Effect of hydrogen on the amorphous structure of the alloy Mg<Subscript>65</Subscript>Cu<Subscript>25</Subscript>Y<Subscript>10</Subscript> under electrochemical saturation

Thin ribbons of the metallic glass Mg₆₅Cu₂₅Y₁₀, obtained by spinning, were saturated with atomic hydrogen from electrochemical decomposition of water. The maximum amount of absorbed hydrogen was 4 mass %. The hydrogen content was determined by hot extraction. We studied the microstructure of samples with different hydrogen contents by x-ray phase analysis (from the change in the diffuse maximum), atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. When the hydrogen content increases up to 3.6 mass %, the amorphous structure of the Mg₆₅Cu₂₅Y₁₀ alloy is converted to a nanocrystalline structure, with formation of magnesium and yttrium hydrides at room temperature.     

Multistage Devitrification Behavior of Mg<Subscript>65</Subscript>Cu<Subscript>25</Subscript>Tb<Subscript>10</Subscript> Bulk Metallic Glass

The devitrification of Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) has been studied by time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) simultaneously. By analyzing the interference peaks on SAXS patterns and the Bragg peaks on WAXS patterns, it is found that devitrification initiates by activation of quenched-in short-range orders. Crystallization proceeds in three stages. During stage I, icosahedral clusters are formed that transforms to a quasi-crystalline 1/1 approximant during stage II, accompanied by the formation of cubic TbMg₃. In stage III, the 1/1 approximant transforms to a 2/1 approximant. The orthorhombic CuMg₂ phase is formed at a higher temperature when the quasi-crystalline phase starts to decompose. Pair distribution functions were evaluated to demonstrate these structural evolutions in real space. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Shape memory effect in a rapidly quenched Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy

The structural and thermomechanical properties of rapidly quenched layered amorphous–crystalline Ti₅₀Ni₂₅Cu₂₅ composite materials with various ratios of amorphous and crystalline phases are studied. These layered composite materials are shown to exhibit the two-way shape memory effect accompanied by bending deformation without additional thermomechanical treatment. The ratio of amorphous and crystalline phases is found to affect the reversible change in the shape of the composite material.     

Influence of heat treatment on the efficiency of impact fragmentation of amorphous alloy Fe<Subscript>73.5</Subscript>Cu<Subscript>1</Subscript>Nb<Subscript>3</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript> ribbons

The mechanism of impact fracture of soft magnetic amorphous alloy Fe_73.5Cu₁Nb₃Si_13.5B₉ ribbons in a disintegrator after heat treatment at a temperature from the range 300–700°C and the fractional composition of the formed powder are studied. The temperature ranges of a change in the mechanism of ribbon fracture are determined. The particle size distribution is shown to change weakly within the revealed temperature ranges.     

Interpretation of precipitation crystallography of Mg<Subscript>17</Subscript>Al<Subscript>12</Subscript> in a Mg-Al alloy in terms of singular interfacial structure

This work applies a constrained coincidence site lattice/constrained complete pattern shift lattice (CCSL/CDSCL) model and secondary O-lattice model to simulate the interfacial structure in the major side interface of Mg₁₇Al₁₂/Mg. The result shows that, at the orientation relationship (OR) with a small deviation (∼0.5 deg) from the Burgers OR, the secondary misfit in the interface normal to the parallel Δg vectors can be completely accommodated by the steps. The secondary dislocation networks in the habit plane and major side facet have been calculated. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee.     

Effect of Planar Flow Melt Spinning Parameters on Ribbon Formation in Soft Magnetic Fe<Subscript>68.5</Subscript>Si<Subscript>18.5</Subscript>B<Subscript>9</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> Alloy

The effect of planar flow melt spinning (PFMS) parameters on the continuity, surface quality, and structure of 10-mm-wide Fe_68.5Si_18.5B₉Nb₃Cu₁ ribbons has been investigated. The change in shape and stability of the melt puddle as a function of the processing parameter was studied using a high-speed imaging system and was correlated to ribbon formation. A window of process parameters for obtaining continuous ribbons with good surface quality has been evaluated. It has been observed that thinner ribbons are found to be more continuous because of higher ductility. The higher melt temperature leads to the formation of crystalline phase in as-spun ribbons, and this deteriorates the soft magnetic properties on annealing. The experimental results are corroborated with the numerical estimates, which suggest that the critical thickness for amorphous phase formation decreases with increasing initial melt temperature.     

