R-phase Research Articles

Electron transport properties of lithium and phase transitions at high pressures

The electric resistivity and thermopower of lithium have been precisely measured at high pressures (up to 8 GPa) and temperatures from room temperature to 100°C. Transition to the fcc phase of lithium has been analyzed. The hysteresis of the direct and inverse transitions is 0.3 GPa at room temperature, decreases slightly with an increase in the temperature, and is almost independent of the prehistory of the sample. The phase transition line on the P-T diagram has a positive slope of dP/dT = 0.03 GPa/K. It is assumed that the fcc phase of lithium, which is stable at a high pressure, can appear for kinetic regions from the 9R phase, which is intermediate in energy between the bcc and fcc modifications.

Mechanical behavior under cyclic loading of the 18R-6R high-hysteresis martensitic transformation in Cu-Zn-Al alloys with nanoprecipitates

Mechanical damping applications could benefit from the large hysteresis, large pseudoelastic strain and the fact that the transformation stresses of the 18R↔6R martensite–martensite transformation depend very little on temperature in Cu-based alloys. This work presents the 18R↔6R mechanical cycling behavior of CuZnAl shape-memory alloy single crystals with electronic concentration e/a=1.48. A fine distribution of gamma phase nanoprecipitates is introduced to prevent plastic deformation of the 6R phase. Results show that, although significant 6R stabilization is observed at very low frequencies (below 10−2Hz), it is possible to obtain more than 1000 stable pseudoelastic cycles with only minor changes in transformation stresses and hysteresis width at frequencies above 10−1Hz. A more pronounced decrease in transformation stresses is observed after 1000 cycles. Nevertheless, the decrease in hysteresis is small up to 2000 cycles. Reported and present results indicate that pair interchange of atoms can explain the stabilization of 6R under quasistatic experimental conditions. However, at higher frequencies of cycling, stabilization of this martensite shows additional features, leading to a dynamic stabilization with slight effects on the mechanical behavior at the required frequency and number of cycles. On the whole, the behavior of this transformation is unique and very promising.

High resolution transmission electron microscopy characterization of fcc → 9R transformation in nanocrystalline palladium films due to hydriding

Sputtered nanocrystalline palladium thin films with nanoscale growth twins have been subjected to hydriding cycles. The evolution of the twin boundaries has been investigated using high resolution transmission electron microscopy. Surprisingly, the ∑3{112} incoherent twin boundaries dissociate after hydriding into two phase boundaries bounding a 9R phase. This phase which corresponds to single stacking faults located every three {111} planes in the fcc Pd structure was not expected because of the high stacking fault energy of Pd. This observation is connected to the influence of the Hydrogen on the stacking fault energy of palladium and the high compressive stresses building up during hydriding.

Growth and transformation mechanisms of 18R and 14H in Mg–Y–Zn alloys

The growth of and transformation between 18R and 14H phases in a Mg–8Y–2Zn–0.6Zr (wt.%) alloy have been examined using conventional transmission electron microscopy and atomic resolution high-angle annular dark-field scanning transmission electron microscopy. The growth of both 18R and 14H within the α-Mg matrix occurs via a ledge mechanism, with the thickness of the particle increasing by the height of the ledge as it propagates. The unit height of the growth ledges or disconnections is 1.563nm for 18R and 1.824nm for 14H, and the displacement vector is a/3〈1¯010〉α. 18R transforms in-situ to 14H during prolonged heat treatment at 500°C. The 18R to 14H transformation is shown to occur most readily in regions where the 18R structure has irregularities in the building block stacking, in particular where a pair of adjacent building blocks is separated by four rather than two α-Mg layers. It is proposed that the diffusional-displacive 18R to 14H transformation rate is controlled by the diffusion rate of Y and Zn atoms into the segregation layers.

Improvements in the mechanical properties of the 18R↔6R high-hysteresis martensitic transformation by nanoprecipitates in CuZnAl alloys

Abstract The 18R ↔ 6R martensite–martensite transformation in Cu-based alloys exhibits large hysteresis, large pseudoelastic strain and weak transformation stress dependence on temperature. However, concomitant plastic deformation taking place in the 6R phase inhibits the use of these properties for applications. A novel approach to minimizing or even suppressing 6R plastic deformation during the 18R–6R transformation in CuZnAl shape-memory alloy single crystals with electronic concentration e/a = 1.48 is presented. The method is based on a thermal treatment that introduces nanoprecipitates in the alloy. Results suggest that the role of CuZnAl shape-memory alloys in engineering should be reconsidered, as many energy damping applications could benefit from the huge hysteresis associated with the 18R–6R transformation, once the 6R plastic deformation is suppressed.

