A dislocation-based constitutive law for pure Zr including temperature effects

A crystal plasticity model for hexagonal close packed (HCP) crystals including twinning and de-twinning mechanisms

The plastic properties of anisotropic polycrystalline aggregates and polyphase materials are in general non-homogeneous and, as a consequence, so is the local plastic deformation. We present in this work a model that describes the plastic behaviour of non-homogeneous materials composed of anisotropic regions (grains or phases). Our model is based on describing each region as a viscoplastic inclusion embedded in the effective medium represented by the other grains, and incorporates explicitly the grain interaction with its surroundings and the plastic anisotropy of grain and matrix. Within the model the grain response is coupled to the overall response of the polycrystal and the grain deformation may differ from the polycrystal's. A characteristic of our approach is that those deformation systems with lower critical resolved shear stress tend to be more active, and less than five systems per grain are sufficient to accomodate the imposed overall deformation. In this work we explore the consequences and the limits of the model, and its dependence on the assumed rate sensitivity as well. We combine the self-consistent formulation with a volume fraction transfer scheme for treating the reorientation due to twinning, and simulate rolling textures of brass (f.c.c.), Zircaloy (h.c.p.), calcite (trigonal) and uranium (orthorhombic). We compare the results with experimental measurements and Taylor-type predictions, infer information concerning the microscopic deformation mechanisms and discuss the limits of applicability of the approach.

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A self-consistent viscoplastic model: prediction of rolling textures of anisotropic polycrystals

The important role of deformation twinning in plastic deformation of h.c.p. metals and alloys is emphasized and the twin nucleation mechanisms are examined. A bifurcated homogeneous nucleation model is presented on the basis of the classical nucleation theory, the shape bifurcation theory and the elastic inclusion model. The activation energy for twin formation depends most sensitively on the twin-boundary energy. The structures and energies of (1122) and (1011) coherent twin boundaries are obtained by atomistic simulations using the Lennard-Jones potential for a model h.c.p. metal. The anisotropic coupling effect of normal stresses on twin disolocation mobility is determined over a wide temperature range (0-1200 K). The available experimental data on titanium and zirconium are consistent with the prediction from the proposed nucleation model and the temperature-dependent mobility of twin dislocations. Alloying effects on twinning and future research areas are discussed.

Deformation twinning in h.c.p. metals and alloys

A review is presented on the role of dislocation cores and planar faults in activating the nonbasal deformation modes, pyramidal slip and deformation twinning, in hcp metals and alloys and in D019 intermetallic compounds. Material-specific mechanical behavior arises from a competition between alternate defect structures that determine the deformation modes. We emphasize the importance of accurate atomistic modeling of these defects, going beyond simple interatomic energy models. Recent results from both experiments and theory are summarized by discussing specific examples of Ti and Mg single crystals; Ti-, Zr-, and Mg-base alloys; and Ti3Al ordered alloys. Remaining key issues and directions for future research are also discussed.

A. Akhtar

Papers

Compression of zirconium single crystals parallel to the c-axis

The nature of -γ-hydride in crept zirconium single crystals