Band 106, Heft 10

It is well known that a good crystallographic compatibility between austenite and martensite in Ni–Ti-based shape memory alloys results in narrow thermal hystereses (e.g. Ball and James, Arch. Ration. Mech. Anal., 1987). The present work suggests that a good crystallographic fit is moreover associated with a small mechanical hysteresis width, observed during a forward and reverse stress-induced transformation. Furthermore, shape memory alloys with a good crystallographic fit show smaller transformation strains. The results obtained in the present study suggest that these correlations are generic and apply to binary Ni–Ti (with varying Ni contents) and quaternary Ni–Ti–Cu– X ( X = Cr, Fe, V) alloys. For binary Ni–Ti, it was observed that Ni-rich compositions (good lattice fit) show a lower accummulation of irreversible strains during pseudoelastic cycling.

The effects of external compressive stress below the yield strength of austenite on the kinetics of low temperature bainitic transformation and microstructure in a superbainite steel were studied through dilatometry, scanning electron microscopy and X-ray diffraction analysis simultaneously. The results show that even under a relatively low isothermal temperature of 300°C, compressive stress less than yield stress can clearly accelerate bainitic transformation because of the increase in total driving force for bainitic transformation and anisotropic transformation strain. Bainite ferrite becomes longer, the orientations of bainite sheaves become more uniform and the transformation strain becomes anisotropic under external compressive stress because of variants selection. The amount of bainite increases while the amount of martensite decreases with the application of external stress.

The compressing pressure and microstructure and their relationship to mechanical and physical properties have been investigated in tungsten powder compacts infiltrated with copper. In this research, high pressure within the range of 250 to 550 MPa was used to produce high density green specimens (60–80%) by cold isostatic pressing while sintering was carried out in a hydrogen atmosphere. The tungsten skeletons were infiltrated with molten copper at 1300 °C. Scanning electron microscopy and tensile tests were conducted with due allowance for the participation of contiguity in microstructure. It was illustrated that the application of high compressing pressure has a direct influence on the tensile strength and conductivity. The measured tensile strength of prepared composites in this study was in the range of 400–640 MPa.

Three-dimensional dendritic growth of an Al-4 wt.% Cu alloy during solidification in the presence of forced flow was simulated by using a three-dimensional cellular automaton model. The three-dimensional Lattice–Boltzmann method was used in computing the flow dynamics. The effects of forced flow on the growth Péclet number, dendrite tip selection parameter, and secondary arm spacing were investigated. The simulated relationships of growth Péclet number and dendrite tip selection parameter with the forced flow are in good agreement with the Oseen–Ivantsov solution. The results indicate that with the increase in forced flow velocity, the secondary arm spacing first increases and then decreases.

Lubricated friction and wear properties of Zn-25Al-3Cu alloy were investigated in comparison with SAE 65 bronze over a range of contact pressure (1–8 MPa) and sliding distance (0–80 km) using a block-on-disc type test machine. The friction coefficients of the alloy and the bronze decreased, but their working temperature and wear volume increased with contact pressure. Their friction coefficient and the working temperature showed opposite changes with sliding distance before they reached steady-state levels. However, the wear volume of both the alloy and the bronze increased continuously with sliding distance. Although the hardness and compressive strength of the alloy were lower than those of the bronze, it exhibited higher wear resistance. This was related to their wear behaviour since smearing played a great role in the wear of the alloy while abrasion dominated the wear of the bronze.

The growth mechanism and electrochemical properties of an oxide film on AISI 304 grade stainless steel were studied in 0.01 and 0.1 mol L −1 fluoride solutions with different pH values (4.5, 5.5, 6.5) by means of electrochemical techniques. The anodic growth and stability of the oxide film on the stainless steel were characterized using cyclic voltammetry. Potentiodynamic analysis suggests that the oxide film growth occurs according to the high-field mechanism. Electric field strength, high-field growth exponential law constants, ionic conductivity through the film and half jump distance were determined. The electrochemical properties of the oxide film, formed spontaneously at the open circuit potential, were studied using electrochemical impedance spectroscopy. The results showed that the fluoride concentration has more considerable influence on the dissolution rate and the resistance of the oxide film than the pH .

