658 research outputs found
Crystal nucleation in glass-forming alloy and pure metal melts under containerless and vibrationless conditions
The undercooling behavior of large spheroids of Pd40Ni40P40 was investigated. By surface etching, supporting the specimens on a fused silica substrate, and successive heating and cooling, crystallization can be eliminated, presumable due to the removal of surface heterogeneities. By this method samples up to 3.2g with a 0.53 mm minor diameter, were made entirely glassy, except for some superficial crystals comprising less than 0.5% of the volume. These experiments show that a cooling rate of approximately 1 K/sec is adequate to avoid copious homogeneous nucleation in the alloy, and that by eliminating or reducing the effectiveness of heterogeneous nucleation sites, it is possible to form bulk samples of this metallic glass with virtually unlimited dimensions
Dynamics of Shear-Transformation Zones in Amorphous Plasticity: Formulation in Terms of an Effective Disorder Temperature
This investigation extends earlier studies of a shear-transformation-zone
(STZ) theory of plastic deformation in amorphous solids. My main purpose here
is to explore the possibility that the configurational degrees of freedom of
such systems fall out of thermodynamic equilibrium with the heat bath during
persistent mechanical deformation, and that the resulting state of
configurational disorder may be characterized by an effective temperature. The
further assumption that the population of STZ's equilibrates with the effective
temperature allows the theory to be compared directly with experimentally
measured properties of metallic glasses, including their calorimetric behavior.
The coupling between the effective temperature and mechanical deformation
suggests an explanation of shear-banding instabilities.Comment: 29 pages, 11 figure
Shear-transformation-zone theory of plastic deformation near the glass transition
The shear-transformation-zone (STZ) theory of plastic deformation in
glass-forming materials is reformulated in light of recent progress in
understanding the roles played the effective disorder temperature and entropy
flow in nonequilibrium situations. A distinction between fast and slow internal
state variables reduces the theory to just two coupled equations of motion, one
describing the plastic response to applied stresses, and the other the dynamics
of the effective temperature. The analysis leading to these equations contains,
as a byproduct, a fundamental reinterpretation of the dynamic yield stress in
amorphous materials. In order to put all these concepts together in a realistic
context, the paper concludes with a reexamination of the experimentally
observed rheological behavior of a bulk metallic glass. That reexamination
serves as a test of the STZ dynamics, confirming that system parameters
obtained from steady-state properties such as the viscosity can be used to
predict transient behaviors.Comment: 15 pages, four figure
Athermal Shear-Transformation-Zone Theory of Amorphous Plastic Deformation I: Basic Principles
We develop an athermal version of the shear-transformation-zone (STZ) theory
of amorphous plasticity in materials where thermal activation of irreversible
molecular rearrangements is negligible or nonexistent. In many respects, this
theory has broader applicability and yet is simpler than its thermal
predecessors. For example, it needs no special effort to assure consistency
with the laws of thermodynamics, and the interpretation of yielding as an
exchange of dynamic stability between jammed and flowing states is clearer than
before. The athermal theory presented here incorporates an explicit
distribution of STZ transition thresholds. Although this theory contains no
conventional thermal fluctuations, the concept of an effective temperature is
essential for understanding how the STZ density is related to the state of
disorder of the system.Comment: 7 pages, 2 figures; first of a two-part serie
Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding
Using molecular dynamics simulation, we examine the dynamics of crystal,
polycrystal, and glass in a Lennard-Jones binary mixture composed of small and
large particles in two dimensions. The crossovers occur among these states as
the composition c is varied at fixed size ratio. Shear is applied to a system
of 9000 particles in contact with moving boundary layers composed of 1800
particles. The particle configurations are visualized with a sixfold
orientation angle alpha_j(t) and a disorder variable D_j(t) defined for
particle j, where the latter represents the deviation from hexagonal order.
Fundamental plastic elements are classified into dislocation gliding and grain
boundary sliding. At any c, large-scale yielding events occur on the acoustic
time scale. Moreover, they multiply occur in narrow fragile areas, forming
shear bands. The dynamics of plastic flow is highly hierarchical with a wide
range of time scales for slow shearing. We also clarify the relationship
between the shear stress averaged in the bulk region and the wall stress
applied at the boundaries.Comment: 17 pages, 15 figures, to appear in Physical Review
Dynamics of Viscoplastic Deformation in Amorphous Solids
We propose a dynamical theory of low-temperature shear deformation in
amorphous solids. Our analysis is based on molecular-dynamics simulations of a
two-dimensional, two-component noncrystalline system. These numerical
simulations reveal behavior typical of metallic glasses and other viscoplastic
materials, specifically, reversible elastic deformation at small applied
stresses, irreversible plastic deformation at larger stresses, a stress
threshold above which unbounded plastic flow occurs, and a strong dependence of
the state of the system on the history of past deformations. Microscopic
observations suggest that a dynamically complete description of the macroscopic
state of this deforming body requires specifying, in addition to stress and
strain, certain average features of a population of two-state shear
transformation zones. Our introduction of these new state variables into the
constitutive equations for this system is an extension of earlier models of
creep in metallic glasses. In the treatment presented here, we specialize to
temperatures far below the glass transition, and postulate that irreversible
motions are governed by local entropic fluctuations in the volumes of the
transformation zones. In most respects, our theory is in good quantitative
agreement with the rich variety of phenomena seen in the simulations.Comment: 16 pages, 9 figure
Direct Observation of Entropic Stabilization of bcc Crystals Near Melting
Crystals with low latent heat are predicted to melt from an entropically stabilized body-centered cubic symmetry. At this weakly first-order transition, strongly correlated fluctuations are expected to emerge, which could change the nature of the transition. Here we show how large fluctuations stabilize bcc crystals formed from charged colloids, giving rise to strongly power-law correlated heterogeneous dynamics. Moreover, we find that significant nonaffine particle displacements lead to a vanishing of the nonaffine shear modulus at the transition. We interpret these observations by reformulating the Born-Huang theory to account for nonaffinity, illustrating a scenario of ordered solids reaching a state where classical lattice dynamics fail.This work was supported by the National Science Foundation (DMR-1310266, DMR-1206765), the Harvard Materials Research Science and Engineering Center (DMR-1420570), and NASA (NNX13AQ48G)
Boundary lubrication with a glassy interface
Recently introduced constitutive equations for the rheology of dense,
disordered materials are investigated in the context of stick-slip experiments
in boundary lubrication. The model is based on a generalization of the shear
transformation zone (STZ) theory, in which plastic deformation is represented
by a population of mesoscopic regions which may undergo non affine deformations
in response to stress. The generalization we study phenomenologically
incorporates the effects of aging and glassy relaxation. Under experimental
conditions associated with typical transitions from stick-slip to steady
sliding and stop start tests, these effects can be dominant, although the full
STZ description is necessary to account for more complex, chaotic transitions
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