Papers by Author: Hugo Ricardo Zschommler Sandim

Abstract: Reduced activation ferritic-martensitic steels are considered for future applications in fusion power plants. The thermal stability of two steels (ODS-Eurofer and Eurofer-97) was evaluated in cold-rolled samples annealed below 800°C. These grades have similar chemical compositions except for the presence of nanosized Y₂O₃ particles in ODS-Eurofer steel. This nanosized dispersion is very effective to prevent recrystallization. On the other hand, full recrystallization occurs in Eurofer-97 steel when annealed above 700°C. The low volume fraction of recrystallized grains (< 0.1) in ODS-Eurofer steel annealed at 800°C can be explained by nucleation at prior grain boundaries and around large M₂₃C₆ particles. Further growth of these nuclei is impeded by Zener drag and concurrent recovery. Static recovery is the main softening mechanism in the ODS-steel below 800°C.

Abstract: To understand and model grain refinement in severe plastic deformation, some analysis of Nb single crystals has been carried out in previous work. To bridge the gap with normal polycrystalline materials, supplementary experiments on large polycrystals, deformed at moderate strains appear to be necessary to explain the grain subdivision step by step. In the present work, successive uniaxial compression tests have been carried out on a large grained Niobium polycrystal up to height reductions of 30% with small strain increments. Electron backscatter diffraction (EBSD) analysis was done after each compression step to characterize the evolution of orientation and microstructures. It is observed that a “rotation front” forms inside the grain and moves with increasing strain from one side to the other side of the grain. In one grain, this process results in a grain boundary affected zone in the vicinity of the grain boundary. Both static orientation evolution inside the grain and historical evolution of the average orientation have been studied, which indicates that the grain orientation rotates around one of the (110) poles at low strain.

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: Polycrystalline iron was deformed by eight ECAE passes using the route Bc to a total strain of 9.2. After deformation the material was annealed at temperatures up to 800oC. Scanning electron microscopy (SEM) and high-resolution electron backscatter diffraction (EBSD) were used to characterize both deformed and annealed structures. In the as-deformed state, the mean grain size is 650 nm and the volume fraction of high angle boundaries (VHAB) is 56%. Upon annealing there is a pronounced softening above 300oC. At the beginning of recrystallization, at about 400oC, the VHAB increases to 71%. The results indicate that discontinuous recrystallization is the main softening mechanism in severely deformed iron.

Abstract: Metals with a bcc crystalline structure such as Ti-13V-11Cr-3Al alloys have their physical properties significantly changed through the addition of interstitial elements such as oxygen and nitrogen. These metals can dissolve substantial amounts of interstitial elements forming solid solutions. Mechanical spectroscopy measurements constitute a powerful tool for studying interactions of these interstitial elements with other elements that make up the alloy. From these measurements, it is possible to obtain information regarding diffusion, interstitial concentration, interaction between interstitials, and other imperfections of the crystalline lattice. In this paper, Ti-13V-11Cr-3Al alloys with several amount of nitrogen, in a solid solution, were studied using mechanical spectroscopy (internal friction) measurements. The results presented complex internal friction spectra which were resolved in a series of constituent Debye peaks corresponding to different interactions and interstitial diffusion coefficients. Pre-exponential factors and activation energies were calculated for nitrogen in theses alloys.

Abstract: This work discusses on the structural evaluation of mechanically alloyed Ti-Nb powders. The Nb amount was varied between 20 and 50 wt-%. The milling process was carried out in a planetary Fritsch P-5 ball mill under Ar atmosphere. The structural evaluation was conducted by scanning electron microscopy, X-ray diffraction, and energy dispersive spectrometry. During ball milling it was noted an excessive agglomeration of ductile Ti-Nb powders on the balls and vial surfaces, and the final amount of remaining powders was then drastically reduced into the vials. This fact was more pronounced with the increased Nb amount in starting powders. Typical lamella structures were formed during ball milling, which were refined for the longest milling times, and fine and homogeneous structures were formed in Ti-Nb (Nb=20-50wt-%) powders. XRD results indicated that the full width at half maximum values of Ti peaks were continuously increased while that the crystallite sizes were reduced for longer milling times due to the severe plastic deformation provided during ball milling of Ti-Nb powders. However, the EDS analysis revealed the presence of Nb-rich regions in Ti-Nb powders after ball milling. The critical ball milling behavior of ductile Ti- Nb powders contributed for reducing the yield powder and increasing the structural heterogeneity.

