their mechanical properties rather than the presence of nanocrystals or chemical variations in the material. The researchers suggest that this could be the result of the presence of crystal- and icosahedral-like structures in BMGs. The atoms in crystal-like regions tend to take up a face-centered-cubic (FCC) like arrangement, which is softer than icosahedral regions. FCC-like MRO clusters also initiate the deformation of the harder, less ordered matrix, the researcher believe.
“Our findings present a new picture of the structural hierarchy existing in BMGs and provide a significantly improved understanding of their deformation mechanisms and how the glassy structure connects processing and mechanical properties,” says Kruzic. “This knowledge will be extremely useful in creating BMGs with controllable and reliable mechanical properties for applications in aerospace, transportation, biomedicine, and consumer products.”