Materials Science Forum Vol. 879

Abstract: During hot rolling, austenite recrystallization determines the grain size evolution and the extent of strain accumulation, and therefore, it can be used to tailor the microstructure and mechanical properties of the final product. However, at the moment, models describing the recrystallization kinetics of high-Mn steels are scarce and they do not take into account the effect of the alloying elements present in these steels. The aim of this work is to provide a quantitative model for the determination of the static recrystallization kinetics valid for a wide range of high-Mn steel compositions. Softening data determined for steels with different Mn (20 to 30%), Al (0 to 1.5%) and C (0.2 to 1%) levels at different strain, strain-rate and temperature conditions were analyzed. Static recrystallization of the investigated high-Mn steels follow Avrami’s law, with n Avrami exponents which are temperature dependent and lower than those determined for low C steels. A dependence of the t0.5 (time for 50% fractional softening) on the carbon content has been also observed and it was incorporated into an equation for the calculation of this parameter.

Abstract: An Al-3% Mg-0.2% Sc alloy was subjected to annealing or solution treatment and further processed by HPT at room temperature. Microhardness measurements were taken along the middle-sections of the discs and they demonstrated that a very substantial hardening is achieved during HPT processing regardless of the initial heat treatment. Hardness values of ~200 Hv were recorded at the edge of the samples although the microhardness distribution remained inhomogeneous along the diameters of the discs after 20 turns of high-pressure torsion. In addition, the microhardness of the solution treated Al-Mg-Sc samples continued to increase with the equivalent strain imposed by the anvils even after 30 turns of HPT processing whereas the hardness at the edges of the annealed discs saturated after 10 turns. These differences in the hardness evolution are attributed to the higher Mg content in solid solution in the case of the solution treated samples and its influence on delaying the recovery rate of this aluminium alloy.

Abstract: The effect of the nature of the second phases in extruded Mg-TM-Y-CeMM (TM refers to a transition metal) alloys reinforced by intermetallic MgRE (RE refers to a rare earth element) compounds and LPSO-phase on their superplasticity has been evaluated between 300 and 400°C at the strain rate of 10-4 s-1. The data have been compared with those of alloys containing a similar volume fraction of the LPSO-phase. The results evidence that no superplasticity below 350°C was found in the alloys containing exclusively the LPSO-phase while the alloys containing both MgRE compounds and LPSO-phase deform superplastically by grain boundary sliding from 300°C. These differences are related to the different behaviour of MgRE compounds and LPSO-phase in the course of superplastic regime. MgRE compounds assist to accommodate the deformation more easily than LPSO-phase, reducing tendency to develop cavities and extending the time for the occurrence of necking. The size and volume fraction of the respective phases are critical in order to promote enhanced superplastic behaviour. Maximum elongations are attained in the alloys combining similar volume fractions of MgRE compounds and LPSO-phase in which their size is reduced to the maximum. An increase in the particle size of the second phases, especially in the case of the LPSO-phase, hinders the grain boundary sliding mechanism in the alloys.

Abstract: Texture and substructure changes in cladding tubes from Zr-based alloys under neutron irradiation in the atomic reactor during 6 years were investigated by use of X-ray methods. For this aim in the “hot” laboratory the automated X-ray diffractometer was set and the technique to handle with irradiated samples was elaborated. The residual radiation activity of samples correlates with integral texture Kearns parameters of tube, so that the texture weakens as the doze of neutron irradiation grows. Because of the initial substructure inhomogeneity of cladding tubes its changes under neutron irradiation are also inhomogeneous: relatively perfect grains of texture maxima become more distorted, whereas initially distorted grains of texture minima becomes more perfect.

Abstract: Cost-effective, modified, self-hardening Al-based alloy is proposed for automotive and aircraft industries. AlZn10Si8Mg is produced by permanent mould casting technique, and the obtained material is re-melted to refine and modify its microstructure and to develop a mechanically more efficient alloy. Ti as grain refiner, in form of TiB, and modifier, in forms of AlSr, were added to the basic alloy composition. Microstructural analysis and impact toughness evaluation were performed at room temperature and up to 180°C. The results obtained confirm that the proposed alloy reveal good properties in the considered temperature range, and demonstrate their applicability for structural components development in the aforementioned areas and in a wide range of temperature.

