Materials Science Forum Vols. 638-642

Abstract: In situ sulphur doping of cubic boron nitride (cBN) films was investigated by adding H2S into a plasma-enhanced chemical vapor deposition system. It was found that the nucleation of cBN was suppressed severely with a very low H2S concentration, while cBN could be grown continuously even at a H2S concentration as high as that of the boron source after its nucleation. Accordingly, S was incorporated into cBN films meanwhile keeping the cubic phase concentration as high as 95%. And a rectification ratio of approximately 10 5 was observed at room temperature for heterojunction diodes prepared by depositing S-doped cBN films on p-type silicon substrates, which suggests the possibility of an n-type-like doping. Moreover, 1500K post annealing of cBN films in H2 atmosphere was found to be able to release the residual compressive stress evidently. Thus, film adhesion strength increased markedly, and cBN films reached a thickness over 200 nm without peeling off from silicon and quartz substrates in air after 9 months.

Abstract: Modification of electric and magnetic properties of ZnO thin films was achieved by low energy Eu ion irradiation. The desired doping levels as well as the depth distribution of the dopant was controlled by the ion energy and the ion flux, following a simulated interaction between the doping ion and the host ZnO matrix of epitaxial ZnO (0001) films of approximatelly 200nm, grown on c-Al2O3 by PLD. The properties of the doped ZnO film depend in a critical way on the homogeneity of the doped ions throughout the entire film. The doping levels and the depth distribution of dopants were measured by elastic recoil detection analysis (ERDA). The results show a uniform depth distribution of Eu, as well as some level of Al diffusion from the substrate and the presence of some low levels of H, N and O. PACS code: 68.49Sf; 74.78Bz

Abstract: The work hardening and tensile behaviour of AISI 316L austenitic stainless steel deformed at temperatures between 600 and 1000°C has been investigated. The alloy has been heat-treated according two different roots: the first solutioning treatment was imposed at 1100°C and the second was designed at 875°C to reduce the dynamic ageing that occurs at temperatures up to 650°C in AISI 316L. In the solutionised material the work hardening data presented two linear regions: at high stresses the linearity has been described as the conventional Stage III of work hardening, whilst at low stresses the linearity has not been rationalised in any conventional stage of work hardening. In the second heat treatment alloy the work hardening data showed a single linear region at high stresses, whilst no linear stage occurred at low stresses. Therefore, the work hardening and tensile behaviour of AISI 316L has resulted to be significantly affected by the two different heat treatments and dynamic aging has been proved to influence work hardening behaviour well beyond the range of temperatures in which serrated yielding occurs.

Abstract: Joint alloying of 0.85 to 1.1 mass% C + N raises the strength and cold work hardening of steels with 18 to 19 mass% Cr and Mn each and allows to produce them at atmospheric pressure. A yield strength of 600 MPa is combined with a true fracture stress of almost 2500 MPa and ≈ 70 % elongation. However, there is a risk of carbide/nitride precipitation during quenching of thicker cross sections after solution annealing. The addition of Mo and Cu affects the corrosion resistance as well as the precipitation. Submersion test and current density/potential tests in several aqueous solutions characterize the corrosion behaviour. Tests on intercrystalline corrosion are used to detect the precipitation as a function of quenching rate. It is shown that the C/N ratio is of key importance in improving the properties.

Abstract: This paper concerns the phase transformation induced by heat treatment and cold rolling in four duplex stainless steel. In 2205 and 2507 , during the isothermal heat treatments, chi-phase precipitates as small particles at the ferrite/austenite boundaries, followed by sigma precipitation. At the lowest temperature the formation kinetic of chi-phase is favoured, with the increasing of time and temperature a progressive transformation of chi to sigma occurs and the kinetic of sigma is favoured. During continuous cooling, the chi -phase appears at low cooling rates. In low Ni grades the grain boundaries precipitation of chromium nitrides were detected , but no sigma and chi. In 2101 the austenite transforms to martensite both after cold rolling and quenching

Abstract: The martensite induced in three types of austenitic stainless steel, which indicate the different stability of the austenitic phase (γ), were estimated by the resistivity measured during the tensile deformation or compressive deformation at the temperatures 77, 187 and 293 K. The resistivity curves were strongly dependent on the deformation mode. The volume fraction of the martensite (α’) was also affected by the deformation mode. The ε phase, which is the precursor of the martensite and is induced from the commencement of the deformation, decreased the resistivity. However, lots of defects generated by the deformation-induced martensite increased the resistivity. The experimental facts and the results shown by the modified parallelepiped model suggested a complicated transformation process depending on each deformation mode. The results shown by the model also suggested a linear relation between the resistivity and the martensite volume at the region of the martensite formation. The fact denoted that the resistivity is mostly not controlled by the austenite, ε phase and martensite, but by the defects induced due to the deformation-induced martensite.

