Papers by Alexandru Stoica
Journal of Applied Crystallography, 2014
The problem of calculating the inverse pole figure (IPF) is analyzed from the perspective of the ... more The problem of calculating the inverse pole figure (IPF) is analyzed from the perspective of the application of time-of flight neutron diffraction toin situmonitoring of the thermomechanical behavior of engineering materials. On the basis of a quasi-Monte Carlo (QMC) method, a consistent set of grain orientations is generated and used to compute the weighting factors for IPF normalization. The weighting factors are instrument dependent and were calculated for the engineering materials diffractometer VULCAN (Spallation Neutron Source, Oak Ridge National Laboratory). The QMC method is applied to face-centered cubic structures and can be easily extended to other crystallographic symmetries. Examples include 316LN stainless steelin situloaded in tension at room temperature and an Al–2%Mg alloy, substantially deformed by cold rolling andin situannealed up to 653 K.
Scientific Reports, 2013
The glass-forming ability (GFA) of alloys with a high-solvent content such as soft magnetic Fe-ba... more The glass-forming ability (GFA) of alloys with a high-solvent content such as soft magnetic Fe-based and Al-based alloys is usually limited due to strong formation of the solvent-based solid solution phase. Herein, we report that the GFA of soft magnetic Fe-based alloys (with .70 at.% Fe to ensure large saturation magnetization) could be dramatically improved by doping with only 0.3 at.% Cu which has a positive enthalpy of mixing with Fe. It was found that an appropriate Cu addition could enhance the liquid phase stability and crystallization resistance by destabilizing the a-Fe nano-clusters due to the necessity to redistribute the Cu atoms. However, excessive Cu doping would stimulate nucleation of the a-Fe nano-clusters due to the repulsive nature between the Fe and Cu atoms, thus deteriorating the GFA. Our findings provide new insights into understanding of glass formation in general. M etallic glasses with a high solvent content have shown unique physical, magnetic and mechanical properties, which make them attractive for widespread uses as energy and aerospace materials 1-7. For example, Fe-based metallic glasses such as commercial FINEMET and NANOPERM alloys exhibited a great potential to substitute the silicon steel for saving energy in the electric appliances due to their excellent magnetic softness and low core loss 8-10. Lightweight Al-based metallic glasses show the good prospect in aerospace application due to the promising mechanical properties 5-7. To further extend the application range, it imminently demands to improve the glass-forming ability (GFA) simultaneously based on attaining the excellent soft-magnetic properties or outstanding mechanical properties in high-solvent based alloys 11-13. Nevertheless, the truth is that the high Fe contents in Fe-based amorphous alloys necessary for ensuring large saturation magnetization will inevitably induce structural heterogeneity, usually the a-Fe nano-clusters precipitated in the amorphous matrix, which reduces the GFA 14-18. Thus, obtaining high saturation magnetization in the Fe-based amorphous alloys is always at the expense of the GFA, as shown in the FINEMET and NANOPERM alloys 8-10. Similarly, high concentration of the solvent in Al-based amorphous alloys normally also induces the strong formation tendency of the a-Al clusters and then dramatically deteriorates the GFA 5-7. Therefore, understanding glass formation and then finding a suitable route to suppress occurrence of the structural heterogeneity in the amorphous matrix will be very crucial for enhancing GFA for such high-solvent based alloys, particularly in the Fe-and Al-based amorphous alloys. Recently, the element copper, which has a positive mixing enthalpy with Fe (113 kJ/mol), was often added into the Fe-based amorphous alloys to enhance the soft-magnetic propertie 14-20. It was argued that copper additions can stimulate nucleation of a-Fe and effectively refine the a-Fe grain sizes to 20 nm or below. Although these Cucontaining Fe-based metallic glasses have a large saturation magnetization close to that of the conventional Fe-Si alloys, their GFA is reduced and the maximum attainable size is limited to the micrometer level 14-21 , which is consistent with the results in the Fe-Co-based alloys 22-24. In our study, however, we surprisingly found that minor doping of the element Cu is actually very effective in destabilizing the a-Fe nano-clusters and promoting the GFA. As a result, soft-magnetic Fe-based bulk amorphous alloys with high GFA and large saturation magnetization simultaneously have been successfully developed. In this report, the relevant experiments are described in detail and the correlation between the nano-clustering and glass formation is discussed. Moreover, the strategy for enhancing the GFA by minor doping the elements, which have positive enthalpy of mixing with the main constituents, is also extended into Al-based amorphous alloys.
Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment, 2009
The neutron optics design and performance for VULCAN—the SNS engineering diffractometer—is presen... more The neutron optics design and performance for VULCAN—the SNS engineering diffractometer—is presented. A gradually tapered focusing neutron guide is used to deliver desired beam divergence at sample position. Selection of instrument resolution is achieved with an interchangeable guide–collimator system in the last 3m of the neutron guide system. The effect of focusing on instrument resolution was evaluated and the strategy for detector deployment is discussed.
