The length-scale effects on the load bearing capacity of reinforcement particles in an ultrafine-grained metal matrix composite (MMC) were studied, paying particular attention to the nanoscale ...effects. We observed that the nanoparticles provide the MMCs with a higher strength but a lower stiffness compared to equivalent materials reinforced with submicron particles. The reduction in stiffness is attributed to ineffective load transfer of the local stresses to the small and equiaxed nanoparticles.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In this study equal channel angular pressing (ECAP) with accommodated backpressure was used as a technique for consolidation of fine atomized monocrystalline (
d
50
=
1.3
μm) Al 99.7% powder. The ...effect of ECAP on consolidation behavior of powder, microstructure and mechanical properties of subsequent compacts in comparison to conventional direct extrusion (DE) is presented. Consolidation was realized via simple processing route, where complicated step of gastight encapsulating of vacuumed powder was avoided. Both ECAPed and extruded compacts featured sound ultra-fine grained (UFG) microstructures, consisting of elongated grains of intensively sheared powder particles. Grain boundaries of compacts were stabilized with homogenously redistributed nano-scale particles of torn oxide surface layers of initial Al powder particles. UFG character of ECAPed and extruded compacts resulted in their relatively high strengths at room temperature (up to
R
m
=
316
MPa) and especially at elevated temperatures (up to
R
m
=
188
MPa at 300
°C). Inferior ductility of ECAPed compacts (
A ∼2%) compared to extrudants (
A ∼10%) was due to their disadvantageous microstructure texture reflecting deformation along ECAP shearing plane. Owing to grain pinning effect of broken oxide particles, compacts exhibited excellent structural stability with no major microstructural changes and deterioration of mechanical properties determined after 350
°C/20
h annealing. The trade-off between simple processing route and effect of residual porosity on high temperature mechanical properties is discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Powder metallurgy (PM) fabricated HITEMAL is a near- and sub-micrometer aluminum (Al) material that is stabilized by a small fraction of a nano alumina (Al2O3) secondary phase. This phase forms in ...situ from surface Al2O3 films present on as-atomized Al powders during compaction. HITEMAL showed unique mechanical properties and creep performance at high homologous temperatures owing to its ultra-fine Al grain structure effectively stabilized by Al2O3. However, conventional fusion welding is generally not applicable to PM Al. This study reports on friction stir welding (FSW) that was applied to join bars extruded from Al powders of different particle size. As PM Al performance crucially depends on the Al2O3 phase, we focused on changes to this phase that were, induced by FSW. Thermal stability, mechanical properties, and deformation behavior of the weld materials were studied versus changes in the morphology and distribution of the Al2O3 phase during FSW. The study confirmed that FSW can be successfully used to join PM Al, although some detrimental transformation of the Al2O3 phase occurred upon FSW. This transformed Al2O3 phase did not provide sufficient stabilization under severe plastic deformation induced by a rotating tool action. The transformation involved dynamic recrystallization and minor Al grain growth in weld zones. This led to a decrease in the mechanical strength of the weld compared with that of the respective base materials. While this negative effect was pronounced for HITEMAL with the finest Al grain size, it was negligible in the coarse-grained material produced from the coarsest Al powder.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Metallic implant materials are biomaterials that have experienced major development over the last fifty years, yet some demands posed to them have not been addressed. For the osseointegration process ...and the outcome of endosseous implantation, it is crucial to reduce the stress shielding effect and achieve sufficient biocompatibility. Powder metallurgy (PM) was utilized in this study to fabricate a new type of titanium (Ti) + magnesium (Mg) bioactive composite to enable stress-shielding reduction and obtain better biocompatibility compared with that of the traditional Ti and Ti alloys used for dental implants. Such composites are produced by well-known cost-effective and widely used PM methods, which eliminate the need for complex and costly Ti casting used in traditional implant production. The relation between the microstructure and mechanical properties of as-extruded Ti + (0-24) vol% Mg composites was investigated with respect to the Mg content. The microstructure of the composites consisted of a biodegradable Mg component in the form of filaments, elongated along the direction of extrusion, which were embedded within a permanent, bioinert Ti matrix. As the Mg content was increased, the discrete filaments became interconnected with each other and formed a continuous Mg network. Young's modulus (E) of the composites was reduced to 81 GPa, while other tensile mechanical properties were maintained at the values required for a dental implant material. The corrosion behavior of the Ti + Mg composites was studied during immersion in a Hank's balanced salt solution (HBSS) for up to 21 days. The elution of Mg pores formed at former Mg sites led to a further decrease of E to 74 GPa. The studied compositions showed that a new Ti + Mg metallic composite should be promising for load-bearing applications in endosseous dental implants in the future.