Search

We have synthesized FINEMET alloy by a melt spinning technique and studied in detail its crystallization behavior and low-temperature magnetic properties. The crystallization behavior is characterized by transmission electron microscopy and Mössbauer spectroscopy. At early stages bcc solid solution precipitates from the amorphous matrix. At later stages, they order to yield DO[SUB3] ordered Fe[SUB3]Si coexisting with a small amount of Fe[SUB2]B. The analysis of Mössbauer spectra supports this observation. The temperature dependence of the magnetization in the temperature range 10-300 K of the FINEMET alloy in its as-quenched state follows the relation M(T)=M[SUB0](T)(1 - BT[SUP3/2]-CT[SUP5/2]-…), which is indicative of the presence of spin wave excitations in the alloy. The value of the C/B ratio and the mean-square value of the range of exchange interaction 〈r[SUP2]〉 are found to be characteristic of the noncrystalline ferromagnets. The small value obtained for the exchange stiffness constant D is an indication of the softening of the exchange interaction. [ABSTRACT FROM AUTHOR]

The electromagnetic levitation technique has been used to systematically study microstructure evolution and growth rate as a function of undercooling in concentrated Fe-18 at% Ge alloy. The samples are undercooled to a maximum of 240 K. Growth-rate analysis and transmission electron microscopy reveal that, beyond an undercooling of 120 K, the primary phase to solidify is disordered. Microstructural investigations show a decrease in grain size with increasing undercooling. Orientation-imaging microscopy using electron back-scattered diffraction (EBSD) and microhardness measurements have been used to show that recovery and recrystallization play a significant role in the evolution of final microstructure. Microstructural evolution has also been discussed in light of current models of dendrite growth and grain refinement. [ABSTRACT FROM AUTHOR]

The effect of size on the alloying and transformation behavior of multicomponent metallic systems has received very little attention. The present study reports a preliminary result in this direction involving Pb–In nanosized alloy particles embedded in an aluminum matrix. With the help of various techniques including in situ electron microscopy, we show that at small sizes, the Pb–In alloys particles are single-phase solid solution having fcc structure at the composition range covering both Pb and In rich regions. The results of in situ microscopy also indicate that the melting point and roughening behavior of these particles differ from particle to particle. The results are compared with that available for pure Pb particles. [ABSTRACT FROM AUTHOR]

Nanoscaled (In, Sn) alloy inclusions in Al matrix have been synthesized via rapid solidification processing route. Preliminary characterization of the samples has been done by X-ray diffraction. The microstructure of these alloy inclusions has been analyzed using transmission electron microscopy. Both In and Sn are immiscible with Al, and the miscibility gap can be accessed under rapid solidification conditions. In and Sn react with each other to form two intermediate phases, viz. β and γ respectively, which together form a eutectic. The morphology and orientation relationship of this β–γ eutectic system in nanoscale is reported here. TEM study reveals that majority of the alloy inclusions possess near cuboctahedral symmetry and that they are bicrystalline. Distinct two-phase contrast of these phases is observed in most cases. Melting and solidification characteristics of these alloy inclusions have been investigated via DSC. A melting peak at higher temperature is observed in the first cycle in addition to the eutectic melting event. Multiple solidification exotherms in the DSC is observed with maximum undercooling of 5 °C in the present case. [Copyright &y& Elsevier]

We report the formation of an amorphous phase in nanosized Bi particles embedded in an Al-based glassy alloy matrix. High-resolution electron microscopy (HREM) has been used to show that the particles contain crystalline and amorphous portions. A depression of the melting point by more than 100K of the crystalline portion of the Bi particles was found by differential scanning calorimetric studies and by in-situ electron microscopy using a heating stage. The same techniques established the absence of an amorphous phase in the particles when the matrix is crystallized. It is shown that the formation of the amorphous phase and the depression of the melting point cannot be explained by the pressure developed by the volume change during solidification in this constrained system. [ABSTRACT FROM AUTHOR]

The Co-19%Zr and Co-8%Fe-19%Zr (all in atomic percentage) alloys contain two magnetic phases Co 11 Zr 2 and Co 23 Zr 6 , with Fe substituting Co in the ternary alloy. The presence of the phases is confirmed by the M(T) measurements with an applied field of 20 Oe. Transmission electron microscopy further supports the above observation as well as provide evidence of the presence of antiphase domains in both the alloys. The saturation magnetization values of the two alloys are 86 emu/g and 93 emu/g respectively while the 0.2% proof stresses of these alloys are determined to be 1.4 and 1.1 GPa respectively. The addition of Fe in the ternary alloy not only has improved the magnetic properties but also affected a pronounced improvement in ductility by 7%. Thus magnetic properties are governed by the magnetic anisotropy of Co 11 Zr 2 . However, the softening of the binary alloys achieved in the ternary by the addition of Fe. [ABSTRACT FROM AUTHOR]

