Materials Science Forum Vol. 891

Abstract: Despite nanosegregation of solutes at grain boundaries has been intensively studied in the past decades and numerous theoretical data as well as experimental values on characteristic energies and/or enthalpies and entropies exist some questions remain still unanswered. In this paper some of these questions – Which energetic quantities obtained in different ways, experimentally and theoretically, can be mutually compared What is the segregation site of a solute in the grain boundary core – are discussed in more detail. It will be shown that the entropy of grain boundary segregation plays an important and indisputable role in responding some issues and that understanding of its role will help us to elucidate fundamentals of the grain boundary segregation.

Abstract: The low hardness and poor tribological performance of titanium alloys restrict their wide applications in automotive fields. Nitriding is widely used to improve tribological properties, wear resistance, and corrosion resistance of steel and titanium alloys. Plasma nitriding is becoming increasingly popular because of its high nitrogen potential, short treatment time, and low environmental impact. Recently, considerable interest has been devoted to alternative nitriding methods such as active screen plasma nitriding (ASPN). In this study, a Ti-6Al-4V titanium alloy was nitrided by ASPN using a titanium double screen in order to investigate the effect of applying the double screen on the microstructure of the nitriding layer. The Ti-6Al-4V sample was placed on the sample stage in a cathodic potential. A titanium double screen was mounted on the cathodic stage around the sample stage. The sample was treated for 1-25 hours at 600oC under 200 Pa in 75% N₂ + 25% H₂ atmosphere. After nitriding, glow discharge optical emission spectroscopy (GD-OES) revealed that the thickness of the nitriding layer composed of TiN tended to increase with increasing the nitriding time. The Vickers microhardness of the sample surface nitrided for 25 hours reached approximately 1300 HV. Ball-on-disk wear test revealed that a wear loss of nitrided sample considerably decreased than that of untreated sample.

Abstract: An advanced high strength steel (0.08 %C, 1.79 %Mn, 0.23 %Si) was subjected to different post-weld heat treatments by quenching & tempering treatments (Q&T) after laser welding to reduce the risk of martensite formation in a few seconds based on an idea of quench and partitioning (Q&P), mechanism. The thermal stability of retained austenite, microstructure development and mechanical properties have been studied at 2 tempering temperatures of 440°C (Ms) and 636°C (Bs), both for 15 minutes, by means of electron microscopy, dilatometry, hardness profile and tensile tests. Dilatometer study unveiled that redistribution of carbon atoms and precipitation of transition carbides occur around 150°C and austenite decomposition occur at 600°C. Tempering at 636°C resulted in notable effect on the mechanical properties, while no significant difference was detected at 440°C, except a slight hardness drop. The strength increased up to 12% for the different specimens without significant loss in ductility for all specimens tempered at 636°C, which may be caused by precipitation hardening and recrystallization of martensite lath boundaries during tempering around 600°C.

Abstract: Udimet 520 is a low precipitation strengthened nickel-based superalloy, which was designed and developed to be gas turbine blades at elevated temperatures. However, after long-term service under high stresses and temperatures, the microstructure of the turbine blades could be continually degraded. Therefore, the mechanical properties could be worse than the new ones. The rejuvenation heat treatment of degraded turbine blades, which were made of cast Udimet 520, was following by solution treatment at 1,121oC / 4 hours and then double aging processes including primary aging at 843 oC / 24 hours and secondary aging at 760oC / 16 hours, respectively. However, in practical reheat treatment processes, the temperature during solution treatment could be dropped by error or malfunction of high temperature heating furnace because the furnace has to be operated continually at very high temperature for very long time resulting in final reheat treated microstructures in many nickel base superalloys. To simulate this effect, the droppings of temperature during solution treatment are chosen and performed for 3 levels; 840oC, 800oC and 760oC, which could happen in practical working then heated up again immediately to solution temperature level. The maximum number of temperature dropping during the single solution treatment is up to 3 times. Received results show that the effect of temperature dropping during solution treatment has influenced on the final rejuvenated microstructures slightly due to the low precipitation behavior of the alloy. The long term heating at 800oC and 900oC / 1000 hours provided much effect in gamma prime particle coarsening.

