S. Scudino

Production of Porous β-Type Ti-40Nb Alloy for Biomedical Applications: Comparison of Selective Laser Melting and Hot Pressing

We used selective laser melting (SLM) and hot pressing of mechanically-alloyed β-type Ti–40Nb powder to fabricate macroporous bulk specimens (solid cylinders). The total porosity, compressive strength, and compressive elastic modulus of the SLM-fabricated material were determined as 17% ± 1%, 968 ± 8 MPa, and 33 ± 2 GPa, respectively. The alloy’s elastic modulus is comparable to that of healthy cancellous bone. The comparable results for the hot-pressed material were 3% ± 2%, 1400 ± 19 MPa, and 77 ± 3 GPa. This difference in mechanical properties results from different porosity and phase composition of the two alloys. Both SLM-fabricated and hot-pressed cylinders demonstrated good in vitro biocompatibility. The presented results suggest that the SLM-fabricated alloy may be preferable to the hot-pressed alloy for biomedical applications, such as the manufacture of load-bearing metallic components for total joint replacements.

High strength hexagonal structured dendritic phase reinforced Zr–Ti–Ni bulk alloy with enhanced ductility

A Zr74.9Ti10.5Ni14.6 nanostructure-dendrite composite was prepared by copper mold casting. X-ray diffraction, and scanning and transmission electron microscopies, reveal a microstructure comprising ultrafine-scale dendrites with hexagonal structure and a two-phase interdendritic matrix, consisting of nanocrystalline hexagonal and tetragonal phases. Room temperature compression tests show evident yielding and 6% plastic strain together with work hardening up to 1622MPa. These results indicate that composite materials based on hexagonal phases might be an alternative to composites containing body-centered cubic phase reinforcements.

Is the energy density a reliable parameter for materials synthesis by selective laser melting

ABSTRACT The effective fabrication of materials using selective laser melting depends on the process parameters. Here, we analyse the suitability of the energy density to represent the energy transferred to the powder bed, which is effectively used to melt the particles and to produce the bulk specimens. By properly varying laser power and speed in order to process the powder at constant energy density, we show that the equation currently used to calculate the energy density gives only an approximate estimation and that hatch parameters and material properties should be considered to correctly evaluate the energy density. GRAPHICAL ABSTRACT IMPACT STATEMENT Al-12Si SLM samples were fabricated at constant energy density. The laser power and laser scan speed combination variation was used to demonstrate the significant changes needed with energy density equation.

Microstructure evolution upon devitrification and crystallization kinetics of Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 melt-spun glassy ribbon

The crystallization behavior of glassy Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 produced by melt spinning was investigated by differential scanning calorimetry, x-ray diffraction, electron microscopy, and atom probe investigations. The devitrification of the as-spun ribbon occurs by primary crystallization of a metastable nanoscale quasicrystalline phase during the first stage of the crystallization process, followed by successive transformation into intermetallic compounds at higher temperatures. The kinetics investigation reveals that quasicrystal formation is characterized by two overlapping processes: the first step probably linked with the redistribution of one or more elements, most likely Al and Zr, between the quasicrystalline phase and the remaining amorphous matrix, and the second step corresponding to the crystallization itself. Furthermore, a higher value of the activation energy for quasicrystal formation compared to other quasicrystal-forming alloys suggests an increased complexity of the crystallizati...

Structural features of plastic deformation in bulk metallic glasses

Spatially resolved strain maps of a plastically deformed bulk metallic glass (BMG) have been created by using high-energy X-ray diffraction. The results reveal that plastic deformation creates a spatially heterogeneous atomic arrangement, consisting of strong compressive and tensile strain fields. In addition, significant shear strain is introduced in the samples. The analysis of the eigenvalues and eigenvectors of the strain tensor indicates that considerable structural anisotropy occurs in both the magnitude and direction of the strain. These features are in contrast to the behavior observed in elastically deformed BMGs and represent a distinctive structural sign of plastic deformation in metallic glasses.

