Epitaxial Ni-Al thin films on NaCl using a Ag buffer layer

Epitaxial nanoscale [001] films of Ni _x Al_100-x (x = 62.5) have been prepared by physical vapour deposition on to a thin film of Ag [001] on NaCl (001) faces with occasional hillocks. The Ag film contains numerous dislocations and stacking faults and has a rms surface roughness of 2 nm. The Ni-Al film is ordered in the B2 structure and reveals many dislocations as well as antiphase boundaries between ordered domains. The formation of subgrains in the Ni-Al film results in severe height variations up to 30 nm across the surface. A cross-sectional model for the growth of both films is presented.     

Onset of plasticity in a Σ = 5 GaAs compliant structure

Compliant structures have been fabricated in which a thin GaAs layer (thickness between 10 and 20nm) was bonded on top of a GaAs substrate with a large twist angle (about 37). This twist angle value was chosen so that the energy of the boundary (coincident boundary of type =5 (001)) was minimized. The structure of the interface was characterized and the onset of plasticity in such a compliant substructure was investigated using nanoindentation that allowed the low-load deformation regime to be observed. The results are compared with those obtained under the same conditions on a GaAs bulk substrate alone. No plastic zone was observed by transmission electron microscopy in the compliant structure under loads below 0.25mN while, under the same loads, plastic deformation was observed in the bulk substrate. For higher loads (2mN), plastic-flow enhancement was observed in the compliant structure. The results are discussed in the light of the arrangement of dislocations observed in the plastic zones.     

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.     

Effect of applied stress on precipitation of Guinier–Preston zones in a Cu–0.9 wt.% Be single crystal

A stress-oriented formation of plate-shaped Guinier–Preston (GP) zones in single crystals of a Cu–0.9?wt.%?Be alloy aged at 200°C has been found. A compressive stress applied in the [001] direction assists the formation of the GP zones perpendicular to the [001] axis, while a tensile stress results in the preferential formation of the GP zones parallel to the stress axis. The applied stress strongly affects the nucleation of the GP zones. Free ageing at 275°C after compressive-stress ageing at 200°C produces plate-shaped γ′′ precipitates perpendicular to the stress axis, indicating a continuous change in the structure of the GP zones into that of the γ′′ phase. From length-change measurements during ageing, the misfit strains of the GP zone or γ′′ precipitate in directions parallel and perpendicular to the plate plane are estimated, respectively, as ?0.01 and ?0.10 for the GP zone and ?0.01 and ?0.11 for the γ′′ precipitate.     

Hydrogen-assisted damage in austenite/martensite dual-phase steel

For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1?1?0}_M localized slip and {1?1?2}_M twin and (2) cracking of martensite–martensite grain interfaces. The former resulted in nanovoids along the {1?1?2}_M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking.     

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.     

Microstructure and martensitic transformation behaviors of a Ti–Ni–Hf–Cu high-temperature shape memory alloy ribbon

The Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon undergoes a B2 ? B19′ transformation upon cooling and heating. When the Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon is annealed at 873 K for 1 h, the spherical (Ti, Hf)₂Ni particles with a diameter of 20–40 nm precipitate in the grain interior. The fine (Ti, Hf)₂Ni precipitates improve the stability of phase transformation temperatures and cause martensite domains, with (001) compound twins in three orientations dominant instead of (011) type I twins. {111}-, {113}- and (001)//{111}-type boundaries are observed among these martensite domains. When the (Ti,Hf)₂Ni precipitates coarsen, (011) type I twins become main martensite structures in the ribbon annealed at 973 K for 1 h.     

Crystallographic analysis of the fcc-to-fct martensitic transformation in an In22.73at.%Tl alloy

Analytical solutions for various crystallographic parameters have been obtained from the application of the infinitesimal deformation (ID) approach to the fcc M fct martensitic transformation with the (101) g [101] g twinning shear as the lattice invariant shear deformation. All crystallographic parameters were calculated for an In-22.73at.%Tl alloy. In order to compare numerical solutions between the ID approach and phenomenological crystallographic theory, the corresponding crystallographic parameters were also calculated using the Wechsler-Lieberman-Read (WLR) theory. Agreement between the two results obtained from the ID approach and the WLR theory is found to be excellent.     

A novel lattice correspondence of B19 → B19′ transformation in Ti–Ni–Cu shape memory alloy thin film

In this study, we found a novel lattice correspondence of the B19–B19′ transformation in a Ti–Ni–Cu thin film: (1?1?1)_B19′//(0?0?1)_B19, [0, 1, 1]_B19′//[1?0?0]_B19. Near the coarse precipitate and the grain boundaries, the B19′ martensite forms with the novel lattice correspondence to product the (1?1?1) type I twinning instead of the usual (0?0?1) compound twinning. Crystallographic analyses show that the novel lattice correspondence results from the local stress concentration.     

