Critical length scales for the deformation of amorphous metals containing nanocrystals

Abstract

Shear localization is studied in simulated amorphous systems containing individual nanocrystalline inclusions. Systematic variation of the inclusion diameter and the shear band thickness reveals a crossover in length scales that separates distinct plastic flow mechanisms in and around the nanocrystalline inclusion. When considered relative to the shear band thickness, small inclusions deform via heterogeneous, interface-dominated mechanisms, while large inclusions yield via the homogeneous nucleation of dislocations in the nanocrystal interior; nanocrystals roughly twice as large as the shear band width are required for the strongest interaction.The mechanisms and mechanics of deformation in amorphous materials have been the subject of extensive investigation by both experiment and simulation (see, for example, [1][2][3][4][5][6][7][8][9][10][11][12]). The process of shear localization is of particular interest in this regard, as shear banding is often the dominant means of plastic strain accommodation in amorphous solids and, therefore, governs the macroscopic mechanical response. From recent works on amorphous metals in particular, it is becoming clear that crystalline inclusions embedded within an amorphous system can also play a critical role in the system mechanics [13][14][15]. Such crystallites may be present either as an unintended consequence of the processing or mechanical history of a specimen [16][17][18], or perhaps by design, with the intent to manipulate the mechanical properties of a glass [19][20][21][22][23][24].Although there is a growing experimental database of macroscopic mechanical properties for amorphous metals containing nanocrystal inclusions, the mechanisms by which embedded nanocrystals influence glass plasticity are unclear. For example, it is generally known that partial devitrification increases the hardness of amorphous metals [17,21,22], although it is unclear whether the hardening mechanism is, for example, a chemical effect stemming from phase separation, a mechanical suppression or dispersion of shear bands, or some combination thereof [21,23].

Criteria for tensile plasticity in Cu–Zr–Al bulk metallic glasses

“…Moreover, further computer simulations have revealed that especially large nanocrystals (about twice as large as the shear band thickness) can effectively interact with nascent shear bands [38]. The grain sizes observed after fracture are on the right order of magnitude to ensure an interaction between the crystals and the local deformation concentrated in shear bands ( Fig.…”

Critical Crystallization for Embrittlement in Metallic Glasses

“…However, no clear trend has been observed for the fracture strength. In some cases the competing crystal's fracture strength has been found to be higher [3,14,[52][53][54] and in others lower [40,47,48,51,[55][56][57][58] than the fracture strength of the metallic glass. Therefore, we consider two cases here: (i) , and (ii) , .…”

“…For the majority of BMG forming alloys, partial crystallization has been shown to degrade properties [12][13][14][15][16][17]. However, it has also been reported that in some cases, partial crystallization enhances fracture toughness [3,14,18,19]. To date, a quantitative understanding of the effect of partial crystallization of metallic glass forming alloys on their fracture toughness and hence, a mechanistic understanding, is lacking.…”

“…crystal), as well as their interface [35,36]. For example, the secondary phase can either increase or decrease the fracture toughness of a composite, which depends on the mechanical properties, volume fraction, size of the secondary phase, and interfacial strength between the matrix and the secondary phase [14,16,18,[37][38][39]. In order to develop a mechanistic understanding of the effect of crystallinity on the fracture toughness of BMGs, we calculated the crystals' contribution to the strength of the composite.…”

Deformation behavior of metallic glass composites reinforced with shape memory nanowires studied via molecular dynamics simulations

“…Moreover, in the presence of nanoscale precipitates multiple SBs form, uniformly distributed through the BMG composite, increasing in this way the resistance against catastrophic crack propagation along one dominant SB. 6 A detailed atomistic understanding of the underlying mechanism has been provided by molecular dynamics (MD) simulations, [7][8][9][10][11] showing that multiple SBs are nucleated at the amorphous-crystalline interface and are blocked by crystalline particles.…”