Superior hydrogen storage kinetics of Mg 12YNi alloy with a long-period stacking ordered phase

The hydrogen absorption/desorption kinetics of the Mg 12 YNi alloy with an 18R-type long-period stacking ordered phase was investigated. It was found that the hydrogen-induced decomposition of the 18R phase with a grain size of 200–300 nm occurs at 573 K, leading to the formation of YH 2 and YH 3 particles with an average size of 10 nm. In the subsequent hydrogen absorption/desorption process, the fine YH 2 and/or YH 3 particles act as catalysts that enhance remarkably the hydrogen storage kinetics.

PHASE FIELD SIMULATION OF EFFECTS OF PORES ON B2-R PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOY

Recently, porous NiTi shape memory alloys (SMAs) have drawn great interest in vari- ous engineering fields, in particular for biomedical applications as one of the promising biomaterials for hard-tissue replacements and orthopedic implants. It is well known that the porous NiTi SMAs exhibit three transformations, B2-B19 � , B2-R and R-B19 � . Among these phase transformations B2-B19 � and R-B19 � involve high lattice distortion and large transformation hysteresis. Consequently, these distortion and transformations usually introduce structural defects which may result in degradation of mechanical stability for the functional application. On the contrary, B2-R transformation is governed by small lattice distortion which indicates less damage to the microstructure and lower sensitivity to the defects, and thus generates higher reversibility and mechanical stability. Due to these unique virtues, it is of great importance to study the R-phase transformation behavior in porous NiTi SMAs, since the pore has a significant influence on B19 � martensitic transformation as well as the R-phase transforma- tion. In this paper, a three-dimensional phase field model aiming at accounting for the pore effect on phase transformation in NiTi SMAs was developed to study the B2-R phase transformation behavior in porous NiTi SMAs. The model was applied to characterize the microstructure evolution of B2-R phase

Influence of Isothermal Aging on the Embrittlement of 00Cr25Ni7Mo4N Super Duplex Stainless Steel

The effect of an isothermal aging treatment on the embrittlement of 00Cr25Ni7Mo4N super duplex stainless steel was studied by combining metallographic, SEM and TEM observation. The results showed that thermal aging treatment below 500°C and above 1050°C did not have any effect on the impact toughness. The observed embrittlement behaviour was mainly attributed to the generation of intermetallic phases in the steel matrix, which included the R phase formation below 600°C, the R phase and especially the σ phase formation at 700°C, and the formation of the σ phase in the range of 800-1000°C. The impact toughness decreased to a minimum after a thermal aging for 30 minutes in the temperature range of 800°C to 900°C. A trend of decreasing impact ductility with increasing amount of precipitated σ phase was observed. The impact energy started to decrease as the amount of the σ phase reached about 10-15 volume-% and reached a minimum when the amount of the σ phase increased to about 30 volume-%.

Dislocation structures of Σ3 {112} twin boundaries in face centered cubic metals

High resolution transmission electron microscopy of nanotwinned Cu films revealed Σ3 {112} incoherent twin boundaries (ITBs), with a repeatable pattern involving units of three {111} atomic planes. Topological analysis shows that Σ3 {112} ITBs adopt two types of atomic structure with differing arrangements of Shockley partial dislocations. Atomistic simulations were performed for Cu and Al. These studies revealed the structure of the two types of ITBs, the formation mechanism and stability of the associated 9R phase, and the influence of stacking fault energies on them. The results suggest that Σ3 {112} ITBs may migrate through the collective glide of partial dislocations.

Rhombohedral (9R) and hexagonal (6H) perovskites in barium silicates under high pressure

Rhombohedral (9R) and hexagonal (6H) perovskites in BaSiO 3 were observed by using in-situ X-ray diffraction methods in high-pressure conditions. The 9R perovskite was found after successive laser heating at 27.9 GPa, and the 6H phase was identified at 48.5 GPa after laser annealing. No intermediate phase between the 9R and 6H phases was found in the pressure range between 27.9 and 48.5 GPa. These phases could not be recovered during decompression at room temperature and became amorphous at ambient pressure. The 9R and 6H structures are characterized by the periodicity of the stacking sequence of edge- and corner-sharing SiO 6 octahedra. In terms of the length of the O-O distance, the face-sharing SiO 6 octahedron in BaSiO 3 perovskite is the most distorted among oxide hexagonal perovskites.

