Cun Yu

A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength

S-T-R-E-T-C-H Me Most metals show elastic strain limits well below 1%, beyond which permanent plastic deformation occurs. Metal nanowires can be elastically stretched to much higher strains, on the order of 4 to 7%. However, when placed inside a metal matrix to form a composite, these nanowires can no longer be stretched to the same extent, even when the nanowires are well distributed and show good bonding with the matrix. Hao et al. (p. 1191; see the Perspective by Zhou) used a shape memory alloy as the matrix material to produce a much better (more elastic) composite. The use of a shape-memory metal alloy as a matrix better exploits the inherent elastic properties of niobium nanowires. [Also see Perspective by Zhou] Freestanding nanowires have ultrahigh elastic strain limits (4 to 7%) and yield strengths, but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. We exploited the intrinsic mechanical properties of nanowires in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the true elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we developed an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Youngs modulus of ~28 gigapascals, and a high yield strength of ~1.65 gigapascals. Our elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be exploited in bulk materials.

The ultrahigh mechanical energy-absorption capability evidenced in a high-strength NbTi/NiTi nanocomposite

A nanocomposite composed of NbTi nanowires uniformly embedded in NiTi matrices was fabricated, which exhibits an ultrahigh mechanical-damping capability. The absorption energy measured under an applied 8% strain is up to 54 MJ/m3, which is over three times higher than that (∼16 MJ/m3) found in the well-known Ni-Ti alloys. In-situ synchrotron x-ray diffraction reveals that a redistribution of stress between the nanowires and matrices was evidenced from an abrupt change in residual lattice strains. The ultrahigh mechanical-damping property is attributed to a combination of the strong interaction of nanowires and matrices and the plastic deformation occurring in NbTi nanowires during deformation causing large energy dissipation.

Texture evolution during nitinol martensite detwinning and phase transformation

Nitinol has been widely used to make medical devices for years due to its unique shape memory and superelastic properties. However, the texture of the nitinol wires has been largely ignored due to inherent complexity. In this study, in situ synchrotron X-ray diffraction has been carried out during uniaxial tensile testing to investigate the texture evolution of the nitinol wires during martensite detwinning, variant reorientation, and phase transformation. It was found that the thermal martensitic nitinol wire comprised primarily an axial (1¯20), (120), and (102)-fiber texture. Detwinning initially converted the (120) and (102) fibers to the (1¯20) fiber and progressed to a (1¯30)-fiber texture by rigid body rotation. At strains above 10%, the (1¯30)-fiber was shifted to the (110) fiber by (21¯0) deformation twinning. The austenitic wire exhibited an axial (334)-fiber, which transformed to the near-(1¯30) martensite texture after the stress-induced phase transformation.

A biopolymer-like metal enabled hybrid material with exceptional mechanical prowess

The design principles for naturally occurring biological materials have inspired us to develop next-generation engineering materials with remarkable performance. Nacre, commonly referred to as natures armor, is renowned for its unusual combination of strength and toughness. Natures wisdom in nacre resides in its elaborate structural design and the judicious placement of a unique organic biopolymer with intelligent deformation features. However, up to now, it is still a challenge to transcribe the biopolymers deformation attributes into a stronger substitute in the design of new materials. In this study, we propose a new design strategy that employs shape memory alloy to transcribe the “J-curve” mechanical response and uniform molecular/atomic level deformation of the organic biopolymer in the design of high-performance hybrid materials. This design strategy is verified in a TiNi-Ti3Sn model material system. The model material demonstrates an exceptional combination of mechanical properties that are superior to other high-performance metal-based lamellar composites known to date. Our design strategy creates new opportunities for the development of high-performance bio-inspired materials.

In situ synchrotron X-ray diffraction study of deformation behavior and load transfer in a Ti2Ni-NiTi composite

The deformation behavior and load transfer of a dual-phase composite composed of martensite NiTi embedded in brittle Ti2Ni matrices were investigated by using in situ synchrotron x-ray diffraction during compression. The composite exhibits a stage-wise deformation feature and a double-yielding phenomenon, which were caused by the interaction between Ti2Ni and NiTi with alternative microscopic deformation mechanism. No load transfer occurs from the soft NiTi dendrites to the hard Ti2Ni matrices during the pseudoplastic deformation (detwinning) of NiTi, which is significantly different from that previously reported in bulk metallic glasses matrices composites. (c) 2014 AIP Publishing LLC.

