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.     

Preparation and thermal stability of Zr-Ti-Al-Ni-Cu amorphous powders by mechanical alloying technique

This study examined the amorphization feasibility of Zr_70−x−y Ti _x Al _y Ni₁₀Cu₂₀ alloy powders by the mechanical alloying (MA) technique. According to the results, after 5 to 7 hours of milling, the mechanically alloyed powders were amorphous basically in the ranges of 0 to 12.5 at. pct Ti and 2.5 to 17.5 at. pct Al. These ranges are larger than those of bulk amorphous alloys prepared by a squeeze mold casting technique. Most of the amorphous mechanically alloyed powders exhibited a wide supercooled liquid region of more than 60 K before crystallization. The glass-transition and crystallization temperatures of mechanically alloyed samples were different from those prepared by squeeze casting. It is suspected that different thermal properties arise from the introduction of impurities during the MA process. The amorphization behavior of Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ was examined in detail. The X-ray diffraction and extended X-ray absorption fine structure (EXAFS) results show the fully amorphous powders formed after 5 hours of milling. A kinetically modified thermodynamic phase transformation process was observed for the glass-transition behavior in the Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ amorphous powder.     

<Emphasis Type="Italic">In-Situ</Emphasis> Synthesis of Cu/Cr-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocomposite by Mechanical Alloying and Heat Treatment

In this article, a novel method has been used to prepare a copper matrix nanocomposite containing Cu-10 wt pct Cr-10 wt pct Al₂O₃ by heat treatment of the mechanically activated Cu, Al, and Cr₂O₃ powder mixture. Structural evolutions were investigated using the X-ray diffraction (XRD) technique. The microstructure of samples was examined using scanning electron microscopy (SEM). It was found that during the milling process, Cu(Al) solid solution and Cu₉Al₄ phase were formed as the intermediate products, and therefore, Al activity was decreased. Hence, the reduction of Cr₂O₃ with Al was prevented during the ball milling stage. Further heat treatment carried out under argon atmosphere at 900 °C for 8 hours resulted in completion of Cr₂O₃ reduction by Al.     

The Effect of Low Concentrations Nb and C on the Structure and High-Temperature Strength of Fe<Subscript>3</Subscript>Al Aluminide

The Fe₃Al iron aluminide alloyed by low concentrations of Nb and C (c _Nb, c _C) is studied. The influence of the c _Nb/c _C ratio on the structure and high-temperature yield strength of iron aluminide was investigated. The structure and phase composition were studied by scanning electron microscope equipped with EDS and EBSD. The strengthening mechanisms are detected as strengthening by incoherent precipitates of NbC and as a solid solution hardening by Nb atoms.     

Grain-size control in Ti-48Al-2Cr-2Nb with yttrium additions

A gas-atomized (GA) prealloyed powder of the Ti-48Al-2Cr-2Nb intermetallic and 1.6 wt pct Y were mechanically alloyed (MA) and hot isostatically pressed (hipped) to produce a fully dense nanocrystalline material. Mechanical alloying of the as-blended powder for 16 hours resulted in the formation of a disordered fcc phase. Hipping of the alloy powder produced a single-phase nanocrystalline TiAl intermetallic, containing a distribution of 20 to 35-nm-sized Al₂Y₄O₉ particles. The formation of oxide particles occurred by the chemical combination of Al and Y with oxygen, which entered as a contaminant during milling. Oxide particles increased the hardness of the intermetallic compound and minimized grain growth even at 0.8 T _m , where T _m is the melting point of the compound.     

Ni-Nb-Sn Bulk Metallic Glass Matrix Composites Fabricated by Microwave-Induced Sintering Process

Using a gas-atomized Ni_59.35Nb_34.45Sn_6.2 metallic glassy alloy powder blended with Sn powder of various contents, Ni-Nb-Sn bulk metallic glassy matrix composites were fabricated by a microwave (MW)–induced sintering process in a single-mode 2.45 GHz MW applicator in a separated magnetic field. The Ni_59.35Nb_34.45Sn_6.2 glassy alloy powder and its mixed powders containing Sn particles could be heated well in the magnetic field. The addition of Sn particles promoted densification of the sintered Ni_59.35Nb_34.45Sn_6.2 metallic glassy powder. Bulk samples without crystallization of the glassy matrix and with good bonding state among the particles were achieved at a sintering temperature of 833 K.     

