Metallographic Preparation Research Articles

Nanoscale Depth Profiling of Residual Stresses Due to Fine Surface Finishing

AbstractMechanical polishing is commonly used for both surface finishing and metallographic sample preparation for a broad range of materials. However, polishing causes local deformation and induces residual stress, which has an important effect on many surface phenomena. Until recently, it has not been possible to quantify the nanoscale depth variation of polishing‐induced plastic deformation (eigenstrain) and the associated residual stress. In this study, the magnitude and depth of polishing‐induced residual stress are evaluated directly by focused ion beam milling and digital image correlation using the micro‐ring‐core geometry method. Depth‐resolved residual stress profiles are obtained with sub‐micrometer resolution at the surface of a titanium alloy sample that is subjected to various polishing steps. It is found that electrochemical polishing and polishing with colloidal silica do not induce any significant residual stress. However, polishing with diamond slurry leads to the formation of compressive residual stresses of up to 300 MPa, which extend deeper into the material when larger diamond particles are used. This study paves the way for further research on polishing and its effect on surface properties.

Evaluation of the performance of an automated metallographic samples analysis system by means of Petri Nets modelling

This paper presents the results of the research about the automation of the metallographic samples preparation and analysis processes. At the University of Oradea the authors designed and made an automated metallographic samples preparation and analysis system. This automated system can achieve polishing, attack by reagents and metallographic analysis. The flexible cell (the automated system) is made of two subsystems. The first subsystem, which is served by an industrial robot, performs the polishing and the sample surface assessment. The second subsystem, also served by an industrial robot, performs the reagent attack and finishes the metallographic analysis in order to determine the microstructure of the sample. The authors have designed a timed Petri nets model in order to assess the performance of the automated system and to identify the possible bottlenecks. The model is made for the case that the system is required to analyse two kinds of samples. The sequences performed by the industrial robots are modelled by transitions with associated timings that are equal to the real time periods taken by the robots to execute the corresponding actions. Other activities, such as grinding polishing, finishing polishing, reagent attack, washing, microscope examining, have been also modelled by means of transitions. The execution of a transition (the completion of a real sequence) is possible if certain conditions are met. These conditions are modelled into the Petri net by means of positions. The number of samples that can be analyzed in a certain time period can be determined by simulation.

Influence of transverse stress exerted at room temperature on the superconducting properties of Nb3Sn wires

Results on the irreversible degradation of Rutherford cables caused by transverse stress applied at room temperature have been recently published in the frame of the Future Circular Collider study for the 16 T dipole development programme. In order to investigate the degradation process of Nb3Sn composite superconductors, experiments based on wires of the same type as used in the cable have been launched. Nb3Sn wires produced by the Restacked-Rod Process were exposed to transverse stress at room temperature and the degradation of their properties was measured. Transport current measurements near the critical temperature at self-field and magnetisation measurements with a SQUID magnetometer were performed to determine changes in performance. In addition, metallographic preparations in combination with scanning electron microscopy were carried out to reveal cracks in sub-elements. In order to determine the stress distribution within the wire, especially the stress on the sub-elements, a finite element method analysis based on results from x-ray tomography was performed. The influence of transverse stress exerted at room temperature on the critical current density and intrinsic properties is analysed and compared with the microscopic observations.

Liquid Metal Embrittlement of Copper Brazed Plate Heat Exchangers

Plate heat exchangers are widely used for heating and cooling activities in domestic and industrial applications. The plates of all these components were made of stainless steel (e.g. type X2 CrNiMo 17-12-2) and they were brazed with pure copper. After cutting and metallographic preparation, the samples were investigated by light optical microscopy (LOM) to measure the depth of copper penetration into the plates (liquid metal embrittlement (LME)). For seven heat exchangers the penetration depth was between 25 and 60 μm, but for one the depth was about 150 μm. At the steel-copper interfaces an additional phase was identified as chromium-rich ferrite by electron beam micro analysis (EBMA). To get information about the sensitivity against corrosion electrochemical tests were performed in two electrolytes (HCO3- + SO42- and HCO3- + SO42- + Cl-). It can be summarized that in the Cl- free electrolyte the copper was attacked mainly. In case of the Cl- containing electrolyte beside the copper braze, the LME zones in the steel were corroded additionally.

