Mattias Thuvander

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti(2)AlC grains in different materials as well as on large M(23)C(6) precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the (13)C isotope, and calculating the concentration of (12)C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti(2)AlC an additional difficulty arises from the overlap between C(2)(+), C(4)(2+) and Ti(2+) at the mass-to-charge 24 Da.

Microstructure of a boron containing high purity nickel-based alloy 690

Abstract A high purity model alloy with major composition Ni–30Cr–10Fe–0.024C–0.006B (wt.%), corresponding to the commercial Alloy 690, was investigated using secondary ion mass spectroscopy, atom probe analysis and transmission electron microscopy in combination with energy dispersive X-ray analysis and electron energy loss spectroscopy. Solution annealing at 1200°C for 20 min put most carbon and boron into solution, but some intergranular borides and carbides, probably formed during quenching, were observed. Heat treatment at 700°C for 1 and 100 h resulted in substantial intergranular precipitation of Cr 23 C 6 and Cr 2 B. The influence of boron on the precipitation process in Alloy 690 is discussed and compared with precipitation in Alloy 600.

Quantitative evaluation of spinodal decomposition in Fe-Cr by atom probe tomography and radial distribution function analysis.

Nanostructure evolution during low temperature aging of three binary Fe-Cr alloys has been investigated by atom probe tomography. A new method based on radial distribution function (RDF) analysis to quantify the composition wavelength and amplitude of spinodal decomposition is proposed. Wavelengths estimated from RDF have a power-law type evolution and are in reasonable agreement with wavelengths estimated using other more conventional methods. The main advantages of the proposed method are the following: (1) Selecting a box size to generate the frequency diagram, which is known to generate bias in the evaluation of amplitude, is avoided. (2) The determination of amplitude is systematic and utilizes the wavelength evaluated first to subsequently evaluate the amplitude. (3) The RDF is capable of representing very subtle decomposition, which is not possible using frequency diagrams, and thus a proposed theoretical treatment of the experimental RDF creates the possibility to determine amplitude at very early stages of spinodal decomposition.

Atomically resolved tissue integration

In the field of biomedical technology, a critical aspect is the ability to control and understand the integration of an implantable device in living tissue. Despite the technical advances in the development of biomaterials, the elaborate interplay encompassing materials science and biology on the atomic level is not very well understood. Within implantology, anchoring a biomaterial device into bone tissue is termed osseointegration. In the most accepted theory, osseointegration is defined as an interfacial bonding between implant and bone; however, there is lack of experimental evidence to confirm this. Here we show that atom probe tomography can be used to study the implant-tissue interaction, allowing for three-dimensional atomic mapping of the interface region. Interestingly, our analyses demonstrated that direct contact between Ca atoms and the implanted titanium oxide surface is formed without the presence of a protein interlayer, which means that a pure inorganic interface is created, hence giving experimental support to the current theory of osseointegration. We foresee that this result will be of importance in the development of future biomaterials as well as in the design of in vitro evaluation techniques.

Detailed Analysis of the Microstructure of the Metal/Oxide Interface Region in Zircaloy-2 after Autoclave Corrosion Testing

Two varieties of Zircaloy-2, with different second phase particle (SPP) size distributions and different corrosion resistance, were oxidized in a steam autoclave. Transmission electron microscopy (TEM) of large thin-foil cross-sections of the oxide and the adjacent metal shows an undulating metal/oxide interface in both materials with a periodicity of slightly less than 1 μm and an amplitude of around 100 nm. The SPPs oxidize slower than the surrounding metal, and the absence of volume increase leads to void and crack formation as the SPPs become embedded in the oxide. On SPP oxidation, iron diffuses out of the particles into the surrounding oxide. A sub-oxide with an oxygen content of approximately 50 at. % and a layer thickness of about 200 nm was observed close to the metal/oxide interface. There is a 200 nm oxygen concentration gradient into the metal, from the level close to the sub-oxide of about 30 at. % down to a few atomic percent. All tin in the matrix is incorporated in the sub-oxide, and no segregation to the metal/oxide interface was found.