Abstract: Duplex Stainless steels (DSS) are biphasic austeno-ferritic steels in which the best combination of mechanical and corrosion resistance properties is achieved for almost equal volume fraction of the phases. These steels are classified according to their pitting corrosion resistance, assessed by the PREN index (Pitting Resistance Equivalent Number) which, although qualitatively, is widely employed as comparison. The present work is aimed to study the pitting resistance of four DSS grades (SAF 2101, 2304, 2205 and 2507) in the as-received condition and after isothermal aging in the critical range 750°C-900°C, to highlight the effect of secondary phases precipitation on the corrosion behavior. The materials were potentiodynamically tested in artificial seawater (pH7) at room temperature and the corresponding Critical Pitting Temperatures (CPT) were determined according to ASTM G150. Secondary phase precipitation mainly affected the lean duplex grades whereas the high-alloyed DSS were more stable even if large precipitation occurred.

Abstract: A simple mesoscale model has been developed for discontinuous dynamic recrystallization. Each grain is considered in turn as an inclusion, embedded in a homogeneous equivalent matrix, the properties of which are obtained by averaging over all the grains. The model includes: (i) a grain-boundary migration-equation driving the evolution of grain size via the mobility of grain boundaries, which is coupled with (ii) a single-internal-variable (dislocation density) constitutive model for strain hardening and dynamic recovery, and (iii) a nucleation equation governing the total number of grains by the nucleation of new grains. All the system variables tend to asymptotic values at large strains, in agreement with the experimentally observed steady-state regime.With some assumptions, both steady-state stress and grain-size are derived in closed forms, allowing immediate identification of the mobility of grain boundaries and the rate of nucleation. An application to Ni–Nb-pure-binary model alloys and high-purity 304L stainless steel with Nb addition is presented. More specifically on one hand, from experimental steady-state stresses and grain sizes, variations of the grain boundary mobility and the nucleation rate with niobium content are addressed in order to quantify the solute-drag effect of niobium in nickel. And on the other hand, the Derby exponents were investigated varying separately the strain rate or the temperature.

Abstract: Evolution of structure and properties of Ni under severe plastic deformation by equal-channel angular pressing (ECAP) and dynamic channel-angular pressing (DCAP) has been studied by transmission and scanning electron microscopy and microhardness measurements, and the differences in formation of submicrocrystalline structure under different deformation techniques have been revealed. The thermal stability of structure obtained by these methods is compared. It is demonstrated that under the DCAP processing the material is strengthened faster, by lesser number of passes, and microstructure’s thermal stability is somewhat lower after ECAP compared to that after DCAP, although after equal number of passes ECAP results in a more homogeneous microstructure.

Abstract: We present successful examples of CALPHAD thermodynamics-based precipitation simulations for three important alloy groups: Single-crystal Ni-base superalloy, austenitic stainless steel and hardenable Al-alloy. Underlying physical models for special features, such as, energies of diffuse interfaces between coherent precipitates and matrix, precipitation of incoherent particles at grain boundaries, evolution of excess vacancies during quenching and continuous aging and their role for metastable precipitate nucleation, are discussed.

Abstract: The present study is focused on the design and fabrication of novel functional 3D-hydrogel scaffolds for regenerative medicine. In order to critically analyzing the effect of the microarchitecture of 3D scaffolds for driving the cellular fate and diffusion of progenitor stem cells we have fabricate a number of scaffolds with different geometry, stiffness and composition. The physical characteristics of the scaffold determine indeed, as well the biochemical factors, the fate of the cells. We use an innovative composite material consisting of hydrogel with different molecular weight and with suitable accordion-like and woodpile structures in order to tailor stiffness and elasticity conferred to the final structure.These novel 3D bioinspired scaffolds were obtained by both single- (1PP) and two-photon polymerization (2PP) processing. In particular, 2PP scaffolds represent a great advantage with respect to previous achievements based on traditional methods. 3D-structures were fabricated with lateral resolution of some microns, allowing an advanced control of pore microarchitecture of defined tensile strength, and the inclusion of albumin microspheres with various functionalities. The morphological, biochemical and functional characteristics are discussed. Moreover, the effects of the structured hydrogel scaffolds on the proliferation and differentiation of adult stem cells is analyzed in view of the fabrication of portion of contractile cardiac muscle to be obtained In Vitro.