Thermodynamic Basis for Glass Formation in Cu-Zr Rich Ternary Systems and Their Synthesis by Mechanical Alloying

Topological factors such as mismatch entropy and configurational entropy, along with thermodynamic entity such as enthalpy of chemical mixing, are found to control glass formation in metallic systems. Taking both these factors into consideration, a parameter called P _HS was proposed to correlate glass forming ability successfully in the Cu-Zr-Ti system. The parameter P _HS (=?H _chem × ?S _σ /k _B ) is a product of enthalpy of chemical mixing and mismatch entropy. Our study indicates that the more negative is the P_HS value within the configurational entropy (?S _config/R) range of 0.9 to 1.0, the higher is the stability of glassy phase resulting in a larger diameter of bulk metallic glass rods. Observed theoretical predictions are supported by experimental results in which the compositions with high negative P _HS resulted in easy amorphous phase formation in comparison with less negative P _HS compositions by mechanical alloying. This criterion was extended to Cu-Zr-Al and Cu-Zr-Ag systems as well, thus establishing a strong correlation between P _HS and the glass forming ability of alloys. The role of size effect, probability of atomic arrangements, and heat of formation among constituent elements in obtaining a larger dimension bulk metallic glasses was addressed in this study.     

Influence of atomising gas on particle characteristics of Al,Al–1 wt-%Li,Mg, and Sn powders

Abstract

The influence of physical and flow properties of atomising gas on the particle characteristics of gas atomised Al, Al–1 wt-%Li, Mg, and Sn powders was investigated in a pilot plant gas atomiser with IN4/ON18/3·5–4·0 type ‘confined design’ nozzle. In the tests, Al powders were produced under high and low pressures of argon, under air, (N₂ + O₂ ) mixture, nitrogen, and helium; Al–1 wt-%Li binary alloy powders were produced under argon and helium; Mg powders were produced under high and low pressures of argon and helium; and finally Sn powders were produced under argon, nitrogen, and helium. The morphology, size, size distribution, and surface features of the powders used in the present study were examined under SEM together with dry and wet sieving, used for sizing the powders. It was observed that high gas velocities and/or low atomising gas densities not only affect powder particle size, but also shape and surface texture. The oxygen content of the atomising gases also has an influence on the powder particle shape. In this context, powders produced under helium are finer in size owing to efficient secondary breakup; more spherically shaped in their fine size fraction in non-oxidising or difficult to oxidise atomising liquids (such as Sn and Al), because the time to breakup is shorter than that for solidification; and more irregularly shaped in their coarse size fraction in oxidising atomising liquids (such as Mg and Al–Li) owing to oxygen (the time to breakup is longer than that for solidification) compared with other atomising gases such as argon, air, (N₂ +O₂ ) mixtures, and pure nitrogen.     

Production and Mechanical Properties of Roll Bonded Bulk Metallic Glass/Aluminum Laminates

Twin roll casting has been used to produce sheet of Mg₆₀Cu₂₉Gd₁₁ bulk metallic glass (BMG). Sheet can be produced with thicknesses between 1 and 4 mm, the width of sheet produced can be between 25 and 75 mm. The dimensional stability of the produced sheet in a cast run is ±1 mm in the width direction and ±0.05 mm in the thickness direction. As with all magnesium-based BMGs the sheet produced is strong yet brittle at room temperature. The maximum flexural stress of a twin roll cast Mg₆₀Cu₂₉Gd₁₁ BMG strip is 150 MPa with a flexural strain of 0.005. The Charpy impact energy of a Mg₆₀Cu₂₉Gd₁₁ BMG strip is 0.02 J. In order to improve the toughness values of the Mg₆₀Cu₂₉Gd₁₁, BMG strip laminates of BMG and aluminum alloy (UNS A91100) were produced via roll bonding. The introduction of aluminum layers to the sheet structure provides a barrier to shear band movement stopping the sudden catastrophic failure of the sheet. After rolling the BMG was examined via X-ray diffraction (XRD) to confirm that the BMG layer remained amorphous. The flexural stress, flexural strain, and Charpy impact energy properties of BMG-Al laminates are improved when compared to monolithic glass properties. The flexural stress values for laminates compared to the monolithic glass improve by 60 pct from 150 to 250 MPa. The flexural strain values improve by over an order of magnitude from 0.005 to 0.14. The Charpy impact energies increase by 2 orders of magnitude from 0.02 to 2.5 J.     

