Metastable alloys based on the Ti–Nb system are considered promising candidates to replace the biomaterials currently used in medicine. Several very promising alloys like TNTM, TNZT, etc., were ...recently developed by adding Zr, Ta, Mo, Fe, or other elements to the Ti–Nb system. However, only a few fundamental studies were devoted to the structural characterisation of Ti–Nb alloys in the as-cast state. In this study, we analyze the microstructure, the phase transformations, the lattice parameters, the interaction of different phases, and Young's modulus of a wide range of Ti–Nb alloys produced by suction casting. The structure of experimental samples was investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and synchrotron X-ray diffraction (SXRD) analysis. The samples with a Nb content of less than 17.5 wt % consisted of α′ phase. The pure α’’ phase was observed only in the alloy with 17.5 wt % Nb. The alloys with an intermediate content of Nb (20–30 wt %) contained a mixture of α’’, β and ω phases. The samples with 30–35 wt % Nb consisted of a mix of β and ω phases. The samples with 37.5 and 45 wt % Nb included two types of β phase. In addition to the regular β phase, a little detectable amount of bcc phase was observed with a larger lattice parameter named βminor. The mechanical properties strongly depended on the phase composition. We found that for α’’-dominated alloys, there is a correlation between Young's modulus and orthorhombicity and c/a ratio of α’’ phase. The lowest Young's modulus (47 GPa) was observed in the alloy with 17.5 wt % Nb, which has orthorhombicity close to unity and c/a ratio close to 1.58.
•High orthorhombicity and low c/a ratio of α’’ phase result in low Young's modulus.•Small volume change in β→ω transformation causes a high Young's modulus.•Two different β metastable phases have been observed in Nb rich alloys.•Lattice strains in β→ω transformations are smaller than in β→α′/α″ transformations.
During an explosion, the interfaces of welded materials experience fast heating due to high strain rate severe plastic deformation. This leads to the formation of local zones, where melting and ...mixing of welded materials is observed. These zones are frequently referred to as vortexes, eddies or swirls, due to the specific rotational movement of materials during mixing. This study is primarily devoted to the discussion of the structures that appear in these zones. Simple approaches to estimate the heating and cooling rates at the interfaces between explosively welded materials were proposed. It was concluded that the heating rate at the interfaces was of the order of 109 K/s, while the cooling rate achieved 107 K/s. Several combinations of explosively welded alloys (steel/steel, Ti alloy/steel, Zr/Cu, Zr/Ni, Ta/Cu, Al/magnesium alloy and Cu/brass) were thoroughly analyzed using scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. In most of these combinations, metastable crystalline, quasicrystalline or glassy phases were observed. The formation of different types of metastable phases is discussed with respect to the compositions of the welded alloys. It was concluded that solidification conditions at the interfaces of explosively welded materials are similar to those during rapid solidification. Thus, the results of numerous experiments on rapid solidification of alloys could be applied to analyze the structures that appear in mixing zones.
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In this study, using a combination of several experimental and computational methods, an attempt to improve the understanding of some important phenomena accompanying the process of explosive welding ...is made. The high-speed shooting was used to observe the formation of a re-entrant jet. Various methods of the materials characterization were used to estimate the morphology of the interface, the distribution of the liquid phase and analyze the evolution of the structure in the process of high-velocity oblique collision. Simulating the process of high-velocity collision using the smoothed particles hydrodynamics (SPH) method allowed us to accurately reproduce formation of the wave boundary, vortex zones, as well as the formation of a jet moving ahead of the collision point. Based on the simulation results, several significant modifications of the Bahrani-Black-Crossland model of the waves formation were done, and a new explanation for the vortex zones formation was proposed. Numerical simulation of the cooling process showed that the solidification of the liquid phase occurs under conditions of the rapid solidification during melt spinning. Combining several numerical-based approaches, a welding window for the steel-steel system was built. The results obtained show a good agreement with currently existing concepts of welding during the high-velocity collisions.
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In this study, forty-layered Ti-Al composites were fabricated in a single-shot explosive welding process. The structure of the composites was thoroughly investigated using scanning and transmission ...electron microscopy. Particular attention was paid to the structure of the mixing zones (vortexes) arising at the interfaces during explosive welding. The complicated process of mixing and rapid solidification of these zones led to formation of different stable and metastable structures. The vitrification of vortexes, formation of “disordered” Ti3Al and ordered solid solution of Al in hexagonal-Ti and Ti in FCC-Al were observed and discussed with respect to the conditions of solidification. Subsequent heat treatment was carried out at 640°C under atmospheric and at 3MPa pressure. For comparison, there were produced reference samples by reaction sintering at the same conditions as for the heat treatment. The heat treatment and reaction sintering promoted the formation of stable Al3Ti phase between Ti and Al. It was found that preliminary explosive welding accelerated the formation of Al3Ti layer and made heat treatment duration four times shorter. The application of pressure was found to play an important role at the final stage of heat treatment after explosion welding to avoid formation of defects between the plates.
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•Single shot explosive welding was used to bond 40 Ti and Al-1Mn plates.•Metastable crystalline and amorphous intermetallics were formed at Ti-Al interfaces.•Al3Ti layer was formed at the interfaces after annealing at 640°C.•Preliminary explosive welding accelerates the formation of Al3Ti layer.•Application of pressure during annealing is essential for healing the defects in Al3Ti layer.
