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Plasticity in hexagonal close-packed zirconium is mainly controlled by the glide of dislocations with 1 / 3 〈 1 2 ¯ 10 〉 Burgers vectors. As these dislocations cannot accommodate deformation in the [0001] direction, twinning or glide of 〈 c + a 〉 dislocations, i.e. dislocations with 1 / 3 〈 1 2 ¯ 13 〉 Burgers vector, have to be activated. We have performed in situ straining experiments in a transmission electron microscope to study the glide of 〈 c + a 〉 dislocations in two different zirconium samples, pure zirconium and Zircaloy-4, at room temperature. These experiments show that 〈 c + a 〉 dislocations exclusively glide in first-order pyramidal planes with cross-slip being activated. A much stronger lattice friction is opposing the glide of 〈 c + a 〉 dislocations when their orientation corresponds to the ⟨ a ⟩ direction defined by the intersection of their glide plane with the basal plane. This results in long dislocations straightened along ⟨ a ⟩ which glide either viscously or jerkily. This ⟨ a ⟩ direction governs the motion of segments with other orientations, whose shape is merely driven by the minimization of the line tension. The friction due to solute atoms is also discussed. [ABSTRACT FROM AUTHOR]

We investigate the mobility of screw dislocations decorated by carbon solutes in body-centred cubic iron based on density functional theory (DFT) calculations and transmission electron microscopy (TEM) observations of in-situ straining experiments. We focus on the high-temperature domain, between 500 and 800 K, where plasticity is controlled by the slow glide of decorated screw dislocations with a mobility similar to the Peierls mechanism existing below 300 K, at variance with the athermal regime observed at intermediate temperatures. We propose that due to the strong pinning of reconstructed dislocation lines by carbon solutes, dislocation glide occurs by the formation and migration of kinks, both controlled by the jumps of carbon atoms. We calculate the associated kink-pair nucleation and formation energies as well as the migration energies of isolated kinks and use these DFT values to predict the dislocation velocity as a function of temperature, applied stress and dislocation length. The validity of the mobility law is assessed by the comparison with dislocation velocities measured in TEM observations at different temperatures and local shear stresses. The quantitative agreement between the experimental measurements and the analytical model supports the proposed glide mechanism and the DFT values obtained for kink energies. [Display omitted] [ABSTRACT FROM AUTHOR]

The ease of a metal to deform plastically in selected crystallographic planes depends on the core structure of its dislocations. As the latter is controlled by electronic interactions, metals with the same valence electron configuration usually exhibit a similar plastic behaviour. For this reason, titanium and zirconium, two transition metals of technological importance from the same column of the periodic table, have so far been assumed to deform in a similar fashion. However, we show here, using in situ transmission electron microscopy straining experiments, that plasticity proceeds very differently in these two metals, being intermittent in Ti and continuous in Zr. This observation is rationalized using first-principles calculations, which reveal that, in both metals, dislocations may adopt the same set of different cores that are either glissile or sessile. An inversion of stability of these cores between Zr and Ti is shown to be at the origin of the profoundly different plastic behaviours. [ABSTRACT FROM AUTHOR]

Dislocation mechanisms responsible for the high-temperature mechanical properties of a Zr alloy have been investigated using in situ straining experiments between 250 °C and 450 °C. At 250 °C and 300 °C, the results show a steady and homogeneous dislocation motion in prismatic planes, with little cross-slip in the pyramidal and/or basal planes. At 350 °C, the kinetics of mobile dislocations becomes very jerky and inhomogeneous, in agreement with a dynamic strain aging mechanism. Above this temperature, the motion is again steady and homogeneous. Extensive cross-slip forms super-jogs which are efficient pinning points against the glide motion. These super-jogs move by glide along the Burgers vector direction, never by climb. The glide velocity between super-jogs is linear as a function of the total driving stress (applied stress minus line-tension stress due to dislocation curvature), in agreement with the solute dragging mechanism. The origin of the stress–strain rate dependence with an exponent larger than unity is then discussed. [ABSTRACT FROM AUTHOR]