Abstract: The investigated super duplex steel belongs to the group of stainless steels which exhibits an austenitic-ferritic microstructure with a phase fraction of about 50% austenite and 50% ferrite. The alloy shows excellent general corrosion resistance as well as a good resistance against stress corrosion cracking, corrosion fatigue and erosion corrosion. Due to these outstanding properties, the super duplex alloy is used in components for sea or waste water applications and in the offshore and chemical industry. In addition, the investigated super duplex steel exhibits a good weldability and a high strength in comparison to pure austenitic steel grades In order to optimize the production process and to provide a suitable microstructure to satisfy the customer’s requirements multiaxial forging test at various temperatures were performed in the Gleeble Maxstrain system. The force and the displacement after each anvil stroke were measured and used to distinguish the mechanical behaviour in the forging process at different thermal conditions. The recorded force and displacement is also compared with a multi step compression test to show the influence of change in the deformation direction. A certain number of samples were exposed an in-situ heat treatment after the deformation while other samples were immediately quenched after the forging to preserve the deformed microstructure, which was measured by optical microscopy and electron microscopy. Furthermore, electron back scatter diffractions scans were applied to characterize the degree of dynamic recrystallization during the forging process.

Abstract: This study focuses on the entire shape of the  twin-precipitate in two phase stainless steel, Fe-25Cr-6Ni in terms of the interphase and elastic strain energies generated between the precipitate and matrix. An investigation of this precipitation is important not only in terms of microstructure control but also for improving the mechanical properties of materials. Firstly, the three-dimensional near-coincidence site lattice (3D-NCS) model, which is based on the atomic matching model, is applied for estimating the preferred habit planes by evaluating the result of geometrical atomic matching. Subsequently, the precipitate shape is determined from candidates of the preferred habit plane produced by the 3D-NCS model, and the elastic strain surrounding and within the precipitate is investigated by FEM analysis, which can calculate the anisotropic elastic strain depending on the shape of the precipitate. The results are compared with observed precipitate by TEM.

Abstract: Ferritic stainless steels (FSSs) have excellent corrosion resistance and good mechanical properties. Applications include heaters, houseware, and automotive exhaust systems. Alloying, even in small amounts, affects the recrystallization behavior of FSSs by selective dragging or pinning effects. In the present study, we present the main results regarding the recrystallization of a coarse-grained Nb-containing AISI 430 ferritic stainless steel. The material was processed by hot rolling and further annealed at 1250oC for 2 h to promote secondary recrystallization. Following, the material was cold rolled to a 80% reduction in thickness and annealed at 400-1000oC for 15 min. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to characterize the microstructure. Recrystallization of this steel begins at 700oC. Important orientation effects were observed in both as-rolled and annealed conditions. Recrystallization kinetics was strongly dependent on the initial orientation of the coarse grains. Results show that grain boundaries, transition bands and coarse Nb(C,N) particles are preferential sites for nucleation at moderate annealing temperatures.

Abstract: Using the full-potential-full-electron-linearized-augmented-plane-wave (FLAPW) method, we carried out ab initio calculations of the electronic structure for austenitic steels of different compositions. The total set allows to find a system of equations for obtaining the value and sign of the interatomic bond energy between Fe-Fe, Fe-Mn, Fe-Cr, Fe-Ni; Ni-Ni; Cr-Cr, Mn-Mn and Mn-Cr. It is obtained that both Cr-Cr and Cr-Fe bonds are attractive, however the Cr-Cr bonds are stronger. The interaction between interstitial carbon (nitrogen) and metallic atoms is calculated. The interstitial atoms change the metal-metal interaction between the nearest neighbors.