Volume 101, Issue 6

The effect of temperature and strain rate on the mechanical properties of three Fe – Mn austenitic steels with Mn contents of 20 mass.% or above has been studied by tensile tests performed at strain rates ranging from 3 × 10 −3 to 200s −1 and at temperatures between −160 and +140 °C. In these steels mechanical properties and strain hardening behaviour result from the competition between different deformation mechanisms including homogeneous or inhomogeneous dislocation glide, strain induced twinning or martensite formation. Thus, the microstructure evolution during deformation was investigated. For two of the steels, a relation of the deformation mechanisms (twinning and dislocation glide) at room temperature with the grain size was found. On the other hand, for the third steel at temperatures lower than 60 °C martensite is progressively formed during deformation. For the three steels, a decreasing volume fraction of twins or martensite with increasing testing temperature was found leading consequently to a decrease in flow stress and strain hardening.

Mesoscale strain hardening phenomena and the effect on the macroscale behavior (yield under tension, earing in deep drawing) are investigated in Fe-3 % Si and Cu. Fe-3 % Si shows a smooth upper and lower yield point depending on grain size and sheet thickness which can be attributed to a lack of mobile dislocations. A free surface effect (reduced strain hardening of near-surface regions) is found for sheets of 300 μm and 960 μm thickness, but limited to small and medium strains. By means of the new orientation gradient mapping, which is correlated with local strain hardening, the local deformation microstructure is investigated in incremental tensile tests and incremental deep drawing for Fe-3 % Si and Cu. The orientation gradient distribution significantly depends on the material. Based on these results phenomenological descriptions of the microstructure at the mesoscale are proposed. From orientation gradient mapping, geometrically necessary and statistical orientation gradients are evaluated which agree well with results of transmission electron microscopy investigations of geometrically necessary boundaries and incidental dislocation boundaries. A constitutive equation correlating orientation gradient mapping and local dislocation density is proposed.

Longevity of materials, especially technological parts, is influenced by microstructure development during long-time annealing of such parts during their industrial service. Important relations between the volume fractions of phases and the time of service, especially the precipitation of intermetallics need to be revealed. The present study describes the time dependence of the structure development of two corrosion resistant steels with chromium content of 27 and 32 wt.% and various additions of nickel, molybdenum, manganese, silicon, copper and nitrogen. The microstructure was studied on samples annealed at 700 °C for 500, 3 188 and 6 170 h, revealing the kinetics of the equilibrium process. The homogeneity of samples was checked before heat treatment. The experimental microstructures are compared with the predicted phase composition. The effect of using different databases is also discussed.

An asymmetry in tensile–compressive yield behaviour of extruded 99.95 % pure magnesium at two different strain rates was investigated. The yield asymmetry was observed at both strain rates, and the compressive yield strengths were over 50 % lower than the corresponding tensile yield strengths. Examinations with transmission electron microscopy revealed that twin bands were observed in compression specimens, and the twinning system was {0112}<0111>. Fibre structures (or textures) were examined using both X-ray pole-figures, which enabled one to identify that the extrusion axis was parallel to <1010> directions. The Schmid factors on slip systems and the twinning system were estimated, i. e., the Schmid factor for {1101}<1120> slip system was maximum in tension, and that for the twinning system was highest in compression. The origin of the yield asymmetry in extruded pure magnesium is associated with both the highest Schmid factor for the twinning system in compression and the critical resolved shear stress for twinning, which is lower than that for slip.

A neutron diffraction study on liquid Al – Ni alloys close to the liquidus temperature is presented. By applying an aerodynamic levitation technique associated with CO 2 laser heating, it was possible to investigate the high-melting alloys around the equi-atomic composition, too. We observe a change in the local chemical atomic arrangement from preferred environing of like atoms in Al-rich alloys to hetero-coordination in the Ni-rich alloys. It is furthermore demonstrated that the apparent pre-peak can be attributed to a distinct Ni–Ni short-range order.

An X-ray diffraction study on liquid As – Se alloys is reported. The average co-ordination number and the mean distance of nearest neighbour atoms obtained from the diffraction experiments and their relation to the composition of the alloy are in good agreement with earlier neutron diffraction results. After applying a model of micro-heterogeneous alloys it is suggested that As – Se alloys with less than 40 at.% As consist of structural units of As 40 Se 60 stoichiometry in a matrix of Se-like structure. An assumption on the local arrangement of the nearest neighbours is deduced.

Current technology provides means of fabrication of spherical micro-particles, either hollow or compact, for all sorts of engineering materials. Such spherical particles can be further embedded into another material to build up either random dispersions or close-packed arrays, according to the production route and the degree of anisotropy intended for the ultimate composite material. In this study, a simple analytical formula for the composite stiffness is derived from an early micromechanics model to describe the actual reinforcement of ductile matrices by a random dispersion of uniform spherical ceramic particles. Predictions from this model are checked against some other relevant models, and specific features arising from its theoretical derivation are pointed out. The basic model can also predict the stiffness of syntactic composites, whose reinforcements are hollow microsphere dispersions. An application of this new model is demonstrated for the assessment of the ductility of brittle composites reinforced by compact spherical particles.

