Semiquantitative analysis of surface relief due to martensite formation in Fe-Mn-Si-based shape memory alloys by atomic force microscopy

Semiquantitative analysis of the surface relief caused by martensite formation in Fe-Mn-Si-based shape memory alloys has been performed by atomic force microscopy. It is found, for the thermally induced martensite transformation, that all three possible variants of martensite with the same {111} habit plane appear while, for the stress-induced martensite transformation, only one martensite variant is likely to form. For the former case, martensite plates with various variants are formed on the same habit plane in such a way that the shape strains of those plates are self-accommodated macroscopically, but each martensite plate itself is a single variant and not a multivariant plate as has been reported in the literature. For the latter case, it is especially emphasized that all the martensite plates formed in the well 'trained' sample have the same variant, which is one of the most important conditions for exhibiting a good shape memory effect.     

Structure of twins in GaAs nanowires grown by the vapour–liquid–solid process

Semiconducting crystalline materials that are poor conductors of heat are important as thermoelectric materials and for technological applications involving thermal management. A combination of neutron scattering, lowtemperature ultrasonic attenuation and thermal conductivity measurements are reported on single crystals of the semiconductors Sr₈Ga₁₆Ge₃₀ and Ba₈Ga₁₆Ge₃₀. Taken together, these measurements suggest specific structural features that result in a crystal with the lowest possible thermal conductivity, namely that of a glass with the same chemical composition. Weakly bound atoms that 'rattle' within oversized atomic cages in a crystal result in a low thermal conductivity, but the present data show that both 'rattling' atoms and tunnelling states are necessary to produce a true glass-like thermal conductivity.     

Deformation mechanism crossover and mechanical behaviour in nanocrystalline materials

We use molecular dynamics simulations to elucidate the transition with decreasing grain size from a dislocation- to a grain-boundary-based deformation mechanism in nanocrystalline fcc metals. Our simulations reveal that this crossover is accompanied by a pronounced transition in the mechanical behaviour of the material; namely, at the grain size where the crossover occurs (the 'strongest size'), the strain rate under tensile elongation goes through a minimum. This simultaneous transition in both the deformation mechanism and the corresponding mechanical behaviour offers an explanation for the experimentally observed crossover in the yield strength of nanocrystalline materials, from Hall-Petch hardening to 'inverse Hall-Petch' softening.     

Effect of Mg content on Portevin–Le Chatelier band strain in Al–Mg sheet alloys

Experimental measurements of stress drops and band strains in type-B Portevin–Le Chatelier (PLC) bands were carried out for 5xxx series Al–Mg sheets with Mg content between 1.8 and 4.5?wt%. While the stress drops increase with global strain, the band strain values in all the samples follow a common linear relationship with global strain. The results indicate that the type-B PLC band strain is independent of solute content at given strain rate.     

Valence band structure of the NiAl–Mo alloy from the photoelectron spectrum

The valence band structures of the NiAl–Mo alloy was investigated by photoelectron spectroscopy. The valence band spectra of the NiAl–Mo alloy was shifted away from the Fermi level so that the Ni-d-band centroid moved to a higher energy by 0.22 eV as Mo was added. A possible explanation lied in the overlap of Ni-d bands in the energy with Mo-d and Al-p bands. The participation of Mo-d bands was correlated with the site preference of Mo in NiAl alloys.     

A structural model for surface-enhanced stabilization in some metallic glass formers

A structural model for surface-enhanced stabilization in some metallic glass formers is proposed. In this model, the alloy surface structure is represented by five-layer Kagomé-net-based lateral ordering. Such surface structure has intrinsic abilities to stabilize icosahedral-like short-range order in the bulk, acting as ‘a cloak of liquidity’. In particular, recent experimental observations of surface-induced lateral ordering and a very high glass forming ability of the liquid alloy Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 can be united using this structural model. This model may be useful for the interpretation of surface structure of other liquid alloys with a high glass forming ability. In addition, it suggests the possibility of guiding the design of the surface coating of solid containers for the stabilization of undercooled liquids.     

Quasi-two-dimensional or hcp palladium nanoparticles with host-guest interaction prepared at room temperature

Quasi-two-dimensional palladium nanoparticles with an average lateral dimension of 7 nm have been prepared by reduction of a PdCl₂ graphite intercalation compound precursor by lithium-diphenylide in tetrahydrofuran at room temperature. Selected-area electron diffraction patterns provide evidence that the palladium nanoparticles are hcp single-crystal particles. Owing to the template effect of the graphite lattice, the lattice parameter of palladium was found to have a strong relation with the graphite, and a 3a _graphite superstructure was inferred. The palladium structure is rotated by 30^o with regard to the carbon host lattice. Raman spectroscopy on this sample showed that a charge transfer between carbon and palladium occurs. The sample can be considered as a common Pd-graphite intercalation compound with palladium nanoparticles as guest. This behaviour is different from palladium nanoparticles prepared by hydrogen reduction at higher temperatures from the same precursor material. These particles may represent an early stage of nanoparticle formation.     

Crystal growth and defect study of BiSbTe3

BiSbTe₃ single crystals have been grown by the Bridgman technique. Microscopic observations of the as-grown crystals reveal typical features, such as striations on the top free surface, which are attributed to the effect of growth conditions. A nitric-acid-based reagent capable of revealing dislocations has been developed and tested. Etch pits are produced at the dislocation sites, but some discrepancies have been observed on matched cleavage surfaces. The structural difference between the matched cleavage surfaces is discussed.