Closure-affected fatigue crack propagation behaviors of powder metallurgy-processed Al-Li alloys in various environments

The environment-affected fatigue crack propagation (FCP) behavior of rapid solidification-processed (RSP) Al-2.6Li-1.0Cu-0.5Mg-0.5Zr (RSP 644-B) and mechanically alloyed (MA) Al-4.0Mg-1.5Li-1.1C-0.8O₂ (MA 905-XL) were examined in air, in vacuum, and in an aqueous 3.5 pct NaCl solution at R=0.1 and a sinusoidal frequency of 20 Hz. The emphasis was placed on the effect of environment-sensitive crack closure on the FCP behavior of fine-grain-sized powder metallurgy (P/M)-processed Al-Li alloys. The present study suggests that closure is extremely sensitive to environmental factors and significantly alters the environment-affected da/dN-ΔK relationships for both alloys. In the submicron grain-sized MA 905-XL, for example, increased corrosion product-induced closure in aqueous NaCl appeared to overwhelm the detrimental environmental effects in low- and intermediate-ΔK regimes. The environment-sensitive closure contribution alone, however, cannot completely explain the FCP behavior of P/M-processed Al-Li alloys. The intrinsic environmental effects also need to be considered for further understanding of this behavior.     

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.     

Dynamic phase transformation during superplastic deformation of Nb/Nb<Subscript>3</Subscript>Al <Emphasis Type="Italic">in-situ</Emphasis> composite

Nb_ss/Nb₃Al in-situ composite with the nominal composition of Nb-16 mol pct Al-1 mol pct B, consisting of bcc niobium solid solution (Nb_ss) and A15 ordered Nb₃Al, was synthesized by arc melting, homogenization annealing, and isothermal forging, and their superplastic deformation behavior was investigated by tensile tests and microstructure observations. Maximum superplastic elongation over 750 pct was obtained at 1573 K and at a strain rate of 1.6 × 10⁻⁴ s⁻¹ for as-forged specimens. Phase transformation from Nb_ss to Nb₃Al was observed to occur during superplastic deformation. Dynamic phase transformation during superplastic deformation progresses more quickly than static phase transformation during annealing without applied stress. Dynamic phase transformation is accompanied by phase-boundary migration, which operates as an accommodation process of grain-boundary sliding. Dislocation creep dominates deformation and grain-boundary sliding is inhibited at a high strain rate, while grain-boundary sliding and cavity formation are promoted at a low strain rate because of insufficient accommodation of grain-boundary sliding arising from sluggish dynamic phase transformation. It is concluded that there exists an optimum strain rate that guarantees the grain-boundary sliding and the rapid dynamic phase transformation to achieve maximum superplastic elongation.     

Study of the Ni<Subscript>41.3</Subscript>Ti<Subscript>38.7</Subscript>Nb<Subscript>20</Subscript> wide transformation hysteresis shape-memory alloy

The shape-memory characteristics in the Ni_41.3Ti_38.7Nb₂₀ alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M _s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ _s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M _s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni_41.3Ti_38.7Nb₂₀ alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.     

