Volume 106, Issue 4

Alloys based on the titanium–tantalum system are considered for application as high-temperature shape memory alloys due to their martensite start temperatures, which can surpass 200 °C. In the present work we study the evolution of microstructure and the influence of creep on the phase transformation behavior of a Ti 70 Ta 30 (at.%) high-temperature shape memory alloy. Creep tests were performed in a temperature range from 470 to 530 °C at stresses between 90 and 150 MPa. The activation energy for creep was found to be 307 kJ mol −1 and the stress exponent n was determined as 3.7. Scanning and transmission electron microscopy investigations were carried out to characterize the microstructure before and after creep. It was found that the microstructural evolution during creep suppresses subsequent martensitic phase transformations.

In this study we investigated the microstructural changes after a variety of heat treatments of the quasicrystalline Al–Mn–Be–Cu alloy. In addition, we report on Vickers microhardness measurements and tensile-test results for the same materials. The samples were produced in a conventional manner, i. e., melting in an electrical resistance furnace in air and a gravitational casting process using a round copper die, which was in the form of a tensile-test specimen with a diameter of 5 mm. After the casting, some of the samples were just solution treated (annealed) and then quenched in water, while others were additionally aged (artificially – T6, or naturally – T4) or directly aged after the casting. In comparison to the as-cast state, the Vickers microhardness values of the aluminum-based matrix and the tensile properties of the samples decreased when just the solution treatment, T4 or T6 treatment was performed. The tensile properties also decreased after the heat treatments. A microstructural inspection revealed that the microstructural changes occurred already during the solution treatment, i. e., the formation of the phases Be 4 Al(Mn, Cu) and τ 1 -Al 29 Mn 6 Cu 4 on the approximant H-Al 4 Mn and quasicrystalline i-phase particles' edges and the occurrence of precipitates in the α Al matrix. The precipitates that would additionally contribute to the hardening of the alloy did not form. The directly aged samples showed little or no increase in microhardness values in comparison to the as-cast samples, but possibilities of θ″ precipitates being formed from the already saturated matrix after the casting could not be excluded. After all the heat treatments the quasicrystalline i-phase, as a primary and eutectic phase, was preserved.

Gibbs energy modeling for sulfur solving liquid and solid iron–chromium phases with body- and face-centered cubic structure has been carried out using a substitutional approach. Experimental data available from the literature on sulfur potentials in the temperature range 1 525 to 1 755 °C for the liquid metallic phase and 1 000 to 1 300 °C for the solid alloys have been taken into consideration. Recent thermodynamic evaluations of the Fe–S and Cr–S binary subsystems served as basis for the presented work. The obtained models allow a satisfactory reproduction of the majority of the sulfur potential data as well as the prediction of an isothermal partial section at 1 300 °C. Consistent embedding of the optimized Gibbs energies within a recent thermodynamic modeling of the complete Cr–Fe–S system is achieved.

This paper presents experimental results on four thermophysical properties of pure polycrystalline ruthenium samples over a wide temperature range. Specific heat capacity and specific electrical resistivity were measured from 250 to 2 500 K, while hemispherical total emissivity and normal spectral emissivity at 900 nm were measured from 1 300 to 2 500 K. All the properties were obtained by using the pulse calorimetry technique. The 200 mm long specimens were in the form of a thin rod, of about 3 mm in diameter. For necessary corrections, literature data on thermal linear expansion were applied. The results are compared with available literature data and discussed. The specific heat capacity and specific electrical resistivity measurements did not indicate any allotropic transformation of the samples over the entire temperature range.

Nanocrystalline and amorphous Mg 2 Ni-type Mg 20– x Y x Ni 10 ( x = 0, 1, 2, 3 and 4) electrode alloys were prepared by mechanical milling. The structures of the as-cast and milled alloys were determined by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The electrochemical hydrogen storage performances of the alloys were tested by an automatic galvanostatic system. The electrochemical impedance spectra, Tafel polarization curves and potential-step curves were plotted by an electrochemical workstation. The results indicate that a nanocrystalline structure can successfully be obtained through mechanical milling. The substitution of Y for Mg facilitates glass forming and leads to an obvious change in the phase composition. The substitution of Y for Mg dramatically improves the cycle stability of the as-milled alloys, while the mechanical milling more or less impairs the cycle stability of the alloys. The discharge capacity of the alloys first augments and then declines with increasing Y content and milling time. Furthermore, the high rate discharge ability, charge transfer rate, limiting current density and diffusion coefficient of hydrogen atomic all decrease with Y content and milling time increasing.

