Particle rearrangement and softening contributions to the nonlinear mechanical response of glasses

Abstract

Amorphous materials such as metallic, polymeric, and colloidal glasses, exhibit complex preparation-dependent mechanical response to applied shear. In particular, glassy solids yield, with a mechanical response that transitions from elastic to plastic, with increasing shear strain. We perform numerical simulations to investigate the mechanical response of binary Lennard-Jones glasses undergoing athermal, quasistatic pure shear as a function of the cooling rate R used to prepare them. The ensemble-averaged stress versus strain curve σ(γ) resembles the spatial average in the large size limit, which appears smooth and displays a putative elastic regime at small strains, a yielding-related peak in stress at intermediate strain, and a plastic flow regime at large strains. In contrast, for each glass configuration in the ensemble, the stress-strain curve σ(γ) consists of many short nearly linear segments that are punctuated by particle-rearrangement-induced rapid stress drops. To explain the nonlinearity of σ(γ) , we quantify the shape of the small stress-strain segments and the frequency and size of the stress drops in each glass configuration. We decompose the stress loss (i.e., the deviation in the slope of σ(γ) from that at σ(0) ) into the loss from particle rearrangements and the loss from softening (i.e., the reduction of the slopes of the linear segments in σ(γ)), and then compare the two contributions as a function of R and γ. For the current studies, the rearrangement-induced stress loss is larger than the softening-induced stress loss, however, softening stress losses increase with decreasing cooling rate. We also characterize the structure of the potential energy landscape along the strain direction for glasses prepared with different R, and observe a dramatic change of the properties of the landscape near the yielding transition. We then show that the rearrangement-induced energy loss per strain can serve as an order parameter for the yielding transition, which sharpens for slow cooling rates and in the large system-size limit.

Intrinsic dissipation mechanisms in metallic glass resonators

“…We find that K r ∼ N −2k , where k ≈ 0.68 for R = 10 −2 and ≈ 0.60 for R = 10 −5 . K r is smaller for rapidly compared to slowly cooled glasses, since U * decreases with increasing R [21,39,40]. These results emphasize that Q can be increased by making resonators smaller and preparing them using slower cooling rates.…”

“…In metallic glasses, which lack crystalline order, intrinsic losses are envisioned to stem from irreversible, collective atomic rearrangements, or shear-transformation zones (STZs) [18]. A number of studies have characterized the role of collective atomic rearrangements in determining the mechanical properties of metallic glasses, including ductility, yielding, and shear-band formation [19][20][21][22][23].…”

Universal $L^{-3}$ finite-size effects in the viscoelasticity of confined amorphous systems

“…Finally, it would be interesting in future work to study the interplay between confinement or boundary effects like those presented here and other low-k phenomena in condensed matter such as hyperuniformity 40 and its ramifications 41 . Also, our ∼ L −3 law for the shear modulus, derived from the viewpoint of nonaffinity, may connect to the 1/N (where N ∼ L 3 is the number of particles) finite-size correction to the shear modulus observed near the jamming transition of random jammed packings [42][43][44] , a connection that should be explored in future work.…”

Enhancing ductility in bulk metallic glasses by straining during cooling

“…Assuming 2% strain as a critical value to cause a "significant" structural change is based on previous work where this strain level has been observed to cause shear transformation zones to become unstable and cause a "significant" structural irreversible change 55 . However, it has also been pointed out that "weak" shear transformations occur at a much lower strain levels 1,18,56,57 , however significantly less in number.…”