Effect of grain orientation and local strain on the quality of polycrystalline YBa 2 Cu 3 O 7 superconductive films

The critical current densities of superconducting thin films and their dependence on the film structural characteristics has been a major research interest for more than a decade. Controlling this relationship is crucial if large-scale high-quality YBa 2 Cu 3 O 7 (YBCO) tapes are to be produced. Two major keystones of information have been established in this field. Firstly, there is a direct relationship between the critical current density and the grain-boundary angle in polycrystalline YBCO films. Grain boundaries with a mismatch angle higher than 5° usually result in reduced critical current densities. This detrimental effect of large-angle grain boundaries to the quality of YBCO films has been attributed to strain fields resulting from such grain boundaries. Secondly, the quality of the YBCO film can be enhanced by straining its lattice in specific direction. Here, we report, for the first time, direct experimental results coupling local grain orientation and local strain maps of thin YBCO films deposited on a (001) biaxially textured nickel substrate. These results were correlated to the quality of the film and showed how grain structure in the nickel substrate affects the grain structure in the YBCO films even in the presence of several buffer layers. More importantly, the data show that highquality films with high critical current densities can be produced, in spite of large-angle grain boundaries, if the film is compressed in the range of 0.5% strain normal to the a axis.     

Heterophase polydomain structure and metal-semiconductor phase transition in vanadium dioxide thin films deposited on (1010) sapphire

Vanadium dioxide (VO₂) thin films deposited on ₍1010₎ sapphire are composed of two mixed monoclinic phases, namely M1 and M2. The M1 phase is unstable because of the existence of a larger misfit strain in the ₍102₎ VO₂ film. The reduction of misfit strain in the film favours the formation of the M2 phase. The X-ray diffraction and pole figure results show that both M1 and M2 phases are well aligned with the substrate and both contain twinned structures. Therefore, the microstructure of the film can be regarded as being a transversely modulated heterophase polydomain. A higher electrical resistivity ratio of the semiconductor phase to the metallic phase (rho_s/rho_m) can be achieved only in single-phase VO₂ thin films, either the M2 or M1 phase. Phase mixing degrades the ratio of rho_s/rho_m. The film with a single M2 phase exhibits a lower transition temperature of 58 C without any degradation of the rho_s/rho_m ratio.     

Microstructure of icosahedral Al-Pd-Mn quasicrystals deformed at room temperature in an anisotropic confining medium

Plastic deformation of Al-Pd-Mn icosahedral quasicrystals has been achieved at room temperature using a high-confining-pressure medium. The deformation microstructure, investigated by transmission electron microscopy, is characterized by long straight bands of dislocations. A detailed analysis of the dislocation configurations indicates that the plastic deformation is controlled by dislocation glide.     

Structure of a phase induced with Xe-ion irradiation-implantation in γ-TiAl

A phase transformation in γ-TiAl intermetallic alloy was found to be induced with 50keV Xe-ion irradiation-implantation at doses larger than 2.2 x 10¹⁸ ions m^-2 at room temperature. The structure and the chemical composition of the induced phase were investigated with high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The zones of the induced phase have sizes up to about several tens of nanometres. The phase has a hexagonal structure with a = 0.286 nm and c = 0.462nm. The crystallographic orientation relationship between the phase (P) and the gamma-TiAl matrix is (001)_P//(111)_γ and \[100]_P//[011]_γ. The \[Al]/[Ti] atomic composition ratio in the phase is analysed to be 56/44, slightly different from that of the matrix, 51/49. These results suggest that the induced phase is an Al solid solution of α-Ti alloy phase, which has different structural parameters and chemical composition from those of the reported phase. It is suggested that the size of the ions is important in the phase transformation.     

Stress anomaly in Al-rich Ti-Al single crystals deformed by the motion of 1/2«110] ordinary dislocations

The microstructure and plastic deformation behaviour of Al-rich Ti-Al single crystals containing 54.7 and 58.0 at.% Al have been examined, focusing on the effect of chemical ordering of a Al5Ti3 superstructure on anomalous strengthening. Fine precipitates with the Al5Ti3 superstructure were developed in the L10 matrix of the Ti-58.0 at.% Al alloy. The size and volume fraction of the precipitates varied depending on temperature. An anomalous increase in the yield stress of the two alloys appeared at around 800oC. This strengthening mechanism is discussed on the basis of the difference in antiphase-boundary energies on (111), (110) and (001) planes in the Al5Ti3 phase.     