Abstract: The microstructure of the Ni₅₀Mn₂₅Ga₂₅ Heusler alloy (HA) rod cast to a copper mould and melt-spun ribbon was studied. According to X-ray diffraction analyses in both cases, the solidified alloy is a single-phase. In suction casting, the radial columnar crystals grow by cellular or dendritic growth, which leads to chemical inhomogeneity of the whole cast rod on the microscopic scale. The melt-spun ribbon exhibits a homogeneous alloy at the wheel side and an inhomogeneous alloy on the free surface side of the ribbon formed by cellular and dendritic growth. At both types of casting, the columnar crystals growth along the <100> crystal direction. According to texture analyses by EBSD, the fine grain structure at the wheel side of the ribbon exhibits no texture, while the columnar grain structure on the free surface side exhibits the <100> fibre texture with a declination by about 10 degrees in the spinning direction. Melt-spinning leads to significant refinement of the HA microstructure.

Abstract: The Late Bronze Age copper mining site “Gasteil Cu I” (ca 1050–850 BC) is located in the district of Neunkirchen, Lower Austria. Since 2010 five excavation campaigns took place and many artifacts as well as corroded metallic droplets were discovered. Two of these droplets were investigated by metallography. At the droplet ́s surface tin was measured, indicating that these droplets are formed during bronze casting. The small droplet is severely corroded but in its core the original bronze alloy is still present. The analysis showed about 10 wt.% Sn. The microstructure is characterized by a Cu-Sn solid solution and a Cu-Sn intermetallic phase. Additionally small amounts of Cu₂S were observed in the metallic core. The corroded rim contains oxides and hydroxides of Cu and Sn and other impurities like Ca, P, Si, S and Fe. Analyzing the corrosion products, Sn concentrations up to 40 wt.% were measured. This enrichment can be explained by a simultaneous formation of insoluble SnO₂ and Cu ions, which were transported to the surface. Surprisingly, the other large droplet has a corroded core and metallic phases are remaining at the rim. The microstructures of the corrosion products reflect the original casting microstructures of the bronze.

Abstract: Cold drawing of steel tubes refers to a forming operation where a precision steel tube is being formed in a die while reducing tube cross-section, altering the wall thickness and making the final tube much longer than the original hollow. The forming itself takes several drawing passes (a.k.a. draws) depending on the final tube dimension requested. The selection of relative reduction for a particular tube diameter plays an important role because an excessive reduction during drawing causes stresses that induce substantial plastic deformation, eventually leading to cracking. In this paper we evaluate the effect of selected reductions on the longitudinal grain boundary orientation in cold drawn tubes, taking tube sample measurements and making necessary calculations of grain boundary orientation in selected planes.

Abstract: Fibre orientation in short fibre reinforced thermoplastics depends on injection moulding parameters. There are a lot of different parameters that must be established and controlled to achieve proper injection moulding of a plastic part. These parameters fall within four major areas: pressure, temperature, time, and distance. The aim of this article is estimation of fibre orientation in injection moulding plastics parts and comparison of these results with numerical simulated ones. Stereological metallography was used for estimation of experimental orientation of fibres. The orientation of simple fibre may be defined by the two angles θ and Φ. In a Short Fibre Reinforced Thermoplastic (SFRT) component there are frequently millions of fibres, therefore each individual fibre orientation specifying is very impractical. The fibres orientation in space can be described by the probability distribution function (PDF), Ψ(θ, Φ). Numerical modelling of fibre orientation was realised using MOLDEX3D software. Moldex3D is the CAE product for the plastics injection moulding industry. This software allows to view results of fibre orientation as an orientation of the X direction, Y direction, Z direction, the total orientation and orientation at surface. These first three orientations are relevant for the establishment of second-order orientation tensor. They belong to tensor ́s values a₁₁, a₂₂ and a₃₃. Utilization of stereological metallography for short fibre orientation in plastic matrix is very similar to its utilization for estimation of grain boundaries orientation in polycrystalline alloys cased by plastic deformation. In the case of short glass fibres reinforced thermoplastics it’s structure consist of thermoplastic matrix and reinforcing fibres, which has some preferred orientation in most of cases – the structure is anisotropy. The way of scalar measurement of structure anisotropy is determination of degree of orientation. The anisotropic microstructure is decomposed into isotropic, planar or linear oriented components using stereology methods.

Abstract: During a long-term operation of nuclear power plants (NPP), the changes of structural material properties occur. To ensure the safe and reliable operation, it is necessary to monitor and evaluate these changes mainly on components from primary circuit of NPPs. One of the dominant ageing mechanisms of NPP components besides the radiation embrittlement and the fatigue loads is the thermal ageing. The thermal ageing is the temperature, material and time dependent degradation mechanisms due to long-term exposure at the operating temperature of 570 K.This paper describes the project for thermal ageing monitoring at primary piping in NPP Bohunice Unit 3. There are summarized the results obtained from evaluation of original primary piping material.