Under imposed stresses, plastic deformation in a crystalline material takes place by the movement of lattice imperfections, i.e. dislocations. As plastic deformation proceeds, the density of dislocations increases and their interaction results in hardening of the material [1]. Hardening occurs in already deformed regions that become resistant to further deformation. In amorphous materials plastic flow takes place in plate-like zones called shear bands by local atomic rearrangements that accommodate plastic strain [2]. The origin of strain localization is considered to be shear softening: the plastically deformed region becomes softer than its surrounding and consequently more susceptible to subsequent flow [3]. The total hardness of the glass metal decreases as a result of softening. This means that bulk metallic glass shows normally strain-softening instead of strain-hardening [4].

In order to understand and manipulate the deformation of glass metals, it’s important to know what are the prerequisites for hardening in an amorphous material and how it can be manipulated by controlling parameters in the deformation. Therefore, specially designed nanoindentation experiments using unloading-reloading cycles were carried out in as-cast and structurally modified CuZr based bulk metallic glasses. It is observed that a hardening effect after each cycle of unloading and reloading occurs. The effect of different parameters like unloading values, unloading and reloading rates and total time has been investigated.

In addition, during nanoindentation the exerted stress is relieved through displacement serrations or pop-ins which means displacement change without load increase [5]. Therefore, also the size and distribution of pop-ins after unloading -reloading sections has been evaluated and compared with the results obtained during standard loading.

References:

[1] A.P. Mouritz, Introduction to Aerospace Materials, Woodhead Publishing, 2012, 57-90

[2] C.A .Schuh, T.G. Nieh, Acta Mater. 51 (2003), 87

[3] A.L. Greer et al., Mat. Sci. Engng. R 74 (2013) 71.

[4] J. Pan et al., Nature, 578 (2020) 559

[5] A.L. Greer et al., Materials Science and Engineering: A, 357-377 (2004) 1182