Some interesting papers

March 19, 2011

[1] Study of spinodal decomposition and formation of nc-Al2O3/ZrO2 nanocomposites by combined ab initio density functional theory and thermodynamic modeling

S H Sheng et al

Using ab initio density functional theory, the equilibrium properties, such as the total energy, the molar volume, the bulk modulus and its first derivative, as well as the formation enthalpy of monoclinic ZrO2 and hexagonal α-Al2O3 phases, were calculated and compared with the published theoretical and experimental data. Based on the good agreement of these data, we calculated the lattice instabilities of hypothetical binary hexagonal Zr2O3 and monoclinic AlO2, and the interaction parameters of ternary Zr1−xAlxOy solid solutions. The binodal and spinodal diagrams were then constructed to predict the possibility of the formation of oxide-based nanocomposites which may display hardness enhancement above that of the solid solutions. It is shown that exponential dependence of the interaction parameter on temperature yields the most reliable results. The system should undergo spinodal phase segregation within the composition range that is relevant for the formation of hard or superhard nanocomposites with high thermal and oxidation stability, which are important for their applications.

[2] Direct characterization of phase transformations and morphologies in moving reaction zones in Al/Ni nanolaminates using dynamic transmission electron microscopy

J S Kim et al

Phase transformations and transient morphologies are examined as exothermic formation reactions self-propagate across Al/Ni nanolaminate films. The rapid evolution of these phases and sub-micrometer morphological features requires nanoscale temporal and spatial resolution that is not available with traditional in situ electron microscopy. This work uses dynamic transmission electron microscopy to identify intermetallic products and phase morphologies, as exothermic formation reactions self-propagate in nanolaminate films grown with 3:2, 2:3 and 1:1 Al/Ni atomic ratios. Single-shot diffraction patterns with 15 ns temporal resolution reveal that the NiAl intermetallic forms within not, vert, similar15 ns of the reaction front’s arrival in all three types of films and is the only intermetallic phase to form, as the reactions self-propagate and quench very rapidly. Time-resolved imaging reveals a transient cellular morphology in the Al-rich and Ni-rich foils, but not in the equiatomic films. The cellular features in the Al-rich and Ni-rich films are attributed to a cooling trajectory through a two-phase field of liquid + NiAl.

[3] Crystal plasticity modeling of texture evolution and heterogeneity in equal channel angular pressing of aluminum single crystal

C Lu et al

A crystal plasticity finite element method (CPFEM) model has been developed to investigate the texture evolution and heterogeneity during equal channel angular pressing (ECAP) of an aluminum single crystal. The developed model has been validated by comparison with experimental observations. The simulation results show that the lattices rotate predominantly around the Z axis (transverse direction) during ECAP. After deformation the billet is subdivided into three matrix bands along the thickness by the Z-axis rotation. The Z-axis rotation angles within three matrix bands are about 60°, 0° and 90°, respectively. It has been found that the die geometry plays a very important role on the texture evolution and heterogeneity in ECAP. In the large strain gradient region, multi-slip can be activated and the material rotation induced by slips is negligible. The lattices must rotate to accommodate the whole material rotation required by deformation. When the strain gradient is small, the single dominant slip is the main slip mechanism. The material remains roughly at the initial orientation after deformation. There is a rigid-rotation region in the lower part of the die corner where the lattices rotate by the die angle Φ.

[4] In situ synchrotron analysis of lattice rotations in individual grains during stress-induced martensitic transformations in a polycrystalline CuAlBe shape memory alloy

S Berveiller et al

Two synchrotron diffraction techniques, three-dimensional X-ray diffraction and Laue microdiffraction, are applied to studying the deformation behaviour of individual grains embedded in a Cu74Al23Be3 superelastic shape memory alloy. The average lattice rotation and the intragranular heterogeneity of orientations are measured during in situ tensile tests at room temperature for four grains of mean size not, vert, similar1 mm. During mechanical loading, all four grains rotate and the mean rotation angle increases with austenite deformation. As the martensitic transformation occurs, the rotation becomes more pronounced, and the grain orientation splits into several sub-domains: the austenite orientation varies on both sides of the martensite variant. The mean disorientation is not, vert, similar1°. Upon unloading, the sub-domains collapse and reverse rotation is observed.

► 3DXRD, Laue microdiffraction measurements of grain rotation in a shape memory alloy. ► During stress-induced martensitic transformation, the austenite grains rotate. ► This rotation reverses with the reverse transformation. ► The austenite grains splits into various orientations with martensite formation.