Fabrication and Response of Al70Y16Ni10Co4 Glass Reinforced Metal Matrix Composites

Al70Y16Ni10Co4 glass was developed through powder metallurgy route. Al-based glass-reinforced metal matrix composites were produced by hot pressing and simultaneous extrusion of these powders.The consolidation parameters were selected from systematic calorimetric studies. The response of these composites is superior mechanical properties, and these properties increases with glassy reinforcement. Remarkable improvement in the wear properties were observed when subjected to abrasion test. The abrasive wear rate has been decreased from 2.46 × 10−4 m3/m for commercial Al-Mg-Si (6061) extruded alloy to 0.373 × 10−4 m3/m for 50 vol% glass reinforced 6061 composite. Similar improvements were observed with glass-reinforced aluminum matrix composites. Plowing as well as pullout of particles during abrasion tests corresponds to the wear of these composites.

Formation of quasicrystals by partial devitrification of ball-milled amorphous Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5

Amorphous Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 powder with low oxygen contamination was prepared by ball milling of crystalline intermetallic compounds. The comparison with the corresponding alloy produced by melt spinning shows that the first crystallization phase does not depend on the way of preparation. In fact, devitrification of both powder and ribbon is characterized by the formation of a metastable nanoscale quasicrystalline phase during the first stage of the crystallization process. This suggests that both the amorphous ball-milled powder and the melt-spun ribbon have the same short-range order and that if this short-range order is icosahedral, it can be achieved also by solid-state processing.

Strain-induced structural transformation of single-phase Al–Cu–Fe icosahedral quasicrystal during mechanical milling

A single-phase stable icosahedral quasicrystalline sample of high quality with the composition Al62.5Cu25Fe12.5 was produced by the spray forming technique. The material was further investigated by mechanical milling under an argon atmosphere to avoid oxidation during milling. At the initial stages of milling (within 5 h) a significant broadening of the diffraction peaks was observed, indicating a reduction of crystallite size and the introduction of lattice strain, which can be linked to phason strain of the quasilattice. Line broadening was noticed to increase with increasing milling time and in the material milled for longer time only a few broad diffraction peaks, which can be identified as a nanoscale bcc phase (i.e. disordered B2 phase, a ∼ 2.9 Å), were visible. At this stage the diffraction signals belonging to the quasicrystals were no longer observable, indicating a complete transformation of the quasicrystals into the bcc phase. Finally, the bcc phase formed during milling transformed back to the quasicrystalline phase during subsequent annealing treatment. The microhardness measured on the milled powders was found to decrease with increasing milling time, most likely as a consequence of the increased volume fraction of the ductile bcc phase. Attempts are made to rationalize the structural transformation.

Quasicrystal formation in mechanically alloyed Zr–Ti–Nb–Cu–Ni–Al glassy powders

Different from the glassy Zr62Ti7.07Nb2.21Cu12.28Ni9.81Al6.62 melt-spun ribbon that forms a quasicrystalline phase upon devitrification, the corresponding alloy produced by mechanical alloying of elemental powder mixtures does not clearly show quasicrystal formation. However, the addition of an appropriate amount of elemental zirconium to the mechanically alloyed powder changes the crystallization behavior inducing the formation of an icosahedral quasicrystalline phase as the first crystallization product. This indicates that for this multicomponent metallic glass quasicrystal formation in the mechanically alloyed powder is crucially linked to the composition rather than to the question whether there is a special quenched-in short-range order.

Mechanochemical route to the synthesis of nanostructured Aluminium nitride

Hexagonal Aluminium nitride (h-AlN) is an important wide-bandgap semiconductor material which is conventionally fabricated by high temperature carbothermal reduction of alumina under toxic ammonia atmosphere. Here we report a simple, low cost and potentially scalable mechanochemical procedure for the green synthesis of nanostructured h-AlN from a powder mixture of Aluminium and melamine precursors. A combination of experimental and theoretical techniques has been employed to provide comprehensive mechanistic insights on the reactivity of melamine, solid state metal-organic interactions and the structural transformation of Al to h-AlN under non-equilibrium ball milling conditions. The results reveal that melamine is adsorbed through the amine groups on the Aluminium surface due to the long-range van der Waals forces. The high energy provided by milling leads to the deammoniation of melamine at the initial stages followed by the polymerization and formation of a carbon nitride network, by the decomposition of the amine groups and, finally, by the subsequent diffusion of nitrogen into the Aluminium structure to form h-AlN.