Abstract: Metals that present bcc crystalline structure, when receiving addition of interstitial atoms as oxygen, nitrogen, hydrogen and carbon, undergo significant changes in their physical properties, being able to dissolve great amounts of those interstitial elements, thus forming solid solutions. Niobium and most of its alloys possess bcc crystalline structure and, as Brazil is the largest world exporter of this metal, it is fundamental to understand the interaction mechanisms between interstitial elements and niobium or its alloys. In this paper, mechanical spectroscopy (internal friction) measurements were performed in Nb-2.0wt%Ti alloys containing nitrogen in solid solution. The experimental results presented complex internal friction spectra and with the addition of substitutional solute, it was observed interactions between the two types of solutes (substitutional and interstitial), considering that the random distribution of the interstitial atoms was affected by the presence of substitutional atoms. Interstitial diffusion coefficients, pre-exponential factors and activation energies were calculated for nitrogen in the Nb-2.0wt%Ti alloys.

Abstract: PM 1000 is a nickel-based oxide dispersion strengthened (ODS) superalloy used for high-temperature applications. The primary recrystallization of a <100>-fiber textured coarsegrained oxide dispersion strengthened nickel-based superalloy (PM 1000) has been investigated. The annealing behavior of this alloy is quite complex. Even when annealing is performed at high homologous temperatures (e.g. 0.9 Tm, Tm is the melting point), recrystallization is partial. In order to understand such a behavior, the microstructure of specimens in both the as-received, deformed, and annealed conditions has been imaged in detail using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron backscatter diffraction (EBSD). In the annealed state we observe a significant volume fraction of tiny crystals in the interior of the recovered grains. These tiny grains are elongated and grow mostly along the existing low angle dislocation boundaries (anisotropic growth). In the present paper we propose a twinningassisted nucleation mechanism to clarify their origin during recrystallization.

Abstract: High-purity niobium single crystals were deformed by equal-channel angular extrusion (ECAE) at room temperature to an equivalent Von Mises strain of about 1.15. Deformed samples were annealed in vacuum from 500 to 800oC for 1 hour to investigate their microstructure evolution. The microstructure of deformed and annealed samples was characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Vickers microhardness testing. The deformed structure after one ECAE pass is rather inhomogenous and consists of parallel sets of coarse shear bands whose spacing varies from one region to another in the cylindrical billet. the microstructure within the shear bands consists of elongated subgrains with sizes below 3 μm and lamellar boundaries. The remaining non-sheared regions display a coarser subgrain structure. Recrystallization is virtually absent in samples annealed at 500oC for 1 hour. Nucleation begins mostly within shear bands. The new grains with sizes ranging from 10 to 50 μm are arranged in clusters rather than being homogenously distributed. The recrystallized volume fraction also varies from one region to another indicating an inhomogenous distribution of stored energy. At 700oC, recrystallization is complete after annealing for 1 hour resulting in a structure with a mean grain size of about 100 μm.

Abstract: Porous Ti-Nb alloys are promising candidates for biomedical applications. In the present study, alloy powders containing 60 wt-% Nb were prepared by high-energy milling of Nb, Ti, and/or TiH2 powders. The high-energy milling process was carried out in a planetary ball mill. The starting and as-milled materials were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). Elemental (Nb, and Ti) and TiH2 powder mixtures with composition Nb-40wt%Ti were mechanically alloyed for 2 to 30 h. The formation of a BCC Nb(Ti) solid solution by high-energy milling using elemental Ti powder to produce Nb-40Ti was observed after milling for 30 h. A HCP-Ti solid solution was formed after milling for 30 h due to the partial decomposition of titanium hydride powder mixture during high-energy milling.