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.     

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.     

Thermomechanical Behavior of Cu<Subscript>50</Subscript>Hf<Subscript>41.5</Subscript>Al<Subscript>8.5</Subscript> Bulk Metallic Glass after Sustained Elastic Deformation

Thermomechanical analysis (TMA) was conducted in a temperature modulated mode to analyze the effect of static and dynamic elastic compression on a Cu₅₀Hf_41.5Al_8.5 bulk metallic glass. The nonreversible length changes clearly demonstrate that the elastic loading affects the thermomechanical behavior of the metallic glass. A sustained static elastic compressive load increases the relative length decrease, while a dynamic elastic load to the same maximum load and for the same time reduces the length decrease. A preliminary interpretation suggests that the static compression raises the defect of free volume level, but the dynamic compression mimics annealing and reduces the free volume level. Elastic compression thus emerges as a novel tool to control the free volume level of metallic glasses.     

Preparation and Properties of Ti<Subscript>50</Subscript>Cu<Subscript>28</Subscript>Ni<Subscript>15</Subscript>Sn<Subscript>7</Subscript> Bulk Metallic Glass by Vacuum Hot Pressing

Amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders were synthesized by a mechanical alloying (MA) technique. Differential scanning calorimetry (DSC) results showed that, after 7 hours of exposure to the milling process, amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders exhibit a wide supercooled liquid region of 61 K. Consolidation of amorphous powders were performed at a temperature slightly higher than the glass transition temperature under a pressure of ∼1.2 GPa, and bulk metallic glass (BMG) discs can be prepared successfully. However, we noticed partial crystallization during the hot pressing process and were not able to achieve full densification of BMG. The Vickers microhardness of Ti₅₀Cu₂₈Ni₁₅Sn₇ BMG was 634 kg/mm², and the trace of the indentation revealed that pre-existing particle boundaries or interfaces between nanocrystals and amorphous matrix may serve as the crack initiation sites. Thus, typical brittle failure of Ti₅₀Cu₂₈Ni₁₅Sn₇ BMG was observed and resulted in relatively low fracture stress compared to that estimated by the microhardness. 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.     

Oxidation Behavior of a Pd<Subscript>43</Subscript>Cu<Subscript>27</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> Bulk Metallic Glass and Foam in Dry Air

The oxidation behavior of both Pd₄₃Cu₂₇Ni₁₀P₂₀ bulk metallic glass (Pd4-BMG) and its amorphous foam containing 45 pct porosity (Pd4-AF) was investigated over the temperature range of 343 K (70 °C) to 623 K (350 °C) in dry air. The results showed that virtually no oxidation occurred in the Pd4-BMG at T < 523 K (250 °C), revealing the alloy’s favorable oxidation resistance in this temperature range. In addition, the oxidation kinetics at T ≥ 523 K (250 °C) followed a parabolic-rate law, and the parabolic-rate constants (k _p values) generally increased with temperature. It was found that the oxidation k _p values of the Pd4-AF are slightly lower than those of the Pd4-BMG, indicating that the porous structure contributes to improving the overall oxidation resistance. The scale formed on the alloys was composed exclusively of CuO at T ≥ 548 K (275 °C), whose thickness gradually increased with increasing temperature. In addition, the amorphous structure remained unchanged at T ≤ 548 K (275 °C), while a triplex-phase structure developed after the oxidation at higher temperatures, consisting of Pd₂Ni₂P, Cu₃P, and Pd₃P.     

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.     

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.     

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.     

Phase equilibria and thermodynamic properties of the Cu<Subscript>2</Subscript>O-CaO-Na<Subscript>2</Subscript>O system in equilibrium with copper

The liquidus surfaces of the Cu₂O-CaO, Cu₂O-Na₂O, and Cu₂O-CaO-Na₂O phase diagrams in equilibrium with metallic Cu were measured by thermal analysis at compositions varying from approximately 0 to 35 wt pct Na₂O and 0 to 15 wt pct CaO. Solubilities in the solid binary terminal solutions were also measured by wavelength dispersive X-ray spectrometer analysis. Copper oxide activities in binary liquid slags were determined from the measured oxygen content of the metallic copper equilibrated with the slags. The ternary system is a simple eutectic system. No ternary compounds were observed. The Cu₂O-CaO binary eutectic was measured at 1140 °C±10 °C at 10±1 wt pct CaO and the Cu₂O-Na₂O binary eutectic was measured at 803 °C±15 °C at 28±2 wt pct Na₂O. The liquid slag was thermodynamically modeled with the modified quasi-chemical model, while the solid Cu₂O-rich solution was treated as Henrian ideal. All data from the present work and from the literature (phase diagrams and activities) for the binary systems were evaluated simultaneously by least-squares optimization in order to obtain the best model parameters. With only these binary parameters, the calculated ternary liquidus surface is in very good agreement with the measurements. Finally, using the model, the liquidus projection of the Cu₂O-CaO-Na₂O system in equilibrium with Cu was calculated as well as the oxygen content of the equilibrated Cu as a function of slag composition.     

