Volume 103, Issue 9

Crystallization of highly undercooled materials either from the melt during cooling or from an amorphous phase during heating can yield novel phases with useful properties, but a reliable control of phase selection has been elusive. In bulk volumes fluxing is a common method to promote high undercooling due to nucleant removal or deactivation, but the mechanism is not defined clearly. From experiments on Au with well controlled conditions the operation of fluxing has been identified as a nucleant refining process. Moreover, structural studies have discovered that fluxing can also affect the melt structure and dynamics in multicomponent alloys. During primary nanocrystallization of amorphous Al alloys, solute substitution has a significant effect on the glass forming ability and crystallization kinetics primarily through the change in diffusion that impacts the crystal nucleation rate through transient effects.

The cooling rate during material processing until glass transition rate is the key parameter for the production of bulk metallic glasses. But in the past, little attention has been paid to advanced production techniques such as deposition of molten metal sprays or spray forming, which offer elevated cooling rates. In this work, cooling conditions during spray forming were investigated due to its utmost importance for producing amorphous structures. Spray forming is treated in this work as a three step cooling process consisting of droplet flight phase, splat phase and deposit phase. All cooling steps were simulated for different droplet sizes. The surface temperature of the deposit was found to play an important role in the production of metallic glasses via spray forming. The simulation model can be used to find suitable spray conditions for the generation of bulk metallic glasses.

In this work we report the formation and microstructure of Ni 62- x Nb 38 Ti x ( x = 0, 3, 6, 10 at.%) bulk metallic glasses (BMGs). The addition of Ti to Ni–Nb binary alloys is effective for the increase in stabilization of supercooled liquid and glass-forming ability (GFA). The maximum supercooled liquid region (Δ T x ) of 46 K and the reduced glass transition temperature ( T rg ) of 0.61 were obtained for Ni 56 Nb 38 Ti 6 , leading to the formation of the bulk glassy alloys. The best GFA was obtained for Ni 56 Nb 38 Ti 6 alloy which was located near the eutectic composition of the ternary component alloy, and glassy alloy samples up to 0.5 mm were fabricated through wedge copper mold casting. The new Ni–Nb-based bulk glassy alloys exhibit large Δ T x of 35 to 46 K. The Ni 62- x Nb 38 Ti x ( x = 0, 3, 6, 10 at.%) BMGs showed high Vickers microhardness in the range of 988 to 1140 HV representing good mechanical properties.

The compositional features of bulk metallic glasses were statistically analyzed with an s–d E f–d L p M –p MLD tetrahedral composition diagram comprising s-, p-, d- and f-blocks in the periodic table, early- and late-transition metals and Metal–Metalloid. The s–d E f–d L p M –p MLD diagram was filled with 25 BMGs with critical diameter over half-inch (∼12.7 cm), 264 BMGs with the critical diameter ranging 0.2–12 mm, 6150 amorphous alloys from 351 ternary systems and 35400 compounds. The analysis revealed that BMGs as well as the ternary amorphous alloys are principally plotted on s–d E f–d L p M and d E f–d L p M –p MLD faces and their edges and vertices, whereas no plots are seen in s–p MLD edge and deep inside the diagram of each component of s, d E f, d L p M or p MLD ≥10 at.%. The s–d E f–d L p M –p MLD tetrahedral composition diagram is a useful tool for alloy design of bulk metallic glasses.

Anomalous X-ray scattering experiments were performed on Zr 63 Ni 25 Al 12 bulk metallic glass. The results were analyzed using reverse Monte Carlo modeling to obtain local- and intermediate-range atomic configurations of this good metallic glass former. Although the Al-related partial information is not reliable due to the small values of their weighting factors in X-ray scattering, good partial information could be obtained from a reverse Monte Carlo analysis. The obtained structural information is given by parameters such as bond angle distributions, bond orientation order parameters, the Warren–Cowley order parameters, and Voronoi polyhedra. Results are compared to the previous results from Fukunaga et al. using X-ray and neutron total scattering.

The as-cast Zr 57 Ti 8 Nb 2.5 Cu 13.9 Ni 11.1 Al 7.5 glass–quasicrystalline composite exhibits high strength, good elasticity but no macroscopic room temperature plastic deformability. Significant improvement in plastic deformability to a strain of about 5% can be achieved after cold-rolling of the as-cast material. This behavior can be ascribed the anisotropic and inhomogeneous stress distribution developing during the mechanical test along with the creation of an extensive system of shear bands.

