Materials Science Forum Vols. 825-826

Abstract: Within the large variety of different additive manufacturing technologies stereolithography excels in high precision and surface quality. Using the Digital Light Processing (DLP) Technology a stereolithography-based system was developed, which is specifically designed for the processing of highly filled photopolymers.The powder-filled suspension enables the 3D-fabrication of a so called ceramic green part. In order to get a dense ceramic structure, subsequent thermal processing steps after the 3D-printing process are necessary. First, the polymer-ceramic composites heated up to 400°C. During this processing step, called debinding, the organic components are burned out. The resulting part, consisting of powder particles stabilized by physical interactions, is further heated to sinter the particles together, and the final, fully dense ceramic part is obtained.The debinding step is the most critical process. The used components have different evaporation or decomposition temperatures and behaviors. Thereby a reduction in weight and also in dimension occurs, which depends on the portion and composition of the organic components and especially on the temperature cycle. Furthermore, the physical characteristics of the ceramic powder, such as the particle size and the size distribution influence the debinding behavior. To measure the changes in weight and dimension a thermo-gravimetric (TGA) and a thermo-mechanical analysis (TMA) can be used. To avoid too high internal gas pressures inside the green parts a preferably constant gas evolution rate is seeked. Also the ‘surface-to-volume ratio’ affects the debinding characteristics. Therefore, optimized debinding cycles for specific geometries allow the crack-free debinding of parts with a wall thickness up to 20 mm.

Abstract: High temperatures (HT) are essential for many production methods. When operating with abrasives from natural sources, e.g. ores, abrasive wear is limiting the lifetime of core components. Material degradation and wear is accelerated in HT environments; wear protective solutions are necessary to minimize maintenance efforts. Especially, cost efficient solutions are needed in the industry, thereto in this research different Fe-based materials with prospective good HT wear pro­perties were chosen: i) austenitic stainless steel, ii) martensitic hot work tool steel, iii) carbide-rich ferrite MMC (metal matrix composite) and iv) complex alloyed hardfacing MMC. The materials were investigated by conducting high stress abrasion tests at temperatures up to 700°C. Wear rates were calculated and wear reducing effects by formation of mechanically mixed layers (MML) were studied. Two different technical approaches were carried out: SEM investigation of the surface coverage by abrasive embedding and optical microscopy analysis on cross-sections to determine the penetration depth of the abrasives. All materials except the hardfacing showed significant MML formation. Results showed increased coverage and penetration depth with ascending temperature. A wear reducing effect predominantly at HT is suspected, as MML forming materials showed shallower wear increase with temperature compared to the hardfacing.

Abstract: High temperature squeeze casting enables the fabrication of composite structures for alloys with high melting points, suitable for structural and tribological applications at high temperatures. Nickel-chromium alloys with chromium contents of about 20 wt% are extremely resistant to corrosion and exhibit operation temperatures up to 1000 °C due to the high solubility of chromium in nickel and its high melting point. The creep and wear resistance of the metal matrix composite material (MMC) is achieved by a stable ceramic backbone of Alumina with a bimodal pore structure. The fabrication of interpenetrating nickel-chromium/alumina MMCs, namely NiCr8020/Al₂O_3/50_pp, at temperatures above 1550 °C is shown. A special infiltration tool geometry has been used to withstand thermal and mechanical strain, necessary for the infiltration of preforms with 40 mm in height. The infiltration was performed in a unique high temperature squeeze casting device with tool temperatures of about 700 °C. Infiltration duration (pore filling) was as long as about 12 s. For this, the thermal management of the tool is the demanding part. Whereas the inner cavity has to be as hot as possible to enable infiltration and to prevent premature solidification. In contrast the outer side of the infiltration tool has to be as cold as possible to withstand the infiltration pressure applied directly to the squeeze casting tool.