High Temperature Oxidation of Zircaloy-4 Under Conditions Simulating a Loss of Cooling Accident in Spent Fuel Pools Examined with Raman Imaging and 18O Tracer Techniques

Since the Fukushima accident, an increased attention is continuously paid to the vulnerability of the Spent Fuel Pools (SFPs). In SFPs, for different reasons, degradation of the fuel rods induced by oxidation in air-steam mixtures after dewatering is a major safety concern: (i) the cladding material is the only barrier left against fission product dissemination, (ii) the heat released by the oxidation reactions can overcome the residual decay heat of the fuel and may become the main driving force to the accident escalation, (iii) hydrogen production is a strong concern because of the presence of steam, (iv) presence of air in the atmosphere is known to be an aggravating factor because of the “catalytic” role of nitrogen on the oxidation mechanism. In this study, high temperature oxidation tests in oxygen, air and air - steam atmospheres were performed with Zircaloy-4 cladding tubes and plates specimens. Because spent fuels are concerned, the presence of the corrosion oxide scale formed in reactor during in-service operation was also considered. The tests described in this study were restricted to the 700 – 950°C range, and mostly performed at 850°C. Raman imaging was used to examine the specimens after the oxidation tests, either directly at the oxide surface, or on metallographic preparations for cross-section examinations. We first used new opportunities given by Raman imaging to analyze, at least qualitatively, the structural and mechanical phenomena involved in this particular corrosion phenomena. Indeed, this method can provide information on the nature of phases that are present in the scale and on the stress level with sub-micron spatial resolution. The different methods used to extract this information that is present in the Raman data will be described and discussed. Moreover, isotopic substitution was also considered. Actually, Raman spectroscopy has also the ability to quantitatively extract the 18O distribution in a zirconia oxide scale after a two stage experiment with a fair accuracy. Raman imaging gave clear evidence for different characteristic distributions of 18O in the scales. Some of them have been correlated with the development of cracks and porosity in the oxide which allows the corroding medium to penetrate locally in the scales. Nitrogen appears to have no or limited influence on the oxygen diffusion, but is observed to reach the metal/oxide interface much faster than oxygen. Thus, we also emphasize the potential of this method to investigate the 18O distribution in such complex corrosion scales.

Ultrasonic Detection of Microstructure Features in Ti-Alloyed Ductile Iron

Ultrasonic detection is used as a nondestructive testing method to check the quality of the ductile iron castings. In the current study, ductile iron was alloyed by Ti up to 0.3 wt% and the influence of alloying on the microstructure and graphite morphology was investigated using a step-like mold of 5, 10, 20 and 30 mm thickness. The results were then confirmed by microstructure investigation of the samples using ultrasonic waves. It was found that the speed of ultrasonic waves decreased from 5556 to 5452 m/s when Ti addition increased from 0.0 to 0.3 wt%, which decreased the nodularity from 80 to 45 at 30 mm thickness. Also the amplitude of the echoes decayed more rapidly with increasing Ti content. Referring to the microstructure changes with Ti addition, it was found that increasing Ti up to 0.3 wt% distorted the graphite nodules and decreased their count in the as-cast parts. This decreased nodularity resulted in the decrease in the speed of ultrasonic waves and affected their attenuation as well. It was concluded that the effect of changing the amount of Ti as alloying additive within a narrow range (0.0–0.3) could be successfully detected by ultrasonic waves without destroying the cast part for metallographic preparation.

Immediate development of processing windows for selective electron beam melting using layerwise monitoring via backscattered electron detection

The availability of a reliable processing window is the basic requirement for processing new materials via selective electron beam melting (SEBM). Typically, these processing windows are derived by a time-consuming procedure comprising fabrication, metallographic preparation and analysis of standardized specimens for every parameter set. This study demonstrates the immediate development of a processing window during one single SEBM process. An electron optical image acquisition system provides the necessary information for evaluation and subsequent adaption of process parameters. It is shown that this immediate approach delivers processing windows with sufficient accuracy, while the required time is substantially reduced from weeks or even months to several hours.

Examinations on the Grain Size/Scattered Light Value Relation of Extruded Aluminium AIMSi0.5

Abstract The quality of technical surfaces depends significantly on the resource structures of the semi-finished product. To confirm the impact of the crystalline composition on the measuring results of scattered light technology we suggest in this article on the example of aluminium to put the measuring results in relation to the micro properties via metallographic preparation and microscopy. The main goal is to be able to determine the grain size by scattered light technology. The results determined show that a coarse grain test during an in-situ examination of aluminium is possible. The measuring method also offers the potential for the surface examination of other types of material.

