Abstract
Deformation and fracture of metallic glasses are often modeled by stress-based criteria which often incorporate some sorts of pressure dependence. However, detailed mechanisms that are responsible for the shear-band formation and the entire damage initiation and evolution process are complex and the origin of such a pressure dependence is obscure. Here, we argue that the shear-band formation results from the constitutive instability, so that the shear-band angle and arrangements can be easily related to the macroscopic constitutive parameters such as internal friction and dilatancy factor. This is one reason for the observed tension-compression asymmetry in metallic glasses. The free volume coalescence leads to precipitous formation of voids or cavities inside the shear bands, and the intrinsic “ductility” is therefore governed by the growth of these cavities. Based on a generalized Stokes–Hookean analogy, we can derive the critical shear-band failure strain with respect to the applied stress triaxiality, in which the cavity evolution scenarios are sharply different between tension-controlled and shear/compression-dominated conditions. This is another possible reason for the tension-compression asymmetry. It is noted that diffusive-controlled cavity growth could also be the rate-determining process, as suggested by the recent measurements of shear-band diffusivity and viscosity that turn out to satisfy the Stokes–Einstein relationship. This constitutes the third possible reason for the tension-compression asymmetry.
1 Introduction
Based on the above discussions, this work aims to determine the critical condition for the catastrophic failure of an array of cavities (circular or lenticular or other shaped) lying on a narrow planar band. This problem resembles the intergranular creep fracture in polycrystalline materials at elevated temperatures [15–19]. As shown in Fig. 3(a), these cavities can grow due to the creep flow of the surrounding material or because of the diffusive process along the grain boundaries. In our problem, we have a shear band that has much higher shear strain rate than outside the shear band, and oftentimes the temperature inside is high so that the shear band behaves liquid like. How and what knowledge from intergranular creep fracture analysis can be transferred here will be thoroughly discussed in this paper.
2 Prediction Based on Creep-Constrained Cavity Growth
As explained in Introduction, the problem that dictates the failure process will be the growth of a planar array of cavities on the shear band, subjected to faraway applied stress fields. These cavities are nucleated from the unstable free volume field and quickly coalesce into feature sizes that are comparable to the shear-band thickness. Therefore, their growth should be controlled by the creeping response of the nearby material.
The modified Sham–Needleman equation in Eq. (5) clearly shows the dependence on the stress triaxiality, T = σm/σe. We have added the term 1 − fcavity to the left-hand side of Eq. (5), without which the original version in Ref. [16] only works when fcavity is less than about 0.6.
If the applied loading condition is pure shear (T = 0) or shear/compression-dominated (T < 0), the modified Sham–Needleman model fails. Their simulations were performed mostly for T > 1/3, noting that the uniaxial tension corresponds to T = 1/3. Analytical solutions for two-dimensional circular or elliptical holes exist for linear elastic Hookean solid or Newtonian viscous material [20–23] and for perfectly plastic solids [24]. Even if we can follow the same solution approaches, these solutions are for small deformation and will not change the cavity closure result if T < 0. In shear/compression-dominated situations, as explained in Fig. 3(b), the cavities will be flattened out to micro-cracks, and then rotate and elongate until they interact with the neighboring micro-cracks. This is a large-deformation result that shows the long-term evolution of these cavities, which are only amenable to detailed finite element simulations.
Predictions in Eqs. (9) and (13) are plotted in Fig. 5 for representative n values, which provides an explanation of the tension-compression asymmetry of “intrinsic ductility” in metallic glasses. It should be reminded that the shear band angle only shows a very weak dependence on the stress state, as explained in the Introduction. In contrast, metallic glasses show little ductility under tension, but moderate ductility under compression. According to Fig. 5, under tension-dominated loading conditions (i.e., T > 1/3), there is an inverse dependence of shear-band failure strain on T. When the applied stress is shear/compression dominated (e.g., T < 0), the complex evolution of cavities from flattening, rotating, elongating, and coalescing leads to considerable critical strain for failure. This can only be properly explained in Eq. (13), as opposed to the modified Sham–Needleman model in Eq. (9).
3 Prediction Based on Diffusion-Controlled Cavity Growth
The forms of Gdiffusion(T) and Gtension(T, n) are similar, while c1 mainly depends on initial value of fcavity and c3 additionally on the initial cavity size (i.e., the value of a3/(δsbR2)). For this reason, Gdiffusion(T) is not plotted on Fig. 5. When the cavities are small, the concerted contributions from both diffusive processes and creep deformation could lead to a very fast growth rate, so that the resulting intrinsic ductility can be very small. For shear/compression-dominated loading conditions, T < 0 and the applied normal stress is negative. The Hull–Rimmer process shall not contribute to the cavity growth; in fact, the compressive normal stress will lead to cavity closure—a process denoted as solid-state bonding [31]. Consequently, the diffusive-controlled cavity growth provides yet another reason for the tension-compression asymmetry of “intrinsic ductility” in metallic glasses.
4 Summary
Because the free volume field inside the shear band is highly unstable and quickly leads to the nucleation of nanometer-sized cavities, it is the cavity growth that dictates the intrinsic ductility, namely, the critical strain that the shear band can sustain prior to its transition into fracture failure. Prior studies on intergranular creep fracture are incomplete for small and negative T values. Noting the generalized Stokes–Hookean analogy, this work is able to present a comprehensive description of the dependence of the shear-band failure strain on T. The cavity evolution under shear/compression-dominated stress states provides a mechanistic explanation of the contrasting ductility under tension versus compression conditions. The contribution from diffusive processes offers another complementary scenario for cavity growth.
Acknowledgment
The author would like to dedicate this work to Professor Kyung-Suk Kim at Brown University for the occasion of his 70th birthday and in this special issue that honors his contributions in nano-, bio-, and fracture mechanics across a wide range of spatiotemporal scales. Financial support is provided by the National Science Foundation (IIP 2052729).
Conflict of Interest
The author declares that there are no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data Availability Statement
Data contained in this paper are available upon request to the corresponding author.