Nanocomposite-like structure in an epitaxial CaCu3Ti4O12 film on LaAlO3 (001)

We report the detailed microstructural study of a CaCu₃Ti₄O₁₂ (CCTO) thin film using transmission electron microscopy (TEM). The CCTO thin film studied in this work was deposited on a (001)-oriented LaAlO₃ (LAO) substrate by pulsed-laser ablation and has a high dielectric constant of about 10⁴ at 1?MHz at room temperature; however, the mechanism for such a dielectric property is not yet understood. Plan-view TEM studies show that the CCTO samples have orthogonal domain structures with the edge nearly parallel to either the [100] or the [010] direction of the CCTO. A minor anatase TiO₂ phase was found at the domain boundaries. The CCTO and the TiO₂ phases are separated by an amorphous-like layer that has a thickness of several nanometres. Cross-sectional TEM studies reveal that both CCTO and TiO₂ in the films are c axis oriented with a very sharp interface to the LAO-(001) substrate and possess a unique crystallographic orientation relationship of (001) _CCTO //(001)_{TiO 2} //(001) _LAO and [100] _CCTO //[100] _{TiO 2} //[100] _LAO. The Rutherford back-scattering ion chanelling studies suggest a composition of Ca_1.5Cu₃Ti_5.5O₁₆ for the film, in which the extra calcium, titanium and oxygen form the anatase TiO₂ phase and amorphous calcium oxide layer that separates the CCTO and TiO₂ phases. Such nanocomposite-like structures may provide an important clue to the mechanism of the dielectric property of these films.     

Energy contributions in the martensitic transformation of shape-memory alloys

Abstract

The thermoelastic martensitic transformation in shape-memory alloys is studied thermodynamically. Calorimetric experiments on the Cu─Zn─A1 alloy system reveal that the transformation takes place with a practically negligible entropy production. The usual hysteretic subloop behaviour during partial cycling is obtained for the first time by calorimetry. An analysis of the measurements gives the quantitative behaviour of elastic and dissipative energies with the volume fraction of martensite.     

Equivalent and effective strains during severe plastic deformation (SPD)

ABSTRACT

This paper considers the characteristics of severe plastic deformation (SPD) to estimate its efficacy and to compare different processing techniques. In contrast to effective strains by von Mises and Hencky, the rotation component of the strain rate is included in the analysis as a mode of deformation, which ranges from simple shear to pure shear. Distortions of material elements during a uniform plane plastic flow are calculated using a kinematic approach. For the fixed deformation mode, the current state is defined by the accumulated shears that are identical to the von Mises effective shear strains. In specific cases of pure shear and simple shear, the accumulated shears match the specific distortions of round or square elements prescribed by Hencky or von Mises approach, respectively. The mode of deformation is important to the structural effects of SPD. In general, two separated characteristics, accumulated shear and the coefficient of deformation mode, are necessary to describe strains during SPD.     

Voidites in pure type IaB diamonds

Abstract

Electron microscopy of five pure type IaB diamonds (showing no trace whatever of a B′ infrared absorption peak due to {001} platelets) reveals voidite distributions unlike any reported before. In three of the crystals the voidites are distributed at random, and are not associated with degraded platelets, which themselves are not present in these specimens. These observations show that voidite formation and platelet degradation are distinct independent phenomena. The other two speamens showed only arrays of dislocations that patently resulted from plastic deformation: no voidites were seen. The non-formation of platelets in diamonds showing such an advanced stage of nitrogen aggregation is unexpected, and places these diamonds in a category distinct from that called ‘irregular’, in which platelets have indeed formed, but then experienced, to some degree, a process of transformation to perfect dislocation loops.     

Transition from diffusive to ballistic capture related to hydrogen incorporation in amorphous silicon

Three intersection mechanisms with the gliding planes (111)_TB, (001)_TB and (115)_TB respectively have been observed by high-resolution electron microscopy in the type-I twin intersection of gamma-TiAl. It was found that the intersection mechanism that occurred was related to the thickness of the incident twin. The accommodation mechanism on the (111)_TB atomic plane is preferred when the incident twin becomes very thin. The dislocation dissociations of the (111)_TB plane accommodation are the most energetically unfavourable of the dissociations of the three intersection mechanisms; however, the resultant dislocations on the (111)_TB planes are the easiest to propagate away from the intersection area. Accordingly, (111)_TB atomic plane accommodation is considered to be the only mechanism allowing shear transmission under the small local stress of the pile-up of the incident twinning partials.     

Transformation of Shockley into Frank stacking faults in a ZnS 0.04 Se 0.96 /GaAs (001) heterostructure

We report the transformation of Shockley partial dislocations (PDs) into Frank PDs in lattice-matched ZnS 0.04 Se 0.96 /GaAs(001) as investigated by transmission electron microscopy. The ZnS 0.04 Se 0.96 layers, with a nominal thickness of 70 nm, were grown on GaAs(001) by metal-organic chemical vapour deposition at 350°C. We mainly find stacking-fault pairs on the (111) and planes that are bound by Shockley PDs with Burgers vector . Different reactions are observed between PDs taking place in situ in the electron microscope, leading to the transformation of Shockley PDs into Frank PDs with and stacking faults on the or planes.