W. Z. Han

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.

High strength and utilizable ductility of bulk ultrafine-grained Cu-Al alloys

Lack of plasticity is the main drawback for nearly all ultrafine-grained (UFG) materials, which restricts their practical applications. Bulk UFG Cu–Al alloys have been fabricated by using equal channel angular pressing technique. Its ductility was improved to exceed the criteria for structural utility while maintaining a high strength by designing the microstructure via alloying. Factors resulting in the simultaneously enhanced strength and ductility of UFG Cu–Al alloys are the formation of deformation twins and their extensive intersections facilitating accumulation of dislocations.

Combined effects of crystallographic orientation, stacking fault energy and grain size on deformation twinning in fcc crystals

The combined effects of crystallographic orientation, stacking fault energy (SFE) and grain size on deformation twinning behavior in several face-centred cubic (fcc) crystals were investigated experimentally and analytically. Three types of fcc crystals, Al single crystals, Cu single crystals and polycrystalline Cu–3% Si alloy with different SFEs and special crystallographic orientations, were selected. The orientations of the Al and Cu single crystals were designed with one of the twinning systems just perpendicular to the intersection plane of equal-channel angular pressing (ECAP). For Al single crystals, no deformation twins were observed after a one-pass ECAP, although a preferential crystallographic orientation was selected for twinning. For Cu single crystals, numerous deformation twins were found even when strained at room temperature and at low strain rate. For Cu–3% Si alloy, deformation twins were only observed in some grains; however, others with different orientations were full of dislocations, although it has the lowest SFE value of the three fcc crystal types. The experimental results provide evidence that SFE and crystallographic orientation have a remarkable influence on the behavior of deformation twinning in fcc crystals. The observations were subsequently analyzed based on fundamental dislocation mechanisms and the grain-size effect. The deformation conditions required for twinning and the variation in twinning stress with SFE, crystallographic orientation and grain size in fcc crystals are also discussed.

Role of interfaces in shock-induced plasticity in Cu/Nb nanolaminates

We investigate deformation of pure Cu, pure Nb and 30 nm Cu/30 nm Nb nanolaminates induced by high strain rate shock loading. Abundant dislocation activities are observed in shocked pure Cu and Nb. In addition, a few deformation twins are found in the shocked pure Cu. In contrast, in shocked Cu/Nb nanolaminates, abundant deformation twins are found in the Cu layers, but only dislocations in the Nb layers. High resolution transmission electron microscopy reveals that the deformation twins in the Cu layers preferentially nucleate from the Cu(112)//Nb(112) interface habit planes rather than the predominant Cu(111)//Nb(110) interface planes. Our comparative study on the shock-induced plastic deformation of the pure metals (Cu and Nb) and the Cu/Nb nanolaminates underscores the critical role of heterogeneous phase interfaces in the dynamic deformation of multilayer materials.

Synthesis of single-component metallic glasses by thermal spray of nanodroplets on amorphous substrates

We show that single component metallic glasses can be synthesized by thermal spray coating of nanodroplets onto an amorphous substrate. We demonstrate this using molecular dynamics simulations of nanodroplets up to 30 nm that the spreading of the nanodroplets during impact on a substrate leads to sufficiently rapid cooling (1012–1013 K/s) sustained by the large temperature gradients between the thinned nanodroplets and the bulk substrate. However, even under these conditions, in order to ensure that the glass transition outruns crystal nucleation, it is essential that the substrate be amorphous (eliminating sites for heterogeneous nucleation of crystallization).

Anisotropic compressive properties of iron subjected to single-pass equal-channel angular pressing

The anisotropic compressive properties and shear deformation mechanism of iron subjected to equal-channel angular pressing (ECAP) with single-pass have been investigated. It was found that the anisotropic compressive properties can be attributed to the effect of the ECAP shear plane. It is suggested that the ECAP shear plane induced by the first pass of ECAP is a relatively weak plane in terms of resisting subsequent shear deformation.

From “Smaller is Stronger” to “Size‐Independent Strength Plateau”: Towards Measuring the Ideal Strength of Iron

Prof. W.-Z. Han, Dr. L. Huang, Q.-J. Li, B.-Y. Liu, Prof. J. Li, Prof. E. Ma, Prof. Z.-W. Shan Center for Advancing Materials Performance from the Nanoscale and Hysitron Applied Research Center in China State Key Laboratory for Mechanical Behavior of Materials Xi’an Jiaotong University Xi’an 710049 , P. R. China E-mail: zwshan@mail.xjtu.edu.cn Prof. S. Ogata, Prof. H. Kimizuka Department of Mechanical Science and Bioengineering Osaka University Osaka 560-8531 , Japan Prof. S. Ogata Center for Elements Strategy Initiative for Structural Materials Kyoto University Kyoto 606-8501 , Japan Prof. Z.-C. Yang Department of Mechanical Engineering and Materials Science and Department of Bioengineering University of Pittsburgh Pittsburgh , PA 15261 , USA Prof. C. Weinberger Sandia National Laboratories Albuquerque , NM 87185 , USA Prof. C. Weinberger Mechanical Engineering and Mechanics Department Drexel University Philadelphia , PA 19104 , USA Prof. X.-X. Zhang Division of Physical Science and Engineering King Abdullah University of Science & Technology Thuwal 23955-6900 , Saudi Arabia E-mail: xixiang.zhang@kaust.edu.sa Prof. J. Li Department of Nuclear Science and Engineering and Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge , MA 02139 , USA Prof. E. Ma Department of Materials Science and Engineering Johns Hopkins University Baltimore , MD 21218 , USA

Shock-induced consolidation and spallation of Cu nanopowders

A useful synthesis technique, shock synthesis of bulk nanomaterials from nanopowders, is explored here with molecular dynamics simulations. We choose nanoporous Cu (∼11 nm in grain size and 6% porosity) as a representative system, and perform consolidation and spallation simulations. The spallation simulations characterize the consolidated nanopowders in terms of spall strength and damage mechanisms. The impactor is full density Cu, and the impact velocity (u_i) ranges from 0.2 to 2 km s^(−1). We present detailed analysis of consolidation and spallation processes, including atomic-level structure and wave propagation features. The critical values of u_i are identified for the onset plasticity at the contact points (0.2 km s^(−1)) and complete void collapse (0.5 km s^(−1)). Void collapse involves dislocations, lattice rotation, shearing/friction, heating, and microkinetic energy. Plasticity initiated at the contact points and its propagation play a key role in void collapse at low u_i, while the pronounced, grain-wise deformation may contribute as well at high u_i. The grain structure gives rise to nonplanar shock response at nanometer scales. Bulk nanomaterials from ultrafine nanopowders (∼10 nm) can be synthesized with shock waves. For spallation, grain boundary (GB) or GB triple junction damage prevails, while we also observe intragranular voids as a result of GB plasticity.