Fangmin Guo

Achieving large linear elasticity and high strength in bulk nanocompsite via synergistic effect

Elastic strain in bulk metallic materials is usually limited to only a fraction of 1%. Developing bulk metallic materials showing large linear elasticity and high strength has proven to be difficult. Here, based on the synergistic effect between nanowires and orientated martensite NiTi shape memory alloy, we developed an in-situ Nb nanowires -orientated martensitic NiTi matrix composite showing an ultra-large linear elastic strain of 4% and an ultrahigh yield strength of 1.8 GPa. This material also has a high mechanical energy storage efficiency of 96% and a high energy storage density of 36 J/cm3 that is almost one order of larger than that of spring steel. It is demonstrated that the synergistic effect allows the exceptional mechanical properties of nanowires to be harvested at macro scale and the mechanical properties of matrix to be greatly improved, resulting in these superior properties. This study provides new avenues for developing advanced composites with superior properties by using effective synergistic effect between components.

Fabrication and Property of W/Tininb Shape Memory Alloy Laminated Composite

W/TiNiNb shape memory alloy laminated composites were fabricated by vacuum hot press, forging and rolling. The microstructure, transformation behavior, mechanical behavior and damping capacity of the laminated composite were investigated by SEM, DSC, DMA and bending test, respectively. The results showed that the W layer and the TiNiNb shape memory alloy layer in the composite was about 15 μm and 5 μm, respectively. The TiNiNb alloy in the composite exhibited the reversible martensite transformation. The composite also had high damping capacity (tanδ=0.03). The three-point bending test showed various plateaus in the stress–strain curve due to delamination processes, which are suitable for enhancing the fracture toughness of the laminates. The flexure strength of the laminated composite was 1260 MPa.

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.

Preparing TiNiNb shape memory alloy powders by hydriding–dehydriding process

High-quality TiNiNb shape memory alloy (SMA) powders were prepared by hydrogenation of cold-worked TiNiNb SMA wire composed of amorphous and nancrystalline microstructure, by mechanical pulverization and vacuum dehydrogenation. It is revealed that abundant structural defects introduced by cold-work greatly promoted hydrogen diffusion, which significantly decreased hydriding temperature and shortened hydriding time. After hydrogenation, the hydrogenated sample composed of TiNiH and NbH with high brittleness can be easily ground into ultra-fine powers. The TiNiNb powers obtained by following vacuum dehydrogenation exhibit almost the same reversible phase transformation behavior as that of the original TiNiNb SMA before cold-work. Moreover, a TiNiNb part was obtained by hot-pressure sintering the hydrogenated powders, where sintering and dehydrogenation are carried out in one single step. The sintered TiNiNb part shows most the same reversible phase transformation behaviors as that of the original TiNiNb SMA and there is no visible additional brittle phase appearance.

Nanocrystalline NiTi shape memory alloys with huge superelasticity and high mechanical damping

Abstract Superelasticity and mechanical damping are important functional properties of nickel–titanium (NiTi) shape memory alloys (SMAs). Owing to the generation and accumulation of dislocation during loading, the recovery strain of commercial NiTi SMAs is usually smaller than 10%, which limits their ability to dissipate energy. In this paper, the superelasticity and mechanical damping of a nanocrystalline NiTi SMA was studied. The results show that the nanocrystalline NiTi SMA alloy possessed a large recovery strain of about 14%, greater than that of the commercial NiTi SMAs, and a high level of absorbed energy, or toughness, of 111 MJ m−3, which is higher than the highest value (about 81 MJ m−3) of all SMAs reported so far. The transmission electron microscopy (TEM) studies suggest that few full dislocations were generated in the nanocrystalline NiTi alloy during loading. Instead, the dominant deformation modes after stress induced martensitic transformation were elastic deformation and detwinning. The detwinning process decreased the twin boundary energy, which stabilised the martensitic phase.