Overview of constitutive laws, kinematics, homogenization and multiscale methods in crystal plasticity finite-element modeling: Theory, experiments, applications

Overview no. 42 Texture development and strain hardening in rate dependent polycrystals

Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B

Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1443&context=ameslab_manuscripts

Additively manufactured hierarchical stainless steels with high strength and ductility

During dendritic solidification of castings and ingots, a number of processes take place simultaneously within the semisolid region. These include crystallization, solute redistribution, ripening, interdendritic fluid flow, and solid movement. The dendritic structure which forms is greatly affected by convection during the early stages of solidification. In the limit of vigorous convection and slow cooling, grains become spheroidal. Alloys with this microstructure possess rheological properties in the semisolid state which are quite different from those of dendritic alloys. They behave thixotropically, and viscosity can be varied over a wide range, depending on processing conditions. The metal structure and its rheological properties are retained after solidification and partial remelting. The semisolid alloys can be formed in new ways, broadly termed «semisolid metal (SSM) forming processes». Some of these are now employed commercially to produce metal components and are also used to produce metal-matrix composites

Behavior of metal alloys in the semisolid state

Material rate dependence and localized deformation in crystalline solids

An analysis of nonuniform and localized deformation in ductile single crystals

Publisher Summary This chapter focuses on micromechanics of crystals and polycrystals. In Section II of the chapter, a brief outline of only some of the important features of the micromechanics of crystalline plasticity is given. The discussion is confined to plastic flow caused by dislocation slip, and face-centered-cubic crystals are used in the examples of dislocation mechanisms. Particular attention is paid to kinematics and to the phenomenology of strain hardening, because these are shown to play dominant roles in macroscopic response. In Section III, constitutive laws for elastic-plastic crystals are developed. The framework draws heavily on Hill's analysis of the mechanics of elasticplastic crystals, but the theory is extended by incorporating the possibility of deviations from the Schmid rule of a critical resolved shear stress for slip. Deviations from the Schmid rule are motivated by micromechanical models for dislocation motion and are shown to lead to deviations from the “normality flow rule” of continuum plasticity. The implications of these “non-Schmid effects” regarding the stability of plastic flow are brought out via some examples of models for kinks bands and shear bands in Section IV. In Section IV some examples of analyses of elastic-plastic deformation in crystals are discussed. The chapter concludes with some suggestions for fruitful research. These involve extensions of the theory to finite-strain rate-dependent polycrystalline models.

Micromechanics of Crystals and Polycrystals

Strain localization in ductile crystals deforming by single slip was analyzed. The plastic flow is modeled as rate-in-sensitive; and localization, viewed as a bifurcation from a homogeneous deformation mode to one which is concentrated in a narrow ''shear band'', is found to be possible only when the plastic hardening modulus for the slip system has fallen to a certain critical value, h/sub cr/, where h/sub cr/ is sensitive to the precise form of the constitutive law governing incremental shear. The general form of this constitutive law is developed. Incorporated within it is the possibility of deviations from the Schmid rule of a critical resolved shear stress. It is shown that h/sub cr/ may in fact be positive when there are deviations from the Schmid rule. It is suggested that micromechanical processes such as ''cross-slip'' in crystals provide specific cases for which stresses other than the Schmid stress may influence plastic response and, further, there is an experimental association of localization with the onset of large amounts of cross-slip. The specific form of h/sub cr/ is given for a constitutive model that corresponds to the non-Schmid effects in cross-slip, and a dislocation model of the process is developed from which anmore » estimate of the magnitude of the parameters involved is made. The work supports the notion that localization can occur with positive strainhardening, h/sub cr/ greater than 0, and the often-invoked notions of the attainment of an ideally plastic or strain softening state for localization may be unnecessary. 6 figures.« less

http://esag.harvard.edu/rice/066_AsaroRice_SingleCryst_JMPS77.pdf

R.J. Asaro

Papers

Overview no. 42 Texture development and strain hardening in rate dependent polycrystals

Material rate dependence and localized deformation in crystalline solids

An analysis of nonuniform and localized deformation in ductile single crystals

Micromechanics of Crystals and Polycrystals

Strain localization in ductile single crystals