Electronic and vibrational properties of an Al-Cu-Ru icosahedral quasicrystal deduced from heat capacities

Heat capacity measurements have been made in the temperature range 1-310 K for a phasonless perfect quasicrystal Al65Cu20Ru15, an Al65Cu20Ru15 quasicrystal with phason disorder, and a crystal approximant Al70Cu20Ru10. The electronic heat capacity of the perfect quasicrystal was found to be essentially zero. The existence of some excitation associated with an energy of about 1.6 meV is proposed. The vibrational properties of the quasicrystal are distinctly different from those of the crystal approximant in the region T < ThetaD/10, indicating that the densities of states are significantly different at the corresponding energies.     

Formation of prismatic loop alignments in crystals deformed in single slip

In γ-TiAl deformed at room temperature in single slip, prismatic loops ar often organized in a staircase-like configuration, called strings whose generation by cross-slip is facilitated by the dislocation tendency to form screw locks as described recently by Grégori and Veyssière. The present letter is aimed at showing that strings may be encountered in virtually any crystal provided that the two impinging dislocations exhibit significantly different velocities. The crossslip annihilation mechanism of crossed dislocations considered by Tetelman is revisited and shown to evolve into a configuration significantly distinct from that originally predicted.     

Cooling-rate-dependent microstructure and mechanical properties of a CuZrAlAg alloy

A Cu₃₆Zr₄₈Al₈Ag₈ alloy rod with a diameter of 10 mm was fabricated using a copper-mould suction casting method. Structural characterization revealed that the rod has different microstructures along the casting direction, including a complete amorphous structure and an amorphous/crystalline composite structure with different amount of B2 CuZr phase. Nanoindentation tests showed that the hardness and the elastic modulus of the crystalline phase are lower than those of the amorphous matrix. The hardness and the elastic modulus of the amorphous matrix decrease with decreasing crystalline proportion of the alloy, while the Vickers hardness of the alloy increases with a reduction in the crystalline proportion.     

Planar faults in MoSi2 single crystals deformed at high temperatures

Abstract

Dislocation structure and planar faults have been examined in MoSi₂ single crystals deformed at high temperatures. Pure stacking faults were found in a crystal deformed at 900°C. The formation of the stacking fault is closely related to the phase stability of the C11_b and C40 ordered structures. Profuse stacking faults with increasing deformation temperature assist the ductility improvement of the MoSi₂ above about 1200°C. The critical resolved shear stress for {110}(331) and {013}(331) slip is determined in the temperature range 1000 to 1500°C.     

Nanoindentation measurements on deformation-induced α’-martensite in a metastable austenitic high-alloy CrMnNi steel

The hardness of deformation-induced α’- martensite and parent austenitic matrix in high-alloy CrMnNi steel was investigated by nanoindentation measurements inside scanning electron microscope using picoindenter. After the indentation, the microstructure was investigated by electron backscattered diffraction measurements. The hardness values for α’-martensite are only 24% higher than those of austenite. Thus, the increase in strength during the formation of deformation-induced α’-martensite is rather caused by the small grain size of α’-nuclei resulting in a dynamic Hall–Petch effect than by its “intrinsic” hardness.     

Comparative study of different welding wires on T92 welded joints

ABSTRACT

A comparative study of the effect of two different welding wires (W2 and Thermanit MTS-616 welding wire W1) on T92 welded joints has been made. Both wires showed good weldability and no defects in the welds were found. The hardness of the welded joint using W2 wire was lower than using W1. The toughness and high temperature (600°C) strength of the weld using W2 wire were higher than those using W1. The two welding wires produce joints having little difference in plasticity at room and high temperature. Large pores in both joints are related to the second phase of the larger particles in the matrix. The second phase of particles in the matrix easily caused cracks, which is unfavourable for the improvement of plasticity. Precipitation of the second phase of large particles in the matrix should be minimised for optimised welds.     

The fatigue limits of copper single crystals cyclically deformed at constant plastic strain amplitudes

Based on systematic surface observations, experimental measurements on the fatigue limits of differently oriented copper single crystals cyclically deformed at constant plastic strain amplitudes are summarized, and an orientation dependence of the fatigue limit is sketched. It was found that the fatigue limits of crystals depend mainly upon the low-strain-amplitude dislocation structures, which are characteristic of the particular crystal orientations. When multiple-slip deformation is the main mode of deformation and ultimately produces stable lowenergy dislocation structures such as labyrinth and cell structures, the fatigue limit can be improved notably.     

Plastic behaviour of an AlAs/GaAs superlattice with a short period

Nanoindentation has been used to investigate the plastic behaviour of an AlAs/GaAs superlattice with a short period. The sample was grown on a (001) surface of a GaAs single crystal by metal-organic vapour-phase epitaxy. The mechanical response of the superlattice to nanoindentation testing was compared with that of a (001) surface of bulk GaAs. The indents formed were observed by transmission electron microscopy. The plastic zone size associated with each indent was measured as a function of the maximum load. Finally the arrangement of the dislocations generated by the indenter has been analysed and compared with the arrangement observed in deformed bulk GaAs.     