Effects of Alloying Elements on the Thermal Stability and Corrosion Resistance of an Fe-based Metallic Glass with Low Glass Transition Temperature

The effects of alloying elements on the thermal stability, glass-forming ability (GFA), corrosion resistance, and magnetic and mechanical properties of a soft magnetic Fe₇₅P₁₀C₁₀B₅ metallic glass with a low glass transition temperature (T _g) of 723 K (450°C) were investigated. The addition of Mo, Ni, and Co significantly increased the stabilization of supercooled liquid, GFA, and corrosion resistance in the H₂SO₄ solution. The maximum critical diameter (d _c) of 4 mm for glass formation was obtained for the Fe₅₅Co₁₀Ni₅Mo₅P₁₀C₁₀B₅ alloy, which shows the largest supercooled liquid region (ΔT _x ) of 89 K (89 °C). The substitution of Cr for Mo further enhanced the corrosion resistance of the Fe₅₅Co₁₀Ni₅Mo₅P₁₀C₁₀B₅, while the ΔT _x and d _c decreased. The (Fe, Ni, Co)₇₀(Mo, Cr)₅P₁₀C₁₀B₅ bulk metallic glasses showed low T _g of 711 K to 735 K (438 °C to 462 °C), wide ΔT _x of 67 K to 89 K, high saturation magnetization of 0.79 to 0.93 T, low coercive force of 2.36 to 6.61 A m^?1, high compressive yield strength of 3271 to 3370 MPa, and plastic strain of 0.8 to 2.3 pct. In addition, the mechanism for enhancing stability of supercooled liquid was discussed in terms of the precipitated phases during crystallization.     

Carbothermal Solid State Reduction of Stable Metal Oxides

The paper considers carbothermal solid state reduction of manganese, titanium and aluminium oxides in argon, helium and hydrogen. The difference in reduction in helium and argon was reflected by different diffusion coefficients of gaseous reactants and products, which are much higher in helium than in argon. When carbothermal reduction took place in hydrogen, it was involved in the reduction process by reducing oxides to suboxides and forming methane. Manganese and titanium oxides were reduced to carbide Mn₇C₃ and oxycarbide Ti(O_xC_1‐x) correspondingly, while products of alumina reduction included carbide Al₄C₃ and vapours of Al and Al₂O, which were re‐oxidised to Al₄O₄C outside the reactor and deposited in the lower temperature zone. Gas atmosphere had a profound effect on the extent and rate of reduction. This effect was different in reduction of different oxides. Reduction of manganese oxides was the fastest in hydrogen, and faster in helium than in argon. Reduction of titania in argon and helium proceeded with about the same rate and was much faster in hydrogen than in the inert atmospheres. The rate and extent of alumina reduction in hydrogen and helium were higher than in argon, although no significant difference was observed in alumina reduction in hydrogen and helium. This reflects differences in reduction mechanisms, which are discussed in the paper.     

Effect of Ti cap layer thickness on microstructures and magnetic properties of Ti/Ni/Ti films

Nanogranular Ti (3 nm)/Ni(30 nm)/Ti(t nm) (t=1, 3, 5, 7, 10) films were prepared by facing magnetron sputtering from Ti and Ni onto glass substrates at room temperature. The structural and magnetic properties of films strongly depended on the Ti layer thickness. X-ray diffraction (XRD) patterns of all as-deposited films showed strong FCC Ni(111) peak. Vibrating sample magnetometer (VSM) measurements indicated that the perpendicular coercivity of the Ti (3 nm)/Ni (30 nm)/Ti (3 nm) film reached about 36 kA/m. With the increase of Co layer thickness, coercivity (H_c) first increased and then decreased. The grain size and magnetic clusters slightly increased and the value of roughness (R_a) was smallest at t=3 nm.     