Phase analysis of multicomponent alloys using an Ni<Subscript>50</Subscript>Co<Subscript>25</Subscript>Mo<Subscript>25</Subscript> alloy as an example

method for phase analysis of three-component alloys is proposed. It is based on a pair interaction model and an experimental determination of the sign of pair chemical interaction energy and includes an electron-microscopic investigation of microstructures above and below the ordering–separation phase transition temperature for each diffusion couple. This method is used to study an Ni₅₀Co₂₅Mo₂₅ alloy. The phases that precipitate in this alloy over the entire heating temperature range, including the liquid state, are detected.     

The effect of heat treatment on Mg<Subscript>2</Subscript>Si coarsening in aluminum 6105 alloy

The coarsening behavior of Mg₂Si intermetallic particles was studied during homogenization of aluminum 6105 alloy. An industrially homogenized 6105 aluminum alloy contained large globular undissolved Mg₂Si. With subsequent heat treatment at 590 °C over 1 week, it was found that the large Mg₂Si precipitates coarsened with time. The coarsening of the large Mg₂Si precipitates was quantified by statistical analysis of micrographs. A linear relationship between and time fits the experimental measurements quite well, indicating that the coarsening is diffusion controlled, with grain-boundary diffusion dominating volume diffusion.     

Effect of Superficially Applied Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating on High-Temperature Corrosion Behavior of Ni-Base Superalloys

Inhibitors and oxide additives have been investigated with varying success to control high-temperature corrosion. Effect of Y₂O₃ on high-temperature corrosion of Superni 718 and Superni 601 superalloys was investigated in the Na₂SO₄-60 pct V₂O₅ environment at 1173 K (900 °C) for 50 cycles. Y₂O₃ was applied as a coating on the surfaces of the specimens. Superni 601 was found to have better corrosion resistance in comparison with Superni 718 in the Na₂SO₄-60 pct V₂O₅ environment. The Y₂O₃ superficial coating was successful in decreasing the reaction rate for both the superalloys. In the oxide scale of the alloy Superni 601, Y and V were observed to coexist, thereby indicating the formation of a protective YVO₄ phase. There was a distinct presence of a protective Cr₂O₃-rich layer just above the substrate/scale interface in the alloy. Whereas Cr₂O₃ was present with Fe and Ni in the scale of Superni 718. Y₂O₃ seemed to be contributing to better adhesion of the scale, as comparatively lesser spalling was noticed in the presence of Y₂O₃.     

Phase transitions in the magnetic superconductor (Dy<Subscript>0.8</Subscript>Y<Subscript>0.2</Subscript>)Rh<Subscript>4</Subscript>B<Subscript>4</Subscript>

A new (Dy_0.8Y_0.2)Rh₄B₄ superconductor (the superconducting transition temperature is T _c ≈ 5.5 K), which has an inherent magnetic subsystem whose properties are determined by the crystal structure of the superconductor, is synthesized at a high pressure (∼8 GPa) and t ≈ 1800°. The magnetic sublattice of the (Dy_0.8Y_0.2)Rh₄B₄ compound is found to substantially affect its superconducting properties and, in a number of cases, to lead to their anomalous variations, namely, to the absence of the traditional Meissner effect and an anomalously abrupt increase in magnetic induction B _k2 (upper critical field) upon a transition of the magnetic subsystem into the antiferromagnetic state. Upon cooling from 250 to 1.6 K, the (Dy_0.8Y_0.2)Rh₄B₄ compound undergoes a number of phase transformations, namely, a paramagnet-ferrimagnet transition at a Curie temperature T _C ≈ 30 K, a superconducting transition at T _c ≈ 5.5 K against the background of a ferrimagnetic order, and a ferrimagnet-antiferromagnet transition (the Neel temperature is T _N ≈ 2.8 K) in the retained superconducting state.     

