Strain-hardening (the increase of flow stress with plastic strain) is the most important phenomenon in the mechanical behaviour of engineering alloys because it ensures that flow is delocalized, enhances tensile ductility and inhibits catastrophic mechanical failure. Metallic glasses (MGs) lack the crystallinity of conventional engineering alloys, and some of their properties—such as higher yield stress and elastic strain limit—are greatly improved relative to their crystalline counterparts. MGs can have high fracture toughness and have the highest known ‘damage tolerance’ (defined as the product of yield stress and fracture toughness) among all structural materials. However, the use of MGs in structural applications is largely limited by the fact that they show strain-softening instead of strain-hardening; this leads to extreme localization of plastic flow in shear bands, and is associated with early catastrophic failure in tension.
In a study published in Nature，collaboration research between Prof. LI Yi’s group at the Shenyang National Laboratory of Materials Science, Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS), and Prof. A.L. Greer’s group at Cambridge University, UK, shows that plastic deformation under triaxial compression at room temperature can rejuvenate bulk MG samples sufficiently to enable strain-hardening through a mechanism that has not been previously observed in the metallic state.
Compared with the as-cast bulk metallic glass, the highly rejuvenated bulk metallic glass shows significantly reduced yield stress, obvious work-hardening and excellent plastic deformation capacity under deformation. Within strain-hardening (a process of homogeneous flow), there would be no cause for shear-banding on the sample surfaces, totally different from traditional metallic glass deforming by shear-banding. As the plastic deformation continues, the hardness increases, accompanying with reduced exothermic heat of relaxation and densification, indicating that strain-hardening is closely related with structure relaxation. Simultaneously, the structural changes indicated by the position of the first halo q1, which shifts back towards higher q by deforming a rejuvenated sample, correlate well with the changes in the exothermic heat of relaxation and hardness, supporting the link between strain-hardening and structural relaxation. In addition, the initial rates of hardening are much higher than those of crystalline alloys, showing a very efficient hardening mechanism. However, the hardening capacity is soon exhausted.
Work-hardening in crystalline metals is mediated by dislocations (defects), which proliferate and increasingly impede each other’s glide under deformation; it is based on an increase in energy, which was proposed by G. I. Taylor eighty-five years ago. However, for bulk metallic glass, the work-hardening is based on a reduction in energy (a transformation from high energy state to low energy state, reducing the defect density), which is opposite with that in crystalline metals, indicating a novel mechanism inducing strain-hardening in the metallic glass. The rejuvenated MGs are stable at room temperature and show exceptionally efficient strain-hardening, greatly increasing their potential use in structural applications.
This work is supported by the National Natural Science Foundation of China, Shenyang National Laboratory of Materials Science, and Institute of Metal Research, Chinese Academy of Sciences.