Carbothermal Reduction of a Primary Ilmenite Concentrate in Different Gas Atmospheres

The carbothermal reduction of a primary ilmenite concentrate was studied in hydrogen, argon, and helium. Ilmenite and graphite were uniformly mixed and pressed into pellets. Reduction was studied in isothermal and temperature-programmed reduction experiments in a tube reactor with continuously flowing gas. CO, CO₂, and CH₄ contents in the off-gas were measured online using infrared sensors. The phase composition of reduced samples was characterized by X-ray diffraction (XRD). Oxygen and carbon contents in reduced samples were determined by LECO analyzers (LECO Corporation, St. Joseph, MI). The main phases in the ilmenite concentrate were ilmenite and pseudorutile. The reaction started with the reduction of pseudorutile to ilmenite and titania, followed by the reduction of ilmenite to metallic iron and titania. Titania was reduced to Ti₃O₅ and even more to Ti₂O₃, which was converted to titanium oxycarbide. Reduction was faster in hydrogen than in helium and argon, which was attributed to involvement of hydrogen in the reduction reactions. The formation of titanium oxycarbide in hydrogen started at 1000 °C and was completed in 300 minutes at 1200 °C, and 30 minutes at 1500 °C. The formation of titanium oxycarbide in argon and helium started at 1200 °C and was not completed after 300 minutes at 1300 °C.     

Fabrication and characterizations of new glassy Co<Subscript>71</Subscript>Ti<Subscript>24</Subscript>B<Subscript>5</Subscript> alloy powders and subsequent hot pressing into a fully dense bulk glass

A single glassy phase of Co₇₁Ti₂₄B₅ alloy has been synthesized by high-energy ball milling the elemental powders at room temperature, using the mechanical alloying method. The synthetic glassy powder obtained after 130 ks of ball milling exhibits good soft magnetic properties with a polarization magnetization and coercivity values of 1.01 T and 2.86 kA/m, respectively. This ternary glassy alloy in which its glass transition temperature (T _g ) lies at a rather high temperature (805 K) crystallizes at 868 K through a single sharp exothermic peak with an enthalpy change of crystallization (ΔH _x ) of −3.28 kJ/mol. The supercooled liquid region before crystallization, ΔT _x of the synthesized glassy powders shows a large value (63 K) for a ternary system. The reduced glass transition temperature (ratio between T _g and liquidus temperatures, T _l (T _g /T _l )) was found to be 0.55. The end product of the glassy powder (130 ks) was compacted in an argon gas atmosphere at 835 K with a pressure of 780 MPa, using the hot-pressing technique. The consolidated sample is fully dense (∼99.5 pct) and maintains its chemically homogeneous glassy structure. The measured polarization magnetization and coercivity values of as-consolidated powders are measured and found to be 0.96 T and 2.92 kA/m, respectively. The Vickers microhardness of the bulk glassy Co₇₁Ti₂₄B₅ sample is measured and found to be in the range between 7.32 and 7.46 GPa.     

Magnetic properties of melt-spun ribbons(Sm_(1-x)Zr_x)(Fe_(0.92)Ti_(0.08))_(10) with ThMn_(12) structure and their hydrides

The structure and magnetic hysteresis properties of the cast Sm_(1-x)Zr_x(Fe_(0.92)Ti_(0.08))_(10)(x = 0-0.3)alloys and melt-spun ribbons prepared from them were studied.In the cast alloy with x0.2, a considerable amount of the eutectic phase is found in the SEM micrographs.Analysis of the temperature dependences of the magnetic susceptibility and XRD patterns allows amorphous state in the as-spun ribbons with x0.2 to be determined.The specific magnetization measured in a field of 17 kOe and remanence decrease with increasing annealing temperature from 800 to 900 ℃ and weakly depend on Zr concentration.The maximal value of coercivity Hc = 4.7 kOe is obtained on the ribbons with x = 0.2 after annealing at 850℃ for 10 min.After additional hydrogenation of the ribbons,both the coercivity and remanence increase by 54% and 7%,respectively.     

Microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys

In this paper, we systematically investigated the microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) analyses. The rod-like NdH_2+x phases with diameters of 10–20 nm are embedded into α-Fe matrix, which hereditarily leads to textured grains in HDDR alloy. The migration of NdH_2+x from Nd-rich region to α-Fe matrix during hydrogen absorption process contributes to the uniform redistribution of Nd-rich phases after HDDR treatment. The HDDR alloy with single domain grain sizes of 200–300 nm exhibits relatively low coercivity of 1.01 T that arises from pinning magnetic domain motion. The weak c-axis orientation of HDDR alloy results in a lower reverse magnetic field (coercivity) to reduce remanence to 0. Moreover, the direct contact of Nd₂Fe₁₄B grains and the high concentration of ferromagnetic elements (Fe content ≈ 66.06 at%, Co content ≈ 0.91 at%) in Nd-rich grain boundary layer lead to strong magnetostatic coupling effect among Nd₂Fe₁₄B grains. The nano-sized α-Fe inside Nd₂Fe₁₄B matrix makes the magnetization reversal easily and decreases the coercivity of HDDR alloy.     

