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.     

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.     

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.     

Effects of Annealing and Pressure on Devitrification and Mechanical Properties of Amorphous Al<Subscript>87</Subscript>Ni<Subscript>7</Subscript>Gd<Subscript>6</Subscript>

Annealing studies at different temperatures, as well as those conducted with 940 MPa hydrostatic pressure, were conducted on amorphous ribbons of Al₈₇Ni₇Gd₆. The studies were performed to investigate the evolution of structure under different conditions and to particularly examine the effects of superimposed hydrostatic pressure during annealing. This amorphous alloy devitrifies at low temperatures via the precipitation of nano-crystalline α-Al particles. The effects of these various exposures on the amount of devitrification have been quantified using a variety of analytical techniques (i.e., X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM)). In addition, the effects of devitrification on the mechanical properties have been quantified using microhardness indentation and uniaxial tension tests. 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.

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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.     

Effect of LiF as Sintering Agent on the Densification and Phase Formation in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-4 Wt Pct Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramic Compound

Different amounts of LiF were added to an Al₂O₃-4 pct Nb₂O₅ basic ceramic, as sintering agent. Improved new ceramics were obtained with LiF concentrations varying from 0.25 to 1.50 wt pct and three sintering temperatures of 1573 K, 1623 K, and 1673 K (1300 °C, 1350 °C, and 1400 °C). The addition of 0.5 wt pct LiF yielded the highest densification, 94 pct of the theoretical density, in association with a sintering temperature of 1673 K (1400 °C). Based on X-ray diffraction (XRD), this improvement was due not only to the presence of transformed phases, more precisely Nb₃O₇F, but also to the absence of LiAl₅O₈. The preferential interaction of LiF with Nb₂O₅, instead of Al₂O₃, contributed to increase the alumina sintering ability by liquid phase formation. Scanning electron microscopy (SEM) results revealed well-connected grains and isolated pores, whereas the chemical composition analysis by energy dispersive energy (EDX) indicated a preferential interaction of fluorine with niobium, in agreement with the results of XRD. It was also observed from thermal analysis that the polyethylene glycol binder burnout temperature increased for all LiF concentrations. This may be related to the formation of hydrogen bridge bonds.     

Deformation and fracture behavior of beams composed of alumium foam core and ceramic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> under monolithic bending

Deformation and fracture mechanisms of sandwich and multilayer beams composed of aluminum foam core and ceramic face sheets under four-point bending condition were investigated in situ by surface displacement analysis (SDA) software. The toughening mechanism of the beams was discussed and a model was given for the computation of the fracture energy of the beams. Beams containing foam core with 5-, 10-, and 20-mm thickness and Al₂O₃ face sheets of 0.5-and 1-mm thickness were prepared. The results show that collapse of the beams is by two basic modes, indentation (ID) and face plate failure (PF). The SDA results illustrated that indentation is localized compression on the portion of the beam adjacent to the loading rollers, where displacement and strain are at the maximum. In PF, the beam entirely bends. It is also found that before collapse of the beams with pure PF mode, the foam core undergoes uniform compressive deformation, which contributes most to the fracture energy of the beams. As for the beams with ID characteristic, the localized compressive deformation plays a key role rather than the uniform compressive deformation in the fracture energy of the beam. The total fracture energy W of a beam under bending condition is proposed as W=W _UC+W _LC+W _CB+W _PF where W _UCis the energy of uniform compressive deformation of the foam core, W _LCis the energy of localized compression of the foam core and W _CBand W _PFare the bending fracture energy of the monolithic foam core and ceramic face sheet, respectively. For the beams with pure PF mode, W _LCis zero. The estimated values of the fracture energy are in good agreement with the measured fracture energy of the beams.     

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.     

Deformation and fracture behavior of beams composed of aluminum foam core and ceramic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> under monolithic bending

Deformation and fracture mechanisms of sandwich and multilayer beams composed of aluminum foam core and ceramic face sheets under four-point bending condition were investigated in situ by surface displacement analysis (SDA) software. The toughening mechanism of the beams was discussed and a model was given for the computation of the fracture energy of the beams. Beams containing foam core with 5-, 10-, and 20-mm thickness and Al₂O₃ face sheets of 0.5- and 1-mm thickness were prepared. The results show that collapse of the beams is by two basic modes, indentation (ID) and face plate failure (PF). The SDA results illustrated that indentation is localized compression on the portion of the beam adjacent to the loading rollers, where displacement and strain are at the maximum. In PF, the beam entirely bends. It is also found that before collapse of the beams with pure PF mode, the foam core undergoes uniform compressive deformation, which contributes most to the fracture energy of the beams. As for the beams with ID characteristic, the localized compressive deformation plays a key role rather than the uniform compressive deformation in the fracture energy of the beam. The total fracture energy W of a beam under bending condition is proposed as

Effect of heat treatments on <Emphasis Type="Italic">In-Situ</Emphasis> Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiAl<Subscript>3</Subscript> composites produced from squeeze casting of TiO<Subscript>2</Subscript>/A356 composites

