Papers by Author: Helmut Klein

Abstract: The precipitation and dissolution behavior of niobium carbo-nitrides is of particular interest for many technical applications. Niobium-microalloyed high strength low alloy (HSLA) steels are widely used in civil construction, automobile and line pipe applications. These steels rely on thermomechanical processing. In this context, coupled processes like thin slab casting and thermomechanical rolling of microalloyed steel grades require most precise information on the precipitation state at the individual processing steps. Reasonable equations for the solubility product at thermal equilibrium can be taken from literature but kinetics is largely unknown. Conventional X-ray technology is not able to detect small volume fractions below 0.1% of nanoscale precipitates. Investigation of nanoscale niobium precipitates by transmission electron microscopy (TEM) analysis or chemical extraction methods is common practice. However, TEM suffers from statistical relevance and chemical extraction will not give information on particle distribution and orientation. Investigation by high energy synchrotron X-ray of about 100 keV offers statistical relevance as volumes of several cubic millimeters are regarded. This large reflecting sample volume allows to detect nanometer-sized particles and provides very high angular resolution leading to an exact determination of the reflection peaks. The wavelength of around 0.12 Å is able to analyze nanometer-sized particles. Due to the high energy of the applied synchrotron radiation, precipitation and dissolution reactions could be observed during thermal treatment inside a soaking furnace. The results establish this technology for analysis of nanoscale niobium carbo-nitride precipitates

Abstract: The exoskeleton of the crustacean Homarus americanus, the American lobster, is a biological multiphase composite consisting of a crystalline organic matrix (chitin), crystalline biominerals (calcite), amorphous calcium carbonate and proteins. One special structural aspect is the occurrence of pronounced crystallographic orientations and resulting directional anisotropic mechanical properties. The crystallographic textures of chitin and calcite have been measured by wide-angle Bragg diffraction, calculating the Orientation Distribution Function (ODF) from pole figures by using the series expansion method according to Bunge. A general strong relationship can be established between the crystallographic and the resulting mechanical and physical properties.

Abstract: Dental enamel is the most highly mineralised and hardest biological tissue in human body [1]. Dental enamel is made of hydroxylapatite (HAP) - Ca5(PO4)3(OH), which is hexagonal (6/m). The lattice parameters are a = b = 0.9418 nm und c = 0.6875 nm [1]. Although HAP is a very hard mineral, it can be dissolved easily in a process which is known as enamel demineralization by lactic acid produced by bacteria. Also the direct consumption of acid (e.g. citric, lactic or phosphoric acid in soft drinks) can harm the dental enamel in a similar way. These processes can damage the dental enamel. It will be dissolved completely and a cavity occurs. The cavity must then be cleaned and filled. It exists a lot of dental fillings, like gold, amalgam, ceramics or polymeric materials. After filling other dangers can occur: The mechanical properties of the materials used to fill cavities can differ strongly from the ones of the dental enamel itself. In the worst case, the filling of a tooth can damage the enamel of the opposite tooth by chewing if the interaction of enamel and filling is not equivalent, so that the harder fillings can abrade the softer enamel of the healthy tooth at the opposite side. This could be avoided if the anisotropic mechanical properties of dental enamel would be known in detail, hence then another filling could be searched or fabricated as an equivalent opponent for the dental enamel with equal properties. To find such a material, one has to characterise the properties of dental enamel first in detail for the different types of teeth (incisor, canine, premolar and molar). This is here exemplary done for a human incisor tooth by texture analysis with the program MAUD from 2D synchrotron transmission images [2,3,4].

Abstract: In order to get information about the transition mechanism, the temperature-induced transformation in the binary com¬pound NiS was investigated. Above 379 °C, a single crystal of millerite -NiS transforms to polycrystalline NiAs type -NiS with a sharp texture. Pole figures of both phases in the same orientation were measured using synchrotron radiation and an imaging plate detector. The Rietveld texture analysis showed that there are at least three components of the high-temperature -NiS phase. The main component shows the following orientation relations: [001]NiAs type  [001]millerite, [100]NiAs type  [210]millerite, [210]NiAs type  [100]millerite. The broad peaks of the recovered polycrystalline millerite occur at the same positions as the reflections of the original single crystal.

Abstract: Microstructure and texture formation in DP steels obtained by thermal treatment at temperatures of 780 °C i.e. between Ac1 and Ac3 and at 900 °C, i.e. above Ac3 and following different cooling techniques were studied by means of X-ray and electron diffraction techniques. The formation of the different structure constituents as well as substructure parameters such as blocks size and misorientation between them induced by thermal treatment was detailed analyzed. Various methods – conventional X-ray methods, high-energy synchrotron radiation and EBSD measuring – the texture of the bcc phase were applied in order to investigate their influence on the results. Beside texture heredity, a softening of the initial texture components induced by cold rolling and of related anisotropy of steels due to thermal treatment was estimated.

Abstract: This investigation aimed at the understanding of texture development during r-value determination by uniaxial tensile testing. Special emphasis was given to examine the texture evolution in the deformation zone of the tensile test specimen during mechanical r-value determination. The textures of different sheet steel grades were investigated in different deformation stages by the orientation distribution function (ODF) and mechanical testing.

Abstract: The new developed “sweeping detector” techniques using high energy synchrotron radiation allow to measure textures and microstructures of materials and their change during heat treatment with high location and orientation resolution. Here we show these new methods applied to cold rolled and subsequently annealed nickel samples. The grain-resolved measurements show, impressively, many details of the recrystallization process which can otherwise not be seen. The results of these measurements can be the base for omprehensive recrystallization theories.

Abstract: Additional to the position of any volume element of a (poly)-crystalline material its crystal orientation must also be known. Both together are described in the six-dimensional orientation-location space. The paper describes the most frequent structures of materials in this space and how these can be imaged with the "Moving Area Detector Method" using hard synchrotron X-rays. This technique is equally well suited for basic reseach in materials science as well as for non-destructive testing of technological parts or even complex structural components.