Pure silver has been rapidly quenched (RQ) in ribbon form by means of planar flow casting with different cooling rates, obtained with different velocities of the cooling wheel. Ingots of pure silver have been subjected to severe plastic deformation (SPD) using constrained groove pressing and equal-channel angular pressing. The grain size of RQ and SPD samples has been analysed using electron microscopy, showing that the higher the solidification rate and plastic deformation, the finer the microstructure. Preferred orientations and quenched-in defect concentration have been determined from X-ray diffraction analysis, showing a correlation between density of defects and cooling rate. Higher defect concentration was found in the samples processed via SPD. Vickers hardness numbers range from 135 for SPD, 70 for RQ and 45 for annealed samples. Differential scanning calorimetry analysis revealed multi-step coarsening of the microstructure at various temperatures for all samples.

An aluminum-based AlNiZr ternary alloy containing 11 at.% Ni and 5 at.% Zr was modified by substitution of 5% of nickel by either iron or vanadium. Melt spun ribbons were cast under a protective helium atmosphere from 99.9% pure elements at 35 m s −1 linear rate of the copper wheel of the melt spinner. X-ray diffraction studies confirmed that most of the volume of all cast ribbons is amorphous showing a broad amorphous halo, however all alloys have shown a small amount of crystalline α(Al) solid solution. The same phase was identified in quaternary alloys. The ribbons showed high microhardness near 500 HV. Transmission electron microscopy studies enabled identification of a large number of spherical crystals of α(Al) solid solution of size near 200 nm in AlNiZr ribbons, containing less solute elements than the amorphous phase. In alloys with iron and vanadium additions crystallites were larger in the form of discontinuous lamellar precipitates. Differential scanning calorimetric studies enabled identification of a multistage crystallization mechanism and the additions of iron and vanadium caused an increase in crystallization temperatures as compared to ternary AlNiZr alloy

A number of metal systems exhibit positive heat of mixing between the constituent elements and consequently they are immiscible and cannot form alloys. Some classical examples of these systems are Ti–Mg, Zr–Nb, W–Cu, Ni–Ag, and Cu–Fe. We have investigated the alloying behavior of the Ni–Ag, Ti–Mg, and Zr–Nb systems through two solid-state non-equilibrium processing techniques, viz., mechanical alloying and high-pressure torsion. Increases in solid solubility limits have been achieved in all the systems, although the magnitude of the increase is different in the different alloy systems. The results obtained are also different depending on the technique employed and the lattice strain introduced into the system. The extent of increase in solid solubility limits has been rationalized in terms of the heat of mixing between the constituent metals and it is shown that the solid solubility limit is higher the smaller the positive heat of mixing.

The mechanism of phase selection during mechanical milling has been investigated first with a large perspective using an exhaustive number of transformations reported in the literature and next with focusing on the specific system of MoSi 2 . The former shows that the phases exhibiting smaller values of the volume per one mole of atoms and the number of atoms in a reduced unit cell are preferred during mechanical milling. The latter involves both experimental and theoretical investigations. The dynamic equilibrium between the stable α and metastable β phases is reached with negligible Fe contamination, which is explained by a transformation mechanism based on synchroshear. The β phase formation is promoted in the presence of Fe, which is attributed to the formation of Fe-substituted MoSi 2 with Si vacancies according to first-principles calculation results.

Al 62 Cu 25.5 Fe 12.5 (A) and Al 65 Cu 20 Fe 15 (B) alloys were fabricated via melting and experimentally studied in both as-cast and annealed conditions. Alloy (B) was also fabricated using mechanical alloying and an annealing treatment. The melted alloys were characterized to investigate the distribution of alloying elements Fe and Cu on the icosahedral and coexisting phases, as well as the hardness of all phases. The as-cast alloys showed the highest amount of icosahedral phase with up to 47% of ingot (A), but this increased after heat treatment to 99% and 75% of ingots in (A) and (B) alloys respectively. During heat treatment the iron content was increased in all phases, while the copper content decreased. As a consequence, the hardness of the icosahedral phase increased by 23% in (A) Al 62 Cu 25.5 Fe 12.5 alloy and 33% in (B) Al 65 Cu 20 Fe 15 alloy. On the other hand, the effect of heat treatment on samples obtained from mechanical alloying caused a modification in the iron and copper contents, leading to the formation of iron-rich phases such as λ-(Al, Cu) 13 Fe 4 phase for all milling times and the icosahedral phase after 35 h.