Evidence for deformation-induced transformations of Cu-rich precipitates in an aged FeCu alloy

We present experimental evidence that deformation can induce the transformation of small Cu-rich precipitates from the coherent bcc phase to the 9R phase in aged binary FeCu alloys. This is in broad agreement with molecular dynamics simulations of the interaction of dislocations with copper precipitates carried out by Bacon and co-workers [Acta Mater. 50 195/209 (2002); Mater. Sci. Eng. A 400/401 353 (2005); J. Nucl. Mater. 329/333 1233 (2004)].

Preparation and characterization of AgI nanoparticles with controlled size, morphology and crystal structure

AgI nanoparticles were prepared by solution-based routes using water-soluble anionic or cationic polyelectrolytes as capping agents. Depending on the polyelectrolytes, AgI nanoparticles with well-defined morphology, size, and phase compositions were obtained: the use of poly(sodium 4-styrenesulfonate) (PSS) resulted in AgI nano-rods of β-AgI in wurtzite structure (2H); with poly(acrylic acid sodium salt) (PAS) truncated-tetrahedron shaped γ-AgI nanoparticles (nanotetrahedra) in zinc-blende structure (3C) were obtained; by employing poly(diallyldimethylammonium chloride) (PDADMAC) plate-like AgI nanoparticles (nano-plates) consisting of unusual polytype phases of AgI (7H and 9R) were formed. Macroscopically unstable γ-AgI and 7H and 9R phases could be stabilized in the form of nanocrystalline powders. They transform reversibly into the high temperature α-AgI phase and exhibit unusually high ionic conductivity and substantially smaller transformation enthalpy values compared to the macroscopic β-AgI.

Effects of Si and Nb additions on CuAINiTi rapidily solidified alloys microstructures

Intensive investigations have been carried out on CuAlNiTi shape memory alloys (SMAs) with 1wt.%Ti and Ni content from 5 to 6 wt%. In this present work, we used a rapid solidification technique to prepare alloys of two compositions: (wt%) Cu 79 Al 13 Ni 5 Ti 2 Si 1 , Cu 79 Al 13 Ni 5 Ti 1 Nb 2 . The ribbons obtained were characterised by means of XRD, DSC measurements and TEM observations. XRD patterns indicated that the first alloy has a two-phase structure with beta and martensite 18R phases, whereas the second one is only beta. DSC measurements gave characteristic transformations temperatures. The Si-containing alloy showed low temperature transformations with M, about -88°C and A f at -41°C in as-quenched state. The Nb-containing alloy exhibited high temperature transformations with A f about 170°C in the as-quenched state, which is quite higher for this alloy family. The transformation temperatures are stabilised after only one cycle at ΔT M =(M s -M f ) constant for both alloys. This stabilisation is likely to be generated, in the case of Cu 79 Al 13 Ni 5 Ti 1 Nb 2 , by nanocrystalline precipitates revealed on TEM micrographs. Narrow and faulted martensite plates have been observed in agreement with previous results on rapidly solidified CuAlNiTi SMAs.

Hrem Observation of the Relaxation of Grain Boundaries in Au At Their Intersection with Free Surfaces

Grain boundaries in fee metals with low stacking fault energy are known to undergo extended relaxations that can at times lead to a thin layer of a different structure. in Cu, for example, it has been found that Σ3{112}} boundaries relax into a 9R phase or even a bec phase. in this work, we have used high resolution electron microscopy to investigate the atomic structure of Σ99 grain boundaries in mazed bicrystal films of Au. On {001} Ge single crystal substrates, Au bicrystals can be grown in two equivalent {110} orientation variants, related to each other by a 90° rotation about the surface normal. There is equal probability to nucleate either orientation variant in early stages of film formation and since there is no reason for preferential growth, the morphology of the bicrystal film consists of many grains in only two orientations. All grain boundaries are of the type Σ99 and display tilt character.Thin films 10-200nm thick were grown by physical vapor deposition on {001} Ge substrate at 280°C and followed by annealing at 200°C to relieve stresses.

