Band 110, Heft 10

Nickel-based superalloys are crucial materials for the development of aero-engine components, since their excellent properties can meet the demands of turbine disks. For the formation of such alloys, powder metallurgy is considered as the ideal method, due to the resultant uniform composition and structure, fine grain, high yield strength, and good fatigue performance. This paper provides a critical review of the development of powder metallurgy nickel-based disk superalloys, their composition, as well as the evolution of the nickel-based superalloys' microstructure in the past few decades. Moreover, the influence of various elements on the material properties and the three major defects of powder metallurgy superalloys are reviewed. The analysis indicates that these defects may be directly or indirectly caused by the quality of the powder. Therefore, the innovative powder techniques of electrode induction melting gas atomization and spark plasma discharge spheroidization are presented, in order to prepare ceramic-free and superfine nickel-based superalloy powders. The powders prepared by electrode induction melting gas atomization and spark plasma discharge spheroidization have been found to be beneficial in improving the properties of powder metallurgy nickel-based disk superalloys.

Global heat transfer computations were performed for the investigation of thermal stress in a Czochralski silicon crystal. The temperature distribution, thermal stress properties including maximum shear stress and von Mises stress distributions at two different axial crystal positions have been investigated with the help of heat transfer simulations. By analyzing the obtained results, the thermal stress maxima during the Czochralski growth process can be controlled by applying optimal crystal rotation of 8 rpm and counter crucible rotations of 5 and 15 rpm for a crystal position of 100 mm and 300 mm. This shows that by applying optimal crystal and counter crucible rotations throughout the growth process, the thermal stress maxima of a growing crystal can be reduced.

The aim of this work was to study the effect of the interaction between tools and workpiece on the plastic behavior and microstructural evolution of a carbon steel during hot deformation of a cylinder with diameter of 600 mm and height of 1 000 mm. The metallurgical processing was simulated applying the finite elements method. Temperature, deformation and strain rate gradients, recrystallized volume fraction, and grain size distribution were determined using this technique. The data obtained shown that the level of friction and the heat transfer between tools and material have strong influence on the deformation and microstructure gradients, creating large grain size heterogeneities.

Interaction of conical carbon nanotubes with lithium during their electrochemical treatment was studied by galvanostatic measurements. The presence of reversible and irreversible reactions during the Li insertion into conical carbon nanotubes was established. The structural changes occurring in the conical walls of the conical carbon nanotubes in consequence of the lithium intercalation were investigated by using X-ray diffraction. The results obtained show that the lithiation of conical carbon nanotubes is partially reversible and leads to a change in the diffraction peak profile (2θ = 26°) corresponding to the interplanar distance in conical carbon nanotubes. Such changes are associated with the lithium insertion into the interplanar spaces of conical carbon nanotubes.

The microstructures and properties of austenitic stainless steel with varying nitrogen content after welding thermal simulation were investigated. The results indicate that the nitrogen fraction has a considerable influence on the phase composition and properties. Low nitrogen fraction leads to formation of δ-ferrite. After a welding thermal cycle, the number of annealing twins of high nitrogen-containing steel decreases, M 23 C 6 precipitates, and fine M 23 C 6 are observed. For low nitrogen-containing steel, large-size M 23 C 6 and elongated δ-ferrite are formed, which deteriorate the ductility. Meanwhile, equilibrium phase calculations also reveal the inhibiting effect of nitrogen on M 23 C 6 and δ-ferrite. Furthermore, susceptibility to intergranular attack by 10 % oxalic acid solution was detected, and the synergistic effect of coherent twin boundaries, M 23 C 6 , and δ-ferrite leads to low resistance to corrosion for low nitrogen-containing steel.

