Papers by Keyword: Atom Probe Tomography (APT)

Abstract: The effects of Sn addition on clustering and age-hardening behavior in an Al-0.6Mg-1.0Si (mass %) alloy were investigated. Addition of Sn delayed the age-hardening in single aging at 170 ̊C. On the other hand, Sn promoted the age-hardening response in 3-step aging process which comprises a pre-aging (PA) at 90 ̊C for 18ks followed by natural aging (NA) for 604.8ks and artificial aging (AA) at 170 ̊C. The characteristics of clusters formed during PA and NA were evaluated by differential scanning calorimetry (DSC) analysis and atom probe tomography (APT). The DSC results show that the endothermic peak at around 160 ̊C to 200 ̊C was observed in the Sn-free alloy. On the other hand, in the Sn-added alloy, endothermic peak was not observed. It is suggested that Sn addition suppresses the formation of the clusters formed during NA. The APT results show that the Sn addition decreases the number density of clusters, especially smaller clusters. No Sn precipitates were found in Mg-Si precipitates formed during AA at 170 ̊C for 3.6ks. It is speculated that suppression of smaller cluster formation by addition of Sn promotes the age-hardening response

Abstract: As-cast Al-Sc alloys and Zr-containing Al-Sc-Zr alloys were aged at 300°C for different times, the Al-Sc-Zr alloy showed more excellent precipitation strengthening. The atom probe tomography (APT) was applied to characterize the precipitates in the two alloys, the results show that the precipitates in Al-Sc-Zr alloy is smaller in size and is higher in number density than that in the Al-Sc alloy at same aging time, it is because that the Zr addition reduces the diffusion rate of Sc, and which retains the growth of precipitates and results in the excellent precipitation strengthening of Al-Sc-Zr alloy.

Abstract: In this study we aim at combining the results from transmission electron microscopy (TEM) and atom probe tomography (APT) to study the early stages of phase decomposition in the age hardening alloy AA 6016. Samples are subjected to different periods of natural ageing or artificial pre-ageing at elevated temperature in order to produce different types of clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their compositions, whereas TEM is applied to analyse and quantify number density and sizes of the particles during artificial ageing at 185°C. It is shown that the two techniques, TEM and APT, are complementary and a combined approach yields more detailed insight into the early stages of phase decomposition in age hardening 6xxx series alloys than possible by the sole use of either technique individually.

Abstract: Mechanical alloying (MA) is an established way to prepare nanocrystalline materials and metastable solutions of materials, which normally have no mutual solubility. This is also the case for oxide dispersion strengthened (ODS) steels with improved mechanical properties at elevated temperatures. It is known that a small addition of yttria (Y₂O₃) has a beneficial effect on high temperature strength and reduces the creep rate in mechanically alloyed ferritic steels by about six orders of magnitude. In this work we present an experimental study using atom probe tomography, X-ray photoelectron spectroscopy, and positron annihilation spectroscopy combined with first principles modeling focusing on the distribution and behavior of yttria in pure iron prepared by mechanical alloying. Atom probe tomography and X-ray photoelectron spectroscopy measurements as well as positron annihilation spectroscopy conducted on powder particles directly after milling have revealed that a predominantly fraction of the yttria powder dissolves in the iron matrix and Y atoms occupy convenient positions, such as vacancies or dislocations. This is supported by ab initio calculations demonstrating that the formation energy for Y substitutional defects in bcc-Fe is significantly lower in the close neighborhood of vacancies.

Abstract: Here we review research in which Vickers hardness tests, optical microscopy, electron backscatter diffraction, and atom probe tomography were used to understand the strengthening effects that can be found with Nb in CASTRIP® steels during thermo-mechanical processing and ageing. Nb addition favours the grain refinement of ferrite by inhibiting the austenite recrystallization when hot rolled and provides a strong cluster-hardening effect during ageing.

Abstract: The early stages of precipitation of Cr rich precipitates were investigated by Atom Probe Tomography (APT) in a Cu-1Cr-0.1Zr (wt.%). This way, quantitative data were obtained about their size, 3D shape, density, composition and volume fraction. Surprisingly, in the early stage of precipitation, nanoscaled precipitates exhibit various shapes (spherical, plates and ellipsoid) and contain a large amount of Cu (up to 50%), in contradiction with the equilibrium Cu-Cr phase diagram. APT data also show that some impurities (Fe) segregate along Cu/Cr interfaces. A precipitation sequence is proposed and the relationship between mechanical properties and microstructure is discussed.

Abstract: The temporal evolution of the microstructure resulting from phase separation into Fe-rich (α), Cr-rich (α¢), and Fe(Ti,Al) (β¢) phases of a Fe-20Cr-6Al-0.5Ti alloy has been analyzed by thermoelectric power measurements (TEP). The early stages of decomposition and the evolution of the three-dimensional microstructure have been analyzed by atom probe tomography (APT). The roles of Cr, Al, and Ti during the decomposition process have been investigated in terms of solute partitioning between the phases. Analysis of proximity histograms revealed that significant Al and Ti partitioning occurs, which is consistent with theoretical calculations. The results indicate that as the α-α¢ phase separation proceeds, Al and Ti are rejected into the α phase, which causes the β¢ phase to nucleate on the surface of the α¢ phase.

Abstract: The dislocation density in ferrite and austenite of a bainitic microstructure obtained by transformation at very low temperature (300 °C) has been determined using transmission electron microscopy. Observations revealed that bainitic ferrite plates consist of two distinctive regions with different substructures. A central region in the ferrite plate is observed with dislocations that may result from lattice-invariant deformation at the earlier stage of bainite growth. As plastic deformation occurs in the surrounding austenite to accommodate the transformation strain as growth progresses, the Ferrite/Austenite interface has also a very distinctive dislocation profile. In addition, atom-probe tomography suggested that dislocation tangles observed in the vicinity of the ferrite/austenite interface might trap higher amount of carbon than single dislocations inside the bainitic ferrite plate.

Abstract: The amount of carbon in solid solution in bainitic ferrite at the early stage of transformation has been directly determined by atom probe tomography at 200 °C, taking advantage of the extremely slow transformation kinetics of a novel nanocrystalline steel. Results demonstrated that the original bainitic ferrite retains much of the carbon content of the parent austenite providing strong evidence that bainite transformation is essentially displacive in nature.

Abstract: This paper presents an overview and some original results about the mechanical properties and phase analysis of a nanostructured (NS) nickel-iron based alloy INCONEL 718. This structure was obtained by severe plastic deformation (SPD) via high pressure torsion (HPT) and multiple isothermal forging (MIF) of the alloy with an initial coarse-grained (CG) structure. Materials before and after SPD were analyzed by scanning, transmission electron microscopes and atom probe tomography (APT). Experimental data indicate that after HPT at room temperature - phase was partly dissolved and that precipitation of the -phase occurs during post deformation aging. A hardness up to 8 GPa was recorded for the NS alloy after SPD and annealing at 600°C.