skip to main content

Title: γ -Surfaces for molecular crystal cyclotetramethylene-tetranitramine (β-HMX)

The γ-surface represents the energetic cost associated with relative, rigid body sliding of crystal planes and contains useful information related to plastic deformation of the respective crystal. Here, we present γ-surfaces for the most active glide planes of the energetic molecular crystal cyclotetramethylene-tetranitramine in the monoclinic β phase, i.e., (101) and (011), at pressures up to 15 GPa. We observe the existence of stable staking faults in both planes and at all pressures and report the increase in the stacking fault energy with pressure. We also report the energetic barriers for sliding along minimum energy paths in various directions contained in these planes as well as the critical resolved shear stress at which the crystal becomes unstable in the absence of crystal defects. [100] traces of the γ-surface for multiple planes such as (001), (010), and (021) are further evaluated in view of the previously reported importance of this slip direction for dislocation cross-slip. It is observed that increasing the pressure does not modify the topology of the γ-surface in an essential way, which implies that although barriers for slip increase, the general phenomenology of dislocation motion is not modified qualitatively by the pressure. The energy barriers increase faster with pressure in the (011) plane, and hence, it is implied that the (101) plane is the most active glide plane at high pressures. The results are generally relevant for studies of plastic deformation in this molecular crystal.

more » « less
Author(s) / Creator(s):
Publisher / Repository:
American Institute of Physics
Date Published:
Journal Name:
Journal of Applied Physics
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    The molecular crystal cyclotetramethylene‐tetranitramine (β‐HMX) is the active ingredient in some broadly used plastic bonded explosives. Plasticity is thought to be essential for reaction initiation and detonation. In this work we use molecular simulations to investigate the conditions under which dislocations nucleate homogeneously in β‐HMX. We evaluate the resolved shear stress leading to homogeneous nucleation of dislocation loops in the planes currently considered to be most important for plasticity in this crystal, (011) and (101), and at pressures up to 20 GPa. The importance of homogeneous nucleation for plastic deformation is discussed in relation to the other plasticity mechanisms operating in HMX. It is concluded that homogeneous nucleation is unlikely to happen at pressures above 5 GPa, while at pressures below this threshold homogeneous nucleation competes with shear localization. The article also presents γ‐surfaces for the (011) and (101) planes, which are used here to interpret the nucleation process and critical stresses, and have broader importance in other contexts. A discussion of the procedure required to compute γ‐surfaces in molecular crystals is included.

    more » « less
  2. Abstract

    For brittle friction and rock deformation, the coefficientαin the general effective stress relationσe = σ − αPpcan be approximated as unity with sufficient accuracy. However, it is uncertain ifαdeviates from unity for semibrittle flow when both brittle and intracrystalline‐plastic deformation is involved. We conducted triaxial and isostatic compression experiments on synthetic salt‐rocks (∼300 ppm water) at room temperature to test the effective stress relation in the semibrittle regime using silicone oil and argon gas as pore fluids. Confining and pore pressures were cycled while their difference (differential pressure) was kept constant, such that changes in the mechanical behavior would indicate deviation ofαfrom unity. Microstructural observations were used to determine the dependence ofαon true area of grain contact from asperity yielding. In triaxial compression experiments, semibrittle flow involves grain boundary cracking and sliding, and intragranular dislocation glide and cracking. Flow strength remains constant for changes in pore fluid pressure of more than two orders of magnitude. In isostatic compression experiments, samples show combined processes of microcracking, grain boundary sliding, dislocation glide, and fluid‐assisted grain boundary migration recrystallization. Volumetric strain depends directly on the differential pressures (i.e.,αequals one). Analysis of grain‐contact area in both experiments indicates thatαis independent of the true area of contact defined by plastic yielding at grain boundaries. The observation ofαeffectively equals one may be explained by operation of pressure‐independent intracrystalline‐plastic mechanisms and transmission of pore pressure at grain boundaries through thin fluid films.

    more » « less
  3. Abstract Magnesium, the lightest structural metal, usually exhibits limited ambient plasticity when compressed along its crystallographic c -axis (the “hard” orientation of magnesium). Here we report large plasticity in c -axis compression of submicron magnesium single crystal achieved by a dual-stage deformation. We show that when the plastic flow gradually strain-hardens the magnesium crystal to gigapascal level, at which point dislocation mediated plasticity is nearly exhausted, the sample instantly pancakes without fracture, accompanying a conversion of the initial single crystal into multiple grains that roughly share a common rotation axis. Atomic-scale characterization, crystallographic analyses and molecular dynamics simulations indicate that the new grains can form via transformation of pyramidal to basal planes. We categorize this grain formation as “deformation graining”. The formation of new grains rejuvenates massive dislocation slip and deformation twinning to enable large plastic strains. 
    more » « less
  4. Abstract

    A novel high‐temperature laser shock peening (HT‐LSP) process was applied to polycrystalline α‐SiC to improve the mechanical performance. HT‐LSP prevents microcrack formation on the surface while induces plastic deformation in the form of dislocation slip on the basal planes, which may be caused by the combination of high shock pressure and a lower critical resolved shear stress at 1000℃. A maximum compressive residual stress of 650 MPa, measured with Raman spectroscopy, was introduced into the surface of α‐SiC by HT‐LSP, which can increase the nanohardness and in‐plane fracture toughness of α‐SiC by 8% and 36%, respectively. This work presents a fundamental base for the promising applications of HT‐LSP to brittle ceramics to increase their plasticity and mechanical properties.

    more » « less
  5. Abstract

    The operation of fracture, diffusion, and intracrystalline‐plastic micromechanisms during semibrittle deformation of rock is directly relevant to understanding mechanical behavior across the brittle‐plastic transition in the crust. An outstanding question is whether (1) the micromechanisms of semibrittle flow can be considered to operate independently, as represented in typical crustal strength profiles across the brittle to plastic transition, or (2) the micromechanisms are coupled such that the transition is represented by a distinct rheology with dependency on effective pressure, temperature, and strain rate. We employ triaxial stress‐cycling experiments to investigate elastic‐plastic and viscoelastic behaviors during semibrittle flow in two distinctly different monomineralic, polycrystalline, synthetic salt‐rocks. During semibrittle flow at high differential stress, granular, low‐porosity, work‐hardened salt‐rocks deform predominantly by grain‐boundary sliding and wing‐crack opening accompanied by minor intragranular dislocation glide. In contrast, fully annealed, near‐zero porosity salt‐rocks flow at lower differential stress by intragranular dislocation glide accompanied by grain‐boundary sliding and opening. Grain‐boundary sliding is frictional during semibrittle flow at higher strain rates, but the associated dispersal of water from fluid inclusions along boundaries can activate fluid‐assisted diffusional sliding at lower strain rates. Changes in elastic properties with semibrittle flow largely reflect activation of sliding along closed grain boundaries. Observed microstructures, pronounced hysteresis and anelasticity during cyclic stressing after semibrittle flow, and stress relaxation behaviors indicate coupled operation of micromechanisms leading to a distinct rheology (hypothesis 2 above).

    more » « less