Dimensional Atom Probe Tomography Research Articles

Effect of solute clusters on plastic instability in magnesium alloys

The interaction between solute atoms and matrix dislocations during plastic deformation of materials strongly influences the flow behavior and can lead to plastic instability at the macroscopic scale. In this study, Lüders band formation and dynamic strain aging phenomena were investigated in a Mg–1%Mn-1%Nd (wt.%) alloy, deformed in uniaxial tension at ambient temperature and 150 °C. The investigated alloy was studied in the initial as-extruded state and after a subsequent heat treatment at 275 °C for 120 min, which allowed for a variation of solute concentration in the microstructure, and thus for a different type/level of solute – dislocation interaction. Microstructure investigations at near-atomic resolution using three dimensional atom probe tomography were carried out in order to visualize the spatial distribution of alloying elements and reveal potential solute clustering trends in connection with the occurrence of plastic instability. Identified nanoscale solute clusters were analyzed on the basis of their size distribution, chemical composition, and statistical significance. Results demonstrate that the number, as well as the types of solute clusters, play an essential role in triggering plastic instability by interacting with deformation carriers at the atomic scale.

Mechanism of anomalous α-Fe formation from stoichiometric Sm2Fe17 jet-milled powder during post-pulverization annealing

Our research group developed a novel low-oxygen powder metallurgy process and succeeded in producing anisotropic Sm2Fe17N3 sintered magnets from finely pulverized high-performance powder without coercivity degradation, but the maximum energy product of the magnets was disappointingly low compared to the excellent performance of the powder. The performance degradation was attributed to some type of surface damage, and more specifically, poor crystallinity and randomly oriented nanocrystalline grains at the particle surfaces caused by the milling process. In this study, the current authors focused on Sm2Fe17 fine pulverized powder instead of Sm2Fe17N3 and explored the possibility of damage recovery to remove those microstructural defects by post-milling annealing/nitridation treatments. This treatment improved crystallinity to some extent but failed to remove the randomized nanocrystalline grains completely. In addition, it was found that α-Fe forms upon annealing of stoichiometric Sm2Fe17, which is inconsistent with the Sm-Fe binary phase diagram at a glance. The mechanism of this unexpected α-Fe formation was investigated by detailed microstructural analyses using transmission electron microscopy and three dimensional atom probe tomography.

High-Accuracy Sample Preparation for Three Dimensional Atom Probe Tomography Using Orthogonal Column Layout FIB-SEM and its STEM function

High-Accuracy Sample Preparation for Three Dimensional Atom Probe Tomography Using Orthogonal Column Layout FIB-SEM and its STEM function

Effects of thermal aging on microstructures of low alloy steel–Ni base alloy dissimilar metal weld interfaces

In this study, the advanced instrumental analysis has been performed to investigate the effect of long-term thermal aging on the microstructural evolution in the fusion boundary region between weld metal and low alloy steel in dissimilar metal welds. A representative dissimilar weld mock-up made of Alloy 690-Alloy 152-A533 Gr. B was fabricated and aged at 450°C for 2750h. The micro- and nano-scale characterization were conducted mainly near in a weld root region by using optical microscopy, scanning electron microscopy, transmission electron microscopy, and three dimensional atom probe tomography. It was observed that the weld root was generally divided into several regions including dilution zone in the Ni-base alloy weld metal, fusion boundary, and heat-affected zone in the low alloy steel. A steep gradient was shown in the chemical composition profile across the interface between A533 Gr. B and Alloy 152. The precipitation of carbides was also observed along and near the fusion boundary of as-welded and aged dissimilar metal joints. It was also found that the precipitation of Cr carbides was enhanced by the thermal aging near the fusion boundary.

Correlative multi-scale characterization of a fine grained Nd–Fe–B sintered magnet

The Nd-rich phases in pressless processed fine grained Nd-Fe-B sintered magnets have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and three dimensional atom probe tomography (3DAP). The combination of the backscattered electron (BSE) and in-lens secondary electron (IL-SE) images in SEM led to an unambiguous identification of four types of Nd-rich phases, NdOx, Ia3 type phase, which is isostructural to Nd₂O₃, dhcp-Nd and Nd₁Fe₄B₄. In addition, the 3DAP analysis of thin Nd-rich grain boundary layer indicate that the coercivity has a close correlation with the chemistry of the grain boundary phase.