Advances in Computational Methods for X-Ray and Neutron Optics, 2004
ABSTRACT Neutron guides are widely used for improving the angular aperture of neutron scattering ... more ABSTRACT Neutron guides are widely used for improving the angular aperture of neutron scattering instruments in a broad band of wavelength. However, the usual guides are not effective enough in the short wavelength range. This is especially critical for time-of-flight instruments, which cannot take advantage of focusing techniques designed for steady-state monochromatic instruments. We discuss alternative ways to shape the reflecting surfaces in order to obtain a maximum angular aperture at the sample position at the expense of beam cross-section reduction. An optimal piecewise solution is proposed and Monte Carlo simulations with the IDEAS package are presented. Simulations for General Purpose Powder Diffractometer (GPPD) at Argonne National Laboratory are presented and the impact of a vertically focusing multiple-stage tapered guide is discussed. The results obtained by simulating the guide system options of engineering diffractometer VULCAN at the Spallation Neutron Source (SNS) are also presented, including vertically and horizontally tapered guide sections. The optimal multi-stage tapered guide design is discussed in terms of instrument figure of merit corresponding to different experimental needs ranging from high Q resolution to high intensity and/or high spatial resolution.
Physical Review Letters, 2012
We show that a variety of bulk metallic glasses (BMGs) inherit their Young's modulus and shear mo... more We show that a variety of bulk metallic glasses (BMGs) inherit their Young's modulus and shear modulus from the solvent components. This is attributed to preferential straining of locally solvent-rich configurations among tightly bonded atomic clusters, which constitute the weakest link in an amorphous structure. This aspect of inhomogeneous deformation, also revealed by our in-situ neutron diffraction studies of an elastically deformed BMG, suggests a rubber-like viscoelastic behavior due to a hierarchy of atomic bonds in BMGs.
Metallurgical and Materials Transactions A, 2011
The focus of the present research is to develop an integrated deformation and recrystallization m... more The focus of the present research is to develop an integrated deformation and recrystallization model for magnesium alloys at the microstructural length scale. It is known that in magnesium alloys nucleation of recrystallized grains occurs at various microstructural inhomogeneities such as twins and localized deformation bands. However, models need to be developed that can predict the evolution of the grain structure and texture developed during recrystallization and grain growth, especially when the deformation process follows a complicated deformation path such as in asymmetric rolling. The deformation model is based on a crystal plasticity approach implemented at the length scale of the microstructure that includes deformation mechanisms based on dislocation slip and twinning. The recrystallization simulation is based on a Monte Carlo technique that operates on the output of the deformation simulations. The nucleation criterion during recrystallization is based on the local stored energy, and the Monte Carlo technique is used to simulate the growth of the nuclei resulting from local stored energy differences and curvature. The model predictions are compared with experimental data obtained through electron backscatter analysis and neutron diffraction.
Journal of Neutron Research, 2004
Time-of-flight neutron diffraction has been used to investigate deformation behaviors due to cycl... more Time-of-flight neutron diffraction has been used to investigate deformation behaviors due to cyclic loading. A study of 316LN .slainless steel shows that in ihe early stage of fatigue life, grain-orientation dependent intergranular stresses develop quickly and oscillate between two extreme states corresponding to the ends of a tensile or compressive half loading cycle. In the late slage. the intergranular strains vanish for tests ending in tension and remain relatively unchanged for tests ending in compression, which poinis to the difference in plastic behaviors during tensile and compressive loadings.
Journal of Applied Crystallography, 1988
ABSTRACT
Applied Physics Letters, 2007
The authors report complementary use of high-energy x-ray and neutron diffraction to probe the lo... more The authors report complementary use of high-energy x-ray and neutron diffraction to probe the local atomic structure in a Zr-based bulk metallic glass. By analyzing the partial coordination numbers, the authors demonstrate the presence of multiple types of solute-centered clusters in the multicomponent glass and efficient packing of the amorphous structure at atomic scale. The authors’ findings provide a basis for understanding how local structures change during phase transformation and mechanical deformation of multicomponent amorphous alloys.
