Silvia Richter

Characterization of dual-phase steel microstructure by combined submicrometer EBSD and EPMA carbon measurements.

Electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) measurements are combined to characterize an industrial produced dual-phase steel containing some bainite fraction. High-resolution carbon mappings acquired on a field emission electron microprobe are utilized to validate and improve the identification of the constituents (ferrite, martensite, and bainite) performed by EBSD using the image quality and kernel average misorientation. The combination eliminates the ambiguity between the identification of bainite and transformation-induced dislocation zones, encountered if only the kernel average misorientation is considered. The detection of carbon in high misorientation regions confirms the presence of bainite. These results are corroborated by secondary electron images after nital etching. Limitations of this combined method due to differences between the spatial resolution of EBSD and EPMA are assessed. Moreover, a quantification procedure adapted to carbon analysis is presented and used to measure the carbon concentration in martensite and bainite on a submicrometer scale. From measurements on reference materials, this method gives an accuracy of 0.02 wt% C and a precision better than 0.05 wt% C despite unavoidable effects of hydrocarbon contamination.

High-spatial-resolution low-energy electron beam X-ray microanalysis

Abstract. Performing X-ray microanalysis at beam energies lower than those conventionally used (< 10 keV) is known to significantly improve the spatial resolution for compositional analysis. However, the reduction in the beam energy which reduces the X-ray interaction diameter also introduces analytical difficulties and constraints which can diminish the overall analytical performance. This paper critically assesses the capabilities and limitations of performing low beam energy, high spatial resolution X-ray microanalysis. The actual improvement in the spatial resolution and the reduction in the X-ray yield are explored as the beam energy is reduced. The consequences for spectral interpretation, quantitative analysis and imaging due to the lower X-ray yield and the increased occurrence of X-ray line overlaps are discussed in the context of currently available instrumentation.

Influence of Si and N additions on structure and phase stability of Ge2Sb2Te5 thin films

The influence of Si and N in Ge(2)Sb(2)Te(5) (space group [Formula: see text]) on structure and phase stability thereof was studied experimentally by thin film growth and characterization as well as theoretically by ab initio calculations. It was found that Si and N most probably accumulate in the amorphous matrix embedding Ge(2)Sb(2)Te(5) grains. The incorporation of Si and N in these samples causes an increase of the crystallization temperature and the formation of finer grains. N is more efficient in increasing the crystallization temperature and in reducing the grain size than Si which can be understood based on the bonding analysis. The incorporation of both Si and N in Ge(2)Sb(2)Te(5) is energetically unfavourable, leading to finer grains and larger crystallization temperatures. While in the case of Si additions no significant changes in bonding are observed, N additions appear to enable the formation of strong Te-N bonds in the amorphous matrix, which are shown to be almost twice as strong as the strongest bonds in unalloyed Ge(2)Sb(2)Te(5).

Chemical strengthening of a dental lithium disilicate glass-ceramic material.

Chemical strengthening of dental ceramics by ion exchange has hitherto only been confirmed for feldspathic porcelains. The objective of this study was to examine whether the strength of lithium disilicate glass-ceramics can be increased by ion exchange as well. A lithium disilicate glass-ceramic material was treated in different molten salts. The concentration gradients of the relevant ions in the surface layer were investigated by means of electron probe microanalysis and secondary ion mass spectroscopy. Characteristic strength and Weibull modulus data were determined. An increase in strength of 25% was achieved by treatment in potassium nitrate. The chemical analyses revealed that the increase in strength resulted from an exchange of potassium for lithium ions. We conclude that ion-exchange treatments can increase the strength of lithium disilicate glass-ceramics. The improved material could be used for highly stressed applications, such as posterior crowns or inlay-retained bridges, with higher mechanical reliability.

Microstructure Analysis of High-Manganese TWIP Steels Produced via Strip Casting

Steels with manganese contents of more than 20% offer a new and favourable combination of material properties like high strength and high ductility. These extraordinary mechanical properties are based on the TWIP effect, which depends on the Stacking Fault Energy (SFE). But there are still problems in the conventional production of high-manganese steels, which prevents their widespread use. Both in casting and subsequent hot rolling difficulties occur, with the consequence that the production is very expensive. One alternative production process of high-manganese steels is strip casting, which basic feasibility was shown in earlier work. Strip casting allows the casting and rolling of hot strip in one combined process. In this way hot strip with a thickness of less than 3 mm could be produced. Characteristic for the strip cast material is the as-cast structure with a fine dendritic structure, which shows pronounced microsegregations with a short wavelength. The pronounced microsegregations can have an impact on the local chemical composition and thus on the dominating forming mechanisms that occur. In this work therefore the microsegregations of strip cast material are investigated by means of electron probe microanalysis (EPMA) measurement. Besides the local element distribution, also the presence and composition of non-metallic inclusions are analysed. Especially oxides from the casting process and sulfides from the raw material are expected. Furthermore, different annealing processes for the elimination of the dendritic as-cast structure are examined. In these experiments the temperatures were varied in the range from 900 to 1150°C at annealing times from several minutes to a few hours.

