Volume 106, Issue 6

Samples of SAF2906 duplex stainless steel were aged at 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C and 950 °C for 1 h, after high temperature solution heat treatment. The morphology of 𝜎-phase in aged SAF2906 duplex stainless steel was then studied by means of scanning electron microscopy, X-ray diffraction and transmission electron microscopy, and the chemical composition of the 𝜎-phase was determined using X-ray energy dispersive spectrometry. The results indicated that the 𝜎 precipitation in SAF2906 duplex stainless steel formed both inside δ-ferrite with high Cr and Mo concentrations, and along δ-ferrite/γ-austenite interfaces; however, relative to the SAF2205 and SAF2507 duplex stainless steels, the content of Cr in 𝜎-phase increased significantly up to 70 %, which was much higher than Fe in the same phase and became the major element of 𝜎-phase.

Recovery and recrystallization processes in cold-rolled Ni are investigated. While recrystallization and grain growth at temperatures above 700 K lead to a significant (threefold) decrease in micro-hardness, recovery at 600 K is shown to cause a measurable micro-hardness increase. This increase in micro-hardness is confirmed by atomic force microscopy observations of Vickers indents, and it is correlated with the vacancy-annihilation peak of the calorimetric signal. Diffusion of impurity–vacancy complexes to the dislocation cores and the resulting pinning of dislocations are surmised to contribute to the measured micro-hardness increase. Self-diffusion along interfaces in cold-rolled and partially recrystallized Ni samples is measured employing the radiotracer serial sectioning technique. Diffusion rates similar to those of general high-angle grain boundaries in well-annealed coarse-grained Ni polycrystals are observed. The results imply that the diffusion rate along the stationary recrystallization front, i. e. the interfaces separating the recrystallized and the as-deformed regions of the material, is similar to that along a general high-angle grain boundary.

The splitting phenomenon was detected in martensitic transformation of X12CrMoWVNbN10-1-1 steel using high resolution dilatometry under certain conditions. In-situ observation of austenite grain growth was carried out. Direct experimental results indicated that this splitting is not connected with the concentration gradient in the austenite resulting from the dissolution of carbonitrides during heating, but instead may be caused by the occurrence of abnormal grain growth.

The functionality of components can be improved by surface modifications such as micro-milling. The micro-milled structures (notches) can cause changes in the mechanical properties of the components. To be able to predict or prevent component failure, the influence of these micro-milled notches on the deformation behaviour of commercially pure titanium under tensile loading is investigated. Quasistatic tensile tests show that the tensile strength is increased by deeper notches and the fracture elongation of the specimens decreases. Metallographic analyses and nanoindentation tests of systematically interrupted tensile tests at different strain rates show the development of the plastic deformation and the inhomogeneous deformation behaviour of the material.

Three different approaches were used for the synthesis of niobium boride using a stoichiometric ratio of Mg–B 2 O 3 – Nb powder mixture: (i) thermal ignition, (ii) mechanochemical and (iii) a combined mechanical and thermal activation process. Phase transformation analysis and structural evaluation were carried out by means of differential thermal analysis techniques, X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. During the thermal synthesis, the reaction between Mg and B 2 O 3 partially occurred (liquid state reaction). The desired final phase (NbB 2 ) was poorly obtained. Phase analysis revealed that the NbB 2 phase was synthesized after 10 h of high energy ball milling. A pre-milling treatment of the powder mixture up to 2 h significantly reduced the reactions temperature (580 °C). Consequently, NbB 2 was prepared simultaneously involving magnesiothermic reduction of B 2 O 3 and reaction between elemental of Nb and boron. Mg 3 (BO 3 ) 2 , as major by-product, was formed in noticeable amounts during the thermal process, while its quantity significantly decreased during mechanochemical synthesis and completely eliminated by using the combined mechanical and thermal synthesis.

Phase equilibria in the ‘Cu 2 O’–‘Fe 2 O 3 ’–MgO – SiO 2 system in equilibrium with copper alloy have been investigated at temperatures between 1 075 and 1 250 °C. A high-temperature equilibration/quenching/electron probe micro-analysis technique has been used to accurately determine the liquid and solid compositions. Results on phase equilibria in the pyroxene and olivine primary phase fields are reported, providing new data on the effect of MgO on the liquidus temperatures in these fields. It was found that increasing MgO concentrations increases the liquidus temperatures in the olivine and pyroxene primary phase fields. The cuprite–delafossite–tridymite–pyroxene eutectic temperature of the quaternary system was determined to be between 1 075 and 1 080 ± 5 °C.

Sintering behavior in both axial and radial directions has been studied for both pure and doped 8 mol.% yttria stabilized zirconia (8YSZ) with different concentrations of lithium nitrate. Detailed measurements were conducted at different positions in radial and axial directions to study the spatial variation in sintering of the powder compact. Doping with lithium nitrate was used to study the effect of varying the sintering rate on the spatial variation of sintering. It was shown that variation in sintering strain in the axial direction is small, whereas a linear gradient is noticeable in the radial direction. The variation in radial strain peaks at relatively low strains (5 – 10 %) and reduces to a minimum at the end of sintering. Comparing average axial and radial strains showed that radial strain is consistently higher than axial strain, implying that sintering is anisotropic especially during the intermediate stage. Density is determined in two ways: (1) Calculated based on measured volumetric strain, (2) calculated assuming sintering is isotropic; the two quantities are compared. It is shown that density divergence can rise to as large as 15 % and nearly vanishes at the end of sintering.