Abstract: Over the last years, new alloys were developed to create metallic glasses showing high crystallization temperatures. Such metallic glasses generally can be embedded into other materials when processing temperatures are lower than crystallization temperatures. As recent studies show, feasible crystallization temperatures may exceed the melting point of common metals and fabrication of metallic glass particle reinforced MMCs is now not only possible by powder metallurgical methods but also by processes using melt infiltration. Hence, these metallic glasses offer a high potential for use as reinforcements in a lightweight metal matrix such as aluminum: By incorporation of metallic glass structures into a ductile matrix, it is possible to utilize its outstanding advantages like high strength and elastic strain limit while circumventing its negative properties like brittleness.The particle reinforced composites in this contribution were produced by gas pressure infiltration. This process includes melt infiltration of a particle filled mold using pressurized gas. To keep a sufficient separation between processing temperature and crystallization temperature, the metallic glass Ni₆₀Nb₂₀Ta₂₀ (T_x = 721 °C) and the eutectic aluminum alloy AlSi12 with a low melting point (T_m = 580 °C) as matrix metal were selected for process. After infiltration, the fabricated MMCs were investigated by micro computed tomography (µCT) to analyze the particle distribution within the composite. Furthermore, mechanical tests and elastic analysis using ultrasound spectroscopy were performed to classify its properties.

Abstract: Ceramic freeze cast preforms based on alumina show an anisotropic behavior due to directional freezing during preform production. Beside the specific characteristics such as alumina content and lamellae spacing also the distribution of the so-called domains – regions with a relatively homogeneous orientation of alumina lamellae – play an important role considering stiffness and strength. The gas pressure infiltration process was used for infiltrating the freeze cast preforms with a eutectic aluminum/silicon alloy with a low melting point. Selected regions taken from the freeze cast preform have been analyzed via X-ray micro computed tomography (µCT) prior to the infiltration due to a higher contrast in comparison to the infiltrated preforms. The orientation of the lamellae has been determined from the three dimensional data with an algorithm which is based on the structure tensor. The mechanical stability - in terms of the strength - of the infiltrated preforms has been quantified via quasi static compression tests on cuboid samples. The results show a good agreement between the orientation of the lamellae distribution and the maximum strength of the preform which could also be verified using an analytical model.

Abstract: The current contribution deals with metal-matrix composites prepared by paper manufacturing technology. In contrast to conventional techniques, this technology is an energy-and cost-efficient process for the shaping of thin sheets using solid powder mixtures. Conventional and pre-ceramic as well as pre-metallic paper-manufacturing have in common that cellulose (pulp) fibres are loaded with inorganic fillers. The present study is focused on the paper web formation using a metastable austenitic steel powder (16-7-3 %Cr-%Mn-%Ni) and a magnesia partially stabilised zirconia powder as inorganic fillers. The paper web formation was investigated. During filtration of the aqueous fibre-filler suspension the steel particles were incorporated in-between the fibre network and steel clusters were formed. Thus, solid retentions of > 90 wt.% were achieved. Calendering had a positive influence on porosity, bulk density, and tensile strength of the green paper sheets. The development of an optimized debinding process is presented and the microstructural changes as well as phase formations during firing are discussed in response to the residual carbon content. The sintered composites attained ultimate tensile strengths of up to 177 N/ mm2 at a total porosity of 66 %. These metal-matrix composites are promising materials for the shaping of light-weight structures.

Abstract: The composite material tungsten fiber-reinforced tungsten (W_f/W) addresses the brittleness of tungsten by extrinsic toughening through introduction of energy dissipation mechanisms. These mechanisms allow the release of stress peaks and thus improve the materials resistance against crack growth. W_f/W samples produced via chemical vapor infiltration (CVI) indeed show higher toughness in mechanical tests than pure tungsten. By utilizing powder metallurgy (PM) one could benefit from available industrialized approaches for composite production and alloying routes. In this contribution the PM method of hot isostatic pressing (HIP) is used to produce W_f/W samples. A variety of measurements were conducted to verify the operation of the expected toughening mechanisms in HIP W_f/W composites. The interface debonding behavior was investigated in push-out tests. In addition, the mechanical properties of the matrix were investigated, in order to deepen the understanding of the complex interaction between the sample preparation and the resulting mechanical properties of the composite material. First HIP W_f/W single-fiber samples feature a compact matrix with densities of more than 99% of the theoretical density of tungsten. Scanning electron microscopy (SEM) analysis further demonstrates an intact interface with indentations of powder particles at the interface-matrix boundary. First push-out tests indicate that the interface was damaged by HIPing.