Amorphous Nb 2 O 5 was synthesized by combining niobium ethoxide with an amine surfactant. The material turned out to have a high surface area, 287 m 2 g −1 , and a homogeneous structure. The material was shown to be amorphous despite the heat treatment performed within a range of 150 to 550°C. The surface area of Nb 2 O 5 reduced to 105 m 2 g −1 after heat treating at 350°C. The activation energy of the Nb 2 O 5 crystallization reaction was found to be 327 kJ mol −1 . The surface area of Nb 2 O 5 reduced significantly with heat treatment but it still improved the hydrogen kinetics since the desorption rate was 1.5 wt.% min −1 . The addition of a small amount of amorphous Nb 2 O 5 to magnesium hydride, throughout milling, helped overcoming limitations inherent to solid hydrogen storage.

Mg 2 FeH 6 is a promising material for hydrogen storage applications, since it presents the highest known volumetric capacity of 150 kg m −3 of H 2 and its metallic constituents are inexpensive. The major drawbacks for its application are the difficulties associated with its synthesis and also its high thermal stability. In this paper, Mg 2 FeH 6 -based nanocomposites were prepared from the elements via reactive milling. A high-yield of the complex hydride synthesis was obtained after a systematic processing study, and the best conditions were successfully extended to the mechanochemical synthesis of Mg 2 CoH 5 . The influence of different additives such as transition metals, transition metal fluorides and graphite on the H-sorption behavior of the Mg 2 FeH 6 -based nanocomposites was evaluated. Mixtures rich in both MgH 2 and Mg 2 FeH 6 hydrides present lower temperature ranges of hydrogen release than those which are rich in only one of these hydrides. The MgH 2 –Mg 2 FeH 6 -based nanocomposite presents ultra-fast H-sorption kinetics at 300°C with partial reversible formation of Mg 2 FeH 6 .

Amorphous ribbons of the systems Fe 60 Co 20 Si 10 B 10 and Fe 50 Co 20 V 10 Si 10 B 10 have been prepared via melt spinning to be used as electrode materials in alkaline water electrolysis. It turned out that the gas atmosphere during melt-spinning has a significant impact on the surface morphology and microstructure of the ribbons. Furthermore, a comprehensive characterization of the microstructure depending on the peripheral speeds of the cooling wheel was conducted. Moreover, the transition from the amorphous to the crystalline state was investigated via differential scanning calorimetry measurements. Preliminary electrochemical investigations suggest a noticeable improvement of the electrocatalytic activity of the melt-spun amorphous ribbons after an anodic–cathodic pretreatment.

Components can be protected with coatings tailored using plasma transferred arc to meet specific demands as mixtures of powders with an almost unlimited chemical composition can be processed. In this study, Ni, Fe and Nb aluminide coatings were processed from mixtures of elemental powders on steel substrates and on a Cu refrigerated mold for comparison purposes. Coatings and mini-billets were characterized by means of microhardness, X-ray diffraction and laser confocal microscopy. Examination of the solidification structures showed that for each alloy system coatings are influenced by the exothermal synthesis of the aluminides and the interaction of the plasma arc with the substrate and with the feedstock. The poor flowability of the Ni–Al powder mixture determined its interaction with the plasma arc and strongly affected the dilution of coatings.

The modified oxalate precursor method was used for the preparation of nanocrystalline CoFe 2 O 4 . Structural investigations of as-produced powders performed by means of high-resolution transmission electron microscopy and X-ray diffraction revealed an average particle size of 8 nm. The nanocrystallites are pure CoFe 2 O 4 and present a well crystallized spinel structure with lattice constant of 8.3583 Å. The particles are superparamagnetic exhibiting a blocking temperature around room temperature. The magnetization and the coercive field values obtained at 9 T are 56.8 emu · g −1 and 17.0 kOe (at 4.2 K), respectively. The crystalline anisotropy determined from the coercive field as a function of temperature is 8.0 × 10 5 J · m −3 , which is much higher than that of bulk materials.