Microstructural evolution of a dual hardening steel during heat treatment

Dual hardening steels combine precipitation of both secondary hardening carbides and intermetallic phases in a martensitic matrix. Due to this combination, the carbon content necessary to achieve high hardness levels can be reduced, resulting in a decreased amount of large and embrittling carbides.In this study, the influence of different heat treatments on microstructure evolution and secondary hardness is investigated. Different metallographic preparation methods were tested in order to visualize the microstructure. Carbides were characterized using spot-pattern electron backscatter diffraction.For light optical investigations, preparation with V2A-pickle lead to the best results. Preparation with colloidal silica suspension achieved the best results for investigations by scanning electron microscopy and for carbide characterization using electron backscatter diffraction. It was found that a homogenization treatment prior to austenitization was unable to increase the amount of dissolved carbides, and thus had no effect on secondary hardness. By increasing the austenitization temperature, the amount of carbides and secondary hardness could be increased significantly.

Thermally induced morphology changes of wire arc sprayed copper and corrosion-resistant steel (316L)

This contribution deals with the determination of the mechanical and thermal properties of thermally sprayed materials. To this end, a comprehensive characterization of copper (Cu 98.7) and corrosion-resistant steel (316L) is carried out by temperature-dependent tensile, three-point bending and caloric tests within a temperature range from 293 K up to 1173 K. For this purpose, thick coatings were manufactured by wire arc spraying. The resulting volume was further processed by electrical discharge machining into suitable test samples. Reference specimens from the corresponding solid materials were also tested for evaluating the morphological features of the coatings. During the calorimetric examinations, the thermally applied materials were exposed to temperatures of up to 1173 K. As a result, permanent property changes could be observed. To investigate these heat treatment effects, metallographic preparations and XRD analyses were performed. Based on the results, conclusions on the morphology of the thermally applied materials could be drawn.

Preparation of Polished Sections and Determination of the Phase Composition of Highly Metastable TRIP Steel by Microscopic and Radiographic Methods

Abstract High alloy TRIP steels offer an excellent combination of strength and formability. The pronounced metastability of the γ austenite present in the material in its unstressed condition places high demands on the metallographic preparation of the section as it must ensure a low level of transformation, i. e. a low level of deformation. In addition, these steels contain phases with an identical lattice structure, i. e. the differentiation of the phases is made considerably more difficult when established methods for the phase analysis such as EBSD and XRD are used. In this work, a methodology for a preparation of polished sections offering a low level of transformation is developed and compared to conventional preparation methods. In this context, various phase analysis methods are compared. In addition, a possibility for a quantitative phase analysis of these multi-phase materials by combination of microscopic and radiographic methods is presented.

Handling of Blackplate in Metallographic Preparation and Heat Treatment Optimization via Quenching Dilatometry

Abstract A prerequisite for the successful optimization of a heat treatment for high-alloyed steels is the knowledge about the present initial microstructure. The metallographic preparation and characterization of the initial microstructure poses a challenge, especially for blackplate, due to its low thickness. In this article, a method is presented which facilitates the handling of blackplate during metallographic preparation. With a precise knowledge of the initial microstructure, tests for the optimization of a heat treatment for such blackplate can also be performed. An optimization of the heat treatment process is particularly important for energy and resource efficient manufacturing processes. In this work, a preparation method was developed which allows winding blackplate to hollow cylinders to determine the optimized heat treatment parameters with the help of a quenching and deformation dilatometer. It will be shown that the two methods presented are an efficient but yet reliable way to optimize heat treatment processes of blackplate via laboratory tests.

Microstructure characterization of W-Ni-Fe heavy alloys with optimized metallographic preparation method

Tungsten heavy alloys (WHA) have been widely adopted in many engineering applications due to their excellent physicochemical properties. Microstructure characterization is a very powerful method ranging from testing materials properties to detecting material failures and defects. However, the microstructure of WHA was not well characterized with the sample prepared by conventional method of etching with strong acids, bases or oxidant after polishing for the coarse surface. To solve the problem, the exact characterization of W-Ni-Fe heavy alloys microstructure was conducted using the sample prepared by optimized polishing method. A series of experiments were conducted to find the suitable polishing conditions including polishing pad, abrasive slurry and slurry pH. The results show that a smooth and clear microstructure sample was obtained by polishing with alkaline colloidal silica and IC polishing pad, which contained few defects compared with that obtained by conventional method of etching after polishing. The microstructure of W-Ni-Fe heavy alloys was analyzed by XRD, EDS tests and EPMA detection. The slipping phenomenon was observed in nanoindentation test using the prepared sample for the first time and the mechanism of satisfactory microstructure sample preparation was illustrated.