A novel multifunctional NiTi/Ag hierarchical composite

Creating multifunctional materials is an eternal goal of mankind. As the properties of monolithic materials are necessary limited, one route to extending them is to create a composite by combining contrasting materials. The potential of this approach is neatly illustrated by the formation of nature materials where contrasting components are combined in sophisticated hierarchical designs. In this study, inspired by the hierarchical structure of the tendon, we fabricated a novel composite by subtly combining two contrasting components: NiTi shape-memory alloy and Ag. The composite exhibits simultaneously exceptional mechanical properties of high strength, good superelasticity and high mechanical damping, and remarkable functional properties of high electric conductivity, high visibility under fluoroscopy and excellent thermal-driven ability. All of these result from the effective-synergy between the NiTi and Ag components, and place the composite in a unique position in the properties chart of all known structural-functional materials providing new opportunities for innovative electrical, mechanical and biomedical applications. Furthermore, this work may open new avenues for designing and fabricating advanced multifunctional materials by subtly combining contrasting multi-components.

Nanostructured Nb reinforced NiTi shape memory alloy composite with high strength and narrow hysteresis

An in-situ nanostructured Nb reinforced NiTi shape-memory alloy composite was fabricated by mechanical reduction of an as-cast Nb-NiTi eutectic alloy. The composite exhibits large elastic strain, high strength, narrow hysteresis, and high mechanical energy storage density and efficiency during tensile cycling. In situ synchrotron high-energy X-ray diffraction revealed that these superior properties were attributed to the strong coupling between nanostructured Nb and NiTi matrix during deformation. Furthermore, this study offers a good understanding of the deformation behavior of the nanoscale reinforcement embedded in the metal matrix deformed by stress-induced phase transformation.

Evolution of Intergranular Stresses in a Martensitic and an Austenitic NiTi Wire During Loading–Unloading Tensile Deformation

In situ synchrotron X-ray diffraction testing was carried out on a martensitic and an austenitic NiTi wire to study the evolution of internal stresses and the stress-induced martensite (SIM) phase transformation during room temperature tensile deformation. From the point of lattice strain evolution, it is concluded that (1) for the martensitic NiTi wire, detwinning of the [011]B19′ type II twins and the {010}B19′ compound twins is responsible for internal strains formed at the early stage of deformation. (2) The measured diffraction moduli of individual martensite families show large elastic anisotropy and strong influences of texture. (3) For the austenitic NiTi wire, internal residual stresses were produced due to transformation-induced plasticity, which is more likely to occur in austenite families that have higher elastic moduli than their associated martensite families. (4) Plastic deformation was observed in the SIM at higher stresses, which largely decreased the lower plateau stresses.

Influence of Annealing and Pre-Straining on the Coupling Effect of a TiNi-Nb Nanowire Composite

The influences of different annealing temperatures and pre-strains on the coupling effect between TiNi matrix and the Nb nanowire were studied by means of synchrotron X-ray diffraction in form of variation in lattice strain of the nanowire upon temperature changing for four groups of samples. For every annealing temperature, variation in Nb (110) lattice strains initially increasing with increasing pre-strain and then decreased after 12% pre-strain. A maximum variation in Nb (110) lattice strain was observed approaching 2.4% within the sample 450°C-20min annealed and 12% pre-strained. Such variations in lattice strain of the embedded nanowires upon temperature changing provided the composite great potentials as structural-functional integrated two-way actuators.

Novel Ti3Sn based high damping material with high strength

Abstract In this paper, ductile β-Ti was selected to toughen brittle high damping intermetallic compound Ti3Sn. An in situ Ti3Sn/β-Ti composite with a composition of Ti77Mo3Sn20 was prepared by arc melting. The composite simultaneously exhibited high yield strength (500 MPa), large plasticity (35%) and high damping capacity (tan δ>0·06). In situ synchrotron high energy X-ray diffraction compression testing revealed that the β-Ti mainly accounts for the plasticity, while Ti3Sn provided the strength of the composite.