Abstract: In this article we interpret the mechanical properties of icosahedral quasicrystals with the dislocation theory. After having defined the concept of dislocation in a periodic crystal, we extend this notion to quasicrystals in the 6-dimensional space. We show that perfect dislocations and imperfect dislocations trailing a phason fault can be defined and observed in transmission electron microscopy (TEM). In-situ straining TEM experiments at high temperature show that dislocations move solely by climb, a non-conservative motion-requiring diffusion. This behavior at variance with that of crystals which deform mainly by glide is explained by the atypical nature of the atomic structure of icosahedral quasicrystals. [Copyright &y& Elsevier]

The mechanisms leading to the formation of various types of loops and dipoles by moving dislocations have been investigated in several metals and alloys. Dissociation loops in TiAl and Ti3Al are nucleated by similar processes, involving either glide or climb dissociation mechanisms. Cross-slip loops in TiAl, Ti, and Fe are the result of the collision of kinks gliding in different planes along screw dislocations (cross-kink mechanism). The role of extrinsic small-size obstacles in the initiation of these mechanisms is discussed. New experiments in Ti and Fe containing various amounts of solute elements show that extrinsic obstacles strongly enhance the rate of loop production by the cross-kink mechanism. [ABSTRACT FROM AUTHOR]

Abstract: Loading–unloading cycles have been performed on ultrafine-grained (UFG) aluminium inside a transmission electron microscope (TEM). The interaction of dislocations with grain boundaries, which is supposed to be at the origin of the inelastic behaviour of this class of materials, differs according to the main character of the dislocation segments involved in pile-ups. Pile-ups are formed by spiral sources and lead to the incorporation of dislocations into grain boundaries (GBs) during loading. Upon unloading, partial re-emission of dislocations from GBs can be observed. Stress and strain measurements performed during these in situ TEM loading–unloading experiments are in agreement with the rather large inelastic reverse strains observed during unloading in loading–unloading tests on bulk macroscopic UFG aluminium specimens. [Copyright &y& Elsevier]

Abstract: The literature data on dissociation widths in elemental semiconductors (Si and Ge) deformed under high-stress conditions are re-examined in the frame of a model which considers that moving dislocations are subjected to a periodic Peierls potential. It is concluded that the experimental results can be interpreted in a consistent way by considering that the mobility of a Shockley partial dislocation depends only on its core structure and not on its leading/trailing position. [Copyright &y& Elsevier]

Abstract: The transformation of titanium into several hydride phases has been studied by transmission electron microscopy at the surface of an acid-etched titanium substrate designed for biomedical applications. Four different orientation relationships have been found between face-centred cubic TiH2−ε (ε ≪1), face-centred tetragonal TiH and hexagonal close-packed Ti. A fast method based on the alignment of diffraction spots perpendicular to interfaces (Δg condition) is proposed, to account for the experimental orientation relationships and interface planes. Different paths leading to the formation of the two above-mentioned hydride phases have been thus identified. A mechanism of misfit relaxation has been observed and described. [Copyright &y& Elsevier]

Abstract: The creep properties of an as-cast Ti48Al48Cr2Nb2 alloy have been studied at 750°C and 150MPa. The activation parameters have been measured by stress jumps during the creep test. TEM analyses have been performed to determine the rate-controlling deformation mechanism. Creep is controlled by the mixed climb of ordinary dislocations, which combines climb and glide components. It is shown that that the stress dependence of the creep rate is correlated to this mechanism. [Copyright &y& Elsevier]

The behaviour of ordinary dislocations in TiAl alloy creep-deformed al 750 C has been investigated. Two alloys processed by the cast and powder metallurgy routes were crept under tensile stresses of 150 and 80MP:i, respectively. Transmission electron microscopy was performed on the crept samples ID determine the characteristics of the dislocations. The stress dependence of the dislocation mechanisms was determined from stress jumps performed during creep deformation. Complementary in-situ heating experiments performed on previously crept samples are also presented. From these experimental investigations, it is shown that some ordinary dislocations move by a mixed climb mechanism for which the elementary process is the nucleation and the lateral propagation o( a jog pair. The coherency between this mechanism and the determined activation parameters, as well as the driving force at its origin, are discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] The interplay of screw dislocations with carbon atoms is investigated in tungsten at high temperature using in situ straining experiments in a transmission electron microscope (TEM) and through ab initio calculations. When the temperature is high enough to activate carbon diffusion, above 1373 K, carbon segregates in the core of screw dislocations and modifies their mobility, even for a carbon concentration as low as 1 appm. TEM observations reveal the reappearance of a Peierls mechanism at these high temperatures, with screw dislocations gliding viscously through nucleation and propagation of kink-pairs. The mobility of screw dislocations saturated with carbon atoms is then investigated with ab initio calculations to determine kink-pair formation, nucleation and migration energies. These energies are used in kinetic Monte-Carlo simulations and in an analytical model to obtain the velocity of screw dislocations as a function of the temperature, the applied stress and the dislocation length. The obtained mobility law parametrised on ab initio calculations compares well with experiments. [ABSTRACT FROM AUTHOR]