The effect of depositing Nb-rich Ti 1– x Nb x N coatings on the metallurgical characteristics and machining performance of tungsten carbide tools is investigated. The direct current reactive magnetron sputter deposited Ti 1– x Nb x N thin film was crystalline in the as-deposited state. The surface of the film is characterized by a dense granular structure with very few voids and lower roughness than the pristine tungsten carbide surface. Nanoindentation studies revealed that the Ti 1– x Nb x N coating enhanced the hardness and Young's modulus of the tungsten carbide tool to 35 GPa and 703 GPa, respectively, as compared to 20 GPa and 550 GPa respectively, for the uncoated tool. Scratch tests showed that the Ti 1– x Nb x N coating increased the adhesion strength on the tungsten carbide tool. Similarly, the tool wear, surface roughness and cutting force in turning an EN24 alloy steel component displayed significant improvement due to the Ti 1– x Nb x N coating. The minimum surface roughness, minimum tool flank wear and minimum cutting forces were predicted for Ti 1– x Nb x N coated tools based on the Taguchi experimental design.

Ultrasonic cavitation experiments using a piezoceramic-based apparatus, according to ASTM G32-2010, were performed on heat and thermochemically treated Cr – Mn low alloyed steel samples. The microstructure in annealed, carburized and tempered states as well as following a duplex treatment (carburized, surface induction hardening and tempering) was analyzed before and after the cavitation erosion tests. The results show the advantage of the duplex treatment, with a significant increase of up to 20 times of the cavitation erosion resistance compared to the annealed state and reveal that the main mechanism for surface deterioration is micro-cracking. The observations are important for the improvement of the behaviour for parts used in hydraulic equipment, for which the volume hardening following the carburization can be replaced by cost-efficient surface induction hardening treatments.

A simple chemical approach was employed to reduce graphene oxide in order to fabricate electrode coatings in close correlation with industrial production standards for supercapacitors. The morphology, structure, thermal stability and the residual oxygen functional groups in chemically reduced graphene oxide were analyzed. Cyclic voltammetry and charge/discharge measurements were employed to study the electrochemical performance of the coatings as a function of active material loading. The results showed an increase in the specific capacitance for chemically reduced graphene oxide-based coatings in comparison to commercial activated carbon, while the desired value needs to be optimized with respect to the conductivity of such materials.

Large-area and oriented silicon nanowire arrays have been successfully fabricated through a two-step metal-assisted chemical etching process at room temperature. The effects of key fabrication parameters (AgNO 3 concentration, Fe(NO 3 ) 3 concentration, and etching time) on the silicon nanowire nanostructure were carefully investigated. The Raman spectra of silicon nanowires prepared under different etching times have been recorded and analyzed. The porosity and length of the nanowire increases with the increase in AgNO 3 concentration from 0.002 mol L −1 to 0.1 mol L −1 , which indicates that the re-dissolved Ag + ions would work as the main oxidative species and catalyze the vertical and lateral etching of nanowires, leading to silicon nanowire growth and porous structure formation.

Poly(3-hexylthiophene) (P3HT) dense film and P3HT nanorods and nanotubes were synthesized on ITO and anodic aluminium oxide (AAO) substrate respectively. The growth mechanism of one-dimensional P3HT was investigated by changing the monomer concentration of 5 mM, 15 mM and 30 mM resulting in different polymer morphologies. By applying the transmittance spectra in the wavelength range of 1 100 to 400 nm, band gaps of 2.1, 1.9 and 1.55 eV were obtained for P3HT dense film, nanotubes and nanorods respectively. Tuning the band gap was achieved by changing the monomer concentration in electrochemical synthesis of P3HT on AAO substrate. Also the conductivity of 3.26 S · cm −1 for rod-shape P3HT and 1.06 S · cm −1 for dense film was calculated indicating better charge transport for nanorods.

Nano-SiC/Ni composite coatings on diamond surface were fabricated using an electro-co-deposition method. SiC nanoparticles with an average size of 50 nm were suspended in the plating solution and co-deposited together with Ni on diamond surface. The weight ratio (wt.%) of nano-SiC/Ni composite coating to the diamond was controlled by varying deposition time. The microstructure, composition and compression strength of an as-deposited single diamond particle was investigated. The results reveal that nano-SiC/Ni composite coating possesses a smoother, finer and denser microstructure as compared to a pure Ni coating. Moreover, the compression strength of an as-deposited single diamond particle gradually enhances with increasing nano-SiC/Ni wt.%.

In this study, the friction and wear properties of A356 alloy processed via semi-solid casting and conventional casting were experimentally investigated. Experimental results indicate that under the same experimental conditions the wear of the semi-solid cast A356 in mass does not exceed 85 % of that of the conventionally cast A356 alloy. The friction coefficient curve of semi-solid cast A356 is smoother than that of the conventionally cast A356. The friction coefficient of the semi-solid cast A356 is around 0.53 for stationary friction, less than the 0.59 for the conventionally cast A356. The improvement in the friction and wear properties is mainly a result of the microstructural change induced by semi-solid processing.