Microstructure

Microstructural characterizations are performed to investigate both the atomic structure and the stability of bulk metallic glasses (BMGs). Many characterizations with a variety of instruments are performed to identify the phases produced during preparation (i.e., quenching), annealing, and devitrification of BMG. The typical parameters that are quantified are the size, morphology, composition, crystal structure, and volume fraction of the phases formed. These parameters may be used to improve the alloy design process. Some techniques provide continuous monitoring of time-dependent processes, such as crystallization, as a function of time and temperature. High-resolution techniques are used to investigate the atomic structure of the glass including short- and medium-range order. Specialized characterizations may be performed to investigate the quantity and type of free-volume in the glass. Microstructural characterizations may also be performed in conjunction with mechanical property tests to investigate the type of failure, the source of crack initiation, and the interaction of shear bands that are produced under stress with microstructural features.

In this chapter, many of the techniques that have been used to characterize the microstructures of BMGs are reviewed. The techniques are divided broadly into the routine techniques, such as differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy, that are used to evaluate parameters such as the glass-forming ability, the critical dimension, and examine the general microstructure, and the higher resolution, more specialized techniques, such as high-resolution and fluctuation electron microscopy, field ion microscopy, atom probe tomography, small angle scattering, and positron annihilation spectroscopy, that are used to characterize the nanometerscale structure, phase composition, open volume, etc.