Jonathan Weidow

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.

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.

APT analysis of WC-Co based cemented carbides

A method for quickly producing sharp and site-specific atom probe specimens from WC-Co based cemented carbides was developed using a combination of electropolishing, controlled back-polishing and FIB milling. Also, a method for measuring the amount of segregated atoms to an interface between two phases with a big difference in field needed for field evaporation was developed. Using atom probe tomography, the interface chemistry of WC/WC grain boundaries, WC/(M,W)C phase boundaries and WC/binder phase boundaries was analysed. In addition, the transition metal solubility in WC was determined.

A Method for Producing Site-Specific TEM Specimens from Low Contrast Materials with Nanometer Precision

A method that enables high precision extraction of transmission electron microscope (TEM) specimens in low contrast materials has been developed. The main idea behind this work is to produce high contrast markers on both sides of and close to the area of interest. The markers are filled during the depositing of the protective layer. The marker material can be of either Pt or C depending on which one gives the highest contrast. It is thereby possible to distinguish the location of the area of interest during focused ion beam (FIB) milling and ensure that the TEM sample is extracted precisely at the desired position. This method is generally applicable and enables FIB/scanning electron microscope users to make high quality TEM specimens from small features and low contrast materials without a need for special holders. We explain the details of this method and illustrate its potential by examples from three different types of materials.

The Influence of Heat Treatment on the Microstructure and Machinability of a Prehardened Mold Steel

The machinability performance of a modified AISI P20 steel, heat treated to have the same hardness but three different microstructures, lower bainite, tempered martensite, and primary spheroidized carbides in a tempered martensite matrix, was studied. The microstructures were characterized using light optical and scanning electron microscopy and X-ray diffraction, and mechanical properties were compared by means of tensile and Charpy V-notch impact tests. The influence of microstructure and the resultant mechanical properties on machinability was studied in the context of single tooth end milling operation. The results showed that the material containing primary spheroidized carbides exhibited a superior machinability at the expense of a marginal loss of tensile strength and impact toughness, with comparable yield strength to that of the material containing tempered martensite. By contrast, the material with bainitic microstructure showed the lowest yield strength and the poorest machinability performance while having the highest uniform elongation.

Effects of Alloying Concepts on Ferrite Morphology and Toughness of Lean Duplex Stainless Steel Weld Metals

Alloying concepts for lean duplex weld metals have been explored. Focus was on compositions with 21.5–24 % Cr with different levels of Ni and Mn in combination with N producing a minimum PREn value of 26. Microstructures and properties of weld metals produced with experimental covered electrodes or metal-cored wires were evaluated. The electron backscattered diffraction (EBSD) technique combined with scanning electron microscopy (SEM) was used to explore relations between weld metal morphology and crystallographic orientation relationships between ferrite and austenite. A duplex microstructure with Widmanstätten type ferrite characteristic of a ferritic solidification was found throughout most weld metals. Regions with a morphology suggesting a mixed ferritic-austenitic solidification occurred particularly for higher Ni- and N-contents. EBSD studies showed that most of the austenite formed following a ferritic solidification was near either the Nishiyama-Wasserman (NW) or Kurjdumov-Sachs (KS) relationships with adjacent ferrite. More phase boundaries in the mixed mode solidification regions had a random orientation relationship. Strength, ductility and PREn requirements were readily matched but the combined Mn, Ni and N alloying levels and the ferrite morphology affected toughness significantly. Both a fully ferritic solidification and a sufficient Ni-content were necessary to produce acceptable and reproducible impact toughness at room and sub-zero temperatures. It was concluded that a composition of 23–24 % Cr, 7–8 % Ni and 0.12–0.16 % N was well suited to fulfil requirements.

Atom probe tomography analysis of WC powder

A tantalum doped tungsten carbide powder, (W,Ta)C, was prepared with the purpose to maximise the amount of Ta in the hexagonal mixed crystal carbide. Atom probe tomography (APT) was considered to be the best technique to quantitatively measure the amount of Ta within this carbide. As the carbide powder consisted in the form of very small particles (<1 μm), a method to produce APT specimens of such a powder was developed. The powder was at first embedded in copper and a FIB-SEM workstation was used to make an in-situ lift-out from a selected powder particle. The powder particle was then deposited on a post made from a WC-Co based cemented carbide specimen. With the use of a laser assisted atom probe, it was shown that the method is working and the Ta content of the (W,Ta)C could be measured quantitatively.