HREM Analysis of ∑3 ﹛112﹜ Tilt Grain Boundaries in Au Bicrystals Observed in < 111 > Projection

Abstract Grain boundaries in fee metals with low stacking fault energy are known to undergo extended relaxations that can at times lead to a thin layer of a different structure. In Cu, for example, it has been found that ∑3﹛ 112﹜ boundaries relax into a 9R phase [1]. In this work, we have used high resolution electron microscopy to investigate the atomic structure of ∑3 grain boundaries in mazed bicrystal films of Au. Using ﹛111﹜ Ge surfaces as a template, Au bicrystals can be grown in two orientation variants, related to each other by a 60° rotation about the surface normal. As described previously, such films have a strong tendency to facet onto the coherent twin plane parallel to the substrate [2], also known as “double positioning” [3]. If films are made very thin, the likelihood for such in-plane boundaries to lie in the foil decreases, and it becomes possible to observe the atomic structure of edge-on interfaces along <111>.

Linear-Response Studies of the Electron-Phonon Interaction in Metals

Applications of the first-principles linear-response method for calculating electron–phonon coupling parameters are presented. Calculations show that the electron–phonon coupling in the low-temperature 9R phase of Li is significantly weaker than in the room-temperature b.c.c. phase, which helps explain the observed absence of a superconducting transition down to the mK regime. Results for the electron–phonon interaction in compressed S indicate that the surprisingly high superconducting Tc of 17 K observed at 160 GPa arises from a combination of a moderately large electron–phonon mass enhancement parameter and a very high phonon energy scale for the compressed lattice.

Slip systems in Cu–Zn–Al martensitic phases

Abstract The plastic behavior of Cu–Zn–Al martensitic phases is reported. The 6R phase was obtained by tensile tests, and the plastic deformation occurs during the transformation. The 18R and 2H structures were induced by compression, plastic strain takes place once the transformation is completed and by increasing the compressive stress. The observed slip modes in each structure correspond to shear on close-packed systems.

Synthesis and structure change of Ba 1− xBi xMnO 3

Polycrystalline samples of Ba 1− x Bi x MnO 3 were synthesized over a wide Bi concentration range and examined by X-ray powder diffraction, electron diffraction and compositional analysis. The metal composition of the platelet-like crystals was Ba:Bi:Mn=47.8:2.3:49.9 for nominal composition x=0.05 and 0.1. A hexagonal shaped and platelet-like single crystal with the size of 160×150×15 μm 3 was obtained by a melt and growth technique using (Ba 0.9Bi 0.1)MnO 3 as precursor and Bi 2O 3 as flux. The chemical composition of as-grown crystals was nearly the same as the bulk specimen. The relationship between the average ionic radius of the Ba/Bi cations in the A site and Bi content showed that the 9R phase was stabilized for x=0.05.

Effect of Grain Boundary Dislocations on 9R Stacking Errors at an Incoherent Twin Boundary in Copper

We consider the nature of faults in the dissociated 9R phase at an incoherent twin boundary in copper. Analysis of the dislocation content of a five-layer break in the normal ABC/BCA/CAB stacking sequence of the 9R shows that this defect is associated with a secondary grain boundary dislocation (SGBD) of Burgers vector a/6[211]. Atomistic simulations of SGBDs at the incoherent twin interface show that the sign of the Burgers vector is important to determining the core structure of the dislocation. The resulting atomistic structures and the five-layer width of the fault can be directly understood by analyzing the set of twinning dislocations that make up the boundary. Atomistic calculations show further, in agreement with the experimental observations, that a shear parallel to the interface causes an increase in the fault width that accompanies the growth of the 9R phase.

Stacking defects in the 9R phase at an incoherent twin boundary in copper

In situ HRTEM observations of the growth of 9R stacked material at a Σ=3 incoherent twin boundary in copper are presented. Stacking defects, which break the normal ABC BCA CAB… sequence of the 9R stacking, are observed in the growing 9R layer. These faults are shown to arise from the presence of secondary grain boundary dislocations with Burgers vectors of type a/6〈 211〉 . It is shown that both the sense (positive or negative) and character (90° or 30°) of the grain boundary dislocation determine the characteristics of the 9R fault.