Papers by Keyword: Microstructural Characterization

Abstract: β-Ti alloys have been increasingly explored for biomedical applications due to their attractive combination of mechanical properties, including low elastic modulus, high tensile strength, good fatigue resistance, excellent ductility, and superior corrosion resistance. In this context, and building upon previous research, a metastable β-Ti alloy, Ti-12Mo-13Nb, was developed via hot forging as a potential alternative to the conventional Ti-6Al-4V alloy. This study investigates the microstructure and mechanical behavior of the Ti-12Mo-13Nb alloy processed by hot swaging, followed by aging at 500 °C for 10 min, 4 h, and 24 h. Phase transformations were analyzed using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical characterization was carried out through Vickers microhardness testing and Young’s modulus measurements. The highest microhardness-to-modulus ratio was achieved after aging at 500 °C for 24 h, corresponding to a microstructure composed of fine α precipitates uniformly distributed within the β matrix.

Abstract: In the present study the notched fatigue behavior of two multi-phase medium entropy alloys (MEAs) AlCrFe₂Ni₂ and AlCrFe₂Ni₂Mo_0.1 was characterized by three-point-bending (3-PB), along with a super-duplex steel 1.4517 as a reference material. An analytical approach for characterizing the fatigue notch factor (k_f), based on grain size analysis in combination with finite element modelling (FEM) was used, relating the theory of critical distances (TCD) to the grain size of the material. To validate the approach, for the reference steel, the fatigue notch factor was also determined experimentally by comparing the fatigue behavior of notched and smooth specimens, resulting in an experimentally determined fatigue notch factor (k_f) ~ 1.07. The numerically and analytically estimated notch effects increase with decreasing average grain size and vary between ~ 1.07 for the coarse-grained reference material – in very good agreement with the experimental results – and ~ 1.35 for the much more fine-grained AlCrFe₂Ni₂Mo_0.1 medium entropy alloy. Note that these values are significantly lower than the stress concentration factor (k_t) ~ 1.58, associated with the notch geometry. Fatigue endurance limits were measured at a fatigue stress ratio R ~ 0.1 (unidirectional stress), but were converted to fatigue amplitudes at R = -1 (σ_{a, R-1}, fully reversed stress), to be able to make due comparisons with available literature data, by using the elliptical relationship. The resulting fatigue endurance limit amplitudes for specimens surviving at least 2E+06 cycles for a minimum of three tested samples and including notch effects are σ_{a, R-1} ~ 508 MPa for the AlCrFe₂Ni₂ alloy, σ_{a, R-1} ~ 540 MPa for the AlCrFe₂Ni₂Mo_0.1 alloy modification and σ_{a, R-1} ~ 400 MPa for the reference super-duplex steel, putting the analyzed MEAs into a very competitive position compared to Cobalt containing multi-phase high or medium entropy alloys as well as commercially available steels.

Abstract: The observation, description, and ultimate prediction of causal connections between processing and resulting macroscopic properties stand at the heart of Materials Science and Engineering. To that end, the microstructure is the subject of intense examination, as it is ultimately responsible for the observed emergent behavior. As a result, many of the scientific or technical questions that we strive to answer boil down to quantitatively studying the—sometimes subtle—effects of processing on the microstructure in terms of known or hypothesized thermodynamic and kinetic phenomena. This statement is naturally also true in the case of hot isostatically pressed powder metallurgy tool steels. In the 50 years since the process' popularization, many parameters have been identified as relevant to microstructure formation during consolidation. Among these process variables, the powder solidification structure distribution is probably the last to join the list. Dendritic solidification during the atomization of relatively massive particles produces slightly elongated carbides. On the other hand, cellular solidification in smaller powder particles is responsible for smaller and more angular carbides. Characterizing powder solidification structure as a function of particle size presents two main challenges: First, the assessment relies on examining cross-sections of the powder particles, which are most likely non-diametric. And, second, the manual identification exercise is tedious and highly subjective. In this work, we show how we achieve fast and reliable powder structure solidification distributions using deep learning combined with state-of-the-art stereology reconstruction techniques.

Abstract: We have determined different phase fractions from microscopy images using semi-automated image analysis fitting technique, and in addition we have classified each phase according to its hardness. The distribution of grayscale pixels of different phases is first characterised separately for each phase, which are sampled from the microscope image. After this the distributions of the separate phases are fitted to give the corresponding distribution of the whole image. The microhardness measurement provides reliability on the classification of the different phases to ferrite, bainite or martensite. In addition to describing the applied techniques in detail, we present the results obtained from the analysis for one steel subjected to isothermal holding experiments at different temperatures.

Abstract: The β-titanium alloys have properties such as low elastic modulus associated with good properties mechanical, higher corrosion resistance and biocompatibility properties ideal for orthopedic application. Recent studies showed that the traditional Ti–6Al–4V alloy (α+β type) presented biological toxicity due to the presence of Al and V in its composition. In this scenario the present work aims at the fabrication and characterization of the microstructure and the mechanical properties of the as–cast Ti-12Mo-25Nb alloy. This alloy was produced by arc melting with non-consumable tungsten electrode in argon atmosphere. The material was characterized by X–ray diffraction, optical microscopy, Vickers hardness and elastic modulus by impulse excitation. The results of the microstructural characterization showed the presence of the β single phase, hardness equal to 207HV and the elastic modulus equal to 77GPa. These characteristics shows that this alloy is suitable for biomedical application such as implants.