Advanced Materials, 2009
Fundamental understanding of composition variations and morphology in the nanoscale is essential ... more Fundamental understanding of composition variations and morphology in the nanoscale is essential to the design of advanced materials. Partial crystallization or devitrification of bulk metallic glasses (BMGs) results in novel microstructures, with high density (10 23-10 24 m À3) nanocrystalline precipitates evenly distributed in a glassy matrix. These crystalline precipitates are known to impede the propagation of shear bands, and are promising candidates for improving the mechanical properties of BMG alloys. [1-4] However, it has been an experimental challenge to determine the fine structure of these precipitates, and no one technique can provide all the answers. In this paper, we report the experimental study of a multicomponent BMG alloy, Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 , utilizing several state-of-the-art characterization techniques. Nanoscale solute partitioning due to strong chemical order is revealed at unprecedented detail by a new wide-field atom probe. This level of details is crucial for understanding the interference peaks observed in small-angle X-ray and neutron scattering experiments, an unsolved mystery for over a decade. A core/shell structure is formed as a result of nanoscale solute partitioning, which poisons the growth and helps stabilize the nanocrystalline particles. Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 is a widely studied BMG with excellent glass forming ability. [5-11] Upon devitrification, crystalline precipitates of 10-20 nm diameter emerge, as evidenced by high-resolution transmission electron microscopy. [9] Moreover, Z-contrast imaging, a technique more sensitive to composition distribution, showed high densities of distinct crystalline particles of similar sizes but with fuzzy boundaries. [9] Nanoscale composition fluctuations have been detected by atom probe tomography (APT), and were attributed to nanocrystalline particles. [11] The structure of devitrified Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 has also been investigated by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). While microscopy reveals structural details in a restricted field of view or analysis volume, small-angle scattering yields the average structure over the scattering volume. SANS and SAXS profiles of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 are both characterized by an interference peak. [8-11] However, there has been no satisfactory analysis of the experimental data that could identify the underlying structure. Although composition fluctuations due to spinodal decomposition can produce interference peaks, [12,13] no composition wave with a characteristic wavelength was detected experimentally. Instead, well-defined crystalline particles were reported by microscopy experiments. [9,11] Mathematically, the interference peak could also be generated by second-phase particles with a depleted diffusion zone. [14,15] However, experimental determination of the fine-scale composition variations in zirconium-based alloys is difficult with traditional voltage-pulsed APT, due to their poor electrical conductivities and brittleness at cryogenic temperatures. As a result, the structure of nanocrystalline particles in BMG alloys has remained a mystery. We have recently conducted an experimental study using a new atom-probe equipped with a high-repetition pulsed laser, [16] together with in situ SAXS. With these two complementary techniques, and making use of previously reported SANS data, [8] the structure of nanocrystalline particles of devitrified Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 is established. Two specimens were characterized by APT; one in the as-cast condition, and the other after isothermal annealing for 900 min at 663 K. For the as-cast sample, the APT analysis revealed a homogeneous microstructure characteristic of a completely amorphous state. On the other hand, the sample annealed for 900 min at 663 K showed a two-phase microstructure, consisting of lens-shaped precipitates and the surrounding matrix. Portions of several [9] impinging precipitates, contained in a 70 nm  70 nm  200 nm box, were extracted from the atom-probe data, and are shown in Figure 1a. Many of the precipitates are partially obscured by others, and the full extent of the precipitates is cropped by the bounding box. The red isoconcentration surface is constructed at 52% Zr, a concentration that is representative of the precipitate/matrix interface. This isoconcentration surface reveals the lenticular shape of each precipitate. The size, position, and orientation of the atom-probe data were selected to reveal the full extent of the upper precipitate. The upper portion of the red Zr isoconcentration surface of this precipitate has been removed, in order to reveal the Al-enriched core in the interior of the precipitate. The core region is denoted by the yellow isoconcentration surface, constructed of 5.4% Al. A rough estimate indicates a precipitate number density on the order of 10 23 m À3 .
Advanced Engineering Materials, 2008
The DSC and XRD experiments were carried out at the High Temperature Materials Laboratory in Oak ... more The DSC and XRD experiments were carried out at the High Temperature Materials Laboratory in Oak Ridge National Laboratory. For the DSC measurements, a high-temperature Netzsch DSC 404C calorimeter was used. Disk coupons were cut from a drop-cast Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 rod. For isochronal scans, the sample heating rates were 1, 2, 5, 10 and 20 K/min. The samples' mass were around 180 mg and a 63 mg sapphire was used as a reference. For isothermal tests, the annealing temperatures were 655, 665, 670 and 675 K, COMMUNICATIONS
Acta Crystallographica Section A Foundations of Crystallography, 2005
INSTRUMENTATION AND EXPERIMENTAL TECHNIQUES C144 structures determination and for application of ... more INSTRUMENTATION AND EXPERIMENTAL TECHNIQUES C144 structures determination and for application of the Rietveld method in quantitative phase analysis. The utilization of this instrument is open for the brazilian and latin-american scientific and technological communities. The authors acknowledge the financial supports given by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), under project no. 95/05173-0, and Ministério da Ciência e Tecnologia (MCT), under project no. 62.00007/98-2.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1994
The problem of computing the neutron optics of curved-crystal spectrometers is reviewed. The cond... more The problem of computing the neutron optics of curved-crystal spectrometers is reviewed. The conditions of focusing in scattering and the limitations to achievable resolutions are discussed. Experience with programs for resolution-intensity optimization is presented. Computational examples are given of high resolution configurations with bent crystals and open beams, showing significant intensity gain over conventional techniques. Quasi-elastic scattering measurements at energy transfer resolutions below 10 WeV are shown to be feasible at high luminosity. Curved monochromators for high resolution neutron powder diffraction, combining the beam focusing onto sample with focusing in scattering, are also discussed. III. APPLICATIONS
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Papers by Alexandru Stoica