M. Bruncko

In Situ Monitoring of Vacuum Carburizing

The present article describes development and testing of a new measurement method that enables nondestructive and in situ monitoring of a vacuum carburizing process. The principle of the method is based on monitoring the carbon diffusion during vacuum carburizing by the in-situ measurement of electrical resistance changes in the carburizing sample. Using this method the electrical resistance changes during vacuum carburizing of pure iron were acquired and analyzed. The experiments were performed in a laboratory vacuum furnace under a low pressure acetylene atmosphere (5 mbar) at a temperature of 950°C. The results of monitoring the kinetics of carburized zones obtained by the novel measurement method were compared with kinetics obtained by metallographic analysis of carburized samples.

Microstructural Changes During Internal Oxidation of a Ag-Sn Alloy

Abstract Electrical contact materials are expected to combine improved mechanical and physical properties with good corrosion resistance at increased temperatures. One of the most widely used groups of contact materials are internally oxidized dispersion hardened silver alloys. The principle of their production involves selective oxidation of less noble solute elements (usually Mg, Al, Sn, Cd and Zn) forming the fine dispersion of oxide particles in the solvent silver matrix. In this study the internal oxidation of dilute silver alloy containing 2 at. % of tin was extensively investigated. The morphological evolution of the alloy and the internal oxidation kinetics were determined by several metallographic examinations and in-situ electrical resistance measurements in the air atmosphere and in the temperature range from 600°C to 800°C. The results of experiments and their analyses show that the microstructural changes during internal oxidation of Ag-Sn alloy are strongly dependent on the annealing temperature. At the highest temperatures the perfectly dispersive distribution of the oxide particles SnO2 in the silver matrix was found, while at lower temperatures the formation of inner oxide bands and distribution of oxide particles along the grain boundaries is predominate.

Characterization and properties of niTi(W) and cuAlNi shape memory alloys

Abstract Shape memory wires were trained under constant stress in order to introduce a uniaxial shape memory effect by a thermomechanical treatment. These investigations were carried out on three different alloy systems (NiTi, NiTiW and CuAlNi) with different microstructures (cold-worked and annealed condition, with and without particles, large and small grain size). Several thousand thermal cycles were performed on the trained shape memory elements, continuously observing the changes in the deformation behaviour. The influence of the microstructure on development and stability of the intrinsic two-way shape memory effect has been discussed. Furthermore, this work deals with the production of thin specimens of shape memory alloys by melt-spinning and splat-cooling, the training procedure and their characterization with respect to microstructure and functional properties.

The response of peripheral blood mononuclear cells to shape memory alloys

The aim of this work was to study the response of human peripheral blood mononuclear cells (PB-MNCs) to Cu-Al-Ni and Ni-Ti shape memory alloys (SMAs). The alloys were prepared as rapidly solidified thin ribbons via melt spinning. It has been shown that the ribbons were non cytotoxic for PB-MNCs as determined by the viability and apoptosis assay. In 18 cultures out of 20 cultures of individual donor PB-MNCs cultivated with Cu-Al-Ni SMAs, no significant changes in the production of Th1 (IFN-γ), Th2 (IL-5 and IL-10) and proinflammatory (IL-1s, TNF-α, IL-6 and IL-8) cytokines were detected compared to controls. In two cultures out of 20, a significant increase in the production of Th1 and proinflammatory cytokines was observed. Microstructural analyses confirmed the formation of thin oxide on the surface of both SMA ribbons.

Identification of liquid/solid transformations in eutectic Pb-Sn alloy

Abstract In this paper the applicability and the efficiency of in-situ electrical resistance measurements with a four-probe method for monitoring the solidification process are presented. For this purpose a special measurement cell was developed. It enables simultaneous in-situ electrical resistance and temperature measurements during solidification to characterize the L → S phase transformation of metals and alloys. The method was experimentally demonstrated with a eutectic Pb – Sn (i. e. Pb-61.9 wt.% Sn) alloy. Additionally, the L → S phase transformation was monitored and compared with conventional differential thermoanalysis. The results of our investigation show that the simultaneous in-situ electrical resistance and temperature measurement method is able to determine the exact position of the start and end points of the L → S phase transformations from the resistance curves R(t) and the corresponding temperatures.

