Crystallization kinetics of Zr65Ag5Cu12.5Ni10Al7.5 glassy powders produced by ball milling of pre alloyed ingots

Ball milling was employed to produce Zr65Ag5Cu12.5Ni10Al 7.5 glassy powder from pre-alloyed mixtures of crystalline intermetallic compounds. Differential scanning calorimetry (DSC) in isochronal as well as in isothermal modes was used to study the thermal stability and the crystallization kinetics of the glassy powder. The activation energy for crystallization was calculated using isothermal and isochronal DSC data as well as from viscosity measurements, which lead to values of the activation energy ranging between 298 and 314 kJ/mol. Johnson-Mehl-Avrami analysis shows that the transformation is a diffusion controlled three-dimensional process and the crystallization proceeds with increasing nucleation rate at annealing temperatures within the super-cooled liquid region. To test the effectiveness of the glassy powder as reinforcement in Al-based metal matrix composites, bulk specimens consisting of pure Al powder blended with 50 vol.% of Zr65Ag5Cu12.5Ni10Al 7.5 glassy powder were synthesized by powder metallurgy. Room temperature compression tests reveal that the strength increases from 155 MPa for pure Al to 235 MPa for the composite with 50 vol.% of glass reinforcement

Simultaneous enhancements of strength and toughness in an Al-12Si alloy synthesized using selective laser melting

The effect of selective laser melting (SLM), an additive manufacturing technique employed to produce metallic components, on the mechanical properties of the Al-12Si alloy is investigated, with particular emphasis on understanding the effect of laser track direction on quasi-static tensile, fracture, fatigue crack growth, and unnotched fatigue properties. The effect of post-SLM heat treatment as well as the scanning strategy (linear vs. checker board hatch style) was examined and the results are compared with those obtained on specimens produced through the conventional casting route. The SLM alloys exhibit a mesostructure, in addition to the fine, supersaturated Al-rich cellular structure with Si particles along the boundaries. While the latter imparts high strength at the cost of ductility, the mesostructure, which arises due to the laser track hatching, causes the crack path to be tortuous, and in turn leads to substantial increase in fracture toughness. This imparts significant anisotropy to fracture while tensile properties are nearly-isotropic. The unnotched studies reveal that the tensile residual stresses, shrinkage porosity, and unmelted powder particles, can degrade the unnotched highest fatigue properties considerably and hence need be eliminated for high fatigue strength. The SLM process offers new avenues for material design that would exploit the micro- and meso-structures generated by the process for simultaneous enhancement in strength and toughness. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Selective laser melting of high-strength, low-modulus Ti–35Nb–7Zr–5Ta alloy

The state-of-the-art alloys for load-bearing implant applications lack the necessary functional attributes and are largely a compromise between biocompatibility and mechanical properties. While commercial alloys pose long-term toxicity and detrimental stress shielding effects, the newly developed alloys are closing in on the gaps, however, falling short of the desired elastic modulus necessary to rule out stress shielding. In this work, we report the fabrication of a low modulus β-Ti alloy, Ti–35Nb–7Zr–5Ta (TNZT), by selective laser melting (SLM) with optimized laser parameters. The as-prepared SLM TNZT shows a high ultimate tensile strength (~630 MPa), excellent ductility (~15%) and a lower elastic modulus (~81 GPa) when compared to the state-of-the-art cp-Ti and Ti-based alloys. The mechanical performance of the as-printed TNZT alloy has been examined and is correlated to the microstructure (grain structure, phase constitution and dislocation density). It is proposed that a high density of GND (geometrically necessary dislocations), resulting from rapid cooling, in the as-prepared condition strengthens the alloy, whereas the single phase β-bcc crystal structure results in lowering the elastic modulus. High grain boundary area and a preferred crystal orientation of {200} planes within the bcc crystal lattices contribute to an additional drop in the elastic modulus of the alloy. It is shown that the TNZT alloy, processed by SLM, demonstrates the best combination of strength and modulus, illustrating its potential as a promising biomaterial of the future

Phase formation, microstructure and deformation behavior of heavily alloyed TiNb- and TiV-based titanium alloys

© 2018 Elsevier B.V. The effect of chemical composition on microstructure and mechanical properties of heavily alloyed beta-titanium Ti-Nb(V)-Cu-Co-Al alloys was studied. The alloys were fabricated by casting into a water-cooled copper crucible employing relatively high cooling rates. The microstructure of these alloys consists of primary micrometer-sized bcc-structured (bcc – body centered cubic) dendrites surrounded by a minor amount of intermetallic phases. The morphology and volume fraction of the intermetallic phases are strongly affected by the alloys’ chemical composition. Particularly, the solubility of Cu and Co in the bcc dendrites of Ti-V-Cu-Co-Al is lower compared to that of Ti-Nb-Cu-Co-Al leading to a higher volume fraction of the intermetallic phase in the latter alloy. The high mechanical strength of the Ti-Nb(V)-Cu-Co-Al alloys (yield strength up to 1430 MPa) is mainly attributed to their multiphase nature and solid solution hardening of the supersaturated bcc-structured dendrites. Moreover, the large compressive plastic deformability supported by pronounced strain-hardening reaches several tens of percent. The alloys exhibit a significant strength asymmetry between compressive and tensile loadings, namely, they are weak and brittle under tensile loading. The tensile brittleness is associated with the lattice distortion in the bcc-structured dendrites as well as crack initiation at the interdendritic precipitates.status: publishe