image: These are shear band profiles by contour plots of effective plastic strain fields under a compressive load. view more
Credit: Copyright Science China Press
Introducing heterogeneities into the structure is an effective way to enhance the plasticity of metallic glasses (MGs). The original randomly distributed free volume in MGs, a natural heterogeneity, has been found to promote plasticity. However, the exact correlation between the free volume distribution and mechanical response is still unclear. Professor Dai Lanhong and his group from the State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, set out to tackle this problem. They investigated the shear banding in MGs with different degrees of structural disorder and revealed that both concentration and dispersion of free volume exert critical effects on the plasticity and strength of MGs. Their work, entitled "How does the initial free volume distribution affect shear band formation in metallic glass", was published in SCIENCE CHINA Physics, Mechanics &Astronomy.2011, DOI: 10.1007/s11433-011-4376-z.
Metallic glasses (MGs) constitute an emerging class of materials with excellent properties envisaged for functional and structural applications; however, these applications are currently hindered by the limited plasticity at room temperature. Due to the absence of long-range order or dislocation-like defects as in crystalline alloys, MGs fail by forming intense shear bands which propagate catastrophically without work hardening. In recent years, researchers have developed several effective ways to improve the plasticity of MGs, such as the introduction of residual stress, optimum design of material composites and including heterogeneities such as nano-or micro-particles or via phase separation. These methods produce heterogeneous structures within MGs which act as initiation sites for the formation of shear bands and also as barriers to the rapid propagation of shear bands. Therefore, instead of several dominant shear bands, multiplication of shear bands is observed in modified MGs with good plasticity. Indeed, MGs are inherently heterogeneous because of their randomly distributed free volume, which can also be potential sites for shear processes. By controlling the free volume, greater plasticity can be achieved in MGs. Recently, researchers have reported the enhancement of plasticity in monolithic MGs, attributed to a large amount of randomly distributed free volume induced by varying alloy compositions or changing processing conditions. Dai and his research group also found that an atomic structure with large packing dispersion within MGs leads to a bigger bulk-shear modulus ratio and increases global plasticity. However, critically, a clear relationship between the initial free volume and the macro-plasticity is still to be resolved.
MGs show topological disorder, and inhomogeneous flow has been found to result from a dynamic competition between creation and annihilation of structural disorder or free volume. To derive a quantitative correlation between the original structural disorder and the mechanical response of MGs, MG samples were developed with different initial free volume distributions – varying both concentration and dispersion. Dai and co-workers investigated the shear banding behaviors in these MG samples using numerical simulations, based on the widely accepted free volume model.
Two important indices reflect the structural disorder of MGs: the free volume concentration (FVC) and the free volume dispersion (FVD). Variations in these two factors influence the global plasticity and yield resistance (as seen in the Figure, which shows that the number and distribution of shear bands change greatly in MGs with varying FVD or FVC). A few minor shear bands formed in the sample with narrow FVD, along with a major shear band that should lead to fracture; in the sample with wider FVD (that is, more evenly spread out free volume), the dominant shear bands bifurcate and intersect with other bands - their propagation is then slowed or even completely suppressed, resulting in initiation and multiplication of new shear bands. This shows that shear bands are inclined to form in the structure with a large amount of randomly distributed free volume. The locations with relatively high FVC usually act as sites for shear band initiation and branching. In contrast, if the initial FVD is fixed, the sample with low FVC shows uniform plastic deformation, indicating good plasticity. With increasing FVC, inhomogeneous deformation becomes significant and major shear bands are fully developed, which would probably cause an immediate failure with little further plastic deformation. This indicates that the sample with smaller FVC can accommodate more plastic deformation in a near-homogeneous manner through profuse and concurrent shear-band formation.
Dai and co-workers point out that a great amount of free volume does not always correspond to good plasticity, because increasing the magnitudes of FVC and FVD shows distinct effects on multiple shear banding in MGs. They propose that the structural conditions for multiple shear banding or plasticity of MGs are low FVC and relatively wide FVD.
The randomly distributed free volume, a heterogeneity inherent to MGs, can provide sites for shear band initiation and branching. Wider FVD means more potential sites. Concurrent nucleation of shear bands together with the high branching of bands then restricts the dominant shearing process. However, in an extreme case, if the potential sites of shear banding are everywhere in a sample, small shear bands should easily nucleate and then immediately coalesce, leading to failure. When FVC within a MG varies greatly from one position to another (i.e., higher FVC and smaller FVD), the location with higher FVC is prone to forming a dominant shear band which restrains other small shear processes, and adversely affects plasticity. Usually, MGs of good plasticity can be obtained using a high cooling rate, because during the process more randomly distributed free volume (i.e., wider FVD) is inherited from the liquid. Through compositional changes, Professor Wang Weihua and his group designed super-plastic bulk metallic glasses which are composed of hard regions surrounded by soft regions. They also found that much more atomic-scale open volumes exist in soft regions. This implies that the unique structure may actually result from a special non-uniform distribution of free volume. On the other hand, a large amount of free volume reduces the barrier to atomic movement, and a shear band can form easily at a low yielding point. Therefore, increasing either the magnitude of FVC or FVD leads to a decrease of activation stress of shear band in MGs. An optimum combination of the FVC and FVD helps to design a MG of both good plasticity and high strength.
See the article: Chen Y, Jiang M Q, Dai L H. How does the initial free volume distribution affect shear band formation in metallic glass? SCI CHINA Phys Mech Astron, 2011, DOI: 10.1007/s11433-011-4376-z