The role of pH value and H 2 S concentration on the sulfide stress corrosion cracking of API 2205 duplex stainless steel was studied by means of slow strain-rate tests, polarization curve measurements and surface analysis. It was found that 2205 duplex stainless steel was highly susceptible to sulfide stress corrosion cracking as pH decreased and H 2 S concentration increased. When the pH value was 2.8, the susceptibility stayed at a low level in 500 ppm H 2 S concentration but there existed a critical H 2 S concentration between 500 ppm and 1 000 ppm, above which the susceptibility rapidly increased. The sulfide stress corrosion cracking resulted from hydrogen-induced cracking and exhibited the characteristics of hydrogen embrittlement. The boundary of a phase and γ phase promoted the initiation of stress corrosion cracking, while γ phase inhibited its propagation.

The wear resistance and corrosion behavior of titanium and 316L stainless steel were studied after depositing a composite coating of zirconia and hydroxyapatite using the cathodic plasma electrolytic deposition technique. No sign of adhesive wear was observed on coated samples due to the unique surface morphology of the treated samples, after a pin on disk wear test in Ringer’s solution. The average values of friction coefficient for treated titanium and steel samples were observed to be 0.2 which exhibited a decrease of about 4 fold for titanium and 7 fold for steel with respect to the substrates. As a result of potentiostatic cyclic tests in Ringer’s solution the corrosion current density of coated steel (2.47 × 10 –10 A cm –2 ) was found to be much lower than for the steel substrate (1.87 × 10 –7 A cm –2 ), treated and untreated Ti (6.60 × 10 –6 , 4.17 × 10 –7 A cm –2 ), indicated the increase in corrosion resistance of steel and decrease of it for titanium after coating.

Bimetals targeted for industrial applications are usually designed to combine the properties of two dissimilar metals, e. g. high strength, high hardness and low cost of steels with chemically resistive titanium. In this work the structure of one particular, but for industries very interesting, bimetallic system – Cr/Ni stainless steel clad with titanium has been studied. The material was prepared by explosion welding, a method capable of joining a wide variety of similar or dissimilar materials. Our analysis is based on X-ray micro-diffraction experimentation utilizing hard monochromatic X-rays focused down to micrometer size. In this way the bimetal in bulk form was analyzed and microstructural differences between the joined materials and their interface were determined.

The oxidation kinetics of a 2-D carbon fabric were studied in both an air and a pure oxygen environment. A simple approach using non-isothermal thermogravimetry and differential thermogravimetry was used. The results showed that the non-isothermal oxidation process of the carbon fabric exhibited self-catalytic characteristics in both environments. The oxidation mechanism and kinetic parameters were analyzed with differential and integral models. Evidence indicated that the oxidation process was a surface-induced phenomenon. The oxidation mechanism in oxygen environment was controlled by random nucleation, whereas in the case of air environment it was phase boundary reaction controlled. Different oxidation mechanisms and their correlations with kinetic parameters have been discussed in detail.

A mechanically alloyed mixture of Al-1 wt.% nano-alumina (n-Al 2 O 3 ) composite powders was produced using a planetary ball milling machine. Different milling times were applied to investigate the effect of milling time on the dispersion and microstructure of n-Al 2 O 3 particulate reinforcement within the aluminum matrix. A good homogeneous dispersion of n-Al 2 O 3 particulates was observed after 8 h of milling. Longer milling times had no significant effect on the dispersion and morphology of n-Al 2 O 3 particulates within the aluminum matrix because a steady state had been reached.

In this study, carbon nanotubes, which were produced by using chemical vapour deposition and mechano-thermal methods, were combined with zinc oxide matrix at different ratios and the effects of the nanotubes on electrical and optical properties of zinc oxide were examined. It was observed that electrical conductivity of zinc oxide at room temperature increased with the increase in the reinforcement rate both in carbon nanotube reinforced samples produced by using chemical vapour deposition and those produced by using the mechano-thermal method. Carbon nano-tube reinforced samples produced by using chemical vapour deposition yielded relatively better results for the same reinforcement rate.

The initial stage of shaping a component in metal injection molding is the injection process. Any defects occurring in this stage are transferred to the subsequent stages, namely debinding and sintering. To investigate the jetting phenomenon in the current study, 316L stainless steel feedstock was exploited as the material for fabricating thick cylindrical specimens with a diameter of 20 mm. Regarding the aforementioned thick specimens, injection at the normal velocity of 15 cm 3 s –1 resulted in critical defects, such as folding, weld lines and porosity. It was found that these defects were eliminated in the specimens injected at velocities as low as 1 cm 3 s –1 . Under such conditions, however, due to the increase in the injection time, the flow front rapidly solidifies with ensuing dramatic deterioration of component surface quality. Furthermore, the present study proposes a novel method, entitled inserted metal injection molding, with a double aim both for removing the jetting phenomenon and the consequent defects in injected components and for making the injection process possible at normal velocities allowing fabrication of the components with high surface quality.