This paper presents a study of the creep at 850 °C under 150 MPa of the IRIS alloy (Ti–Al48–W2–B0.1) densified by spark plasma sintering. The dislocation microstructure in a sample strained up to 1.5% was studied by post-mortem transmission electron microscopy. The deformation is mainly due to ordinary dislocations. Several populations of dislocations are evidenced. Their Burgers vectors, the plane in which they are moving and the corresponding deformation mechanisms are determined. In the discussion section, the deformation mechanisms, the factors controlling their activation and the role of tungsten as hardening element are examined. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We propose a simple geometrical definition of metadislocations based on theN-dim description of quasicrystals and their approximants, as being the traces of quasicrystalline dislocations with a non-zero component of theirN-dim Burgers vector in the perpendicular space of the approximant. Examples are given in the octagonal canonical tiling. [ABSTRACT FROM AUTHOR]

Abstract: Biomimetic apatite coatings were obtained by soaking chemically treated titanium in SBF with different HCO3 − concentration. XRD, FTIR and Raman analyses were used to characterize phase composition and degree of carbonate substitution. The microstructure, elemental composition and preferred alignment of biomimetically precipitated crystallites were characterized by cross-sectional TEM analyses. According to XRD, the phase composition of precipitated coatings on chemically pre-treated titanium after exposure to SBF was identified as hydroxy carbonated apatite (HCA). A preferred c-axis orientation of the deposited crystals can be supposed due to the high relative peak intensities of the (002) diffraction line at 2θ =26° compared to the 100% intensity peak of the (211) plane at 2θ =32°. The crystallite size in direction of the c-axis of HCA decreased from 26nm in SBF5 with a HCO3 − concentration of 5mmol/l to 19nm in SBF27 with a HCO3 − concentration of 27mmol/l. Cross-sectional TEM analyses revealed that all distances correspond exactly to the hexagonal structure of hydroxyapatite. The HCO3 − content in SBF also influences the composition of precipitated calcium phosphates. Biomimetic apatites were shown to have a general formula of Ca10−x−y Mg y (HPO4) x−z (CO3) z (PO4)6−x (OH)2−x−w (CO3) w/2. According to FTIR and Raman analyses, it can be supposed that as long as the HCO3 − concentration in the testing solutions is below 20mmol/l, only B-type HCA (0< z <1; ) precipitates. At higher HCO3 − concentration, it can be assumed that AB-type HCA (z =1;) is formed. [Copyright &y& Elsevier]

Abstract: Simulated body fluid (SBF) solutions are widely used for in vitro bioactivity tests and to coat bioinert materials with biomimetic calcium phosphates. In this study, a SBF solution was used to precipitate hydroxy carbonated apatite (HCA) with a general formula Ca10− x (HPO4) x − y (CO3) y (PO4)6− x (OH)2− x on a bioactive titanium surface. X-ray diffraction (XRD) as well as cross-sectional transmission electron microscopy (TEM) analyses revealed that the biomimetically derived crystallites show a preferred growth orientation in direction of their c-axis and perpendicular to the surface of the bioactive substrate. This growth behaviour as well as the measured crystal dimensions ranging from 5×10×20 to 5×20×100nm are very similar to that found in the mineral part of mammalian bone, where the c-axes of plate-shaped apatite crystals are well aligned with the long axes of the collagen fibrils. [Copyright &y& Elsevier]

