C. Bolfarini

Topological instability as a criterion for design and selection of aluminum-based glass-former alloys

This letter proposes a criterion to quantitatively correlate chemical composition with crystallization behaviors of Al-based glass-former alloys. By extending to multicomponent systems the topological instability model of Egami and Waseda [J. Non-Cryst. Solids 64, 113 (1984)], we introduce a “lambda (λ) criterion” according to which amorphous alloys with λ>0.1 exhibit glassy behavior, whereas alloys with λ<0.1 are nanocrystalline. Nanoglassy alloys occur when λ≈0.1. A large number of experimental findings in the literature supports the λ criterion, rendering it a useful tool for the design and selection of glass-former systems and compositions.

Topological instability and electronegativity effects on the glass-forming ability of metallic alloys

The glass-forming ability (GFA) of metallic alloys is associated with a topological instability criterion combined with a new parameter based on the average electronegativity difference of an element and its surrounding neighbours. In this model, we assume that during solidification the glassy phase competes directly with the supersaturated solid solution having the lowest topological instability factor for a given composition. This criterion is combined with the average electronegativity difference among the elements in the alloy, which reflects the strength of the liquid. The GFA is successfully correlated with this combined criterion in several binary glass-forming systems.

Severe Plastic Deformation of Mg-Fe Powders to Produce Bulk Hydrides

We describe in the present work the production of bulk Mg hydrides by hydrogenation treatment of samples processed by severe plastic deformation. The compact bulk samples of Mg-Fe have been obtained by high pressure torsion. The ternary complex Mg2FeH6 and the binary MgH2hydrides have been synthesized by hydrogenation treatment at 350°C, at 3 MPa during 24 h. The average grain refinement after HPT was modest as estimated by XRD. A comparison between the XRD patterns of the powders and of the HPT samples showed the formation, as expected, of a preferred orientation in the latter. The XRD of the hydride HPT samples (H-HPT) showed the presence of Mg, Fe, MgH2 and Mg2FeH6. The first de-hydrating reaction of the alloys (after H-HPT) was studied by differential scanning calorimetry (DSC). These results showed a reduction in the hydrogen dessorption temperature in comparison with commercial MgH2, indicating thermodynamic destabilization of the hydrides as a result of the high density of dislocations in the H-HPT samples.

Atomization and Selective Laser Melting of a Cu-Al-Ni-Mn Shape Memory Alloy

Shape memory alloys (SMAs) are a class of material that undergoes a reversible shape change after a plastic deformation. The recovery of the original shape is possible due to a structural transformation upon heating to a critical temperature. The shape memory effect is related to a martensitic-austenitic transformation from a phase with a low symmetry (martensite) to a high-temperature phase (parent phase) [1]. Cu-based shape memory alloys have the advantage of large thermal and electrical conductivities and the system Cu-Al-Ni alloys are quite attractive due to better stabilisation against aging phenomena [2].

Prediction of good glass formers in the Al-Ni-La and Al-Ni-Gd systems using topological instability and electronegativity

A new criterion has been recently proposed combining the topological instability (λ criterion) and the average electronegativity difference (Δe) among the elements of an alloy to predict and select new glass-forming compositions. In the present work, this criterion (λ.Δe) is applied to the Al-Ni-La and Al-Ni-Gd ternary systems and its predictability is validated using literature data for both systems and additionally, using own experimental data for the Al-La-Ni system. The compositions with a high λ.Δe value found in each ternary system exhibit a very good correlation with the glass-forming ability of different alloys as indicated by their supercooled liquid regions (ΔTx) and their critical casting thicknesses. In the case of the Al-La-Ni system, the alloy with the largest λ.Δe value, La56Al26.5Ni17.5, exhibits the highest glass-forming ability verified for this system. Therefore, the combined λ.Δe criterion is a simple and efficient tool to select new glass-forming compositions in Al-Ni-RE systems.