Deformation and failure of Zr<Subscript>57</Subscript>Nb<Subscript>5</Subscript>Al<Subscript>10</Subscript>Cu<Subscript>15.4</Subscript>Ni<Subscript>12.6</Subscript>/W particle composites under quasi-static and dynamic compression

We have investigated the mechanical behavior of a composite material consisting of a Zr₅₇Nb₅Al₁₀Cu_15.4Ni_12.6 metallic glass matrix with 60 vol pct tungsten particles under uniaxial compression over a range of strain rates from 10⁻⁴ to 10⁴ s⁻¹. In contrast to the behavior of single-phase metallic glasses, the failure strength of the composite increases with increasing strain rate. The composite shows substantially greater plastic deformation than the unreinforced glass under both quasi-static and dynamic loading. Under quasi-static loading, the composite specimens do not fail even at nominal plastic strains in excess of 30 pct. Under dynamic loading, fracture of the composite specimens is induced by shear bands at plastic strains of approximately 20 to 30 pct. We observed evidence of shear localization in the composite on two distinct length scales. Multiple shear bands with thicknesses less than 1 μm form under both quasi-static and dynamic loading. The large plastic deformation developed in the composite specimens is due to the ability of the tungsten particles both to initiate these shear bands and to restrict their propagation. In addition, the dynamic specimens also show shear bands with thicknesses on the order of 50 μm; the tungsten particles inside these shear bands are extensively deformed. We propose that thermal softening of the tungsten particles results in a lowered constraint for shear band development, leading to earlier failure under dynamic loading.     

Kinetics study on non-isothermal crystallization of Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> metallic glass

In this paper, the crystallization kinetics of melt-spun Cu₅₀Zr₅₀ amorphous alloy ribbons has been investigated using differential scanning calorimetry. Moreover, the Kissinger, Ozawa and isoconversional approaches have been used to obtain the crystallization kinetic parameters. As shown in the results, the onset crystallization activation energy E _x is less than crystallization peak activation energy E _p. The local activation energy E _α increases at the crystallized volume fraction α < 0.2 and decreases at the rest, which suggests that crystallization process is increasingly hard (α < 0.2) at first, after which it become increasingly easy (α > 0.2). The nucleation activation energy E _nucleation is greater than grain growth activation energy E _growth, indicating that the nucleation is harder than growth. In terms of the local Avrami exponent n(α), it lies between 1.27 and 8, which means that crystallization mechanism in the non-isothermal crystallization is interface-controlled one- two- or three-dimensional growth with different nucleation rates.     

Microstructural Evolution and Mechanical Properties of Nanointermetallic Phase Dispersed Al<Subscript>65</Subscript>Cu<Subscript>20</Subscript>Ti<Subscript>15</Subscript> Amorphous Matrix Composite Synthesized by Mechanical Alloying and Hot Isostatic Pressing

The structure and mechanical properties of nanocrystalline intermetallic phase dispersed amorphous matrix composite prepared by hot isostatic pressing (HIP) of mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous powder in the temperature range 573 K to 873 K (300 °C to 600 °C) with 1.2 GPa pressure were studied. Phase identification by X-ray diffraction (XRD) and microstructural investigation by transmission electron microscopy confirmed that sintering in this temperature range led to partial crystallization of the amorphous powder. The microstructures of the consolidated composites were found to have nanocrystalline intermetallic precipitates of Al₅CuTi₂, Al₃Ti, AlCu, Al₂Cu, and Al₄Cu₉ dispersed in amorphous matrix. An optimum combination of density (3.73 Mg/m³), hardness (8.96 GPa), compressive strength (1650 MPa), shear strength (850 MPa), and Young’s modulus (182 GPa) were obtained in the composite hot isostatically pressed (“hipped”) at 773 K (500 °C). Furthermore, these results were compared with those from earlier studies based on conventional sintering (CCS), high pressure sintering (HPS), and pulse plasma sintering (PPS). HIP appears to be the most preferred process for achieving an optimum combination of density and mechanical properties in amorphous-nanocrystalline intermetallic composites at temperatures ≤773 K (500 °C), while HPS is most suited for bulk amorphous alloys. Both density and volume fraction of intermetallic dispersoids were found to influence the mechanical properties of the composites.