Accelerating heterogeneous nucleation to increase hardness and electrical conductivity by deformation prior to ageing for Cu-4 at.% Ti alloy

ABSTRACT

Deformation bands, especially shear bands were intentionally formed in supersaturated Cu-4 at.% Ti alloy by groove-rolling with 75% area reduction. Cold deformation prior to the ageing process changed the precipitation behaviour of the alloy. The deformed structure with shear bands was maintained, without showing recovery and recrystallisation, even after prolonged ageing at 450°C for more than one day. The thermally stable shear bands act as nucleation initiation sites and prevent the expansion of discontinuous cellular precipitates. The hardness reached a value of 305?Hv and an electrical conductivity of 15% IACS (IACS?=?International Annealed Copper Standard. 100% IACS is defined as the conductivity corresponding to a volume resistivity at 20°C of 17.241?nΩ), compared to 283?Hv and 13% IACS for conventional solid-solutioned and peak-hardened alloy. In addition, the ageing time to reach the highest hardness was shortened from 720 to 180?min at 450°C.     

Homogeneous nucleation of dislocation loops under stress in perfect crystals

We present an analysis and results on the homogeneous nucleation of a dislocation loop under stress in a perfect crystal. By using a variational boundary integral method in the Peierls-Nabarro framework, we have determined the saddle-point configurations of embryonic dislocation loops and their associated activation energies under stress levels up to the ideal shear strength. The high-energy barriers under the usual levels of applied shear stresses, differing markedly from the ideal shear strength, confirm the widely held view that thermal motion should play no role in such nucleation. The result provides means for more definitive solutions of fundamental problems involving homogeneous nucleation of dislocation loops and has significant implications for models based on the mechanism of nucleation of dislocations from a perfect crystal.     

A measure of time perspective with the TAT and some issues of reliability

In an effort to duplicate high interrater reliability coefficients reported in the use of Epley and Ricks' (1963) time orientation scoring system with the Thematic Apperception Test (TAT), two pairs of judges and two different training procedures were employed. Reliability coefficients considerably lower than those quoted by other researchers were found. One method of using the system was to have judges discuss scoring differences during training and at various times during a research project until perfect agreement was reached. When used as an adjunct with periodic assessment' of reliability as judges scored a large number of stories, reliability coefficients within a range acceptable for research purposes were obtained. This procedure is presented with correlational evidence for the presence of the time factor that the scoring system purports to measure.     

Solid-solution strengthening in ordered In x Ga1 − x P alloys

Ordered In _x Ga_{1??? x} P alloys have been grown on to GaAs(001) substrates by metal–organic vapour-phase epitaxy. Lattice-matched compositions of alloys (In _x Ga_{1??? x} P with x?≈?0.5) were used in order to produce epitaxial layers free of structural defects (threading dislocations). Growth temperature and substrate orientation were adjusted to control the degree η, of order, of the alloys in the range 0.3–0.5. Nanoindentation tests were carried out to measure the mechanical response of the heteroepitaxial layers. Transmission electron microscopy was used to characterize the structure as well as the plastic zones generated into the specimen by nanoindentation. Comparison of the alloys with binary references (InP and GaP bare substrates) showed strengthening, with the hardness and flow-stress values that are much larger than those of InP and of the same order of magnitude as those of GaP.     

Dislocation relaxation in aluminium single crystals at 10 MHz frequency

Abstract

Dislocation relaxation, namely the Bordoni peak, in the three principal planes of a high-purity aluminium single crystal has been studied by measuring the ultrasonic attenuation of longitudinal waves at a frequency of 10 MHz using a conventional ultrasonic single-ended pulse-echo technique. The crystals of <100>, <110>, or <111> orientation were deformed by compression, and the temperature dependence of the ultrasonic attenuation was measured after applying resolved shear stresses ranging up to 60N mm^?2. The results lead to the following conclusions: (i) The temperature of the Bordoni peak is essentially independent of the orientation; (ii) the temperature of the peak decreases up to a resolved shear stress near 28 N mm^?2 and then increases slightly for higher prior stresses; (iii) the peak height increases with increasing deformation; and (iv) a subsidiary peak at the lower temperature side of the Bordoni peak (the Niblett-Wilks peak) appears only at low deformations.     

