Nathan A. Mara

High strain rate superplasticity in a friction stir processed 7075 Al alloy

In this paper, the authors report the first results using friction stir processing (FSP). In the last ten years, a new technique of Friction Stir Welding (FSW) has emerged as an exciting solid state joining technique for aluminum alloys. This technique, developed by The Welding Institute (TWI), involves traversing a rotating tool that produces intense plastic deformation through a stirring action. The localized heating is produced by friction between the tool shoulder and the sheet top surface, as well as plastic deformation of the material in contact with the tool. This results in a stirred zone with a very fine grain size in a single pass. Mahoney et al. observed a grain size of 3 {micro}m in a 7075 Al alloy. This process can be easily adopted as a processing technique to obtain fine grain size. FSP of a commercial 7075 Al alloy resulted in significant enhancement of superplastic properties. The optimum superplastic strain rate was 10{sup {minus}2}s{sup {minus}1} at 490 C in the FSP 7075 Al alloy, an improvement of more than an order of magnitude in strain rate. The present results suggest an exciting possibility to use a simple FSP technique to enhance grain size dependent properties.

Deformability of ultrahigh strength 5 nm Cu/Nb nanolayered composites

In this work, micropillar compression testing has been used to obtain stress-strain curves for sputter-deposited Cu–Nb nanolaminate composites with nominal bilayer thickness of 10nm. In addition to the extremely high flow strength of 2.4GPa, the 5nm Cu∕5nm Nb nanolaminate exhibits significant ductility, in excess of 25% true strain.

High-strength and thermally stable bulk nanolayered composites due to twin-induced interfaces

Bulk nanostructured metals can attribute both exceptional strength and poor thermal stability to high interfacial content, making it a challenge to utilize them in high-temperature environments. Here we report that a bulk two-phase bimetal nanocomposite synthesised via severe plastic deformation uniquely possesses simultaneous high-strength and high thermal stability. For a bimetal spacing of 10 nm, this composite achieves an order of magnitude increase in hardness of 4.13 GPa over its constituents and maintains it (4.07 GPa), even after annealing at 500 °C for 1 h. It owes this extraordinary property to an atomically well-ordered bimaterial interface that results from twin-induced crystal reorientation, persists after extreme strains and prevails over the entire bulk. This discovery proves that interfaces can be designed within bulk nanostructured composites to radically outperform previously prepared bulk nanocrystalline materials, with respect to both mechanical and thermal stability.

Design of Radiation Tolerant Materials Via Interface Engineering

A novel interface engineering strategy is proposed to simultaneously achieve superior irradiation tolerance, high strength, and high thermal stability in bulk nanolayered composites of a model face-centered-cubic (Cu)/body-centered-cubic (Nb) system. By synthesizing bulk nanolayered Cu-Nb composites containing interfaces with controlled sink efficiencies, a novel material is designed in which nearly all irradiation-induced defects are annihilated.

Mechanism for shear banding in nanolayered composites

Recent studies have shown that two-phase nanocomposite materials with semicoherent interfaces exhibit enhanced strength, deformability, and radiation damage resistance. The remarkable behavior exhibited by these materials has been attributed to the atomistic structure of the bimetal interface that results in interfaces with low shear strength and hence, strong barriers for slip transmission due to dislocation core spreading along the weak interfaces. In this work, the low interfacial shear strength of Cu/Nb nanoscale multilayers dictates a new mechanism for shear banding and strain softening during micropillar compression. Our findings, supported by molecular dynamics simulations, provide insight on the design of nanocomposites with tailored interface structures and geometry to obtain a combination of high strength and deformability. High strength is derived from the ability of the interfaces to trap dislocations through relative ease of interfacial shear, while deformability can be maximized by controlli...

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.

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.

Strong and ductile nanostructured Cu-carbon nanotube composite

Nanocrystalline carbon nanotube (CNT)—reinforced Cu composite (grain size <25 nm) with high strength and good ductility was developed. Pillar testing reveals that its strength and plastic strain could be as large as 1700 MPa and 29%, respectively. Compared with its counterpart made under the same condition, an addition of 1 wt % CNTs leads to a dramatic increase in strength, stiffness and toughness without a sacrifice in ductility. Microstructural analysis discloses that in the Cu matrix, CNTs could be distributed either at grain boundaries or inside grains and could inhibit dislocation nucleation and motion, resulting in an increase in the strength.

Atomic-level study of twin nucleation from face-centered-cubic/body-centered-cubic interfaces in nanolamellar composites

We report deformation twinning in Cu within accumulative roll-bonded Cu-Nb nanolamellar composites. Twins appear connected to the Nb{112}//Cu{112} interface with the Kurdjumov-Sachs orientation relationship, which we show to be ordered and faceted. The interface adopts a different faceted structure after twinning. Our analysis suggests that deformation twinning involves facet dissociation and slip-transfer from the Nb layer to the Cu layer due to a geometrically favorable slip transmission pathway.

Emergence of stable interfaces under extreme plastic deformation

Significance Many processing techniques, such as solid-state phase transformation, epitaxial growth, or solidification, can make nanocomposite materials with preferred crystallographic orientation relationships at internal interfaces. On the other hand, metal-working techniques can make composites in bulk quantities for structural applications but typically the resulting bimetal interfaces lack crystallographic order and are unstable with respect to heating. Using a metal-working roll-bonding technique, we find that at extreme plastic strains, the bimetal interfaces develop a remarkably ordered, preferred atomic structure. Using atomic-scale and crystal-plasticity simulations, we study the dynamical stability conditions responsible for this counterintuitive phenomenon. We show that the emergent interface corresponds to a unique stable state, which leads to exceptional mechanical, thermal, and irradiation stability of the nanocomposite. Atomically ordered bimetal interfaces typically develop in near-equilibrium epitaxial growth (bottom-up processing) of nanolayered composite films and have been considered responsible for a number of intriguing material properties. Here, we discover that interfaces of such atomic level order can also emerge ubiquitously in large-scale layered nanocomposites fabricated by extreme strain (top down) processing. This is a counterintuitive result, which we propose occurs because extreme plastic straining creates new interfaces separated by single crystal layers of nanometer thickness. On this basis, with atomic-scale modeling and crystal plasticity theory, we prove that the preferred bimetal interface arising from extreme strains corresponds to a unique stable state, which can be predicted by two controlling stability conditions. As another testament to its stability, we provide experimental evidence showing that this interface maintains its integrity in further straining (strains > 12), elevated temperatures (> 0.45 Tm of a constituent), and irradiation (light ion). These results open a new frontier in the fabrication of stable nanomaterials with severe plastic deformation techniques.