Study on Chemical Durability of BAP System Glass Doped with Rare Earths

The chemical durability of BaO-Al₂O₃-P₂O₅(BAP) system glass doped 10% and 15% Sm₂O₃ was studied by SEM technique. The results indicated that addition of Al³⁺ and Sm³⁺ affected the water resistance ability obviously. The higher of Al³⁺ and Sm³⁺ content, the more stable of glass structure, so well as the better of water resistance ability of the Sm₂O₃ doping phosphate glass. Improvement of the polarize ability (Z²/r) of cations would improve acid resistance ability of rare earth phosphate glass, at the same time, overlays on the glass surface were formed by acid corrosion, which slowered the corrosion rate. In alkali medium, the corrosion mechanism was that the metal ions of phosphate long chains were hydrated, which disentangled the P-O-P bond and formed the orthophosphate units dissolved. And increase of Sm³⁺ ions resulted in decrease of the alkali resistance ability.     

Production of Nd-Fe-B alloy powders using high-pressure gas atomization and their hard magnetic properties

Fine spherical Nd−Fe−B powders with a tetragonal Nd₂Fe₁₄B phase have been produced by high pressure argon or helium atomization. The average size defined by 50 pct cumulative weight fraction is as small as about 25 μm. The Curie temperature of the powders is about 580 K and the intrinsic coercivity (iH _c ) of the bonded products made from the powders increases with decreasing particle size and reaches about 0.581 MA/m for the powder below 25 μm diameter. TheiH _c value increases with an increase in the cooling rate by helium atomization as well as with an increase in neodymium and boron content to 18 at. pct Nd and 12 at. pct B with the highestiH _c value reaching 0.716 MA/m. Annealing in the range of 773 to 1073 K gives rise to a further increase ofiH _c to 1.035 MA/m. The highiH _c value is promising for practical use as isotropic bonded powder magnets. The increase of magnetization for the bonded powder magnets takes place rapidly and the behavior is similar to that for sintered Nd−Fe−B magnets, in goods contrast to a sluggish increase of magnetization for the bonded Nd−Fe−B powder magnets made from the comminuted powders of melt-spun ribbon. From the magnetization behavior, it was presumed that the generation of the large intrinsic coercivity is due to the difficulty of the nucleation of reverse domain rather than the pinning of domain walls.     

Effect of titanium addition on the magnetic property of Nd2Fe14B/α-Fe nanocomposite alloys

Rapidly solidified nanocrystalline α-Fe/Nd2Fe14B alloys with enhanced coercivity were obtained by melt spinning.The effects of Ti addition on the microstructore and magnetic properties of the nanocomposite α-Fe/Nd2Fe14B alloys were investigated by X-ray diffraction(XRD)and superconducting quantum interference device(SQUID)magnetometer.The analysis of XRD showed that Vα-Fe estimated to be about 35.3% in the Ti-free α-Fe/Nd2Fe14B nanocomposites decreased down to 26.5% as the addition of was 5 at.% Ti.Accordingly,adding Ti resulted in relevant improvements of magnetic properties,especially of the coercivity Hc from 595 kA/m up to 1006 kA/m.The dependence of Mirrev(H)/2Mr on the reverse field H indicated that nucleation was the dominating mechanism for the magnetization reversal in these nanocomposites.The analysis of the temperature dependence of the demagnetization curve in the α-Fe/Nd2Fe14B nanocomposite magnets indicated that a reduction of αex could play a leading role in an increase in the coercivity of Ti-doped sample.     

A Highly Fatigue-Resistant Zr-Based Bulk Metallic Glass

The strength-normalized fatigue endurance strength of the bulk metallic glass (BMG) Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (Vitreloy 105) has been reported to be the highest for any BMG; however, to date, there has been no explanation of why this material is so much better than other Zr-based compositions. In this study, the fatigue-crack growth behavior of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ was compared in ambient air vs dry nitrogen environment. The excellent fatigue life behavior is attributed to a relatively high fatigue threshold (ΔK _TH ≈ 2 MPa√m) and a lack of sensitivity to environmental effects on fatigue-crack growth in ambient air, as compared to other Zr-based BMGs. Fatigue life experiments conducted in ambient air confirmed the excellent fatigue life properties with a 10⁷-cycle endurance strength of ~0.24 of the ultimate tensile strength; however, it was also found that casting porosity, even in limited amounts, could reduce this endurance strength by as much as ~60 pct. Overall, the BMG Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ appears to have excellent strength and fatigue properties and should be considered as a prime candidate material for future applications where good mechanical fatigue resistance is required.     