Origin of the initial rapid age hardening in an Al-1.7 at.% Mg-1.1 at.% Cu alloy

The origin of the rapid hardening which occurs during the initial ageing stage of an Al-1.7 at.% Mg-1.1 at.% Cu alloy has been investigated by the threedimensional atom-probe (3DAP) and transmission electron microscopy techniques. Although the rapid hardening reaction occurs within 1 min at 150 degrees C, no evidence for solute clusters or Guinier-Preston-Bagaryatsky (GPB) zones was detected using the 3DAP after this ageing time. When a short-term aged specimen is deformed and then aged at 150 degrees C, further rapid hardening occurs. This suggests that the rapid hardening is associated with a solutedislocation interaction. Uniform dispersions of Cu-Mg co-clusters do not occur until closer to the end of the hardness plateau and it is thought that these evolve into GPB zones during the second stage of the age-hardening process.     

Dudman and the Plans of Mice and Men

V.H. Dudman has argued that a better semantic account of the conditional emerges from placing grammar 'in the driver's seat'. His account of their grammar identifies two main categories, which differ from those postulated by traditional theorists, and which he claims correspond to two very different and deep-rooted styles of thought. I show that it is unlikely that a perfect match exists between styles of thought and grammatical categories in the way that Dudman postulates. I consider arguments by Dale and Edgington in this context. More importantly, however, I also show that even if there were a perfect match, Dudman's account would still obscure semantic similarities between 'if'-sentences which are arguably of greater importance than the ones he highlights. This, I suggest, has implications extending far beyond Dudman's work.     

Nanoindentation study of size effects in nickel–graphene nanocomposites

Metal–graphene nanocomposites find applications in nanoscale devices, as functional materials and can serve as a test bed to gain insight into fundamental deformation mechanisms of metals under geometric confinement. Here, we report full atomistic nanoindentation simulations for nickel–graphene nanocomposites with varied numbers of layers of graphene sheets to investigate the size effects on the hardness, and to understand how emerging dislocation loops interact with the nickel–graphene interface under varied geometric confinements. A detailed analysis of the plastic deformation mechanism shows that as dislocation loops reach the nickel–graphene interface, the local bending of the graphene sheet is altered and further dislocation propagation is blocked. An increase in the number of graphene layers decreases the hardness, but increases the maximum elastic deformation of the nickel–graphene nanocomposites. These findings indicate that the mechanical properties of nickel–graphene nanocomposites can be engineered by controlling the thickness of nickel and graphene layers, respectively.     

Atomistic calculation of the interaction between an edge dislocation and a free surface

By using molecular statics, we compute the variation in the excess energy associated with the gradual approach of an edge dislocation towards the free surface of a crystal. The calculations rely on a phenomenological potential adapted to aluminium and an appropriate constraint procedure that allows investigations of both the extended and the perfect configurations of the dislocation core. Thereby, an estimation of the energy required for the introduction of a dislocation in a thin film is obtained.     

How do slender mineral crystals resist buckling in biological materials?

Mineral crystals in biological materials such as bone, tooth, and sea shell are susceptible to buckling under compressive loading due to their slender geometry with large aspect ratios. How does nature deal with this problem? This question is especially interesting in view of the three orders of magnitude difference in elastic modulus between protein and mineral. As a first analysis of this question, we study the stability of a single mineral platelet confined in an otherwise perfect protein–mineral nanostructure. We find that there exists a transition of buckling strength from an aspect ratio-dependent regime to a lower threshold value, independent of the crystal geometry. Typical values of the aspect ratio of mineral crystals of bone and nacre fall in this transition region. The existence of a buckling strength independent of the detailed geometrical parameters of mineral may be critically important from the point of view of structure robustness as the composite behaviour of biomaterials should not depend sensitively on small variations in crystal size and shape.     

A cluster‐based factor rotation

A new oblique factor rotation method is proposed, the aim of which is to identify a simple and well‐clustered structure in a factor loading matrix. A criterion consisting of the complexity of a factor loading matrix and a between‐cluster dissimilarity is optimized using the gradient projection algorithm and the k‐means algorithm. It is shown that if there is an oblique rotation of an initial loading matrix that has a perfect simple structure, then the proposed method with Kaiser's normalization will produce the perfect simple structure. Although many rotation methods can also recover a perfect simple structure, they perform poorly when a perfect simple structure is not possible. In this case, the new method tends to perform better because it clusters the loadings without requiring the clusters to be perfect. Artificial and real data analyses demonstrate that the proposed method can give a simple structure, which the other methods cannot produce, and provides a more interpretable result than those of widely known rotation techniques.     

Errors in the stereoscopic separation of surfaces represented with regular textures

Stereograms containing two similar or dissimilar linear textures, either on the same surface or at two different depths, were tested on seventy subjects. Whereas random textures usually produced correct percepts, regular textures consistently led to errors of stereoscopic interpretations, including a reversal of hollows into bumps, dissociation of single surfaces into two layers, and errors in relative positioning of two surfaces. Horizontal-vertical textures tended to be seen as flatter and further away from the observer than diagonal ones. Continuous textures tended to be seen closer than discontinuous ones. In the interpretation of the results, the possibility is raised that different textures are processed independently and that the brain has no reliable method for combining the conclusions into a rigorous global percept.