Material Investigations on Guitar Strings

Abstract The present investigations on e-guitar strings were performed aiming at providing an overview of the variety of materials used for strings and evaluating the strings’ influence on sound generation. The main emphasis of these investigations is on determining the used materials and dealing with questions regarding the metallographic preparation of these material compounds made up of a wrap wire and a core wire. The main difficulties during the preparation arise mainly from very thin-walled sample areas which are characterized by different chemical compositions and an arrangement close to one another and their mechanical-technological pretreatments.

Effect of ion and neutron irradiation on oxide of PHWR fuel tube material

The effect of heavy ion irradiation on the stability of oxide phase of autoclaved Zircaloy 4 fuel tube material, has been studied using Glancing angle X-ray diffraction (GIXRD) technique after 306 KeV Ar+9 ions irradiation at a dose of 3 × 1019 Ar+9/m2. To estimate the extent of damage, a simulation was carried out using “Stopping and Range of Ions in Matter (SRIM-2008)” computer program based on the Monte Carlo method. For the first time, the oxide formed in 0n1 irradiated fuel tube after 7600 MWD/T burn up in Pressurized Heavy Water Reactor (PHWR) has been characterized using GIXRD technique. The advantage of this technique is that the stress-induced phase transformation, which normally occurs during metallographic sample preparation for optical and electron microscopy, is eliminated. The un-irradiated autoclaved oxide in the steam environment (415 °C and 500 °C), both uniform as well as nodular oxide has been characterized using GIXRD, X-ray photo-electron spectroscopy (XPS), scanning electron microscopy (SEM) attached with Energy Dispersive spectroscopy (EDS). Cross-sectional transmission electron microscopy has been carried out in uniform oxide before and after heavy ion irradiation. It is observed that heavy ion and neutron irradiation induce monoclinic to tetragonal phase transformation in the oxide. Presence of significant fraction of tetragonal ZrO2 phase as well as sub-oxide Zr3O has been identified in the oxide layer near oxide-coolant interface in 0n1 irradiated in-pile sample whereas in unirradiated autoclaved oxide these phases are present in a small fraction near the oxide-metal interface. XPS analysis indicates the difference in the chemical state of alloying element in the oxide when autoclaving is carried out at different temperatures.

Analysis of cut orientation through half-finished product using WEDM

ABSTRACTWire electrical discharge machining (WEDM) is a non-traditional technology utilized to manufacture parts of complex shapes or machining the materials conventionally difficult to process. It is also applied in the aerospace industry, where Inconel 625, which was the subject of the research in this study based on aDesign of experiment (DoE), is usually machined. In this way, the experiment contained 33 rounds during which the settings of machined parameters were changed systematically. An identical experiment (33 rounds) was performed in which the semi-finished product was rotated by 90° to define the impact of the orientation direction of the cut on the speed of cutting and the quality of the surface and subsurface layers. Subsequently, a complex analysis of the machined surfaces of the individual samples was performed, including both the surface and subsurface layers. Topography was evaluated using a non-contact profilometer, including the creation of a color-filtered 3D surface relief. The subject of further investigation was the analysis of morphology and surface defects including the chemical composition analysis. For a very detailed observation of the distribution of the individual elements in the material, lamella was prepared, which was subsequently studied using a Transmission Electron Microscope (TEM). For the subsurface layer study and its defects, metallographic preparations of all samples, which were observed using both light and electron microscopy, were prepared. On the basis of the analyses performed, the impact of the cutting direction on the cutting speed as well as the machined surface quality was determined.

