[en] This paper investigates the atomic scale buckling of diamond carbon coatings on silicon substrate caused by residual stresses in two orthogonal directions. It was found that different buckling patterns occurred when the ratio of the residual stresses in the two directions were changed. The size of wrinkles increased on going from uniaxial to biaxial compression of the residual stress fields. A telephone-cord like buckling mode took place when the residual stresses were bi-axially equal

[en] The onset of the lamellar decomposition (α→α₂+γ) in a titanium aluminide alloy containing Nb and Zr has been studied by transmission electron microscopy. Samples water-quenched from the solution-treatment temperature of 1350 deg. C show fault-like features resembling those reported previously as the precursors for the formation of the γ lamellae. High-resolution lattice images obtained from such features have revealed that the 'faults' are actually embryonic γ lamellae, just a few atomic layers in thickness, which clearly exhibit the ordered L1₀ structure. This implies that the γ phase is formed directly, rather than via some intermediate disordered face-centred-cubic phase as suggested previously. Moreover, the character and configuration of the interfacial defects is consistent with this occurring in a diffusive-displacive manner with short-range fluxes across the risers of mobile perfect interfacial disconnections

[en] The atomic rearrangement in metallic glass (MG) is a dynamics process. However, this problem has always been investigated within the quasi-static limit of the material. As such, the physical mechanisms of how the local rearrangements nucleate and coalesce into shear bands have not been fully understood. This study is to clarify the issue with the aid of a systematic molecular dynamics analysis. The present study unveils that the underlying mechanism of plastic deformation is through the rearrangement of atoms, characterized by the sudden surge in kinetic energy or strain rate of a local region, and that the shear banding is a stress-driven dynamic coalescence of these rearranging clusters, propagating in the speed of a shear wave. The ratio of the internal strain rate in forming a cluster to the applied strain rate is a measure of the severity of the local atomic rearrangement. The larger the severity, the easier the shear banding forms. The additional kinetic energy associated with the atom rearrangement in clustering is due to the descending of the potential energy after crossing the energy barrier. As temperature increases, the thermal vibration energy becomes larger than the barrier height, leading to thermal activations in MG and hence giving rise to a homogeneous deformation

[en] A tensor representation of the Green's functions is given in the context of the φ^κ field theory. This approach rests on the fact that the interaction term is a tensor on the infinite-dimensional space of field configurations. This representation can be regarded as a simple and clear algebraic form of Feynman diagrams. It is also applicable to a type of non local interaction

[en] Self-aligned GaAs metal-semiconductor field-effect transistor process requires a very thermally stable gate material which must maintain good Schottky contact with GaAs after high-temperature annealing. The electrical characteristics of rf-sputtered ZrN, TiN, and NbN contacts on n-GaAs substrate have been investigated as a function of annealing temperature. We show that all these refractory metal nitride contacts on GaAs have ideality factors very close to unity after annealing at temperatures as high as 850 ⁰C. The barrier height for these contacts increases with annealing temperature and very low reverse leakage current is obtained. We also observe similar behavior from previous work on WN/GaAs contacts. Such barrier height enhancement at elevated temperatures has been attributed to the incorporation of nitrogen into GaAs near the metal/GaAs interface

[en] Highlights: ► Revealed the formation mechanisms and microstructures of ASBs. ► A deformed shear band is due to severe plastic shear deformation. ► A transformed shear band involves a process of microstructural evolution. ► A dynamic rotational recrystallization is the origin of equiaxed grains in ASBs. - Abstract: This paper presents an in-depth investigation into the formation mechanisms and microstructures of adiabatic shear bands (ASBs) in hardened AISI 1045 steel induced by high speed machining (HSM). A systematic analysis, both experimentally and theoretically, shows that a low cutting speed leads to deformed ASBs, and a high cutting speed results in transformed ASBs. The formation mechanisms of the two types of ASBs are fundamentally different. A deformed ASB is simply due to severe plastic shear, but a transformed ASB involves a process of reorientation and elongation of the martensite laths in the hardened steel, and the partitioning of elongated subgrains. It was found that a dynamic rotational recrystallization is likely the origin of the equiaxed grains in the center of a transformed ASB.

[en] Double layer coatings, with celsian-Y₂SiO₅ as inner layer and Y₂Si₂O₇ as outer layer, were prepared by microwave sintering on the surface of carbon fiber reinforced silicon carbide matrix composite. Both celsian, Y₂SiO₅ and Y₂Si₂O₇ were synthesized by in situ method using BAS glass, Y₂O₃ and SiO₂ as staring materials. The sintering temperature was 1500 deg. C, and little damage was induced to the composite. The composition and micrograph of the fired coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The oxidation and thermal shock resistance of samples with doubled-layered coating were characterized at 1400 deg. C in air. After 150 min oxidation and thermal cycling between 1400 deg. C and room temperature for 15 times, the weight loss of double layer-coated sample was 1.22% and there were no cracks in the coating.

[en] Highlights: • Core-shell structured Ti-6Al-4 V is composed of α shells and α + β cores. • Nitrogen solution and core-shell structure significantly enhances the strength. • Core-shell structural alloy performs inhomogeneous deformation. - Abstract: Core-shell (CS) structured Ti-6Al-4 V (Ti64) alloy with single α shells and duplex α + β cores was fabricated by powder nitriding and spark plasma sintering. The mechanical property and microstructure evolution of CS-Ti64 alloy was investigated by compression at temperatures of 950–1100 °C and strain rates of 0.001–1 s⁻¹. The nitrogen-induced solid solution strengthening and the nitrogen-stabilized CS structure significantly enhanced the compressive strength. Microstructure evolution of CS-Ti64, involved inhomogeneous deformation of the core and shell, which was quite different from that of the commercial Ti64. Regions adjacent to shells and central locations of cores suffered large strain and therefore preferentially underwent dynamic recrystallization (DRX). The deformation mechanism of the novel CS-Ti64 alloy was discussed in terms of the α shells, α laths and DRX.

[en] Detailed transmission electron microscopy analysis on a severely deformed Al-Si composite material has revealed that partial dislocation slips and deformation twinning are the major plastic deformation carriers in ultrafine silicon grains. This resembles the deformation twinning activities and mechanisms observed in nano-crystalline face-centred-cubic metallic materials. While deformation twinning and amorphisation in Si were thought unlikely to co-exist, it is observed for the first time that excessive twinning and partial dislocation interactions can lead to localised solid state amorphisation inside ultrafine silicon grains.

[en] Two Sn-containing Ti-based multicomponent alloys, Ti₅₀Cu₃₅Ni₁₂Sn₃ and Ti₅₀Cu₁₈Ni₂₂Al₄Sn₆, were chosen to investigate the amorphization of pre-alloyed fragments under high-energy ball milling. For comparison, the amorphous alloy ribbons with the same compositions were also prepared using melt-spinning method. Amorphous nature of the as-prepared samples was characterized using X-ray diffraction, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For the ball-milled (BM) Ti₅₀Cu₃₅Ni₁₂Sn₃ alloy, the glass formation is predominant but a small fraction of the α-Ti nanocrystals still remains unreacted. In contrast, the complete amorphization can be achieved in the ball-milled Ti₅₀Cu₁₈Ni₂₂Al₄Sn₆ alloy. In both cases, thermal properties of the ball-milled glassy alloys are comparable to those of the melt-spun (MS) glasses, exhibiting a distinct glass transition and wide supercooled liquid region about 60 K. The amorphization extent in the final milled products is associated with whether the α-Ti phase exists in the initial microstructure of the alloys as starting materials