Difference in microstructure of Zr41Ti14Cu12.5Ni10Be22.5 glasses prepared in a 52 m drop tube and by water quenching

A 52 m drop tube has been used to solidify bulk-glass-forming Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 alloy. Glassy balls with different sizes solidified from the droplets whose structural features, glass-transition behaviour and crystallization kinetics have been investigated. The results indicate that the apparent activation energies of the glass transition and main crystallization reaction are significantly different from those of samples prepared by water quenching. The structural difference between the two types of glassy specimen is revealed by compression studies and in situ energy-dispersive X-ray diffraction. The results are important for understanding the structural features of bulk-forming glasses.     

Effects of irradiation with electrons of different energies on the dark conductivity and the network of hydrogenated amorphous silicon films

The effects of irradiation with electrons having energies in the range 0.7–1.7 MeV on the dark conductivity and the network of hydrogenated amorphous silicon (a-Si:H) thin films have been studied. The dark conductivity measurements show that electron irradiation leads to a degradation in the dark conductivity, with the degradation being greater at lower electron energies. The Raman results suggest that the irradiation induces structural defects in the a-Si:H films, with the lower energy electrons producing more disorder in the amorphous network.     

Tensile behaviour of submicrocrystalline ferritic steel processed by large-strain deformation

Mechanical tests have been carried out on Fe–15%Cr ferritic stainless steel with various microstructures. Ultrafine-grained microstructures with grain sizes of 0.2–0.3 µm were developed by large-strain cold-working and light annealing. The effects of severe deformation on the mechanical behaviour of as-processed and recovered steel were evaluated with reference to the same material having conventional work-hardened and recrystallised microstructures. Despite the low dislocation density in the fine grain interiors in the as-processed state, the samples with strain-induced submicrocrystalline structure were characterised by high internal stresses that resulted in a higher strength than could be expected from simple grain-size strengthening. These internal stresses were associated with a non-equilibrium state of strain-induced grain boundaries after severe deformation.     

Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations

High-speed heavy plastic deformation of thin foils of fcc metals, including aluminium, is found to produce a high density of small vacancy clusters, in the form of stacking-fault tetrahedra. The dependences of the density of the clusters on the deformation temperature and deformation rate indicate the production of vacancy clusters from deformation-induced dispersed vacancies. Neither dislocations nor any indication of the reaction of dislocations are present in the regions containing a high density of vacancy clusters. A possible model is proposed that describes, at extremely high strain rates where dislocation generation is difficult, how a high concentration of point defects is produced by a large number of parallel shifts of atomic planes without dislocations.     

Mechanical hysteresis due to microplasticity in alumina with microcracks

Abstract

Stress-strain hysteresis in alumina with microcracks has been investigated by a loading–unloading test in the microstrain range around 10 ^?4 While there remains a permanent strain after the initial loading, steady-state cyclic loading results in a single closed hysteresis loop with a symmetrical shape. Such a stabilized hysteresis loop is responsible for internal friction and can be attributed to the microplasticity associated with a forerunning phenomenon of fracture.     

Nucleation of the C40-to-C11 b transformation in magnetron-sputtered MoSi 2 thin films

Transmission electron microscopy has been used to reveal the microstructure of metastable C40 MoSi 2 thin films produced by annealing amorphous magnetron-sputtered deposits at 700°C. The films contain nanoscale acicular grains elongated parallel to (0001), with extensive basal faulting. The faults are intrinsic with R ´ 1/3[0001] and correspond to thin slabs of the equilibrium C11 b phase. It is proposed that these faults may act as nuclei for the subsequent transformation from C40 to C11 b by a process akin to discontinuous coarsening.     

A study of the spin-Hamiltonian parameters for Cr5+ at the rhombically-distorted tetrahedral P5+ site of Ca2PO4Cl crystal using a two-mechanism model

The complete high-order perturbation formulae of spin-Hamiltonian (SH) parameters (g factors g_i and hyperfine structure constants A_i , where i = x, y, z) containing contributions from both the crystal-field (CF) and charge-transfer (CT) mechanisms (the latter mechanism is neglected in the widely-used CF theory) are established for d¹ ions in rhombic tetrahedra. From these formulae, the SH parameters of Cr⁵⁺ ion at the rhombically-distorted tetrahedral P⁵⁺ site of Ca₂PO₄Cl crystal are calculated. The CF and CT energy levels used in the calculation are obtained from the optical spectra of the studied Ca₂PO₄Cl : Cr⁵⁺ crystal. The calculated results showed reasonable agreement with the experimental values. The signs of the hyperfine structure constants A_i and the relative importance of the CT mechanism to SH parameters are acquired from the calculations.     

Measurement of the size effect in the yield strength of nickel foils

There is a growing consensus that materials become stronger in small volumes and in the presence of large strain gradients. It has not been clear whether this is due to increased resistance to the motion of dislocations, fewer dislocations, or increased difficulty of multiplying dislocations in these situations. A classic experiment by Stölken and Evans (J.S. Stölken and A.G. Evans, Acta metall. 46 5109 (1998)) showed that thin nickel foils under bending display increased strengthening at large plastic strain values and, correspondingly, large plastic strain gradients. We have adapted their technique to small strains, and report preliminary data for the stress–strain curves of thin nickel foils through the elastic–plastic transition. These data show unambiguously that the yield strength is greater in the thinner foils. The strengthening is additive to the Hall–Petch effect, and is consistent with a size effect at the onset of plastic deformation.     

Nature of ferroelectric–paraelectric transition

Ferroelectric (FE) materials directly convert electrical energy to mechanical energy and are critical to applications such as sensors, transducers, and actuators. The giant electromechanical response is the manifestation of the critical point between the first-order and second-order ferroelectric–paraelectric (FE–PE) transitions. For the simple classic FE lead titanate (PbTiO₃), it is commonly accepted that there is a critical point in the temperature–pressure phase diagram separating the first- and second-order FE–PE transitions at zero electric field. Here, we show that the FE-PE transition in PbTiO₃ is second-order at zero electric field. We introduce the concept of the invariant critical points (ICP) among three phases, representing the stability of the PE phase with respect to two FE phases in a three-dimensional electric field-pressure-temperature phase diagram of PbTiO₃. It is pointed out that the electromechanical response near ICPs is larger than that near the line of critical end points (LCEPs) between two phases.     

Polytypes of long-period stacking structures synchronized with chemical order in a dilute Mg–Zn–Y alloy

A series of structural polytypes formed in an Mg–1 at.%Zn–2 at.%Y alloy has been identified, which are reasonably viewed as long-period stacking derivatives of the hexagonal-close-packed Mg structure with alternate AB stacking of the close-packed atomic layers. Atomic-resolution Z-contrast imaging clearly revealed that the structures are long-period chemical-ordered as well as stacking-ordered; unique chemical order along the stacking direction occurs as being synchronized with a local faulted stacking of AB′C′A, where B′ and C′ layers are commonly enriched by Zn/Y atoms.