Correlation between heat- and deformation-induced crystallization of amorphous Al alloys

Deformation-induced crystallization is correlated with thermal-induced crystallization in alloys with different compositions in a single amorphous alloy system. In Al87Y6Ni5Co2 and Al85Y8Ni5Co2 alloys, which undergo primary crystallization during heating, deformation-induced crystallization of fcc-Al has been observed. In Al83Y10Ni5Co2 alloy, which undergoes eutectic-like crystallization, no deformation-induced crystallization was observed. These observations can be explained by the presence or absence of quenched-in nuclei and the work required for the creation of an amorphous/crystalline interface under compressive stress.

Corrosion resistance and glass forming ability of Fe47Co7Cr15M9Si5B15Y2 (M=Mo, Nb) amorphous alloys

In the present work the effect of substituting Mo with Nb on the glass forming ability and corrosion resistance of Fe-Co-Cr-M-Si-B-Y (M=Mo, Nb) ribbons with high corrosion resistance is investigated. The X-ray powder diffraction pattern indicates that the alloy containing both Nb and Mo presented greater glass forming ability than the alloy containing either of these elements separately. The results obtained indicate that Mo is more effective in enhancing corrosion resistance than the Nb in 4.0 M HCl solution. The alloy containing both Nb and Mo presented greater overall corrosion resistance than the alloy containing only one of these elements.

Microstructure Characterization and Kinetics of Crystallization Behavior of Tubular Spray Formed Fe43.2Co28.8B19.2Si4.8Nb4 Bulk Metallic Glass

Abstract In this study the Fe43.2Co28.8B19.2Si4.8Nb4 (at.%) alloy was processed by spray forming in an industrial facility with the aim of investigating the formation of amorphous phases. The thicker layers (5–15 mm) presented partial/fully crystalline microstructure with onset of crystallization of the amorphous phase at Tx = 595 °C. On the other hand, thinner layers (below 2.5 mm) presented fully amorphous structure, and glassy behaviour with glass transition (Tg) at 555 °C. The thermal stability and the crystallization kinetics of the alloy were studied using the isothermal DSC curves, measured at different heating rates and temperatures. Despite its good glass-forming ability (GFA) and high stability against crystallizations, its incubation time for crystallization is almost zero. In order to rule-out the complete crystallization mechanism, in situ temperature resolved studies using TEM were done. In addition, the amorphous part of the deposit was heat treated at various temperatures/times and the results were compared with the in situ crystallization behavior observed by TEM.

Characterization of Atomized Powders and Extruded Samples of an Al-Si-Cu Alloy

In this work, the microestrutural characterization and mechanical properties of atomized Al-9Si-3Cu alloy powders and extruded samples are presented. The microstructure was evaluated by a combination of X-ray diffraction, optical microscopy and scanning electron microscopy. The mechanical properties of extruded samples were also characterized by tensile test and hardness measurements. The results revealed that the powder particles and the extruded samples are constituted by α-Al, intermetallic and metastable phases. The extruded samples obtained by the use of smaller atomized particles show lower ductility than with larger particles. The same behavior was observed with low extrusion temperature than with high temperatures. It was also observed minor variations in the yield strength and hardness with variation in the size of the powder particles.

Rapid Solidification and Laser Cladding of Gas Atomized Ni-Nb-Sn Bulk Metallic Glass

Glassy overspray powders of Ni59Nb35Sn6 (at%) bulk metallic glass (BMG) obtained by spray forming were used in order to produce coatings on AISI 1020 mild steel substrate by laser cladding of the pre-placed powders. Different laser parameters, resulting in a variation of the power density, PD (J/mm2), were tested with a Yb fiber laser (up to 500 W). Gas atomized powders, suction cast sample trough copper mold casting and the laser clad tracks were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and coatings were subjected to measurements of Vickers microhardness. Atomized powder obtained showed no crystalline phases formation up to 425 μm, indicating good glass forming ability (GFA) of Ni59Nb35Sn6 (at%) alloy. Microstructure characterization confirmed maximum glassy dimension of tc =1mm for the Ni59Nb35Sn6 (at%). Laser cladding track showed nanocrystalline phases embedded in a glassy matrix with Vickers microhardness ranging from 336 to 1184 HV.