Abstract: Through the development of new metal matrix composites, the specific strength and stiffness can be increased above the level of conventional light metal alloys and increase their potential for lightweight applications. The composite extrusion process is a promising manufacturing method for reinforced light metal extrusions. Particularly, the reinforcement with ceramic fibers can increase both the specific strength and stiffness which are essential for lightweight purposes. To exploit the full potential of the reinforcement, the interface of this MMC has to be optimized regarding the load transfer between matrix and fiber and therefore has to offer a strong bonding. In this contribution a hybrid composite is produced by using an Al₂O₃-fiber/AlMg0.2 composite wire which is embedded in an EN AW-6082 extrusion profile. Both the characterization of the interface and determination of the influence of processing and heat treatment are presented. For that purpose, the composites are characterized qualitatively by metallographic analysis and quantitatively by micro push-out testing of the ceramic fibers prior and after composite extrusion. To investigate the influence of additional heat treatment the state as fabricated, which equals a T4 state of the matrix material, as well as a T6 state with additional solution annealing and artificial ageing are compared. It was found that the extrusion process has a beneficial influence on the microstructure and the mechanical interface properties and therefore confirms applicability of composite extrusion for manufacturing of alumina reinforced profiles. The heat treatment however showed no significant influence on the embedded composite wire and its interface properties.

Abstract: MMCs consisting of diamonds and highly conductive metal matrices have been produced via gas pressure assisted liquid metal infiltration and their thermal properties have been investigated. Special attention was paid towards the diamond surface termination and its influence on the diamond-metal-interface and the resulting heat transport across this interface. Altering the diamond terminating surface layer can lead to a rather drastic increase in the thermal conductivity, rendering MMCs with pretreated diamonds double the thermal conductivity of the ones with as-received diamonds. The evolution of those terminating layers with different pretreatment conditions and their influence on the thermal conductivity of the resulting MMCs is rather complex and an ever-growing field of interest for diamond heat sink materials.The observed thermal properties of the MMCs produced in this study will be linked with the established diamond surface termination and will demonstrate the potential that lies within the method of diamond surface modification.

Abstract: During technical operation, high performance materials are partially exposed to high frequency cyclic loading conditions. Furthermore, the small strains in the very high cycle fatigue (VHCF)-regime lead to accumulative damage which causes crack initiation related to an appropriate local deformation leading to final fatal fracture. At the same time, quite high requirements with regard to high number of cycles without any damage are demanded for many applications. Fields of application of these light-weight, but expensive materials, are e.g. in the automobile industry (e.g. engine blocks, cylinder heads, brakes).The fatigue behavior of Al-matrix composites (Al-MMCs) reinforced by alumina particles (15 vol.% Al2O3) or short fibers (20 vol.% Saffil), respectively, was already intensively studied in the LCF and HCF range. The present study is focusing on investigations in the very high cycle fatigue regime at stress amplitudes up to 140 MPa to reach fatigue life of about 1010 cycles. All experiments were carried out using an ultrasonic fatigue testing device under symmetric loading conditions (R=-1). Fatigue tests were accompanied by in situ thermography measurements to record the temperature of the whole specimen and to find “hot spots” indicating changes in microstructure and therefore the initiation or growth of cracks. Moreover, the resonant frequency as well as the damage parameter were evaluated to determine the beginning of damage. For a better understanding of the damage mechanism (matrix decohesion, matrix failure or failure of reinforcement) all fractured surfaces were investigated by scanning electron microscopy. The combination of these methods contributes to a better understanding of the underlying mechanism of damage in aluminum-matrix-composites.