Abstract
Contrary to crystalline solids, amorphous solids always become softer when vitrifying the melts under higher cooling rates. Understanding this phenomenon is of utmost importance in providing a basis for the mechanical-performance control of amorphous solids. However, the underlying mechanisms leading to this cooling-rate-induced softening of amorphous solids have remained elusive, especially the dynamic reasons are neglected. Here, we use a colloidal glass as the model system to directly study this issue. Shear modulus is used as the representative parameter to monitor the stress-bearing properties of colloidal glass. The space-spanning immobile particles, whose population is sensitive to the cooling rate, are found to make the dominant contribution to the shear modulus. The rapid solidification induced softening of colloidal glass is observed to originate from fewer immobile particles formed at higher cooling rates.
Highlights
One of the effective ways of hardening crystalline solids is to rapidly solidify the melts into a solid state[1]
The softening of the colloidal glass caused by higher cooling rate originates from fewer immobile particles formed under higher cooling rate
We have built a colloidal glass with a cooling-rate gradient along its height as the model system to directly study the cooling-rate-induced softening of amorphous solids
Summary
One of the effective ways of hardening crystalline solids is to rapidly solidify the melts into a solid state[1]. The hardness and elastic modulus of the Zr50Cu50 bulk metallic glass were found to be reduced by rapid solidification[4] This cooling rate induced softening in amorphous solids is always rationalized with the help of configurationally-looser atomic packing or higher defect concentration[7,8]. Colloidal glass of micrometer-sized hard spherical particles can serve as an ideal model to study this possible scenario, since the larger size and concomitant slower time scale of colloidal particles make them much more experimentally accessible[15,16] These colloidal particles can be directly observed in real time and their three-dimensional (3D) positions can be determined accurately by high-speed confocal microscopy. The rapid-solidification-induced softening of colloidal glass can be attributed to fewer immobile particles formed at higher cooling rates. The origins for the large shear modulus of immobile particles are interpreted in details
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