A.M. Condó

Interaction of twin boundaries with stacking faults in 2H martensite: A high-resolution electron microscopy study

Twinned 2H martensite is observed in copper-based shape memory alloys together with basal and non-basal plane stacking faults. Knowledge of the twin-boundary structure and its interaction with the faults is important to understand the deformation mechanism by twin coalescence. High-resolution electron microscopy coupled with image simulations have been used for these studies. The interface of the type I twinning in 2H martensite shows an atomic configuration with mirror antisymmetry. An atomic plane is shared by the twinned variants in such a way that the distance between the planes, parallel to the interface, is unchanged at the boundary and the mean atomic volume is conserved. The interaction of the boundary with a basal plane fault generates a shift in the interface. A mirror antisymmetrical boundary is maintained by introducing an imperfect interface dislocation.

High-resolution transmission electron microscopy study of the interfaces and stacking defects in superconducting/magnetic perovskite superlattices

We studied the microstructure of GdBa2Cu3O7−δ∕La0.6Sr0.4MnO3 superlattices grown on (001)MgO substrates by high-resolution transmission electron microscopy. The superlattices show two different epitaxial growths. In addition different types of interfaces between the layers were found. Moreover, several kinds of stacking faults and domain boundaries were observed in both materials. The crystallography of the interfaces and the defects was analyzed and their role on the physical properties of the superlattices is discussed.

Formation of epitaxial β-Sn islands at the interface of SiO2∕Si layers implanted with Sn ions

180 nm SiO2 layers on Si (100) were implanted with Sn ions producing a profile with a peak concentration of 3 at. % at the middle of the oxide. After high temperature (900–1100 °C) annealing, an array of β-Sn islands epitaxially attached to the Si was observed at the SiO2∕Si(100) interface due to the migration of the implanted Sn atoms. The breakdown of the planar SiO2∕Si interface and the appearance of the island system is discussed in terms of the Sn–Si equilibrium properties. Our results reveal a new method to create a high density of nanosized islands with good uniformity in size and shape.

The habit plane of non-twinned 2H martensite in thin foils

Well defined interfaces between the matrix and non-twinned 2H martensite plates, formed in thin foils with 〈110〉 orientation, were observed by transmission electron microscopy. Using the crystallographic theory it is shown that a line of no-distortion parallel to the foil and a sequence of interface dislocations piled-up along the foil depth determine the habit plane.

Lattice relaxation around non-basal plane stacking defects in 2H martensite

Abstract Stacking faults produced in the spontaneous martensitic transformation of 2H Cu-Zn-Al alloys have been studied by means of transmission electron microscopy. They were found to lay in basal and inclined {231}-type planes, forming a domain-like structure. Since the extinction criterion was not followed by all the faults, image simulations based on the dynarnical theory in the two-beam approximation were performed. The displacement vectors were by comparison between experimental and calculated images. The basal fault displacement vector was found to be R= [± (⅓δ),½,½] where δ takes into account the orthorhombic distortion of the unit cell. Further relaxations around the inclined faults were observed, giving rise to two types of fault F1 and F2 having additional displacement vectors, ∊1 = {−0·03,0·03, −0·16] and ∊2 = [−0·28, 0·03, −0·16] respectively, depending on the way that they are connected to the basal fault. The difference between displacement vectors of connected faults implied the presence o...

The influence of microstructure on the martensitic transformation in Cu-Zn-Al melt-spun ribbons

The martensitic transformation was investigated in a set of twin roller melt-spun Cu–Zn–Al shape memory alloys, solidified at tangential wheel speeds between 20 and 40 m/s. The resulting microstructures were analyzed using X-ray diffraction, optical and transmission electron microscopy techniques. The characteristic martensitic transformation temperature, M S, was determined for each condition by conventional resistometric methods. The ribbons are homogeneous in shape and for each quenching rate they exhibit a quite uniform M S temperature. By proper thermal treatments, the different factors affecting M S could be separately examined and from temperature measurements, the contribution of L21 antiphase boundaries evaluated. A calculation of this contribution using pair interchange energies is in good agreement with the experimental results.