It was shown that new high-manganese austenitic steels can be used as an alternative to conventional Ni–Cr austenitic steels especially in vacuum vessels. In this study, a nickel-free austenitic steel with chemical composition of 24 wt.% Mn, 10 wt.% Cr, 0.13 wt.% C and trace elements of Si, Ti, V, W, and Al was hot-rolled at 950 and 1100 °C to different strains. Microscopic observations, X-ray diffraction, tensile, impact and microhardness tests were performed to characterize the microstructural aspects and mechanical properties. A small amount of chromium carbide was found on the grain boundaries of this fully austenitic steel. The results showed that the microstructure and mechanical properties of hot rolled manganese austenitic steel are affected by the dynamic recrystallization and twinning phenomena. Also, by changing the grain morphology from equiaxed to elongated, strength and hardness increased, and the impact toughness and ductility were reduced.

In the present work, Al-based metal matrix composites (MMCs) have been developed by the powder metallurgy (PM) route using exfoliated graphite nanoplatelets (xGnP) as nanofillers and their microstructure, microhardness and sliding wear behaviour were investigated. The Al-based MMCs were developed by using nanostructured Al powder developed by mechanical milling for 25 h in a high energy planetary ball mill. The crystallite size and lattice strain of Al after 25 h of milling were found to be 32 nm and 0.383 %, respectively. Al-1, 2, 3 wt.% xGnP composites were developed by the PM route. A significant improvement in both the microhardness and wear resistance of the Al–xGnP up to addition of 3 wt.% of the nanofiller was observed. For Al-3 wt.% xGnP composite developed using as-milled nanostructured Al, a microhardness of ∼ 1 GPa could be achieved, which is ∼6 times higher than that of the pure sintered Al sample (∼ 169.7 MPa). Nanostructured Al also leads to enhancement of the wear behaviour as compared to as-received Al. The wear mechanism in the various composites was found to involve a combination of abrasion, ploughing, delamination, microcracks, deep grooves and pullout of nanofillers.

In the domain of incremental nanotechnology, surface mechanical attrition treatment has been seen as a significant technique to transform the surface of a material into a nano-crystalline layer, while preserving the surface chemistry unchanged. In the present study, a process was investigated to develop a nano-crystalline layer on the surface of titanium using an electromagnetic vibration system. The surface mechanical attrition treatment was carried out on commercially pure titanium for various durations (i.e., 30, 60, 90 and 120 min). The characterization showed that a maximum depth of 15 μm of nanocrystalline layer was obtained after 90 min of treatment. Further increase in time did not contribute towards development of any thicker layer. The crystallite size varied from 140 to 35 nm with increasing treatment durations. Tensile strength was increased from 645 MPa (untreated sample) to 711 MPa (120 min duration); however elongation was decreased by 43 %.

The oxidation behavior of porous Ni-16Cr-9Al alloys at 800 and 1 000 °C was studied using the isothermal temperature oxidation method. The differences in surface morphology, phase and pore structure between oxidized and non-oxidized materials were characterized by means of scanning electron microscopy, X-ray diffraction analysis and mercury intrusion porosimetry. The results revealed that the oxidation rate of the samples which were oxidized for 420 h at 800 °C was 0.012% 2 h −1 and the oxidation products were Al 2 O 3 and Cr 2 O 3 . The oxidation rate of the samples which were oxidized for 390 h at 1 000 °C was 0.415% 2 h −1 and the oxidation products were Al 2 O 3 , Cr 2 O 3 and Ni(Cr, Al) 2 O 4 . All the oxidation kinetics curves obeyed the parabolic law, exhibiting excellent high temperature oxidation resistance.

The present work focuses on the effect of sintering temperature on structural and magnetic properties of bulk MgFe 2 O 4 ferrites prepared by the solid state reaction method and sintered at 1 200 °C, 1 300 °C and 1 400 °C. Single phase cubic spinel structure was confirmed by X-ray diffraction. The bulk density, grain size and initial permeability increase with sintering temperatures. Decrease of hysteresis loops with low coercivity was observed in B–H loop studies. Saturation magnetization increases with sintering temperature and a higher value of magnetization at 5 K than 300 K was observed. In this communication the change in properties of bulk Mg-ferrite due to sintering temperature is explained in detail.