Characterization of Nanoscale Precipitates in 10Ni3MnCuAl Steel by Three Dimensional Atom Probe Tomography

The precipitation hardened steel 10Ni3MnCuAl after solution heat treatment and aging at 510 °C is investigated by three dimensional atom probe (3DAP). The results show that the Ni, Mn, Al and Cu distribute uniformly in the steel after solution heat treatment, but they form multicomponent precipitates after aging and cause the hardness increase. The concentration ratio between Ni and Al in multicomponent precipitates is roughly 1, while the concentration of Fe decreases, and the distance between concentration peak of Cu and those of Ni and Al becomes wider with the aging time, which is related to the precipitation character of Cu in α-ferrite.

THREE DIMENSIONAL ATOM PROBE STUDY OF NI-BASE ALLOY/LOW ALLOY STEEL DISSIMILAR METAL WELD INTERFACES

Three dimensional atom probe tomography (3D APT) is applied to characterize the dissimilar metal joint which was welded between the Ni-based alloy, Alloy 690 and the low alloy steel, A533 Gr. B, with Alloy 152 filler metal. While there is some difficulty in preparing the specimen for the analysis, the 3D APT has a truly quantitative analytical capability to characterize nanometer scale particles in metallic materials, thus its application to the microstructural analysis in multi-component metallic materials provides critical information on the mechanism of nanoscale microstructural evolution. In this study, the procedure for 3D APT specimen preparation was established, and those for dissimilar metal weld interface were prepared near the fusion boundary by a focused ion beam. The result of the analysis in this study showed the precipitation of chromium carbides near the fusion boundary between A533 Gr. B and Alloy 152.

Growth-in-place deployment of in-plane silicon nanowires

Up-scaling silicon nanowire (SiNW)-based functionalities requires a reliable strategy to precisely position and integrate individual nanowires. We here propose an all-in-situ approach to fabricate self-positioned/aligned SiNW, via an in-plane solid-liquid-solid growth mode. Prototype field effect transistors, fabricated out of in-plane SiNWs using a simple bottom-gate configuration, demonstrate a hole mobility of 228 cm2/V s and on/off ratio >103. Further insight into the intrinsic doping and structural properties of these structures was obtained by laser-assisted 3 dimensional atom probe tomography and high resolution transmission electron microscopy characterizations. The results could provide a solid basis to deploy the SiNW functionalities in a cost-effective way.

Evidence of lateral heat transfer during laser assisted atom probe tomography analysis of large band gap materials

Investigation of wüstite (FeO), iron (Fe), and silicon (Si) specimens by infrared laser assisted three dimensional atom probe tomography shows evidence of confined surface absorption of the laser energy as the mechanism of laser assisted field evaporation for high band gap materials. The absorption region is shown to be located on the laser illumination side, close to the specimen apex. A simple model based on thermal diffusivity and spatial extent of the absorption region is derived and applied to the experimental results. The values of thermal diffusivity thus obtained are in good agreement with the published data.

Influence of cooling rate on the development of multiple generations of γ′ precipitates in a commercial nickel base superalloy

Abstract The compositional and microstructural evolution of different generations of γ′ precipitates during the continuous cooling of a commercial nickel base superalloy, Rene88DT, has been characterized by three dimensional atom probe tomography coupled with energy-filtered transmission electron microscopy studies. After solutionizing in the single γ phase field, continuous cooling at a very high rate results in a monomodal size distribution of γ′ precipitates with a high nucleation density and non-equilibrium compositions. In contrast, a relatively slower cooling rate (~ 24 °C/min) results in a multi-modal size distribution of γ′ precipitates with the larger first generation primary precipitates exhibiting close to equilibrium composition, along with the smaller scale secondary γ′ precipitates, exhibiting non-equilibrium composition (excess of Co and Cr, depleted in Al and Ti). The composition of the γ matrix near these precipitates also exhibits similar trends with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates.

低炭素快削鋼を切削した工具摩耗面と鉄凝着物界面の微視的観察

Microstructures and element distribution at the interface between a worn tool surface and a residue of low carbon cutting steel, which was left behind and accumulated on the tool surface during its machining, were observed and analyzed by transmission electron microscopy (TEM) and laser-assisted three dimensional atom-probe tomography (LA3DAP). The residue of the machined steel was composed of fine ferrite grains a few hundred nanometers in size, which were considered to be a result of a grain refinement process by severe plastic deformation. Tungsten carbide (WC) particles embedded in the tool were sharply worn out at the interface, thus the interface was found to be flat and sharp in nanometers scale. Since the deposited steel on the worn tool surface was so tightly bound that the interface was successfully analyzed by LA3DAP in transverse direction. It was shown that tungsten atoms from the WC particles had diffused into the deposited steel with a distance of approximately 10 nm from the interface. It has been suggested that the tool wear upon machining low carbon cutting steels proceeds mainly by the diffusion of tool component elements into the deposited iron, in which accelerated diffusion occurs due to the fast diffusion paths created by severe plastic strains introduced during machining.

