John S. Carpenter

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.

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.

Thermal Stability of Cu-Nb Nanolamellar Composites Fabricated via Accumulative Roll Bonding

In situ annealing within a neutron beam line and ex situ annealing followed by transmission electron microscopy were used to study the thermal stability of the texture, microstructure, and bi-metal interface in bulk nanolamellar Cu/Nb composites (h = 18 nm individual layer thickness) fabricated via accumulative roll bonding, a severe plastic deformation technique. Compared to the bulk single-phase constituent materials, the nanocomposite is two orders of magnitude higher in hardness and significantly more thermally stable, e.g., no observed recrystallization in Cu at temperatures as high as 85% of the melting temperature. The nanoscale h = 18 nm individual layer thickness is maintained up to 500°C, the lamellar structure thickens but is maintained up to 700°C, and recrystallization is suppressed even up to 900°C. With increasing temperature, the texture sharpens, and among the interfaces found in the starting material, the {112}Cu || {112}Nb interface with a Kurdjumov-Sachs orientation relationship shows the greatest thermal stability. Our results suggest that thickening of the individual layers under heat treatment coincides with thermally driven removal of energetically unfavorable bi-metal interfaces. Thus, we uncover a temperature regime that maintains the lamellar structure but alters the interface distribution such that a single, low energy, thermally stable interface prevails.

Engineering Interface Structures and Thermal Stabilities via SPD Processing in Bulk Nanostructured Metals

Nanostructured metals achieve extraordinary strength but suffer from low thermal stability, both a consequence of a high fraction of interfaces. Overcoming this tradeoff relies on making the interfaces themselves thermally stable. Here we show that the atomic structures of bi-metal interfaces in macroscale nanomaterials suitable for engineering structures can be significantly altered via changing the severe plastic deformation (SPD) processing pathway. Two types of interfaces are formed, both exhibiting a regular atomic structure and providing for excellent thermal stability, up to more than half the melting temperature of one of the constituents. Most importantly, the thermal stability of one is found to be significantly better than the other, indicating the exciting potential to control and optimize macroscale robustness via atomic-scale bimetal interface tuning. Taken together, these results demonstrate an innovative way to engineer pristine bimetal interfaces for a new class of simultaneously strong and thermally stable materials.

A wedge-mounting technique for nanoscale electron backscatter diffraction

The practical spatial resolution of electron backscatter diffraction (EBSD) is around 100 nm, which limits the length scales from which phase and orientation relationship characterization can be accomplished. This precludes collection of statistically relevant data on the crystallography of interfaces within nanomaterials where such information is essential for understanding the unique properties of these materials. In this work, we present a wedge-mounting technique that enables EBSD data to be collected for sub-100 nm thick layers of Cu-Nb bimetallic multilayers fabricated via accumulative roll bonding. We present statistics on layer thickness distributions, grain morphology, orientation distributions, twin volume fraction, and interface character for material with an averaged layer thickness of 86 and 56 nm.

Strain rate sensitivity and activation volume of Cu/Ni metallic multilayer thin films measured via micropillar compression

Micropillar compression testing with repeated jumps in strain rate is used to circumvent inherent difficulties associated with nanoindentation and tensile testing of free-standing films. Application to sputtered 21 nm/21 nm Cu/Ni multilayer thin films with a cube-on-cube texture reveals an average strain rate sensitivity (m = 0.014) and activation volume (V = 17 b3), comparable to nanocrystalline face-centered cubic metals. Yet, m increases by ∼50% and V decreases by 70% with increasing strain, opposite to trends reported for nanotwinned Cu. The large, strain-dependent shifts in m and V are dependent on the underlying misfit dislocation structure of Cu/Ni interfaces.

The critical role of grain orientation and applied stress in nanoscale twinning

Numerous recent studies have focused on the effects of grain size on deformation twinning in nanocrystalline fcc metals. However, grain size alone cannot explain many observed twinning characteristics. Here we show that the propensity for twinning is dependent on the applied stress, grain orientation and stacking fault energy. The lone factor for twinning dependent on grain size is the stress necessary to nucleate partial dislocations from a boundary. We use bulk processing of controlled nanostructures coupled with unique orientation mapping at the nanoscale to show the profound effect of crystal orientation on deformation twinning. Our theoretical model reveals an orientation-dependent critical threshold stress for twinning, which is presented in the form of a generalized twinnability map. Our findings provide a newfound orientation-based explanation for the grain size effect: as grain size decreases the applied stress needed for further deformation increases, thereby allowing more orientations to reach the threshold stress for twinning.

An interface facet driven Rayleigh instability in high-aspect-ratio bimetallic nanolayered composites

One limitation hindering the structural and electrical applications of nanostructured metals is the loss of their nanostructure and strength under elevated temperatures. Nanostructured metals often have grain structures that contain a high density of triple junctions, where thermally induced instabilities commonly initiate. Prior work has resulted in fabrication of nanolayered two-phase composites that possess high-aspect ratio grains, a scarcity of triple junctions, and a thermally stable microstructure. In this work, transmission electron microscopy is used to investigate how these composites could eventually breakdown during heating. We reveal an unconventional thermal instability mechanism in this class of materials, which operates without the assistance of triple junctions. The mechanism can be rationalized by that thermally induced pinch off occurs as the result of bimetal interface faceting and can trigger a classic Rayleigh instability.

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.