A methodology to quickly prepare CoFe nanofilms with large magnetic permeability and resonance frequency from simple salt solutions is demonstrated. As the microwave properties of thin films are largely determined by their surface morphology and composition, CoFe film with unique morphology and composition is proposed based on theoretical analysis and subsequently electrodeposited with suitable parameters. This approach reveals that Fe x Co 1 - x (0.3 < x < 0.5 in atomic ratio) films consisting of sub-30 nm spherical nanoparticles, even in the form of bigger aggregated-nanoparticles, usually show a low coercivity (≤4240 A · m −1 ), moderate magnetic anisotropy (2900–8580 A · m −1 ), and high magnetic moment (≥1.4 T), permeability (>200) as well as resonance frequency (>1 GHz). Further experimental analyses show root causes of the phenomena. This methodology also provides useful references to rapidly identify microwave properties of thin films from their surface morphologies and main electrodeposition parameters.

A new piezo-absorbing composite was fabricated with silicone rubber and spherical or flaky carbonyl iron particles (CIPs). The complex permittivity and permeability of the composites, under variable compressive strain, were measured using a vector network analyzer in the frequency of 2–18 GHz and the reflection loss ( RL ) calculated to test the piezo-absorbing effect. The results show that, under compressive strain, the complex permittivity decreased slightly because of the breakdown of the original conductive network, whereas the complex permeability increased, but the enhancement mechanism was different. In the case of spherical CIPs/rubber composite, this was caused by the surface effect of the absorbents, and in the case of flaky CIPs/rubber composite by the orientation of the flaky particles. As the compressive strain was applied, the RL value and the absorbing band changed slightly with thickness of 1 mm or 2 mm. As the applied strains decreased the thickness of the composites, the absorption band ( RL < −10 dB) broadened and the minimum RL decreased.

A ceramic-matrix nanocomposite based on polyaniline emeraldine base and aluminum oxide (PANI-EB/α-Al 2 O 3 ) was obtained by in-situ polymerization. X-ray diffraction pattern presented peaks related to both materials. The level of crystallinity was estimated at about 53%. The average crystallite sizes of PANI-EB and α-Al 2 O 3 were found to be ∼40 Å and 570 Å, respectively. Scanning electron microscopy showed polymerization over ceramic particles. Fourier-transform infrared spectroscopy suggested physical deposition. The electrical conductivity of the PANI-EB/α-Al 2 O 3 nanocomposite was decreased by a factor of 80 when compared with that of pure PANI-EB. Therefore, the polymeric reinforcement and the ceramic matrix maintained their original structural features, but the electrical conductivity in the nanocomposite was reduced.

Four series of Mo 2 NiB 2 -based cermets with Cr 3 C 2 addition of between 0 and 7.5 wt.% in 2.5 wt.% increments were studied by means of scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffractometry. The transverse rupture strength and hardness were also measured. It was found that Cr 3 C 2 completely dissolved in Mo 2 NiB 2 -based cermets. Cr 3 C 2 addition improved the wettability of the Ni binder phase on the Mo 2 NiB 2 hard phase, which resulted in a decrease in the porosity and an increase in the phase uniformity. The cermets with 2.5 wt.% Cr 3 C 2 content showed relatively fine grains and almost full density. A high Cr 3 C 2 content resulted in the formation of M 6 C ( M = Mo, Cr, Ni) phase. In addition, energy dispersive X-ray spectroscopy results showed that the content of Mo in the binder decreased with increasing Cr 3 C 2 content. The cermets with 2.5 wt.% Cr 3 C 2 addition exhibited the highest transverse rupture strength of 2210 MPa, whereas the cermets without Cr 3 C 2 addition exhibited the highest hardness.

In this research, an examination of the effects of fraction of particulate nano-sized alumina (0, 2, 5, 10, 15 and 20 wt.%) on the mechanical properties and fracture mechanism of Al–Al 2 O 3 nano-composite has been carried out. The samples were uniaxially tested at room temperature and sample sections were analyzed using scanning electron microscopy. The results showed that the fracture mode of the composites was changed by volume fraction of the reinforcement particles. The main damage mechanisms observed in the specimens were clustering and microvoid formation, crack branching and crack deflection.

In this article a new method of synthesizing ultrafine powder of vanadium carbide V 8 C 7 by a combination of classic liquid-phase precipitation on a carbon support and low-temperature microwave heat treatment is proposed. An entire spectrum of intermediates obtained during thermolysis, reduction, and carbidization of precursors to the final product has been presented. The structure, morphology and distribution according to the particle size, phase structure and specific surface area of samples has been shown.