The Application of Centrifugal Atomization Method for Preparation of Rapidly Solidified Nd-Fe-B Flakes Used for Production of Permanent Magnets

Nd-Fe-B-type permanent magnets are used in applications that require a high-energy product/volume ratio in order to reduce weight. Automotive industry, hard drives, or wind turbines are just examples of an application where their use can be found. Conventional casting techniques reveal the formation of a high quantity of α-Fe and large Nd-rich regions. On the other hand, new techniques, like strip casting, melt-spinning and centrifugal atomization, produce homogeneous and fine-scaled microstructures. This paper discusses the application of rapid solidification by means of the centrifugal atomization method for preparation of Nd-Fe-B flakes. The effect of alloy composition and various process parameters of centrifugal atomization on the microstructure of rapidly solidified Nd-Fe-B alloy were investigated. The microstructures and the phase composition were examined by metallographic techniques, namely optical and scanning electron microscopy. Additionally, the influence of the processing parameters on the microstructures of as-cast flakes and subsequent magnetic properties of the prepared magnets will be discussed.

Application of in-situ electrical resistance measurements to the study of phase transformations in ferrous alloys

Phase transformations have been studied in a variety of different steels with the use of “in situ” electrical resistance measurements. The results were evaluated by metallography of the initial and final microstructures and with consideration of data from the published literature. On this basis a good correlation has been established and it was shown that this method is suitable for such investigations. It even presents certain advantages, thus providing a more complete understanding of the physical metallurgy of steels. We out-lined the field in which the measurement of electrical resistance is particularly suitable and an example of processes that are difficult to monitor using other commonly used methods.

Characterization of Oxidation Resistance of Stainless Steels at High Temperature by Metallographic Examinations and In-Situ Electrical Resistance Measurements

Abstract Practically all metals and alloys survive high-temperature exposure by growing oxide scales and/or by precipitation of the oxide particles in the matrix. Formed products can grow in shape of external oxide layers on surfaces, or as discrete oxide particles precipitated in a metal matrix. The first case represents external oxidation, and the other case is called internal oxidation. These processes are very important, because they determine the properties and applicability of metallic materials. Generally, they are undesired, because they cause deterioration of the mechanical properties and decomposition of metallic material. On the other side, the controlled process of external oxidation could be used for formation of protective coatings and the internal oxidation for dispersion strengthening of materials. In this paper we present monitoring of high-temperature oxidation of X12Cr13 stainless steel by in-situ electrical resistance measurements at different annealing temperatures in the air atmosphere. We determined the kinetics of oxide scale formation and its morphology with additional metallographic examination made by optical and scanning electron microscopy. The results of this research work show that in-situ monitoring and characterization of high-temperature oxidation present a strong tool that will contribute to a better fundamental understanding of the phenomena that occur during high-temperature oxidation of metallic materials.

Microstructural investigation of melt-spun NiTi based ribbons

Abstract In order to get small-dimensioned shape memory alloys (SMAs) with good functional and mechanical properties, a rapid solidification technique was employed. This paper discusses the use of different melt-spinning procedures for the preparation of proper thin NiTi ribbons. Metallographic characterization of SMAs is an important way to evaluate material properties, therefore the effect of alloy composition and different conditions of melt-spinning process (crucible, melting time and temperature) on the various inclusions and microstructure of NiTi alloys was investigated. Specimens for metallographic studies were cut from the longitudinal cross sections of melt-spun ribbons. The microstructures and different phases were examined by optical, scanning and transmission electron microscopy and the influence of the processing methods on microstructures of ribbons will be discussed.

Microstructural Changes during Vacuum Carburizing of Steels

Abstract Vacuum carburizing is a modern, very efficient and environmentally friendly process for case hardening of steels. Nowadays the technology has replaced atmosphere carburizing in many cases due to better repeatability and uniformity of heat treated samples. The process of vacuum carburizing is performed under oxygen free hydrocarbon gas atmosphere (usually pure acetylene) at partial pressure lower than 20mbar and in conditions which are far from thermodynamic equilibrium. The process consists of the cycles of active (saturation) and passive (diffusion) stages. Desired case depth and corresponding target concentration profile of carbon can be supervised by controlling the duration of these two stages.