Materials Science Forum Vol. 879

Abstract: Advancements in the design, optimization and manufacture of turbine engine hot-section components during the past few decades have contributed enormously to the improvement in power-ratings and efficiency levels of gas turbine engines. Nickel-base superalloys are extensively used to produce the hot-section components as this class of alloys offer improved creep strength and higher fatigue resistance compared to other alloys due to the presence of precipitate-strengthening γ' phases i.e. Ni₃[Ti, Al, Ta etc.] in the normally face centered cubic (FCC) structure of the solidified nickel. Although this second phase is the main reason for the improvement in properties, it also results in increased processing difficulty as these alloys are prone to crack formation. In this work, we demonstrate powder-bed additive manufacturing of René 142 onto René 125 substrates through scanning laser epitaxy (SLE). René 142 is a high strength, nickel-base directionally solidified (DS) alloy that has high rupture strength, excellent resistance to grain boundary cracking, and superior high-velocity oxidation resistance. Successful deposition of René 142 on René 125 provides an avenue to repair legacy hot-section components by depositing superior quality alloys at the damage locations. The microstructure of the deposited René 142 is observed to follow the polycrystalline or EQ morphology of the underlying René 125 substrate. The SLE processed René 142 exhibits dense and crack-free deposits, and microstructure refinement compared to the underlying cast René 125 substrate. This work is sponsored by the Office of Naval Research through grants N00014-11-1-0670 and N00014-14-1-0658.

Abstract: High Pressure Die Casting (HPDC) is a foundry process particularly suitable for high production rates and applied in several industrial fields, but the amount of scrap, caused by defects or incomplete filling, is sometimes very high. Thus it is important to know which are the main causes of defect formation and their effects on microstructure and mechanical properties. This paper presents, within the European MUSIC project, the qualitative and quantitative results of a study conducted on AlSi9Cu3(Fe) alloy castings, referred to as Horse-shoe Reference Castings, specifically designed to generate different kinds of defects with different severity levels. The work focuses on the correlations obtained between the casting mechanical properties, their defect content in terms of porosity and oxide films and the process parameters adopted, mainly second phase plunger velocity and intensification pressure. The three point bending test was carried out on the four specimens obtained from the two appendixes of the casting. The fracture surfaces were studied by scanning electron microscopy (SEM) and optical microscopy (OM) highlighting that the defect content is clearly correlated to the mechanical properties and the process parameter settings.

Abstract: The hot ductility of Ti-Nb microalloyed steel has been investigated to evaluate the sensitivity to surface crack formation during the continuous casting process. Tensile samples were subjected to different thermal treatments and were tested at deformation temperatures ranging from 650°C to 1000°C using a strain rate of 10^-3s^-1. It has been found, that the investigated steel evinced poor ductility over almost the whole testing temperature range characterized by marked grain boundary cracking, irrespective of which thermal cycle has been utilized or whether the samples have been melted or only reheated. Microstructural examinations and supplementary thermo-kinetic computer simulations revealed distinct Ti-Nb precipitation throughout the microstructure being responsible for the deteriorated materials hot ductility.

Abstract: The present contribution gives an overview on innovative methods to characterize cyclic deformation and lifetime behavior of metallic materials and hybrid joints based on high precision measurement of electrical resistance, temperature and magnetic properties during fatigue testing. General aim is to minimize the number of fatigue tests for reliable S-N curve calculation. Moreover, instrumented cyclic hardness tests allow short-time assessment of cyclic hardening in case of limited availability of test material. The methods are applied to a wide range of materials, from carbon steels, over cast iron and metastable austenitic steels to ultrasonically welded Al-alloy/polymer matrix composites.

Abstract: As the lightest structural metal, magnesium alloys have been attractive to reduce vehicle weight and emissions by lightweight design in the automotive industry. Structural crashworthiness is not a physical property itself, but correlates with the material’s ductility and structural design. Magnesium is known to be a material with lower failure strain than other metallic materials. Therefore the use of magnesium in crash-related areas is more challenging compared to steel and aluminum.In structures with a bending load, as in the case of a bumper or the sill, crash properties can be significant improved by filling profiles with a stabilizing core. In order to evaluate the crashworthiness of this hybrid structure under bending loads, both empty and polyurethane foam-filled rectangular section beams were constructed and tested by using the quasi-static/dynamic three-point bending facilities at German Aerospace Centre (DLR) – Institute of Vehicle Concepts.For structures with axial crash loads the normal buckling mode will lead to a very early fracture of the magnesium part. In collaboration with researchers from the University of Windsor and the University of Waterloo, novel technologies for energy absorption which are based on cutting or peeling mechanisms have been developed and investigated, which allow the use of magnesium in these challenging applications. Results of the joint research will be presented.