Identification of phases and precipitates by AFM in duplex stainless steel unions

ABSTRACTAtomic Force Microscopy (AFM), a characterization technique that generates topographic images of surfaces at very high resolutions, works by recording the surface relief details of the material with a cantilever which moves over the sample, while a piezoelectric sensor monitors changes in height. This technique is useful for identifying phases, particles and precipitates on a nanoscale and is therefore proposed for identifying them in the fusion zone of welded components and for early identification of faults due to cracking. This is a technique that does not require conductive samples, or special metallographic preparation of specimens as is the case with electron microscopy. The material studied is a duplex 2205 stainless steel, welded by means of gas tungsten arc welding (GTAW). The welded material was subjected to stress tests. The test specimens were characterized using optical microscopy, scanning electron microscopy, atomic force microscopy and nanoindentation tests. The results focus on the identification of the specific magnetic, topographic and hardness characteristics of each of the phases, particles and areas present in the base metal, the fusion zone and the zone deformed by the fracture.

Impact of the Microstructure of Refractory Metals on their Mechanical Properties – a Multi-Scale Study

Abstract High-performance materials play a dominant role in modern society. Without them, modern manufacturing and transportation technologies would, for instance, be impossible. In order to purposefully improve and optimize the potential of these materials based on their properties, a multi-scale understanding of the interaction of microstructural elements and mechanical properties is essential. Such understanding can be achieved by the specific analysis of the interaction of microstructural components such as interfaces, crystal structures, precipitates, and other defects in the material, as well as of their impact on the underlying deformation processes in complex alloys. For this purpose, not only a precise metallographic preparation of the individual microstructural constituents is performed but also, and in particular, rate- and temperature-dependent plastic deformation processes are determined. On the one hand, this is done on a local scale using micromechanical examination methods such as nanoindentation or uniaxial micropillar-compression experiments. On the other hand, experiments on the global scale with compression and tensile tests are performed. These mechanical characteristics are subsequently correlated with structural and chemical high-resolution analyses based on electron microscopy and atom probe tomography methods. Thus, reliable mechanistic models of the dominating deformation mechanisms of high-performance materials can be created based on these examinations – even under harsh conditions such as elevated temperatures or aggressive environment. This targeted correlative interaction of metallography, high-resolution microstructural analysis, and mechanical deformation experiments is demonstrated by the examples of the refractory metals Mo and Cr as well as on a Mo-Hf-C alloy.

New Steel Combinations Produced by the Damascus Technique

Multilayered forged steel plates, which combine the properties of diverse steel qualities, are referred to as Damascus steels. Since the 3rdcentury AD blades and weapons have been produced by the Damascus technique in Europe. In this work four different steel combinations were investigated. Combining Fe with carbon steel C60 resulted in a ferritic-pearlitic microstructure. By forging two heat-treatable steels C40 and C60 martensite with an inhomogeneous carbon distribution was formed. Combining Fe with an austenitic stainless steel showed ferrite and austenite with grain boundary carbides and segregation bands. The last combination of two cold working steels K110 and K600 led to a complex microstructure with martensite, retained austenite and two special types of carbides. After metallographic preparation and using of different etchants the various microstructures were characterized by light optical microscopy and confirmed by Vicker ́s microhardness measurements. Of high interest are the interfaces and the quality of the weld between the individual steel layers. In some regions oxidation and carbon diffusion were observed.

Dynamic Response and Microstructure Evolution of AA2219-T4 and AA2219-T6 Aluminum Alloys

In this study, the dynamic deformation behavior of AA2219 aluminum alloy was investigated in two different temper conditions: T4 and T6, with a view to determining the effect of heat treatment on the microstructure and flow behavior of the material under high strain rates. Split Hopkinson pressure bar experiment was used in determining the dynamic response of the alloy while a digital image correlation system was employed in visualizing and tracking the surface deformation of the specimens. Optical microscopy and scanning electron microscopy were used to assess the microstructure of the material after following standard metallographic specimen preparation techniques. The results obtained showed heterogeneous deformation of the alloy in the two temper conditions. It was observed that the dynamic mechanical behavior of each sample preparation was dependent on its strength properties due to aging type, which in turn controls the metamorphosis of the strengthening precipitates and the initial microstructure. At the maximum strain rate of 3500 s−1, transformed bands leading to crack nucleation was observed in the AA2219-T4 aluminum alloy while AA2219-T6 had fractured at the same strain rate. The modes of crack formation and growth in the two alloys were found to be similar: nucleation, growth and coalescence of voids. However, shear band bifurcation phenomenon was observed only in the AA2219-T6 alloy.