Prediction of Microstructure and Microsegregation in a Fe-Mn-C Austenitic Steel based on Phase-field Microstructure Simulations

This paper presents an experimental and theoretical investigation of the microstructure formation and segregation behaviour of a new austenitic steel based on the Fe-Mn-C alloy system. In order to accomplish the simulations, a modification of the model by Cha et al. is introduced. It includes a correction which compensates for artificial solute-trapping. A good qualitative agreement is found between the simulated 2D microstructures and the corresponding micrographs. Both simulations and experiments show that this alloy is very prone to manganese segregation. The further investigation on this field is scheduled.

Electron Probe Microanalysis of Ni Silicides Using Ni-L X-Ray Lines

We report electron probe microanalysis measurements on nickel silicides, Ni5Si2, Ni2Si, Ni3Si2, and NiSi, which were done in order to investigate anomalies that affect the analysis of such materials by using the Ni L3-M4,5 line (Lα). Possible sources of systematic discrepancies between experimental data and theoretical predictions of Ni L3-M4,5 k-ratios are examined, and special attention is paid to dependence of the Ni L3-M4,5 k-ratios on mass-attenuation coefficients and partial fluorescence yields. Self-absorption X-ray spectra and empirical mass-attenuation coefficients were obtained for the considered materials from X-ray emission spectra and relative X-ray intensity measurements, respectively. It is shown that calculated k-ratios with empirical mass attenuation coefficients and modified partial fluorescence yields give better agreement with experimental data, except at very low accelerating voltages. Alternatively, satisfactory agreement is also achieved by using the Ni L3-M1 line (Lℓ) instead of the Ni L3-M4,5 line.

Characterization of a heat‐insulating coating on floatglass by sputter‐assisted EPMA

Heat-insulating coatings on window glass consist of thin metallic films embedded in thin dielectric films. Such multilayer systems are produced commercially by physical vapour deposition in a continuous technique. For process control as well as for the development of new types of multilayer systems, surface-sensitive methods of analysis are essential to characterize the structure of the coatings in terms of the thickness and chemical composition of each layer. The present work reports on the first application of electron probe microanalysis (EPMA) in depth profiling of a complex heat-insulating coating on floatglass. It is well known that conventional EPMA is a powerful tool in microbeam analysis to quantify chemical compositions with high accuracy and to detect rather low concentrations even for light elements (B-F). However, the large information depth of x-rays (∼0.1-1 μm) limits the applicability to depth profiling. A great step forward in depth profiling in the submicrometre range was achieved by combining EPMA with surface removal by a sputtering procedure. Two various techniques can be applied: measurement of the emitted x-rays by scanning the electron beam across a sputter crater edge; or sputtering the material in situ with an integrated ion gun. In this work we succeeded in quantifying the film thickness (mass coverage) and chemical composition of each single component of the heat-insulating structure. The great advantage of the new method is its ability to determine the depth scale, as well as its remarkable detection limit. For example, in a depth of 100 nm a thin NiCr layer ∼1 nm thick could be detected. The Ni/Cr ratio of the layer was determined reproducibly to be 85: 15. The minimal thickness that could have been detected was estimated to be 0.15 nm.

Application of sputter-assisted EPMA to depth profile analysis

Abstract. A common problem in depth profile measurement is the calibration of the depth scale. The new technique of sputter assisted electron probe microanalysis offers the possibility of calculating the composition as well as the depth scale solely from the acquired X-ray intensity data without further information, e.g. sputter rates. To achieve a depth resolution that is smaller than the depth of information of the electron probe, i.e. 0.1–1 μm, special deconvolution algorithms must be applied to the acquired data.To assess the capabilities of this new technique it was applied to a Ti/Al/Ti multilayer on Si under different measurement conditions. Quantitative depth profiles were obtained by application of a deconvolution algorithm based on maximum entropy analysis. By comparison of these profiles with AES depth profiles and AFM roughness measurements, it was shown that the limiting factor to the achievable depth resolution is the occurrence of surface roughening induced by the sputtering process rather than the relatively large depth of information of the electron probe.We conclude that for certain applications sputter-assisted EPMA can be regarded as a valid depth profiling technique with a depth resolution in the nm range.

Towards Reliable Quantification of Steel Alloys at Low Voltage

Whether it be inclusions, precipitates or segregation at grain boundaries, the development of new steel alloys and processes hinges on the chemical quantification of submicrometer features. The combined requirements for high spatial resolution and accurate measurements of several elements make field emission electron microprobes suitable instruments for studying steel alloys. For quantitative analyses, the spatial resolution is determined by the x-ray emission volume the dimensions of which depend on the sample composition, the beam energy and the overvoltage ratio. For most elements, lowering the beam energy directly translates into the necessity of using lower energy x-ray lines. However, the improvement of the spatial resolution comes with additional challenges, mainly a lower intensity (worst statistics), a larger influence of contamination and oxidation, and inaccuracies of the quantification.