[5] Twin relationships of 5M modulated martensite in Ni–Mn–Ga alloy

Z Li et al

For Ni–Mn–Ga ferromagnetic shape memory alloys, the characteristic features of modulated martensite (including the number/shape of constituent variants, the inter-variant orientation relationship and the geometrical distribution of variant interfaces) determine the attainability of the shape memory effect. In the present work, a comprehensive microstructural and crystallographic investigation has been conducted on a bulk polycrystalline Ni50Mn28Ga22 alloy. As a first attempt, the orientation measurements by electron backscatter diffraction (EBSD) – using the precise information on the commensurate 5M modulated monoclinic superstructure (instead of the conventionally simplified non-modulated tetragonal structure) – were successfully performed to identify the crystallographic orientations on an individual basis. Consequently, the morphology of modulated martensite, the orientation relationships between adjacent variants and the characters of twin interfaces were unambiguously determined. With the thus-obtained full-featured image on the configuration of martensitic variants, the possibility of microstructural modification by proper mechanical “training” was further discussed. This new effort makes it feasible to explore the crystallographic/microstructural correlations in modulated martensite with high statistical reliability, which in turn provides useful guidance for optimizing the microstructure and shape memory performance.

► We determine orientation relationships of 5M modulated martensite in NiMnGa alloy. ► Accurate EBSD mapping is performed using monoclinic superstructure. ► Four distinct variants mutually twin-related to each other are revealed. ► Three twinning types and full twinning elements are identified. ► Twin interfaces do coincide with respective twinning planes.

[6] Chemistry and structure of core/double-shell nanoscale precipitates in Al–6.5Li–0.07Sc–0.02Yb (at.%)

C Monachon et al

An Al–6.3Li–0.07Sc–0.02Yb (at.%) alloy is subjected to a double-aging treatment to create nanoscale precipitates, which are studied by atom-probe tomography and transmission electron microscopy. After homogenization and quenching, Yb atoms form clusters exhibiting L12-like order. A first aging step at 325 °C leads to a doubling of microhardness as a result of the formation of coherent precipitates with an Al3Yb-rich core and an Al3Sc-rich shell. The core and shell both exhibit the L12 structure and both contain a large concentration of Li, which substitutes for up to 50% of the Sc or Yb atoms at their sublattice positions. These core/single-shell precipitates provide excellent resistance to overaging at 325 °C. Subsequent aging at 170 °C increases the microhardness by an additional 30%, through precipitation of a metastable δ′-Al3Li second shell on the core/single-shell precipitates, thereby forming a chemically and structurally complex core/double-shell structure. The metastable δ′-Al3Li phase is observed to form exclusively on pre-existing core/shell precipitates.

► An Al–Li alloy with dilute Sc and Yb-additions was double-aged for strengthening. ► Due to kinetic effects, core/shell precipitates form during the first aging. ► Lithium, though fast-diffusing, delays overaging of the alloy. ► A second aging treatment resulted in core/double-shell precipitates. ► Distinct hardening events correspond to the core, first shell, and second shell.

[7] An in situ transmission electron microscopy study of interface growth during martensitic transformation in an Fe–Ni–Mn alloy

J Wu et al

Frame-by-frame analysis of the austenite/lath martensite interface during in situ heating using transmission electron microscopy was used to provide direct information on the mechanisms of the martensite interface motion in an Fe–20Ni–5.5Mn (wt.%) alloy. When the temperature was increased to not, vert, similar550 °C, the tip of the lath martensite receded slightly, and then decomposed into ledges on only one side of the lath. With further time at 550 °C, the ledges migrated in a start–stop fashion; the highest velocity observed was 0.79 μm s−1. When the temperature was increased to not, vert, similar580 °C, the interface on the mobile side of the lath preferentially receded within certain transformation twins formed during the earlier quenching treatment. Based on these experimental observations, it appears that the austenite may form an array of parallel twins during the martensitic transformation, which coalesce to form a lath shape. This lath then thickens in one direction only to establish the final morphology by a ledge mechanism displaying start–stop growth behavior.

► Movement of martensite interfaces was investigated using in situ heating in the TEM. ► Martensite interfaces moved by a ledge mechanism rather than advancing uniformly. ► Martensite ledges vary from atomic dimensions to several tens of nanometers in height. ► Martensite lath dissolution occurred preferentially on one side.


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