Corrosion Behavior of Fe<Subscript>41</Subscript>Co<Subscript>7</Subscript>Cr<Subscript>15</Subscript>Mo<Subscript>14</Subscript>C<Subscript>15</Subscript>B<Subscript>6</Subscript>Y<Subscript>2</Subscript> Bulk Metallic Glass in Sulfuric Acid Solutions

An Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk metallic glass with a diameter of 5 mm was prepared with the copper-mold-casting method. The corrosion resistance of this amorphous steel in sulfuric-acid solutions was determined by electrochemical measurements. The passive film formed on the surface of the alloy after immersion in the 0.5-mol/l H₂SO₄ solution for 1 week was analyzed by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements show that the corrosion resistance of the amorphous steel in the 1 mol/l-H₂SO₄ solution is superior to a stainless steel (SUS 321), and is almost the same as Ti6Al4V, which shows that the amorphous steel has an excellent corrosion resistance in sulfuric-acid solutions. As the concentration of the sulfuric-acid solutions increases from 0.5 mol/l to 4 mol/l, the corrosion resistance of the amorphous steel decreases. The XPS result reveals that a bilayer structure of protective film formed on the surface of the amorphous steel in a H₂SO₄ solution. The compositions of the inner part of the film are MoO₂, Cr₂O₃, CoO, and FeO, and those of the outer film are MoO₃, Cr(OH)₃, Co(OH)₂, and Fe(OH)₃.     

Mechanically induced solid-state devitrifications of Zr<Subscript>70</Subscript>Pd<Subscript>20</Subscript>Ni<Subscript>10</Subscript> glassy alloy powders

A single glassy phase of Zr₇₀Pd₂₀Ni₁₀ alloy powder was synthesized by mechanical alloying the elemental powders for 48 hours, using a high-energy ball-milling technique. The obtained glassy phase transformed into a metastable big-cube phase upon increasing the ball-milling time (100 hours). After 150 hours of milling, a complete glass-metastable-phase transformation was achieved, and the end product was nanocrystalline big-cube powder, which has a lattice constant of 1.23 nm. As the ball-milling time was further increased the big-cube phase could no longer withstand the mechanical deformation that was generated by the milling media and transformed into a new metastable phase of nanocrystalline fcc Zr₇₀Pd₂₀Ni₁₀. The lattice constant of this metastable phase was calculated to be 0.455 nm. The reported metastable phases here are new and have never been, so far as we know, reported for the ternary Zr-Pd-Ni system, or its binary-phase relations.     

Structural Transitions and Magnetic Properties of Ni<Subscript>50</Subscript>Mn<Subscript>36.7</Subscript>In<Subscript>13.3</Subscript> Particles with Amorphous-Like Phase

The high-energy ball-milling method was used for fabricating Ni₅₀Mn_36.7In_13.3 fine-sized particles. The as-melt polycrystalline Ni₅₀Mn_36.7In_13.3 alloy exhibits a 14 M modulated martensite structure at room temperature (RT). The atomic pair distribution function analysis together with the differential scanning calorimetry technique proved that the 14 M modulated martensite transformed to a metastable amorphous-like structure after ball milling for 8 hours. Annealing of the ball-milled particles with the amorphous-like phase first led to the crystallization to form a B2 structure at 523 K (250 °C), and then an ordered Heusler L2₁ structure (with a small tetragonal distortion) at 684 K (411 °C). The annealed particles undergo different structural transitions during cooling, tailored by the atomic arrangements of the high-temperature phase. Low-field thermomagnetization measurements show that the ball-milled particles with the amorphous-like structure or the atomically disordered crystalline structure exhibit a magnetic transition from the paramagnetic-like to the spin-glass state with decreasing temperature, whereas the crystalline particles with the ordered Heusler L2₁ structure present a ferromagnetic behavior with the Curie temperature T _c ≈ 310 K (37 °C).     

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.     

[(Fe<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>)<Subscript>0.75</Subscript>B<Subscript>0.20</Subscript>Si<Subscript>0.05</Subscript>]<Subscript>96</Subscript>Nb<Subscript>4</Subscript> Metallic Glasses with Small Cu Additions

Recently, (Fe-Co)-B-Si-Nb bulk metallic glasses (BMGs) were produced. Such BMGs exhibit high glass-forming ability (GFA) as well as good mechanical and magnetic properties. These alloys combine the advantages of functional and structural materials. The soft magnetic properties can be enhanced by nanocrystallization. To force the nanocrystallization, small content of Cu was added to the starting composition. In this article, {[(Fe_0.5Co_0.5)_0.75Si_0.05B_0.20]_0.96Nb_0.04}_100–x Cu _x glassy alloys (x = 1, 2, and 3) were chosen for investigation. The GFA and the thermal stability of these alloys were evaluated. The effects of crystallization during heat-treatment processes on the phase evolution and the magnetic properties, including M _s , H _c , and T _c , in these alloys were investigated. The phase analyses were done with the help of the X-ray diffraction patterns recorded in situ by using the synchrotron radiation in transmission configuration.