Structural origins of the boson peak in metals: From high-entropy alloys to metallic glasses

Abstract

The boson peak appears in all amorphous solids and is an excess of vibrational states at low frequencies compared to the phonon spectrum of the corresponding crystal. Until recently, the consensus was that it originated from "defects" in the glass. The nature of these defects is still under discussion, but the picture of regions with locally disturbed short-range order and/or decreased elastic constants has gained some traction. Recently, a different theory was proposed: The boson peak was attributed to the first van Hove singularity of crystal lattices which is only smeared out by the disorder. This new viewpoint assumes that the van Hove singularity is simply shifted by the decreased density of the amorphous state and is therefore not a glass-specific anomaly. In order to resolve this issue, we use computer models of a four-component alloy, alternatively with chemical disorder (high-entropy alloy), structural disorder, and reduced density. Comparison to a reference glass of the same composition reveals that the boson peak consists of additional vibrational modes which can be induced solely by structural disorder. While chemical disorder introduces fluctuations of the elastic constants, we find that those do not lead to sufficient local softening to induce these modes. A boson peak due to a reduction of density could be excluded for the present metallic system.

Boson Heat Capacity Peak in Metallic Glasses: Evidence of the Same Defect‐Induced Heat Absorption Mechanism in Structurally Relaxed and Partially Crystallized States

“…(see Ref. for a review). Nonetheless, the widely accepted opinion is that the BP originates from certain “defects” of glass although their nature is not well understood …”

“…These “defects” are often understood as free volumes and “soft spots”, i.e., regions of reduced short‐range, lowered elastic moduli and mechanical strength . An interesting study addressed to the BP nature was recently performed by Brink et al who analyzed computer models of a four‐component CuNiCoFe alloy alternatively with chemical disorder (high‐entropy state), structural disorder, and reduced density. They found that density reduction and fluctuations of the elastic constants cannot be responsible for the BP.…”

From metallic glasses to nanocrystals: Molecular dynamics simulations on the crossover from glass-like to grain-boundary-mediated deformation behaviour

“…The per-atom shear moduli were calculated as described in Ref. 54: Different stress states were applied at 0 K and the stiffness tensor calculated with Hooke's law from the strain response for every atom using ovito's atomic strain analysis [57]. Diagonalisation of this tensor yields five shear moduli [55].…”

Relationship Between the Boson Heat Capacity Peak and the Excess Enthalpy of a Metallic Glass

“…The models suggested for the interpretation of the BP can be divided into two groups (for a review, see refs. ). First of all, it is most often assumed that the BP originates from certain “defects” of the glassy structure, which are related to locally disturbed short‐range order and/or decreased elastic constants .…”

“…An important study of the BP was recently reported by Brink et al They performed molecular dynamic simulation of a four‐component CuNiCoFe alloy alternatively with chemical disorder (high‐entropy state), structural disorder, and reduced density. They found that the density reduction, fluctuations of the elastic constants and shift of the van Hove singularity cannot be responsible for the BP.…”