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The oxidation behavior of gas-atomized Al and Al alloy powder green compacts during heating prior to hot extrusion compaction was studied at laboratory and industrial scales by TGA, DSC, DTA, EDX, ...TEM and XRD methods. The effect of the heating of green compacts on the mechanical properties of the powder-extruded samples was assessed. Significant oxidation of Al and Al alloy powder green compacts takes place in the solid state during heating in air. The onset and intensity of oxidation were affected by the Mg content, the surface area of the powder and the volume of the powder green compacts. An exothermic heat associated with the oxidation of Al and Al alloy powders resulted in intense overheating of bulky powder green compacts during heating in air. The samples extruded from the powder green compacts heated in air exhibited reduced strength. The loss in strength was especially pronounced in the case of Mg-containing Al alloy powders. Mg diffuses from a powder metallic core toward the native Al2O3 surface layer present on as-atomized Al alloy powders; it reacts with oxygen present in air and in the Al2O3 surface layer where the MgO phase forms, eventually resulting in the depletion of Mg from the powder core. Materials extruded from Al powders depleted of Mg do not exhibit effective Al-Mg solid solution strengthening or strengthening by Mg-containing precipitates. Economically viable approaches to avoiding the detrimental effects of powder oxidation during the heating of green compacts prior to hot working consolidation are discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The issue of intrinsic microstructural and mechanical instability of Zn-based metals limits their expansion in potential applications of bioresorbable stents and orthopedic fixators. A new concept of ...stabilization of Zn microstructure by a small fraction of nontoxic nanometric ZnO dispersoids is proposed for the first time and demonstrated on the particular bioabsorbable model material. The effect of the ZnO dispersoids on post-processing microstructural stability, deformation and strengthening mechanisms, corrosion, and in-vitro biological behavior are pursued. The ZnO dispersoids arise in situ within deformed Zn structure during the consolidation of fine atomized Zn 99.99wt.% powder by hydro-extrusion. ZnO nanodispersoids (4.75 vol.%; ∼136 nm) form from passivating films present on Zn. They allow formation of ultrafine-grained Zn structure with an average grain size of ∼750 nm and its retention by Zener pinning action during annealing held at 100 °C. The model Zn + ZnO composite shows the superior mechanical properties than those reported for pure Zn materials. The utilized stabilization concept doesn't compromise corrosion and biological responses. Immersion of the Zn + ZnO in DMEM results in a corrosion rate, which complies with the desirable standard rate for biodegradable materials. Electrochemical tests suggest that the Zn + ZnO reaches a similar degradation rate after the first week of immersion and a more uniform corrosion behavior compared to the cast Zn reference. In-vitro cyto/genotoxicity assays performed using DMEM diluted extracts of the Zn + ZnO and cast Zn incubated with L929 cells yield in comparable and non-toxic responses. The presence of ZnO dispersoids induces a small but still significant bacteriostatic activity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
An industrial press was used to consolidate compacted aluminum powder with a nominal diameter in the range of 1 µm. Direct and indirect hot-extrusion processes were used, and suitable process ...parameters were determined from heating conditions, ram speeds and billet temperatures. For comparison, a direct-extrusion press for hot extrusion of a conventional aluminum alloy AA 1050 was used. The extruded Al powder showed better mechanical properties and showed a thermal stability of the mechanical properties after annealing treatments. To increase the theoretical density of the directly extruded Al powder, single-hit hot-compression tests were carried out. Activation energies for hot forming were calculated from hot-compression tests carried out in the temperature range 300–580 °C, at different strain rates. Processing maps were used to demonstrate safe hot-working conditions, to obtain an optimal microstructure after hot forming of extruded Al powder.
Nanomaterials play an important role in metal matrix composites (MMC). In this study, 3.0 wt.%, 6.0 wt.%, and 9.0 wt.% nano-AlN-particles-reinforced AA6061 (nano-AlN/AA6061) composites were ...successfully prepared by pressure infiltration technique and then hot extruded (HE) at 500 °C. The microstructural characterization of the composites after HE show that the grain structure of the Al matrix is significantly refined, varying from 2 to 20 μm down to 1 to 3 μm. Nano-AlN particles in the composites are agglomerated around the matrix, and the distribution of nano-AlN is improved after HE. The interface between AA6061 and nano-AlN is clean and smooth, without interface reaction products. The 3.0 wt.% nano-AlN/AA6061 composite shows an uppermost yield and supreme tensile strength of 333 MPa and 445 MPa, respectively. The results show that the deformation procedure of the composite is beneficial to the further dispersion of nano-AlN particles and improves the strength of nano-AlN/AA6061 composite. At the same time, the strengthening mechanism active in the composites was discussed.
This study is aimed to evaluate the galvanic corrosion of a newly designed TiMg dental implant. The TiMg composite was tested in two forms: (1) cylinders (TiMg cyl) and (2) dental implants (TiMg DI). ...Surface roughness was measured using a confocal microscope. The electrochemical behavior was determined in a Hank’s balanced salt solution (HBSS) by the open-circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) measurements. Surfaces after corrosion were examined by a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). OCP of TiMg cyl and TiMg DI increased in the noble direction, indicating the formation and growth of a base film on their surfaces. After 2-h immersion, TiMg cyl had higher values of
i
Corr
and
R
Corr
compared with those of TiMg DI. After 24-h immersion, a prominent reduction in the values of
i
Corr
and
R
Corr
was observed, and
E
Corr
shifted positively. The Nyquist plots of TiMg cyl and TiMg DI show capacitive loops that tended to close after 2-h exposure. As the time extended, TiMg DI showed higher corrosion resistance compared with that of TiMg cyl, and the linear diffusion response arose from the formation and dissolution of chemically non-stable by-products caused by the hydrogen evolution. In such a case, the involved reaction was controlled by charge transfer and diffusion processes. The formation of a film composed of stable CAp species that attached directly to the surface and chemically non-stable by-products that included (Mg
3
(PO
4
)
2
) and Mg(OH)
2
was demonstrated. To conclude, the obtained results revealed that TiMg DI of a complicated threaded design had a more stable passive film and higher corrosion resistance in HBSS compared with those of TiMg cyl of a cylindrical design. These findings have the utmost importance for the fabrication of dental implants.
Graphical abstract
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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