Quantitative APT analysis of Ti(C,N)

A specially produced Ti(C,N) standard material, with a known nominal composition, was investigated with laser assisted atom probe tomography. The occurrence of molecular ions and single/multiple events was found to be influenced by the laser pulse energy, and especially C related events were affected. Primarily two issues were considered when the composition of Ti(C,N) was determined. The first one is connected to detector efficiency, due to the detector dead-time. The second one is connected to peak overlap in the mass spectrum. A method is proposed for quantification of the C content in order to establish the C/N ratio. A correction was made to the major C peaks, C at 6 and 12 Da, with the (13)C isotopes, at 6.5 and 13 Da, according to the known natural abundance. In addition, a correction of the peak at 24 Da, where C and Ti overlap, is proposed based on the occurrence of single/multiple events for respective element. The results were compared to the results from other techniques such as electron energy loss spectroscopy, chemical analysis and X-ray diffraction. After applying the corrections, atom probe tomography results were satisfactory. Furthermore, the content of dissolved O in Ti(C,N) was successfully quantified.

Hydrogen analysis in APT: Methods to control adsorption and dissociation of H2

Experimental factors that influence adsorption of hydrogen from the residual gas on a nickel-rich alloy during atom probe tomography are investigated. The rate of adsorption has a maximum value at field strengths between 24 and 26 V/nm. It is found that by selecting sufficiently high laser energies, or alternatively high DC fields, it is possible to significantly reduce adsorbed quantities. Some of the physical mechanisms for hydrogen supply to the analyzed area of the tip are discussed, and it is concluded that the dominating supply mechanism is most likely direct adsorption from the gas phase. Low hydrogen adsorption at high fields is attributed to autoionization, and a decline at low fields is explained by reduced field adsorption.

A statistical method to detect ordering and phase separation by APFIM

A method for evaluation of atom probe field ion microscopy (APFIM) data to test if a material exhibits phase separation or ordering tendencies is presented. In this method the standard error (s) of the frequency distribution for a large number of block sizes is compared with the standard deviation (σ) of the binomial distribution (BD). The BD is expected for a solid solution, where the atoms are randomly distributed. For an ordered material the experimental s will be smaller than σ for the BD. A large s, on the other hand, can be the consequence of phase separation. Simulations were used to determine the probability that an observed large or small s, indicating some feature of the material, could have been obtained from a random distribution. Also, simulations were used to reveal the influence of detection efficiency on the possibility to detect ordering. Examples from four materials are presented to illustrate the applicability of the method.

Reduction of multiple hits in atom probe tomography.

The accuracy of compositional measurements using atom probe tomography is often reduced because some ions are not recorded when several ions hit the detector in close proximity to each other and within a very short time span. In some cases, for example in analysis of carbides, the multiple hits result in a preferential loss of certain elements, namely those elements that frequently field evaporate in bursts or as dissociating molecules. In this paper a method of reducing the effect of multiple hits is explored. A fine metal grid was mounted a few millimeters behind the local electrode, effectively functioning as a filter. This resulted in a decrease in the overall detection efficiency, from 37% to about 5%, but also in a decrease in the fraction of multiple hits. In an analysis of tungsten carbide the fraction of ions originating from multiple hits decreased from 46% to 10%. As a result, the measured carbon concentration increased from 48.2 at%to 49.8 at%, very close to the expected 50.0 at%. The characteristics of the multiple hits were compared for analyses with and without the grid filter.

Atom probe analysis of carbonitride grains in (Ti, W, Ta, Mo)(C, N) (CoNi) cermets with different carbon content

Abstract A series of three alloys with varying carbon content has been characterised using atom probe microanalysis, transmission electron microscopy and energy dispersive X-ray analysis. The atom probe was mainly used to determine the composition of the rim of carbonitride grains. It was found that the amount of tantalum decreases with increasing total carbon content, whereas neither the tungsten content nor the N (C + N) ratio in the rim seem to be particularly affected by the overall carbon content. The results can be understood from how the content of tungsten in the cobalt/nickel binder phase is controlled by the total carbon content in the material and from the composition and volume fraction of the different phases in these complex materials.