Abstract: Microalloying elements Ti and Nb are commonly added to high-strength Dual Phase steels as they can provide efficient means for additional strengthening due to grain refinement and precipitation strengthening mechanisms. In the form of solute elements or as fine carbonitride precipitates, Ti and Nb are also expected to have a significant effect on the microstructural changes during annealing and especially on recrystallization kinetics. The present work investigates the influence of microalloying elements Ti and Nb on recrystallization in various cold-rolled Dual Phase steel grades with the same initial microstructure but different microalloying contents. Using complementary experimental and modeling approaches makes it possible to give some clarifications regarding both the nature of this effect and the comparative efficiency of Ti and Nb on delaying recrystallization. It is shown that niobium is the most efficient micro-alloying element to impede recrystallization and that the predominant effect is solute drag.

Abstract: The deformation microstructures and their effects on mechanical properties of austenitic stainless steels processed by cold rolling at ambient temperature to various total strains were studied. The cold working was accompanied by the development of strain-induced martensitic transformation because of meta-stable austenite at room temperature. The strain-induced martensitic transformation and deformation twinning promoted the grain refinement during cold rolling, leading to nanocrystalline structures consisting of a mixture of austenite and martensite grains with their transverse grain sizes of 50-150 nm containing high dislocation densities. The rolled samples experienced substantial strengthening resulted from high density of strain induced grain/phase boundaries and dislocations. The yield strength of austenitic stainless steels could be increased to 2000 MPa after rolling to total strains of about 4. The martensite and austenite provided almost the same contribution to overall yield strength. The dislocation strengthening was much higher than the grain boundary strengthening at small to moderate strains of about 2, whereas the latter gradually increased approaching the level of dislocation strengthening with increasing the strain.

Abstract: In this study, aluminium rods were cold extruded in a direct process by KOBO method in two variants: variant I with varying (decreasing) frequency of die oscillations necessary to maintain a constant extrusion force, and variant II with constant frequency of die oscillations, leading to a decrease in the extrusion force. The tensile test of rods was carried out in a temperature range of 20 - 200°C and at a strain rate from 8xE10^-5 to 8xE10^-1 s^-1. Significant differences in the elongation of the tested rods were observed. It was found that rods extruded at variable die oscillations and stretched at room temperature had similar elongation, independent of the strain rate. With the increase of temperature, the elongation of samples stretched at a low speed was growing from a value of about 8% at room temperature up to 40% at 200°C. At high strain rates, despite the increasing temperature, the elongation remained at the same level, i.e. 5-6%. In rods extruded at constant die oscillations, the elongation at a low strain rate was growing with the temperature from 10% at room temperature up to 29% at 200°C. At high strain rates, the elongation decreased from 28% at room temperature to 11% at 200°C. The results were interrelated with examinations of the structure of rods and fractures of tensile specimens. In the material extruded by KOBO method with constant die oscillations, the beginnings of the recrystallization process were observed, absent in the material extruded at variable die oscillations.

Abstract: Conventional Bioreactor systems for cultivating cells in Life Science have been widely used for decades. An in vitro cell cultivation bioreactor should reliably and reproducibly mimic the in vivo microenvironment of the cultured cells. Normally, mammalian cell cultures are performed in conventional bioreactor devices such as culture flasks and culture-dishes. However, these tools have fundamental limitations due to being inappropriate for high throughput screening and consume a considerable amount of resources and time [1]. Therefore, there is a trend towards miniaturization, disposables and even micro platforms that fulfill increasing demands strongly aiming for production and testing of novel pharmaceutical products. Here we present the development and manufacture of a disposable miniaturized flow-through bioreactor system that can be produced in large numbers at low costs. nanoporous hollow fibers are located at the fluidic sources and drains of the miniaturized bioreactors and retain cells. The necessary mixture of oxygen and carbon dioxide is provided via diffusion through a semi-permeable membrane. Fluidic connections allow the continuous feeding of the cells adding nutrient solution at constant rates at the inlet of the micro bioreactor and removing the solution at the same rate at the outlet. This medium can be collected and used for subsequent analysis. Different designs and concepts for such bioreactors were carried out with varying numbers of plates, and integrated or joined miniaturized reactor chambers. First tests show full technical and biological functionality, cells could successfully be cultivated at high viability rates for some days.

Abstract: Several are the instrumental tests currently available in the diagnosis of retinal diseases. Their outputs are typically images of anatomical portions of the eye. However, these are useful to highlight only few aspects of the characteristic lesions of the occurring pathology. For this reason, the clinician needs to have tools able to perform comparative analysis of the different diagnostic images by using procedures of integration of the different clinical information contained in each of them. In this paper, we will describe a new medical software tool (MicroRetina) tailored for the comparative analysis of images of the fundus oculi, acquired during diagnostic tests that make use of different medical instruments. The developed software is an open system able to manage images acquired by different instruments and it will be able to give a helpful support to one of the existing problem in performing diagnosis by using images of the fundus oculi. In particular, it will allow the clinician to perform in real time the comparative analysis of the different clinical findings allowing him to have further diagnostic information not otherwise available by the analysis of the single images only.