In this study, a laser cladding technique was used to produce a protective TiAl-based coating on TA6Zr4DE near-alpha titanium alloy. Ti48Al2Cr2Nb powder was used as a cladding material. The ...microstructure of samples was characterized using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Fully lamellar microstructure consisting of γ-TiAl (fcc) and α2-Ti3Al (hcp) phases was observed in the coating. Oxidation resistance of substrate and coating was evaluated by heating the samples in an air atmosphere at 700, 800 and 900°С during 100h. The oxidation process of TA6Zr4DE alloy surface led to a formation of multilayered oxide films as result of repeated growth and peeling. The coated samples showed better oxidation resistance in whole temperature range compared to that of the substrate. This behavior was explained by the composition of the cladded layer. Niobium and chromium contained in the cladded powder inhibited the intensive growth of TiO2 (rutile) and contributed to the formation of a protective layer composed mainly of alumina.
•Fully lamellar TiAl+Ti3Al structure was formed in laser cladded coating.•High oxidation rate of TA6Zr4DE titanium alloy above 700°С is due to rapid growth of TiO2.•Multilayered oxide scales was formed on the surface of TA6Zr4DE titanium alloy at 900°C.•High oxidation resistance of Ti48Al2Cr2Nb coating is explained by the presence of Nb and Cr in composition.
Ti-Al3Ti metal-intermetallic laminate (MIL) composites are known as promising structural materials due to the unique combination of their specific properties. However, their application is still ...limited due to the extremely high brittleness of the Al3Ti phase. In this study, we attempt to address this issue by changing the D022 crystal structure of Al3Ti to the more ductile L12 structure by alloying it with silver. To select the best fabrication regimes of Ti-Ti(Al1−xAgx)3 composites, in situ synchrotron X-ray diffraction analysis was performed to reveal the chemical reactions occurring upon heating the Ti-Al-Ag sample. The analysis showed that the highest amount of Ti(Al1−xAgx)3 phase with the L12 structure appears at 930 °C. This temperature was chosen for subsequent spark plasma sintering experiments. Scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffraction analysis revealed that the sintered sample consisted mainly of Ti, Ti(Al1−xAgx)3, and a minor fraction of the Ag-Al compound distributed in the central parts of the intermetallic layers and at the grain boundaries. Modification of the titanium trialuminide crystal structure positively affected the properties of the composite, providing a 60% increase in fracture toughness. The Ag-Al phase also contributed to toughening, causing an additional crack deflection effect.
•Ag stabilizes L12 crystal structure of Al3Ti in Ti-Al3Ti multilayer composite.•The maximum fraction of L12 phase in the composite is reached at 930 °C.•Modification of the Al3Ti provides 60% increase of the composite fracture toughness.•Minor Al-Ag phase causes the additional crack deflection effect.
In this study, the structure of explosively welded Ni–Al multilayer composites was investigated. In particular, the interface between Ni and Al plates was studied using scanning electron microscopy ...(SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). Continuous interlayers of mixed Al and Ni were found at the interface. The Al and Ni in these zones were heated above their melting temperatures, non-uniformly mixed, and rapidly solidified. Several intermetallic phases, including the decagonal phase and metastable Al9Ni2, were observed in these zones using electron diffraction. A cellular dislocation structure formed in the Ni plates and a polygonized dislocation structure formed in the Al plates due to the extremely high strain rate deformation and heating. Subsequent heat treatment at 620°C led to the rapid formation of stable intermetallic layers at the interfaces. The growth of the intermetallic layers was considerably faster in the explosively welded composite than in the reference sample.
•Ni–Al composite was produced by explosive welding.•The decagonal quasicrystals and Ni2Al9 metastable phase were observed at the interface.•Cellular and polygonized dislocation structures were formed in Ni and Al respectively.•The metastable structures were formed due to high pressure and rapid solidification.•Heat treatment at 620°C led to formation of stable intermetallics layers.
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Layered metallic-intermetallic composites formed by explosion welding of nickel and aluminum plates and subsequent annealing of the bimetal for 1 – 100 h at 550, 570, 590 and 610°C are studied. X-ray ...diffraction analysis is used to determine special features of the structure of interfaces of bimetallic billets after welding and to estimate their contribution into the processes of nucleation and growth of layers of nickel aluminide. It is shown that the formation of the intermetallic is determined primarily by volume diffusion. The activation energy of the growth of the nickel aluminide layer depends on the structural state of the diffusion pair and on the presence of oxide films on the interfaces of the metallic plates.
Multilayer composites formed by explosion welding from alternated plates of niobium and aluminum are studied. The samples are subjected to annealing at 700, 800 and 900°C under a load and without ...load. The structure, the ultimate tensile strength, the impact toughness and the Vickers microhardness of the composites are determined. It is shown that mixed zones represented by nonequilibrium phases, intermetallic particles of NbAl
3
and Nb
2
Al, and undissolved niobium volumes are formed near “local melting/rapid solidification” interfaces. Increase of the annealing temperature from 700 to 900°C causes growth of the intermetallic inclusions, appearance of cracks in the reaction layer, and considerable worsening of the mechanical properties. The highest ultimate strength (700 MPa) and impact toughness (86 J/cm
2
) are obtained in the samples annealed under pressure at 900 and 800°C, respectively. Annealing at 700°C at a pressure of 30 MPa is shown to be an optimum treatment producing a favorable effect on the strength characteristics and providing a defect-free structure.
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