Atomic scale characterization of titanium Ohmic contacts to SiC using three dimensional atom probe tomography and high resolution transmission electron microscopy

Three dimensional atom probe tomography coupled with high resolution transmission electron microscopy were used to analyze the structural and compositional transitions across interfaces of Ti Ohmic contacts on SiC. The results conclusively show that during the interfacial reaction carbon diffuses into the Ti layer, with the reaction zone extending beyond the immediate interface. The resultant TiC/TiCx interfacial layer provides a graded work function profile between the Ti and SiC, and facilitates low contact resistance which was measured by the transmission line, current-voltage method.

Effect of Titanium Addition on Composite L1<sub>2</sub> Precipitates in Aluminum-Zirconium Alloy

The particle morphology and composition of nanometer-scale Al3(Zr1-xTix) precipitates formed after aging at 450 °C for 1 h in Al-0.3mol%Zr-0.5mol%Ti alloy were investigated using high resolution electron microscopy (HREM) and three dimensional atom probe tomography (3D-APT). The alloy exhibits an inhomogeneous distribution of solute atoms after casting. After aging, Al3(Zr1-xTix) precipitates with a metastable L12 structure formed within solute-enriched dendrites. The Al3(Zr1-xTix) precipitate particles were spherical with diameters between 3 and 5 nm. The Al3(Zr1-xTix) precipitates were found to be coherent with matrix using HREM. Interfacial segregation of Ti at the matrix/precipitates was also observed in the composite Al3(Zr1-xTix) precipitates. The Zr:Ti atomic ratio in the Al3(Zr1-xTix) precipitates measured by 3D-APT was approximately 7 (x=0.125).

Characterization of the Bake-hardening Behavior of Transformation Induced Plasticity and Dual-phase Steels Using Advanced Analytical Techniques

The bake-hardening (BH) behavior of TRansformation Induced Plasticity (TRIP) and Dual-Phase (DP) steels after intercritical annealing (IA) has been studied using transmission electron microscopy, X-ray diffraction and three dimensional atom probe tomography. It was found for the DP steel that carbon can segregate to dislocations in the ferrite plastic deformation zones where there is a high dislocation density around the “as-quenched” martensite. The carbon pinning of these dislocations, in turn, increases the yield strength after aging. It was shown that bake-hardening also leads to rearrangement of carbon in the martensite leading to the formation of rod-like low temperature carbides in the DP steel. Segregation of carbon to microtwins in retained austenite of the TRIP steel was also evident. These factors, in combination with the dislocation rearrangement in ferrite through the formation of cells and microbands in the TRIP steel after pre-straining, lead to the different bake-hardening responses of the two steels.

Characterization of Precipitation Behavior in a Spray-Formed Al-Zn-Mg Alloy Using a Combination of Transmission Electron Microscopy and Three Dimensional Atom Probe Tomography

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Contingency table techniques for three dimensional atom probe tomography

A contingency table analysis procedure is developed and applied to three dimensional atom probe data sets for the investigation of fine-scale solute co-/anti-segregation effects in multicomponent alloys. Potential sources of error and inaccuracy are identified and eliminated from the technique. The conventional P value testing techniques associated with chi(2) are shown to be unsatisfactory and can become ambiguous in cases of large block numbers or high solute concentrations. The coefficient of contingency is demonstrated to be an acceptable and useful basis of comparison for contingency table analyses of differently-conditioned materials. However, care must be taken in choice of block size and to maintain a consistent overall composition between experiments. The coefficient is dependent upon block size and solute composition, and cannot be used to compare analyses with significantly different solute compositions or to assess the extent of clustering without reference to that of the randomly ordered case. It is shown that as clustering evolves into larger precipitates and phases, contingency table analysis becomes inappropriate. Random labeling techniques are introduced to infer further meaning from the coefficient of contingency. We propose the comparison of experimental result, mu(exp), to the randomized value, micro(rand), as a new method by which to interpret the quantity of solute clustering present in a material. It is demonstrated that how this method may be utilized to identify an appropriate size of contingency table analysis blocks into which the data set is partitioned to optimize the significance of the results.