The microstructure and magnetization reversal behavior of melt-spun (Nd1−xCex)-Fe-B ribbons

In this study,the alloy ingots with nominal compositions of(Nd_(1-x)Ce_x)_(31)Fe_(bal)Co_(0.2)Ga_(0.1)B(x=0, 0.1 wt%,0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%) were prepared and then melt-spun to form nanocrystalline ribbons at the wheel speed of 20 m/s. XRD results show that all melt-spun ribbons exhibit the tetragonal structure(Nd,Ce)_2 Fe_(14)B phase with the space group P42/mmm. The Curie temperature and lattice constant decrease with the increase of Ce content. The Curie temperature decreases gradually from 306 to 247 ℃with the increase of Ce content. Those results indicate that Ce element has been incorporated into Nd_2 Fe_(14)B main crystalline phase and formed(Ce,Nd)-Fe-B hard magnetic phase. It is also found that the remanence ratio(M_r/M_s) decreases from 0.693 to 0.663 and the coercivity(H_c) decreases from 18.7 to14.2 kOe with the increase of Ce content. However, a relatively high coercivity of 18.3 kOe for(Nd_(1-x)Ce_x)_(31)Fe_(bal)Co_(0.2)Ga_(0.1)B(x = 0.2) melt-spun ribbon is achieved. The coercivity is sensitive to microstructure. The AFM patterns show the sample(x = 0.2) has the most uniform and finest microstructure. The magnetization reversal behavior(δM plots) is discussed in detail. The positive δM value is observed in every sample, which confirms the existence of exchange coupling interaction. Evidently, theδM maximum value reaches 0.9 in the sample(x = 0.2). It is indicated that the intergranular exchange coupling effect is the strongest, which is consistent with coercivity enhancing.     

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion(GBD) process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure is more complicated compared to sintered magnets.Here,we investigated the effects of different GBD methods,i.e.,intergranular addition(in-situ GBD 1#),in-situ GBD from magnet surface during hot pressing and hot deformation(in-situ GBD 2#),and conventional GBD,on the magnetic prope...     

Peculiarities of helium porosity formation in the surface layer of the structural materials used for the first wall of fusion reactor

Transmission electron microscopy is used to study the formation of helium porosity in the nearsurface layer of ferritic–martensitic steels and vanadium irradiated by 40-keV He⁺ ions at a temperature of 923 K up to fluence of 5 × 10²⁰ He⁺/m² and, then, by 7.5-MeV Ni²⁺ ions at 923 K up to dose of 100 dpa. Large gas bubbles are found to form in the zone with the maximum concentration of radiation vacancies during He⁺ ion irradiation. Moreover, small bubbles form in some grains at the depths that are larger than the He+ ion range in the irradiated material. Sequential irradiation by He⁺ and Ni²⁺ ions leads to the nucleation of helium bubbles at still larger depths due to helium atom transport via recoil and/or ion mixing. The precipitation hardening of the steels by Y₂O₃ oxide nanoparticles is found to suppress helium swelling substantially.     

Structure and permanent magnetic properties of SmFex （x=3-8） melt spun ribbons during heat treatment

Compounds with the composition SmFex（x=3–8） were prepared by melt spun method at a velocity of 40 m/s and subsequent annealing at temperature between 600–1000 ℃. The crystal structures of the as-quenched and as annealed powders were investigated by XRD methods with following Rietveld analysis. The glass forming ability could be enhanced by the increase of Sm content to x≤5.Metastable Sm5Fe17-type structure existed when 3≤x≤5 and temperature was lower than 800 ℃. SmFe2-type structure could be stable up to 1000 ℃ when x〉3 and temperature was under 800 ℃. The content of SmFe2-type decreased with the increase of x value and increased with temperature lower than 800 ℃, from which SmFe2-type started to bring the transition to SmFe3-type. As for Sm5Fe17-type compounds with x=3.4, the highest coercivity of 33.6 kOe could be obtained under a velocity of 30 m/s and heat treated under 700 ℃×1h.