Volume 109, Issue 8

Development of mechanical properties of titanium alloys is usually achieved by the control of their microstructure (globular, lamellar, bimodal). Dynamic fragmentation and spheroidization of elongated α phase grains, formed from martensite laths, in the microstructure of Ti-6Al-4V alloy are considered in the paper. The effects of α phase grain spheroidization was evaluated in tensile tests over a temperature range of 800–900°C and strain rate range of 10 −3 –10 −2 s −1 . Completed spheroidization of elongated α phase grains and subsequent growth of equiaxed grains for all deformation modes were found. A low value of apparent activation energy for deformation was calculated – suggesting grain boundary sliding was the main deformation mechanism. The results showed good correlation with the kinetic rate equation based on the Zener–Hollomon parameter.

Hot tearing susceptibility ( HTS ) of AZ31, AZ61, and AZ91 magnesium alloys was investigated with the constrained rod casting process at an average cooling rate of 1.5 K s −1 . The experimental results showed that the maximum HTS that was associated with the AZ31 alloy resulted from the coarse dendrites and poor feeding ability during the final stages of solidification. The HTS s in the AZ91 and AZ61 alloys were much smaller, due to the finer microstructure and good feeding ability. The results were verified through examination of the as-cast microstructure, and simulation of the solidification process.

The ternary alloy Zn-27Al-1Cu was produced by permanent mold casting. As-cast alloy was aged for different periods after being subjected to solution treatment followed by water quenching. The microstructure, mechanical properties and fatigue behavior of as-cast and heat-treated alloy were then investigated. The quench-aging eliminated the as-cast dendritic structure and produced a uniform microstructure consisting of large grains combined with precipitates. The hardness and tensile strength generally increased, whereas the ductility slightly decreased with aging time. Under all conditions, the longest fatigue life was obtained from the alloy that had been aged for 0.5 h, while the alloy which had been aged for 10 h exhibited the shortest. The fatigue properties were found to be more dependent on impact toughness and elongation, rather than tensile strength.

The microstructure and mechanical properties of novel β-type Ti 74- x Nb 26 Ni x ( x = 4, 8, 12, 16, 20, and 24 at.%) alloys with a low Young's modulus and containing different amounts of a second phase (Ti 2 Ni and Ni 3 Nb) were investigated. The experimental results show that the relative volume fractions of Ti 2 Ni phase in the Ti–Nb–Ni alloys increased with increasing Ni content. The mechanical properties of Ti–Nb–Ni alloys varied with the amount of second phase. Ti 70 Nb 26 Ni 4 , Ti 66 Nb 26 Ni 8 , and Ti 62 Nb 26 Ni 12 alloys containing a small amount of Ti 2 Ni phase exhibited a higher plastic strain; however, Ti 58 Nb 26 Ni 16 , Ti 54 Nb 26 · Ni 20 , and Ti 50 Nb 26 Ni 24 alloys containing a large amount of Ti 2 Ni phase and a small amount of Ni 3 Nb phase exhibited a small plastic strain. The increase in strength and decrease in plasticity of alloys can be mainly attributed to the increase in high-strength brittle Ti 2 Ni phase and grain refining of the β-phase matrix. In addition, Ti 50 Nb 26 Ni 24 alloy with a low Young's modulus has a high yield strength and elastic energy; this has potential application as biomedical and functional materials.

Microstructure evolution mechanisms of undercooled Ni 80 Cu 20 alloys were systematically studied, and two types of grain refinement were observed in the as-solidified microstructures. The first type of grain refinement appeared in a low undercooling range (i.e., 50 K ≤ Δ T ≤ 90 K). The remelting of primary dendrites during or after rapid solidification was due to liquid/solid interfacial tension and chemical superheating. The second type of grain refinement occurred in a high undercooling range (i.e., Δ T ≥ 165 K), in which the liquid phase pressure drop due to solidification contraction induced the flow of liquid in the inter-dendrite region and produced stress accumulation in the dendrites. At the slow solidification stage, i.e., post-recalescence period, recrystallization was observed. In addition, highly undercooled Ni 80 Cu 20 alloy melts were quenched before rapid solidification by Ga–In liquid alloy, and partially recrystallized microstructures were obtained. Using electron backscattering diffraction and transmission electron microscopy techniques, the occurrence of recrystallization was verified experimentally.

Cu 48- x Zr 48 Al 4 Nb x ( x = 1 or 5 at.%) metallic glass composite rods with diameters of 4 mm, 3 mm and 2 mm were prepared by using copper mold drop-casting. Their microstructures and tensile properties under different cooling rates were investigated. The microstructure and tensile properties of samples were affected by the crystalline phase content. With the increase in Nb content or the decrease of cooling rates, the microstructures of samples crystallized more significantly and the tensile fracture strength reduced. The fracture mode changed from shear fracture to brittle fracture, shear bands on the side surface decreased and vein stripes on the fracture surface were transformed into plastic pit structures as Nb content increased from 1 to 5%. The results may be helpful for the investigation of metallic glass composite mechanical behavior.

The influence of undercooling and a rare earth ( RE ) metal modifier on the solidification behaviour of hypereutectic Al–Si alloys was investigated by the electromagnetic levitation technique combining analysis of high-speed video and scanning electron microscopy. The results show that, for Al–Si alloys without RE addition, the morphology of the primary silicon transforms from dendrites to fragmented bulks and granular grains and then to equiaxed grains with increasing degree of undercooling; the nucleation frequency increases exponentially, while the average grain size decreases exponentially. The effect of undercooling on microhardness was also analysed. On the addition of an RE metal to the Al–Si alloys, the critical undercooling of the morphological transition and the grain size decrease, while the nucleation number and nucleation frequency increase. A comparison of the microhardness before and after the RE addition reveals that the microhardness is also improved at similar degrees of undercooling.