Abstract: The solidification pathways of Nb rich Nb–Si alloys when processed under non-equilibrium conditions require understanding. Continuing with our earlier work on alloying additions in single eutectic composition [1,2], we report a detailed characterization of the microstructures of Nb–Si binary alloys with wide composition range (10–25at% Si). The alloys are processed using chilled copper mould suction casting. This has allowed us to correlate the evolution of microstructure and phases with different possible solidification pathways. Finally these are correlated with mechanical properties through studies on deformation using mechanical testing under indentation and compressive loads. It is shown that microstructure modification can significantly influence the plasticity of these alloys. [Copyright &y& Elsevier]

Abstract: The paper reports the effect of addition of small amount of Mg on the mechanical and oxidation properties of Nb–Nb3Si eutectic composites in Nb–Si system under the condition of suction casting. Mg addition increases the volume fraction of primary dendrites of Nb solid solution. This phase contains significant amount of strengthening precipitates. Two different precipitates are identified. The large plate shaped precipitates are that of hcp phase, while fine coherent precipitates have the structure similar to recently identified δ-Nb11Si2 phase. The Mg addition improves both the strength and ductility of the composite at room temperature (∼1.4GPa and ∼5% engineering strain) as well as at 700°C(∼1.2GPa and ∼7% engineering strain). The presence of Mg results in a complex barrier layer which significantly increases the oxidation resistance up to a temperature of at least 1000°C. [Copyright &y& Elsevier]

CuBO2 nanostructures were synthesized by employing a low-cost hydrothermal technique to combine into the CuBO2-RGO nanocomposite for the first time using chemically prepared graphene sheets. The nanohybrid samples were characterized for structural information using X-ray diffraction (XRD) that revealed the proper crystalline phase formation of CuBO2 unaltered by composite formation with graphene. Raman spectroscopic studies were employed to confirm the presence of graphene. A morphological study with field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) suggested the proper wrapping of RGO sheets over CuBO2 nanocubes. Moreover, the close proximity of lattice planes of CuBO2 and RGO to each other was observed in high-resolution TEM studies that were correlated with the Raman spectroscopic studies. Finally, the samples were characterized to study the field emission (FE) properties of the same using a laboratory-made high-vacuum field-emission setup. Finite-element-based theoretical simulation studies were carried out to explain and compare the field emission properties with the experimental results. The FE properties of the composite samples were found to be tuned by the nature of wrapping the RGO sheets over the CuBO2 nanocubes, which was typically dependent upon the spiky morphology of the nanocubes. [ABSTRACT FROM AUTHOR]

The paper reports the design and development of wrought Al-Cu alloys that retain high strength until 250 °C. It is achieved by minor alloying of Zr (<0.15 at%) and Nb (<0.1 at%) and inducing precipitation of stable nanometric dispersion of L1 2 ordered precipitates during controlled thermomechanical treatment (rolling) of cast alloys before conventional heat treatment (solutionizing and ageing). The rolling temperature was optimized to 450 °C by quantitatively evaluating the size distribution of the dispersions at different rolling temperatures. The dispersions influence the microstructure and the nature of the precipitation of the strengthening θ′ plates during subsequent conventional heat treatment. The microstructure of the processed alloy show stability at high temperatures. The atom probe tomography reveals Zr enrichment at the broad faces of θ′ plates. The presence of Zr plays a critical role in the high-temperature strength by promoting the stability of θ′ precipitates and resisting its coarsening. The composite microstructure of Al-Cu-Nb-Zr alloy displayed yield strength (YS) of 415 MPa and 220 MPa at room temperature and at 250 °C, which are higher by 26 % and 45 % respectively as compared to a commercial Al-Cu based alloy (2219-T851). The present results are promising for developing wrought Al alloys for high-temperature applications. • Effect of Zr and Nb addition in Al-Cu alloy processing through thermo-mechanical route. • Dynamic precipitation of nanometric L1 2 ordered phase in Al matrix during wrought processing • L1 2 ordered precipitate promotes the nucleation of θ′ and retard the growth and coarsening. • APT study reveals solute (Zr/Nb) segregation at the Al/θ′ interface. [ABSTRACT FROM AUTHOR]

Abstract: The Ag–Ni system is characterized by large differences in atomic sizes (14%) and a positive heat of mixing (+23kJmol−1). The binary equilibrium diagram for this system therefore exhibits a large miscibility gap in both solid and liquid state. This paper explores the size-dependent changes in microstructure and the suppression of the miscibility gap which occurs when free alloy particles of nanometer size are synthesized by co-reduction of Ag and Ni metal precursors. The paper reports that complete mixing between Ag and Ni atoms could be achieved for smaller nanoparticles (<7nm). These particles exhibit a single-phase solid solution with face-centered cubic (fcc) structure. With increase in size, the nanoparticles revealed two distinct regions. One of the regions is composed of pure Ag. This region partially surrounds a region of fcc solid solution at an early stage of decomposition. Experimental observations were compared with the results obtained from the thermodynamic calculations, which compared the free energies corresponding to a physical mixture of pure Ag and Ni phases and a fcc Ag–Ni solid solution for different particle sizes. Results from the theoretical calculations revealed that, for the Ag–Ni system, solid solution was energetically preferred over the physical mixture configuration for particle sizes of 7nm and below. The experimentally observed two-phase microstructure for larger particles was thus primarily due to the growth of Ag-rich regions epitaxially on initially formed small fcc Ag–Ni nanoparticles. [Copyright &y& Elsevier]