Abstract: Million tons of ornamental stones residues are produced every year. Most of this residue is disposed without any kind of processing or treatment. Moreover, disposal occurs without a prospective of reuse or recycling. The incorporation into ceramics is a possible alternative for part of this residue. Clay-based ceramics have high capacity of incorporation of industrial residues. This work aimed to conduct a microstructural characterization of clay-based ceramics with incorporation of granite residues in the composition of the ceramic matrix. Specimens were produced with the addition of 0, 10, 20 and 30 wt. % of granite residues. The specimens were prepared by uniaxial pressing and sintered at temperatures of 1050 and 1200°C. For the microstructural analysis were carried out by scanning electron microscopy and X-ray diffraction. The results indicated that the incorporated ceramics sintered at 1200oC presented higher densification and lower porosity as compared with those sintered at 1050oC.

Abstract: A Ti-3Al-2.5V matrix composite reinforced with 8.5 vol.% TiB was produced using a powder metallurgy route. Processing included the mechanical alloying of Ti-3Al-2.5V and TiB₂ powders and Hot Isostatic Pressing (HIP) of the resultant composite powders, to produce a dense billet. These billets were subsequently extruded and/or subjected to various Conversion Heat Treatments (CHT), to complete the transformation of the TiB₂ particles into TiB needles. The CHT was performed either before or after extrusion. Microstructures and tensile properties of the materials at each stage of the processing routes were investigated and compared to those of a non-reinforced Ti-3Al-2.5V material, manufactured by the same powder metallurgy route. It has been demonstrated that the processing routes have a great impact on the mechanical properties, through modifications of the matrix and reinforcement characteristics. Well-chosen processing routes lead to more ductile composites, though this gain in ductility leads to slightly lower stiffness and strength values. This study clearly demonstrates the possibility to produce, at an industrial scale, a ductile version of a highly reinforced titanium matrix composite, showing important application potential.

Abstract: The microstructural characterization of a blade made of Ni-based superalloy was carried out and discussed. The blade was removed from service, of a gas turbine, due to preventive maintenance. This component was studied on different cross sections according to the surface temperature obtained by ANSYS software. The cross sections were characterized by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and High Resolution Scanning Electron Microscopy (HR-SEM). It was determined that the maximum value of total deformation is 0.001717 mm, located in the surface upper section of the blade, which not correspond to the section with the highest value of temperature calculated with ANSYS software. These results were consistent with the rafted microstructure observed at the upper region of the blade. Microcavities close to the MC carbides with a size of about 40x10^-6 m were also observed. The mechanical behavior of the Ni-based superalloy was studied by Rockwell Hardness testing (RHT). So, morphological changes were identified in the occurrence of the strengthening precipitated, γ', according to the operating conditions: stress and temperature. The average radius of the γ' precipitated was obtained by computer image analysis using ImageJ software. No clear relationship was found between the hardness values obtained and the coarsened γ' precipitated. A bimodal occurrence of coarsened γ' particles was identified distributed through γ matrix by HR-SEM. Thus, this study was carried out with the purpose to identify the critical parameters that promote microstructural changes in the Ni-based superalloy and therefore affect the mechanical behavior in this turbine blade.

Abstract: In recent years, steels microalloyed with high Ti levels have received increasing attention due to the interesting combination of high strength and formability because of the dispersion of nanometric sized titanium carbides that can be formed within the matrix. However, one of the problems related to these compositions is that their performance can be highly sensitive to variations in the processing route. To study this, in this work coiling simulations were performed by dilatometry tests with a reference Nb microalloyed steel (0.03%Nb) and a high-Ti steel (0.03%Nb-0.1%Ti) using temperatures from 550 to 675oC. The mechanical behaviour of the samples was characterized using Vickers hardness. A large hardness increase was observed in the high-Ti steel samples, resulting in estimated yield strength increases between 69 and 214 MPa. This improvement in the mechanical behaviour was very dependent on the coiling temperature; the maximum hardness was observed at 625oC-650oC, while this decreased drastically for temperatures from 550oC to 600oC and at 675oC. EBSD and TEM analysis has been performed to study the contribution of microstructural constituents and precipitation to the observed mechanical behaviour.

Abstract: The microstructures of Co–GaSb junctions in samples annealed at 300, 400, 500, and 600°C in a N₂ atmosphere were characterized using transmission electron microscopy (TEM) in combination with energy-dispersive spectrometry (EDS), nanobeam electron diffraction (NBD), and the selected area diffraction patterns (SADPs). The isolated CoSb₃(Ga) phase started to form at the interface of Co/GaSb in the temperature of 400°C and then the CoSb₃(Ga) phase changed to a continues layer at the interface when the annealing temperature was increased to 500°C. Upon increasing the temperature to 600°C, a large amount of Ga from GaSb diffused out toward Co to form a CoGa layer. The specific contact resistivity of Co/GaSb contact was evaluated by circular transmission line model (CTLM) and indicated that the lowest value was 5.410^-4 Ω-cm at annealing temperature of 500°C and possessed high current density of 41.7 A/cm² at 1V. These results indicate that the annealing temperature of the Co/GaSb structure could be maintained below 500°C for the successful formation of low-resistance metal Co/GaSb contacts in GaSb-based p-type metal-oxide-semiconductor field-effect transistors.