Compaction and characterization of mechanically alloyed nanocrystalline titanium aluminides

Blended elemental (BE) Ti-24 at. pct Al-11 at. pct Nb (Ti-24-11) and Ti-55 at. pct Al (Ti-55) powders and prealloyed (PA) Ti-24-11 powders were mechanically alloyed in a SPEX mill or an attritor. After SPEX milling for 10 hours, the BE Ti-24-11 powder contained the B2/bcc phase, while the BE Ti-55 powder showed the presence of an amorphous phase. The PA Ti-24-11 powder containing the B2 phase showed a decrease of crystal size on milling. These powders were consolidated by hot isostatic pressing (“hipping”), Ceracon process, and dynamic methods. On compaction, the B2/bcc phase in the Ti-24-11 sample transformed to a mixture of the B2 and orthorhombic (“O”) phases, while the amorphous phase in the Ti-55 powder crystallized to a mixture of the γ-TiAl and α ₂-Ti₃Al phases. The finest grain size in compacted material was obtained in the dynamically consolidated powder, and the grain size in the hot isostatic pressed (“hipped”) powder became larger with the increasing hipping temperature.     

Densification of Al-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> composite powder by equal channel angular pressing

In this study, equal channel angular pressing (ECAP) with and without back pressure was used for consolidation of fine mechanically alloyed Al-Yttria composite powder. The effect of ECAP on consolidation behaviour of powder, microstructure, density and hardness is discussed. The powder was encapsulated in an aluminium can. Room temperature back pressure aided ECAP results in a nearly full denser (97% of its theoretical density) compact at room temperature. Nano Indentation technique was used to determine the modulus of the ECAP consolidated compact.     

Effects of Zr on the eutectoid decomposition behavior of Nb<Subscript>3</Subscript>Si into (Nb)/Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>

The effect of Zr on the formation of Nb/Nb₅Si₃ lamellar microstructure by eutectoid decomposition reaction of Nb₃Si is investigated. It has been shown that the kinetics of the eutectoid decomposition of high-temperature Nb₃Si phase into Nb and Nb₅Si₃ phases are sluggish in the binary Nb-Si system and that they are enhanced by Zr additions. The time-temperature-transformation (TTT) diagram for the decomposition is experimentally determined and the acceleration of the reaction by small Zr addition of 1.5 at. pct is confirmed by comparison with the reported TTT curves of binary and ternary alloys containing Ti. The role of the ternary element on the decomposition kinetics is discussed in terms of crystallographic orientation relationships (ORs) and Zr distribution in the parent Nb₃Si phase during solidification. 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.     

The role of oxygen and zirconium in the formation and growth of Nb3sn grains

One method of producing Nb₃Sn is to react a molten tin alloy with a solid niobium alloy. Using this process, the addition of zirconium and oxygen to the niobium foil has been found to dramatically reduce the Nb₃Sn grain size and affect the Nb₃Sn superconducting critical current properties. Nb₃Sn grains grow semicoherently on the niobium alloy foil. The initial grain size is about 50 nm. These initial Nb₃Sn grains coarsen rapidly to become equiaxed grains about 0.2 μm in diameter. The equiaxed Nb₃Sn grains away from the Nb/Nb₃Sn interface are completely surrounded by a tin alloy phase that would have been liquid at the reaction temperature. Based on transmission electron microscopy observation and electrical property characterization, it is concluded that ZrO₂ clusters, less than 10 Å in size, form in the niobium alloy foil during processing. These clusters combine at the Nb/Nb₃Sn interface to form ZrO₂ precipitates. The ZrO₂ precipitates are found in all of the Nb₃Sn grains that have formed from a reaction between the liquid tin and the solid niobium at the Nb/Nb₃Sn interface. The precipitates are coherent with their host Nb₃Sn grains. During Nb₃Sn grain growth, the ZrO₂ precipitates dissolve in shrinking grains and reprecipitate in growing grains, as the migrating grain boundary intersects the precipitate. This dissolution/reprecipitation process slows the growth of Nb₃Sn grains. Formerly with GE Corporate Research & Development, Schenectady, NY,     

On suitability of novel fluidised bed technique for separation of metallic powders during commercial powder metallurgical processing

Abstract

Experiments have been performed to test the efficiency with which a novel fluidised bed technique could separate different metallic powders in terms of size and density. The overall aim was to assess the potential of this technique for the commercial separation of defective powder fractions from mechanically alloyed (MA), iron based powders. Separation in terms of size was readily achieved, with the largest powder particles sinking to the bottom of the fluidised bed. In a simulated commercial process, density separation of defective Fe₃Al powder could not be demonstrated as any differences in density were overshadowed by size and morphology differences. However, from a batch of iron based powder (ρ=8·02 g cm^?3), seeded with six other metallic powders, aluminium powder (ρ=2·70 g cm^?3) segregated strongly to the top of the bed from where samples containing 93 vol.-%Al were taken. The process is thought to be sensitive to differences in density of a factor of 1.1–3.     