Slurry aluminide coatings were produced on Ti-based alloy Ti6Al4V in an argon atmosphere. A slurry with a newly-developed composition was applied, consisting of: powders of aluminium and silicon, a fused mixture of salts as an activator and a water solution of a soluble glass as an inorganic binder. The coating production parameters were varied: annealing temperatures of 900, 1000 and 1100°C were used for annealing times of 2, 4 and 6 h. The microstructure, chemical composition and phase composition of the coatings were studied. The correlation between the technological parameters and the coating thickness was also determined. The obtained coatings have a multilayered structure with a phase layout that is desirable from the viewpoint of mechanical and corrosion resistance performance: aluminium-rich phases on the surface and titanium-rich phases near the substrate.

Composites of ultrathin SnO 2 nanorods, ∼20 nm in diameter and ∼100 nm in length, intercalated with reduced graphene oxide nanosheets were synthesized by a simple one-step hydrothermal process. The electrochemical performance of the composites as electrode materials for supercapacitors was studied in 1 M Na 2 SO 4 electrolyte. The experimental results indicated that a maximum specific capacitance of 184.6 F g −1 could be obtained from composites at a current density of 100 mA g −1 , which was much higher than that of pure SnO 2 (62.4 F g −1 ). Furthermore, the composite exhibited excellent cycling stability (the specific capacitance still retained 98% after 6000 cycles when the scan rate was 50 mV s −1 ). The excellent electrochemical performance of the composites was attributed to the synergistic effect of SnO 2 nanorods and reduced graphene oxide, which makes up for the shortcomings of the individual components. These results indicated that the prepared composites are excellent candidates as electrode materials for high performance energy storage devices.

A facile and efficient combustion synthesis method is proposed to synthesize single-crystal silver nanoparticles (AgNPs) by using high chemical activity nano-aluminum powder as a reducing agent and silver oxide as an oxidant. The synthesized samples were characterized by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and ultraviolet–visible spectroscopy analysis. The results indicate that the as-prepared spherical AgNPs are of a single-crystal structure with an average size of about 40 nm. Moreover, the formation mechanism of AgNPs was also investigated by closed bomb test and high-speed video recording. It is observed that a large amount of energy which is released instantaneously makes the reaction occur at a rapid speed, so that the nucleation rate of AgNPs is greater than the growth rate. This study opens up a promising route for high-yield and high-quality AgNPs, as well as various other metal nanoparticles.

The isothermal sections of the Ag–Cr–Zr ternary system at 1000 and 750°C were experimentally established. A series of phases as well as their compositions and crystal structures were characterized on the annealed alloys by means of optical microscopy, electron probe microanalysis, scanning electron microscopy and X-ray diffraction. Our studies have detected four three-phase regions at both 1000 and 750°C. (Ag) was observed to exist in the solid state at 750°C, and in the liquid state at 1000°C. No ternary compound was found in this system. The maximal solubilities of Cr in (Ag), Ag in ZrCr 2 , and Cr in AgZr 2 at 1000°C are 4.12, 2.13, 0.91 at.%, respectively. At 750°C, the solid solubilities of Cr in AgZr, AgZr 2, and that of Ag in αZrCr 2 are determined to be about 1 at.%.

The paper presents the results of thermal studies of carbonized composites based on carbon filled nitrile-butadiene rubber. It was shown that carbon fibers (CF) increase the thermal conductivity of the composites and reduce their linear expansion, whereas carbon nanotubes (CNTs) practically do not change the linear expansion of the composites; instead, they effectively increase their thermal conductivity. The thermal conductivity of the composites with 25 PHR of CNTs exceeds the thermal conductivity of the composites reinforced with 25 and 50 PHR of CF. Thus, CNTs more effectively increase thermal conductivity than CF due to the appearance of additional heat transfer bridges. It was found that the composites have very high values of the storage modulus at room temperature (16–20 GPa), and it is about 4–5 GPa at 300°C, which is almost impossible for traditional polymer matrix composites.

A titanium sample was irradiated with Xe 168 MeV energy at fluences of 1 × 10 14 , 2.2 × 10 14 , and 5 × 10 14 ions · m −2 . Changes induced by the ions with irradiation were analysed with an atomic force microscope, scanning electron microscope, and X-ray diffractometer. The irradiation at a fluence of 1 × 10 14 ions · cm −2 leads to formation of hillocks with a density of 9 × 10 6 hillocks · cm −2 . An increase in the fluence value results in disappearance of the hillocks and formation of void–dislocation lines. The changes in the surface topography are accompanied by a change in the relative content of the hcp (α) and fcc phases. The substantial changes in the crystalline structure (increased content of the fcc phase, reduction of lattice constants, changes in stresses) resulted from irradiation at a fluence of 1 × 10 14 ions · cm −2 . Irradiation of massive titanium samples with xenon ions at a fluence of 168 MeV 1 × 10 14 ions · cm −2 at room temperature leads to a transition of ∼10% of the hexagonally closed-packed phase – hcp (α) into a face centred cubic structure (the fcc phase). Higher Xe ion fluences contribute to further reduction of lattice constants and crystallite sizes as well as reduction of the phase fcc content and lower values of stresses. It has been found for the first time that irradiation at higher fluences leads to partial recovery of the hcp (α) phase. Irradiation changes the texture of∼50% of the fcc phase crystallites.