In-situ Al₂O₃/TiAl₃ intermetallic matrix composites were fabricated via squeeze casting of TiO₂/A356 composites heated in the temperature range from 700 °C to 780 °C for 2 hours. The phase transformation in TiO₂/A356 composites employing various heat-treatment temperatures (700 °C to 780 °C) was studied by means of differential thermal analysis (DTA), microhardness, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). From DTA, two exothermic peaks from 600 °C to 750 °C were found in the TiO₂/A356 composites. The XRD showed that Al₂O₃ and TiAl₃ were the primary products after heat treatment of the TiO₂/A356 composite. The fabrication of in-situ Al₂O₃/TiAl₃ composites required 33 vol pct TiO₂ in Al and heat treatment in the range from 750 °C to 780 °C. The hardness (HV) of the in-situ Al₂O₃/TiAl₃ composites (1000 HV) was superior to that of nonreacted TiO₂/A356 composites (200 HV). However, the bending strength decreased from 685 MPa for TiO₂/A356 composites to 250 MPa for Al₂O₃/TiAl₃ composites. It decreased rapidly because pores occurred during the formation of Al₂O₃ and TiAl₃. The activation energy of the formation of Al₂O₃ and TiAl₃ from TiO₂ and A356 was determined to be about 286 kJ/mole.     

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.     

A Comparative Study on the Microstructure and Mechanical Properties of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn Joints Formed by TLP Soldering With/Without the Assistance of Ultrasonic Waves

In this study, the microstructure and mechanical properties of Cu₆Sn₅ and Cu₃Sn intermetallic joints, formed by the transient liquid phase (TLP) soldering process with and without the assistance of ultrasonic waves (USWs), were compared. After the application of USWs in the TLP soldering process, Cu-Sn intermetallic compounds (IMCs) exhibited a novel noninterfacial growth pattern in the molten solder interlayer. The resulting Cu₆Sn₅ and Cu₃Sn joints consisted of refined equiaxed IMC grains with average sizes of 3 and 2.3 µm, respectively. The Cu₆Sn₅ grains in the ultrasonically soldered intermetallic joints demonstrated uniform mechanical properties with elastic modulus and hardness values of 123.0 and 5.98 GPa, respectively, while those of Cu₃Sn grains were 133.9 and 5.08 GPa, respectively. The shear strengths of ultrasonically soldered Cu₆Sn₅ and Cu₃Sn joints were measured to be 60 and 65 MPa, respectively, higher than that for reflow-soldered intermetallic joints. Ultrasonically soldered Cu₆Sn₅ and Cu₃Sn joints both exhibited a combination of transgranular and intergranular fractures during shear testing.     

Processing,microstructure, and mechanical properties of (Nb)/Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript> two-phase alloys

It has been shown that a fine lamellar structure composed of Nb solid solution, (Nb), and Nb₅Si₃ is formed through eutectoid decomposition in the Nb-Si binary system and its ternary derivatives. Such alloys would exhibit a high strength at over 1400 K, yet showing room-temperature toughness of over 10 to 20 MPa m^1/2 if a proper lamellar spacing is chosen. In the present work, effects of processing on the microstructure evolution and mechanical properties are investigated on the Nb-18 at. pct Si alloys prepared by hot pressing (HP) and spark-plasma sintering (SPS). The powders used in the present work are of pure Nb and Nb₅Si₃ in order for the fabrication to become possible at temperatures higher than the melting point of Si and to reduce the formation of SiO₂. The results show that the SPS yields more uniform two-phase microstructure but the alloy fabricated through HP tends to provide higher elevated temperature strength. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Mechanical behaviour of Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> based alloys and composites

The demand for materials to be used in the components operating above 1100°C in advanced aero-engines drives the development of the silicide-based intermetallic alloys and composites, including the titanium silicides. The mechanical behaviour of Ti₅Si₃ and its composites has been reviewed with emphasis on the microstructure-property relationships. It is found that the grain size is a critical parameter, and smaller grain sizes are desirable for reducing the magnitude of internal residual stress caused by the crystallographic anisotropy in coefficients of thermal expansion. The reduction in grain size leads to significant improvement in hardness, room temperature flexural strength and fracture toughness. On the other hand, the high temperature strength observed at slow strain rates and creep resistance are higher in the samples with the coarser grain sizes. Further improvements in the strength, fracture toughness and high temperature creep resistance are possible, either through the development of multiphase alloys, or by the use of ceramic reinforcements in composites.     

Atomic diffusion and phase equilibria at the interfaces of the CoAl/Ir multilayer on Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>-base alloys

Atomic diffusion and phase equilibria have been investigated at the interfaces of Ir/CoAl and Ir/Nb₅Si₃ to evaluate the suitability of a diffusion-barrier layer of Ir between an oxidation-resistant layer of B2-CoAl and a base material Nb₅Si₃. Diffusion couples were prepared by hot pressing and annealed at 1573 K for up to 178 hours. Diffusion layers of (Ir, Co) solid solution and B2-(Ir, Co)Al were formed at the Ir/CoAl interface. The concentration of Al dramatically dropped at the interface, which indicates that the Ir layer effectively works as the diffusion barrier against the inward diffusion of Al. To quantitatively evaluate the potential of Ir as a diffusion barrier, the Boltzmann-Matano analysis was employed to determine the diffusion coefficient of Al using Ir-8 at. pct Al/Ir diffusion couples annealed at temperatures of 1573, 1673, and 1773 K. For instance, an extremely low value of 7.0×10⁻¹⁹ m²/s is evaluated for Ir-4 at. pct Al at 1573 K. At the Ir/Nb₅Si₃ interface, the intermetallic phases Ir₃Si and Ir₃Nb are formed on the Ir side and the Nb₅Si₃ side, respectively. The formation of Ir₃Si is controlled by the diffusion of Si through Ir₃Nb in which the solubility of Si is limited quite small. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.