[(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.     

Solid-state reactions during heating mechanically milled Al/TiO<Subscript>2</Subscript> composite powders

Solid-state reactions between Al and TiO₂ during heating high-energy mechanically milled Al/TiO₂ composite powders have been investigated by using a combination of thermal analysis, X-ray diffraction (XRD), and various microstructural characterization techniques. When the TiO₂ particles and their interparticle spacing in the Al/TiO₂ composite powder particles are sufficiently large, the reaction between Al and TiO₂ proceeds by two steps. The low-temperature step is an interfacial reaction, which starts at a temperature close to 660 °C. The high-temperature step is a reaction facilitated by bulk diffusion and starts at a temperature above 820 °C. The first phase formed from the reaction is always Al₃Ti irrespective of the starting powder composition or milling time. Al₂O₃ is difficult to form at temperatures below 800 °C. The formation of the α-Ti(Al,O) phase proceeds slowly and requires either continuous heating to a temperature above 1000 °C or holding at a temperature close to 1000 °C for a period of time. Mechanical milling of the Al/TiO₂ powder enhances the interfacial reaction between Al and TiO₂. This enhancement is originated from the establishment and refinement of Al/TiO₂ composite microstructure.     

The effect of Nb and W alloying additions to the thermal expansion anisotropy and elastic properties of Mo<Subscript>5</Subscript>Si<Subscript>3</Subscript>

The directional thermal expansion and elastic properties of Mo₅Si₃, (Mo_0.8Nb_0.2)₅Si₃, and (Mo_0.85W_0.15)₅Si₃ have been studied as a function of temperature through the use of single crystals. Thermal expansion anisotropy was reduced by Nb and W alloying. The decrease in thermal expansion anisotropy by Nb alloying was only found to occur at low temperatures, and thermal expansion anisotropy of (Mo_0.8Nb_0.2)₅Si₃ was similar to that for the other two compounds at 800 °C. Values for the polycrystalline Young’s, bulk, and shear moduli calculated from the measured single-crystal elastic constants are reduced by Nb alloying, and increased by W alloying at all temperatures studied. The elastic modulus E was calculated for the orientations between [100]-[001] and [100]-[010]. In contrast to the effects of Nb on thermal expansion anisotropy, Nb alloying increased the E _[001]/E _[100] elastic anisotropy. 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.     

Structure of mechanically alloyed Ti-Al-Nb powders

Ti-Al-Nb ternary powder mixtures containing 24Al-11Nb, 25Al-25Nb, 37.5Al-12.5Nb, and 28.5Al-23.9Nb (at. pct) were mechanically alloyed in a SPEX 8000 mixer mill using a ball-to-powder weight ratio of 10:1. The structural evolution in these alloys was investigated by X-ray diffraction and transmission electron microscopy techniques. A solid solution of Al and Nb in Ti was formed at an early stage of milling, followed by the B2/body-centered cubic (bec) and amorphous phases at longer milling times. The stability of these phases and their transformation to other phases have been investigated by heat treating these powders at different temperatures. The B2/bcc phase transformed into an orthorhombic (O-Ti₂AlNb) or a mixture of the orthorhombic (O) and hexagonal close-packed (α₂-Ti₃Al) phases, the proportion of phases being dependent on the powder composition. Milling beyond the amorphous phase formation resulted in the formation of an fee phase in all the powders, which appears to be TiN, formed as a result of contamination of the powder. Formerly Graduate Student, University of Idaho