Several dislocation glide mechanisms are studied in Ti and Ti3Al by means of in situ straining experiments in a transmission electron microscope at various temperatures. The prismatic glide of α titanium occurs by the jerky motion of straight screw a-dislocations subjected to a frictional force. An explanation for the discontinuity in the temperature dependence of the corresponding activation area is proposed, on the basis of the experimentally measured variation of the corresponding dislocation jump length. In Ti3Al, superlattice 2 a-dislocations exhibit two different dissociation modes in prismatic planes corresponding to highly different antiphase boundary energies. The properties of these two types of dislocation are compared and discussed. It is shown that the motion of 2 c+ a superlattice dislocations in pyramidal planes is controlled by a new mechanism: the self-nucleation of small-size obstacles as the result of irreversible atomic displacements. The tension/compression asymmetry observed between type 1 and type 2 pyramidal planes is finally discussed. [ABSTRACT FROM AUTHOR]

Dislocation properties have been studied in several Ll2 nickel-based alloys with high (Ni74Al26), medium (Ni3(Al, 0.25% Hf)), and low (Ni3Ga) antiphase boundary (APB) energies respectively, deformed in the temperature domain of the yield-stress anomaly. Several APB energies have been measured and compared with those found in the literature. The peak in the temperature variation of the strain-hardening coefficient has been correlated with the unlocking of incomplete Kear-Wilsdorf locks. The corresponding unlocking stresses, as well as the frequencies of octahedral-cube and cube-octahedral cross-slip, are strongly correlated with the respective fault energies. The local pinning of near-edge dislocations in cube planes appears essential for the observation of the yield-stress anomaly in alloys with high APB energies. [ABSTRACT FROM AUTHOR]

The mechanical behaviour and fracture were studied in a Fe-5Mn-2.5Al-0.2C (in wt-%) Medium Mn steel. Metallographic and magnetic measurements confirm the significant influence of the transformation kinetics of the strain-induced martensite on the mechanical properties and strain heterogeneities (Lüders and Portevin-Le Chatelier-like phenomena). An accurate study of isothermal evolutions of the microstructure, associated with atomistic calculations, complements current thermodynamic databases to quantify the nature and volume fraction of phases at different temperatures. A kinetic approach then predicts the influence of annealing conditions on the composition of retained austenite, key parameter for the martensitic transformation kinetics. This supports quantitative modelling of the influence of the intercritical annealing temperature on the ultimate tensile strength for industrial developments of these new grades. [ABSTRACT FROM AUTHOR]

An extensive DFT search of (meta)stable structures of the screw dislocation in hcp-Ti is presented. It reveals that the stable core structures are never basal but always prismatic. This prismatic core dissociates into two partial dislocations in the same or neighboring prismatic planes depending on the initial position of the dislocation line, leading to either a symmetric or an asymmetric core. An alternative way of defining the core region from an electronic structure point of view is also proposed. It evidences clearly the symmetric or asymmetric character of the cores. We then introduce an ansatz for a straightforward and fast calculation of the excess energy, per unit length of dislocation, of a screw dislocation applicable to DFT calculations, in the cluster approach. The method is first validated on calculations of a screw dislocation in hcp-Ti, performed with an EAM potential from which exact excess energies can be extracted. Then, it is shown that it does work in a DFT calculation, through its application to the same screw dislocation in hcp-Ti with an accuracy of 8.4 meV/Å (1.8% of the excess energy for a cluster of 126 atoms per plane normal to the dislocation line). The comparison of the excess energies of the symmetric and assymmetric cores, calculated with the proposed ansatz, reveals that their energy difference is within the uncertainty of the method, which implies that the potential energy surface is very flat and that there could be many metastable core structures in hcp-Ti. [ABSTRACT FROM AUTHOR]

Abstract: The origin of the improved mechanical properties of sub-micron single crystals and whiskers is still debated, but studies generally concentrate solely on size effects. In comparison, the role of the initial defect content, linked to the crystal size, has been given less consideration. We show using in situ SEM and TEM tensile testing of sub-micron Al fibers prepared using selective etching of a eutectic alloy that multiplication of dislocations through intermittent spiral sources directly causes a power-law increase of the yield stress with decreasing cross-sectional size. The size effect and resulting mechanical response are directly linked to the initial defect density and the distance between the source and the surface. In the absence of dislocations, fibers elastically reach high stresses with limited to no plasticity, reminiscent of whisker behavior. [Copyright &y& Elsevier]