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861.
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. 相似文献
862.
The complete high-order perturbation formulae of spin-Hamiltonian (SH) parameters (g factors gi and hyperfine structure constants Ai , 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 Ai and the relative importance of the CT mechanism to SH parameters are acquired from the calculations. 相似文献
863.
P. Moreau M. Raulic K. M. Y. P'ng G. Gannaway P. Anderson W. P. Gillin 《Philosophical Magazine Letters》2013,93(7):339-343
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. 相似文献
864.
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. 相似文献
865.
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. 相似文献
866.
The transformation of second-phase particles in a Cu–Fe alloy following equal-channel angular extrusion and annealing has been determined by transmission electron microscopy. Equal-channel angular pressing of the Cu–Fe alloy transformed coherent γ-Fe particles in the Cu matrix into incoherent α-Fe. Upon annealing, numerous coherent γ-Fe particles were observed. A dislocation–particle interaction mechanism is suggested to explain the re-precipitation of coherent γ-Fe particles following annealing. 相似文献
867.
The influence of regression (partial dissolution) effects on the temperature dependence of nucleation rate in a binary solid solution has been studied theoretically. The results of the analysis are compared with the predictions of the simplest Volmer–Weber theory. Regression effects are shown to have a strong influence on the shape of the curve of nucleation rate versus temperature. The temperature TM at which the maximum rate of nucleation occurs is found to be lowered, particularly for low interfacial energy (coherent precipitation) and high-mobility species (e.g. interstitial atoms). 相似文献
868.
869.
A sample containing single-wall carbon nanotubes (SWCNTs) has been annealed at 900°C and then irradiated by high-energy electrons in an ultrahigh-vacuum transmission electron microscope. Sequential high-resolution images showed that the structures of SWCNT bundles and individual SWCNT segments first collapsed into disordered and fullerene-like carbon materials and then reorganized into graphitic particles under continuous electron irradiation. The speed of structural evolution is dependent on the flux density of the electron irradiation. 相似文献
870.
The microstructures of Al–1.8 to 92.5?at.% Cu thin films prepared by radiofrequency (13.56?MHz) cathodic magnetron sputtering have been investigated by X-ray diffraction (XRD) and transmission electronic microscopy (TEM). A phase separation occurs in films of nominal Al–66.64?at.% Cu composition, consisting of a fcc Al solid solution phase, a fcc Cu solid solution phase and an unexpected sc Cu3Al ordered phase with a Cu3Au structure and a lattice parameter of about 0.36?nm. 相似文献