Martensitic transformation in as-grown and annealed near-stoichiometric epitaxial Ni2MnGa thin films

Magnetic shape memory nanostructures have a great potential in the field of the nanoactuators. The relationship between dimensionality, microstructure and magnetism characterizes the materials performance. Here, we study the martensitic transformation in supported and free-standing epitaxial Ni47Mn24Ga29 films grown by sputtering on (0 0 1) MgO using a stoichiometric Ni2MnGa target. The films have a Curie temperature of ~390 K and a martensitic transition temperature of ~120 K. Similar transition temperatures have been observed in films with thicknesses of 1, 3 and 4 μm. Thicker films (with longer deposition time) present a wider martensitic transformation range that can be associated with small gradients in their chemical concentration due to the high vapour pressure of Mn and Ga. The magnetic anisotropy of the films shows a strong change below the martensitic transformation temperature. No features associated with variant reorientation induced by magnetic field have been observed. Annealed films in the presence of a Ni2MnGa bulk reference change their chemical composition to Ni49Mn26Ga25. The change in the chemical composition increases the martensitic transformation temperature, being closer to the stoichiometric compound, and reduces the transformation hysteresis. In addition, sharper transformations are obtained, which indicate that chemical inhomogeneities and defects are removed. Our results indicate that the properties of Ni–Mn–Ga thin films grown by sputtering can be optimized (fixing the chemical concentration and removing crystalline defects) by the annealing process, which is promising for the development of micromagnetic shape memory devices.

The spontaneous room temperature reduction of HAuCl4 in ethylene glycol in the presence of ZnO: a simple strategy to obtain stable Au/ZnO nanostructures exhibiting strong surface plasmon resonance and efficient electron storage properties

The room temperature spontaneous reduction of HAuCl4 in ethylene glycol in the presence of pre-formed ZnO nanoparticles is investigated by UV-vis spectroscopy. Analysis by HRTEM demonstrated that the synthesized nanostructures consist of small ZnO nanoparticles (5.0 nm) in contact with bigger (15 nm) spherical Au nanoparticles. The electronic communication between Au and ZnO blocks is confirmed by UV excitation of the colloid. These experiments indicate that ZnO nanoparticles efficiently transfer the electrons to Au nanoparticles in contact, inducing a 15 nm blue shift in the plasmon band. Titration experiments using the methylviologen couple (MV2+/MV˙+) are presented and analyzed to quantify the enhancement in the electron storage capability in the new nanostructure and the negative shift in the Fermi level caused by the Au loading in ZnO.

Microstructural Changes in a Duplex Stainless Steel During Low Cycle Fatigue

Abstract Low cycle fatigue of SAF 2507 duplex stainless steel has been studied at different temperatures in two thermal conditions: unaged (as-received) and aged (100 hours at 475°C). The corresponding cyclic flow stress components, the friction, σF, and the back stresses, σB, have been analyzed in relation to the observed microstructure. σF attains higher values at room temperature than in the intermediate range of temperatures in both thermal conditions due to the influence of the lattice friction stress on screw dislocations in the ferrite phase. The cyclic softening observed at all temperatures in aged samples is caused by the decrease of the back stress σB. The increase of σB in unaged samples at 300°C is consistent with the high dislocation density observed in the ferrite phase ascribed as a typical manifestation of dynamic strain aging.

Cooperative nucleation modes in polycrystalline CoxPd1−x nanowires

Polycrystalline CoxPd1−x (x = 1, 0.60, 0.45, 0.23, and 0.11) cylindrical nanowires (o = 18–35 nm, about 1 μm length) are produced by AC electrodeposition into hexagonally ordered alumina pores. Single-phase nanowires of an fcc Co-Pd solid solution, with randomly oriented equiaxed grains (7–12 nm) are obtained; in all the cases, the grain size is smaller than the wire diameter. The coercive field and the reduced remanence of Co-rich nanowire arrays are hardly sensitive to temperature within the range varying from 4 K to 300 K. On the other hand, in Pd-rich nanowires both magnitudes are smaller and they largely increase when cooling below 100 K. This behavior also depends on the mean grain size. These facts are systematized considering two main aspects: the non-trivial temperature and composition dependence of the crystalline anisotropy and the saturation magnetostriction in Co-Pd alloys; and a random anisotropy effect, which defines a nucleation localization length that may involve more than a single grain...