arXiv subjects

S. Pedrazzini

Publications and source records attributed to S. Pedrazzini.

7 recordsLinked to original sources

arXiv2024

Testing Outlier Detection Algorithms for Identifying Early-Stage Solute Clusters in Atom Probe Tomography

Atom probe tomography is commonly used to study solute clustering and precipitation in materials. However, standard techniques, such as the density based spatial clustering applications with noise (DBSCAN) perform poorly with respect to small clusters of less than 25 atoms. This is a fundamental limitation of density-based clustering techniques due to the usage of Nmin, an arbitrary lower limit placed on cluster sizes. Therefore, this paper attempts to consider atom probe clustering as an outlier detection problem of which KNN, LOF, LUNAR algorithms were tested against a simulated dataset and compared to the standard method, for a range of cluster sizes. The decision score output of the algorithms was then auto thresholded by the Karcher mean to remove human bias. Each of the major models tested outperforms DBSCAN for cluster sizes of less than 25 atoms but underperforms for sizes greater than 30 atoms. However, the new combined kNN and DBSCAN method presented was able to perform well at all cluster sizes. The combined kNN and DBSCAN method is presented as a possible new standard approach to identifying solute clusters in atom probe tomography.

cond-mat.mtrl-sci↗

arXiv2022

Effect of alloying on the microstructure, phase stability, hardness and partitioning behavior of a new dual-superlattice nickel-based superalloy

A novel y-y'-y" dual-superlattice superalloy, with promising mechanical properties up to elevated temperatures was recently reported. The present work employs state of the art chemical and spatial characterization techniques to study the effect systematic additions of Mo, W and Fe and variations in Nb and Al contents have on the phase fraction, thermal stability, elemental partitioning and mechanical properties. Alloys were produced through arc melting followed by heat treatment. Multi-scale characterization techniques and hardness testing were employed to characterize their microstructure, thermal stability and mechanical properties. Alterations in such properties or in elemental partitioning behaviour were then explained through thermodynamic modelling. A modest addition of 1.8 at.% Mo had a strong effect on the microstructure and thermal stability: it minimized microstructural coarsening during heat treatments while not significantly decreasing the y' solvus temperature. A reduction of Nb by 0.6 at.%, strongly reduced the y" volume fraction, without affecting the y' volume fraction. The reduced precipitate fraction led to a significant reduction in alloy hardness. Fe, added to achieve better processability and reduced material cost, decreased the y' solvus temperature and caused rapid microstructural coarsening during heat treatments, without affecting alloy hardness. A reduction of Al by 0.4 at.%, reduced the y' volume fraction and the y' solvus temperature, also without affecting alloy hardness. The addition of 0.9 at.% W decreased the y' solvus temperature but increased both precipitate volume fractions. These data will be invaluable to optimize current alloy design and to inform future alloy design efforts.

cond-mat.mtrl-sci↗

arXiv2020

Effect of bed temperature on solute segregation and mechanical properties in Ti-6Al-4V produced by selective laser melting

Advanced characterisation techniques were used on LPBF Ti-6Al-4V samples produced on a heated base plate. When the substrate temperature is 100°C the elongation is 6\%, which increases and peaks at 10\% at 570°C, then sharply decreases to zero ductility at 770°C. At 100°C, a heavily strained and twinned microstructure, primarily composed of α+α', was observed and it was comparable to asbuilt microstructures obtained by conventional LPBF methods. At higher temperatures, twins are no longer present and instead nano-scale β precipitates are observed within α' and α, as well as dislocation networks (570°C) and tangles (770°C). Solute segregation at crystal defects was observed in all pre-heating conditions. Al and V segregation at microtwins was observed in the 100°C sample, reporting for the first time `selective' and mutually exclusive Al- and V-rich regions forming in adjacent twins. V segregation at dislocations was observed in the 570°C and 770°C samples, consistent with the higher preheating temperatures. High O contents were measured in all samples but with apparent opposing effects. At 100°C and 570°C was estimated to be below the critical threshold for O embrittlement and locally aids in maintaining a strength high by solid solution strengthening, whereas at 770°C it was above the threshold, therefore failing in a brittle fashion. Based on these observations, the initial increase in ductility from 100°C to 570°C is attributed to a reduction in microtwins and the dislocation networks acting as `soft barriers' for slip within a coarser microstructure. The lack of ductility at 770°C was attributed to local solute redistribution causing dislocation pinning and an increase of O content in this sample.

