Abstract
Volume changes accompanying the plastic deformation at 300 K of nanocrystalline samples of α-Fe with a columnar grain structure possessing a ⟨11¯0⟩ random fiber texture has been obtained from molecular dynamics (MD) simulations. The samples were strained in tension along the common axial direction of the columnar grains. After removal of the elastic volume change, the evolution of plastic volume strain was obtained. Small but non-negligible volume dilations or contractions are observed depending on size (samples of very small grain size show volume contraction). The rate of volume change is high during the first 10% plastic deformation and continues at a low pace thereafter; the first 10% deformation represents a transient in the stress–strain behavior too. The complex behavior observed is reasonably explained by the superposition of contributions from different plastically-induced structural changes to the mass density change: Mainly from changes of grain size, grain boundary structure, dislocation density and density of point-defects. The results are of interest for the development of crystal plasticity theories not restricted by the volume conserving assumption.
Highlights
Volume invariance by plastic deformation of fully dense crystalline materials is commonly assumed in continuum theories of deformation
In this note we present results on plastic volume strains incurred by nanostructured αFe samples deformed by unidirectional elongation up to moderate/large total strains of about 50% at room temperature (300 K) under zero hydrostatic external pressure
At first sight the observed behavior may be surprising, but it can be understood bearing in mind mind both; the initial volume strain of the samples relative to the unstressed perfect lattice and the both; the initial volume strain of the samples relative to the unstressed perfect lattice and the subsequent structural changes induced by plastic deformation
Summary
Volume invariance by plastic deformation of fully dense crystalline materials is commonly assumed in continuum theories of deformation. Standard experimental techniques of density measurement (hydrostatic weighing, helium pycnometry) detect small changes after moderate/large strains (e.g., increasing dilations in unidirectional strain paths or in fatigue) These changes are partly (but not exclusively) due to nucleation and growth of microscopic internal discontinuities (cracks or voids) covered by the term “structural damage” (strongly dependent on the material “cleanliness”). The existence and effects of such “structural damage”, linked to initial porosity or to ongoing brittle or ductile local fractures, is effectively considered by ad hoc modifications of continuum plastic theories [1] It is well known, that crystal plasticity micro-mechanisms involve the presence and evolution of concentrations of lattice defects with associated volume strain [2,3,4,5,6]. The last reference is mainly motivated by the implications of plastic volume changes associated with point defects (mainly, vacancies and their clusters); volume changes are Metals 2020, 10, 1649; doi:10.3390/met10121649 www.mdpi.com/journal/metals
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