The present study aims at the computer-aided design of suitably functionalized oxide surfaces for the integration of nanotubes into multi-purpose nano-electronic devices. The adsorption of the nucleotide cytidine monophosphate on the rutile (110) surface is investigated by density-functional-based tight-binding calculations. The nucleotide favors anchoring with two oxygen atoms of its phosphate part. Adsorption occurs preferentially at two neighboring five-fold coordinated Ti atoms along the [001] direction, thus opening a pathway to an ordered adsorption of nanotubes along [001]. The electronic densities of state show that the aromatic part of the cytidine residue remains unchanged upon adsorption on rutile. This implies that no significant changes occur in the nanotube binding capacity by -stacking of the aromatic part, hence, nucleotide-functionalized oxide surfaces are ideal substrates for the ordered, stable and electronically and chemically inert immobilization of nanotubes.

Samples of Ti-8Si, Ti-13.67Si and Ti-23Si (at.%) alloys treated using different cyclic heat treatment processes have been investigated using X-ray diffraction, optical microscopy and scanning electron microscopy. The microstructures, Vickers hardnesses and compressive properties of the as-cast and annealed samples are compared. For the Ti-8Si and Ti-23Si alloys, only the eutectic microstructure in the as-cast alloy is obviously altered after annealed at 900 °C (where β-Ti is stable) for 20 min and then cooled in air with six cyclic treatments, or firstly treated at 900 °C (where β-Ti is stable) for 20 min followed by quenching in air, and then treated at 700 °C (where α-Ti is stable) for 40 min followed by cooling in furnace, with six cycles) cyclic heat treatment. The microstructure of Ti-13.67Si alloy changes from the features of clubbed and larger sized Ti 5 Si 3 grains to finer grain size during different cyclic heat treatment. Vickers hardness of all alloys is significantly increased in the cyclic processing. For the Ti-23Si alloys, particles in the eutectic structure of as-cast alloy were spheroidized after cyclic heat treatment, resulting in increasing of both Vickers hardness and compressive properties.

Temper free induction bending can have a significant impact on the mechanical and corrosion-resistance properties of thick-walled super duplex pipe. Sections from induction bent pipe were solution annealed at 1 120 °C for various periods to sequentially eliminate different deleterious phases. By comparing the results of modified DL-EPR analysis, optical microscopy, mechanical and corrosion testing before and after the various heat treatments, it was shown that the deterioration in properties was due to the precipitation of α′. It was also possible to determine the order in which α′, γ 2 and Cr 2 N had precipitated, and thus to conclude that the unfavourable metallurgy was the result of inadequate post-bend cooling. A likely thermal history for the pipe is proposed that leads to the description of a modified temper free process.

The steady state creep in a rotating disc having variable thickness and made of isotropic aluminium–silicon carbide particulate composite has been investigated. Using threshold stress based creep law, the general expressions for stresses and strain rates in the discs have been obtained. The expressions are used to calculate the distributions of stresses and strain rates in different discs viz (i) disc having constant thickness (ii) disc having linearly varying thickness and (iii) disc having hyperbolically varying thickness. The volume of different discs is kept the same. The study revealed that the stresses and strain rates in the disc could be reduced to a significant extent by varying the disc profile. The linearly varying disc exhibits the lowest values of stresses and strain rates compared to those observed in hyperbolic or uniform thickness disc.

Torsion tests in the temperature range 900 to 1 200 °C and strain rates varying from 0.005 to 5s −1 for a high nitrogen steel were analyzed. The influence of various conversion methods from torsion tests on the stress – strain curves and on the Garofalo equation parameters as a function of strain was studied for this steel. In addition, the creep behavior of the high nitrogen steel was analyzed using the Garofalo equation. The high values of the activation energy and stress exponent using the peak stress are attributed to the microstructure that changes strongly during the first stages of deformation. The analysis of the Garofalo parameters allowed determination of a steady state at strains of about 2.5. At this strain, the values of the activation energy and the stress exponent correlated well with slip creep mechanisms operating in other reinforced steels at high strain rates.

It is well known that ferritic cast irons with 4 to 5 wt.% Si provide good service and low cost in many elevated-temperature applications. In terms of cast iron microstructures, both spheroidal graphite and compacted graphite cast irons are currently used to produce engine exhaust manifolds and turbocharger housings. Between the two graphite morphologies, the mixed graphite microstructure has been proposed and evaluated in the aspects of molten metal treatment, solidification characteristic, microstructural stability, standard tensile testing, long-term hot oxidation and thermal cycling testing. The effects of graphite morphologies on the material properties are presented. The samples with mixed graphite structure showed an improved ductility at medium temperatures and thermal fatigue resistance.

The present study is focused on the effect of tool rotation speed on the microstructure and mechanical properties of butt welded dissimilar pure copper/brass alloy plates by friction stir welding. The joint performances were evaluated by microhardness measurements, tensile and bend tests. The results show that the grain size and the onion ring formation in the weld nugget zone and the mechanical properties of the joints were affected by tool rotation speed. It was found that the tensile strength of the joints was lower than that of the copper and brass base metals, while the bend strength was higher than that of the copper base metal and lower than the brass base metal. The highest mechanical properties were obtained from a tool rotation speed of 800 rpm at a travel speed of 22 mm min −1 . The average microhardness values in the weld nugget zone increased with decreasing the tool rotation speed.

The thermoelectric materials of CoSb 3 -based Skutterudite compounds were prepared by spark plasma sintering. The micrographic images, the elemental compositions and the crystalline structures of the samples were characterized by scanning electronic microscopy, energy spectrum analysis and X-ray diffraction respectively. The molar heat capacities of CoSb 3 , (Co 0.95 Ni 0.05 )Sb 3 and Er 0.1 CoSb 3 were measured by modulated differential scanning calorimetry from 190 to 600 K. The results show that both the transition element Ni and the rare-earth element Er can decrease considerably the molar heat capacities of CoSb 3 -based compounds.