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Strain-Hardening in Metallic Glasses
Alan Lindsay Greer1,Yi Li2
University of Cambridge1,Chinese Academy of Sciences2
Conventional polycrystalline alloys show strain-hardening. This is a key factor underpinning their usefulness in structural applications, delocalizing plastic deformation, enabling ductility, and preventing catastrophic failure. In contrast, metallic glasses (MGs) generally show strain-softening, leading to extreme localization of plastic flow in thin (10–20 nm) shear bands. This shear banding leads to early catastrophic failure in tension, and to unsightly surface steps when significant plastic strains can be achieved in other deformation modes, notably bending.
There have been many attempts at ‘shear-band engineering’, mainly to stimulate as many shear bands as possible to promote more uniform plastic flow, and by such means uniquely high damage tolerance has been achieved . Nevertheless, shear-banding is the key obstacle to the wider use of MGs in mechanical applications . Elimination of shear bands would be highly desirable. Yet there is the concern that shear-banding might be inevitable, given that the yield stress of MGs is a high fraction of the theoretical strength, so that any capacity for hardening must be limited.
In recent years, there has been a focus on the range of MG states that can be obtained at a given composition. As annealing and relaxation often lead to embrittlement, there is particular interest in the reverse process (‘rejuvenation’), by which the MG is taken to higher-energy, less dense states. It is speculated that a sufficiently rejuvenated MG could show strain-hardening , but it has been unclear whether the required rejuvenation could be achieved without destabilizing the MG against, for example, crystallization.
Thermomechanical processing, particularly in compression, can induce extreme rejuvenation in significant (mm-scale) volumes of MG . When these rejuvenated volumes are subjected to uniaxial stress, in tension or compression, they do show strain-hardening, and correspondingly shear-banding is suppressed . With this finding, it is possible that the aim of MG research should shift from shear-band engineering to the elimination of shear bands.
The first well accepted explanation of strain-hardening in polycrystalline metals was that proposed by G.I. Taylor in 1934 : plastic strain involves the generation of dislocations, and as the imposed macroscopic strain increases, internal local strain gradients increasingly impede the motion of those dislocations. In general terms, this has remained the only explanation for strain-hardening until that now offered for MGs. In Taylor’s model, it is intrinsic that strain-hardening involves increased defect density and a higher energy for the system. In contrast, the strain-hardening of MGs involves a lower energy and, effectively a decreased density of defects. We survey the strain-hardening rate of metallic materials, and find that this is particularly high for MGs.
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