cond-mat.mtrl-sci↗

arXiv2020

The effect of manganese and silicon additions on the corrosion resistance of a polycrystalline nickel-based superalloy

The service lives of nickel superalloys are often limited by environmental degradation. The present study compares oxidation, sulfidation and hot corrosion at 750C of three variants of a polycrystalline superalloy: a baseline alloy, a variant containing 1wt% Mn and one containing 0.5wt% Si. Mn reduced the oxidation rate without changing the scale morphology. The MnCr2O4 scale formed proved more protective against sulfidation and hot corrosion, but internal sulfides extended the damage depth. Si modified the oxide morphology to a continuous Cr2O3-Al2O3 dual layer. This provided improved protection, reducing the sulfidation depth by 2/3 and the hot corrosion depth by 1/2.

physics.app-ph↗

arXiv2020

Radiation Damage and Thermal Recovery of Perovskite Superconductor Yttrium Barium Copper Oxide

High temperature superconducting materials are being considered to generate the magnetic fields required for the confinement of plasma in fusion reactors. The present study aims to assess the microstructural degradation resulting from ion implantation at room temperature under two implantation conditions in Yttrium Barium Copper Oxide (YBCO) tapes. Xray Diffraction (XRD) and high resolution characterisation techniques including Atom Probe Tomography (APT) and Transmission Electron Microscopy (TEM) analyses were used to correlate alterations in superconducting behaviour measured using a Magnetic Properties Measurement System (MPMS) to amorphization and recovery caused by ion implantation. TEM analysis was performed to depth profile the degree of crystallinity (or lack thereof) on irradiated samples. SRIM predicted the damage depth at 900 nm below the sample surface of the 2 MeV Xe implanted sample and 450 nm beneath the surface of the 0.6 MeV Xe implanted sample. 2 MeV Xe implantation caused the superconducting temperature to decrease by 10 K and the critical current density to display a 10 fold reduction. Post irradiation heat treatments up to 600C caused recrystallisation of the irradiated layer, but also oxygen loss and alterations in grain size. The recrystallised grain orientation was random in TEM lamellae, however, bulk samples re-grew along the original crystal orientation provided that some of the original material was not amorphized (ie if they nucleated on crystalline YBCO). This is extremely promising for the thermal recovery of tokamak components.

cond-mat.mtrl-sci↗

arXiv2018

High Strain Rate Behaviour of Nano-quasicrystalline Al93Fe3Cr2Ti2 Alloy and Composites

In the present work, we demonstrate for the first time the outstanding dynamic mechanical properties of nano-quasicrystalline Al93Fe3Cr2Ti2 at.% alloy and composites. Unlike most crystalline aluminium-based alloys, this alloy and composites exhibit substantial strain rate sensitivity and retain much of their ductility at high rates of strain. This opens new pathways for use in safety-critical materials where impact resistance is required.

cond-mat.mtrl-sci↗

arXiv2018

On the effect of Ti on Oxidation Behaviour of a Polycrystalline Nickel-based Superalloy

Titanium is commonly added to nickel superalloys but has a well-documented detrimental effect on oxidation resistance. The present work constitutes the first atomistic-scale quantitative measurements of grain boundary and bulk compositions in the oxide scale of a current generation polycrystalline nickel superalloy performed through atom probe tomography. Titanium was found to be particularly detrimental to oxide scale growth through grain boundary diffusion.

cond-mat.mtrl-sci↗