Thomas Nizolek

Anomalous Basal Slip Activity in Zirconium under High-strain Deformation

In this letter, we reveal anomalous basal slip activity in zirconium under high strains. The frequently reported classical rolling texture of Zr is shown to develop as a result of substantial amounts of basal slip. The reason is not that physical barriers to basal slip have become easier but that over a large straining period, easy prismaticslip has significantly strain-hardened and crystallographic texture has evolved to be more favorable for basal slip. Basal slip is, therefore, an important deformation mechanism in Zr at room temperature under high to severe strain-deformation conditions.

Tensile behavior and flow stress anisotropy of accumulative roll bonded Cu-Nb nanolaminates

The flow stress, ductility, and in-plane anisotropy are evaluated for bulk accumulative roll bonded copper-niobium nanolaminates with layer thicknesses ranging from 1.8 μm to 15 nm. Uniaxial tensile tests conducted parallel to the rolling direction and transverse direction demonstrate that ductility generally decreases with decreasing layer thickness; however, at 30 nm, both high strengths (1200 MPa) and significant ductility (8%) are achieved. The yield strength increases monotonically with decreasing layer thickness, consistent with the Hall-Petch relationship, and significant in-plane flow stress anisotropy is observed. Taylor polycrystal modeling is used to demonstrate that crystallographic texture is responsible for the in-plane anisotropy and that the effects of texture dominate even at nanoscale layer thicknesses.

The Suppression of Instabilities via Biphase Interfaces During Bulk Fabrication of Nanograined Zr

Severe plastic deformation (SPD) is a common method to fabricate nano-grained metals. However for Zr, a structural metal for nuclear applications, obtaining a nanoscale grain structure via SPD has been problematic due to deformation twinning and phase transformations. Here, nanostructured hcp Zr is fabricated through a refinement process via the introduction of a biphase interface. Despite mechanical and thermal conditions known to chemically mix Zr and Nb, no intermixing is observed and the heterophase interfaces appear resistant to phase transformations and twinning. Increasing the density of chemically sharp Zr–Nb interfaces is a very different refinement mechanism than substructure development, stacking fault formation, or alloying.

A multi-scale model for texture development in Zr/Nb nanolayered composites processed by accumulative roll bonding

Recently it has been demonstrated that nanolayered hcp/bcc Zr/Nb composites can be fabricated with a severe plastic deformation technique called accumulative roll bonding (ARB) [1]. The final layer thickness averaged to approximately 90 nm for both phases. Interestingly, the texture measurements show that the textures in each phase correspond to those of rolled single-phase rolled Zr and Nb for a wide range of layer thickness from the micron to the nanoscales. This is in remarkable contrast to fcc/bcc Cu/Nb layered composites made by the same ARB technique, which developed textures that strongly deviated from theoretical rolling textures of Cu or Nb alone when the layers were refined to submicron and nanoscale dimensions. To model texture evolution and reveal the underlying deformation mechanisms, we developed a 3D multiscale model that combines crystal plasticity finite element with a thermally activated dislocation density based hardening law [2]. For systematic study, the model is applied to a two-phase Zr/Nb polycrystalline laminate and to the same polycrystalline Zr and polycrystalline Nb as single-phase metals. Consistent with the measurement, the model predicts that texture evolution in the phases in the composite and the relative activities of the hcp slip modes are very similar to those in the phases in monolithic form. In addition, the two-phase model also finds that no through-thickness texture gradient develops. This result suggests that neither the nanoscale grain sizes nor the bimetal Zr/Nb interfaces induce deformation mechanisms different from those at the coarse-grain scale.

Kink Band Formation in High-strength Bulk Metallic Nanolaminates

1. Materials Department, University of California Santa Barbara, Santa Barbara, CA 93106, USA 2. Institute for Materials Science and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3. Material Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 4. Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA