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Kolonieverhalten und Koloniewechselwirkung in Metall-Keramik-Verbundwerkstoffen auf Basis von Freeze-Casing-Preforms

Fachliche Zuordnung Metallurgische, thermische und thermomechanische Behandlung von Werkstoffen
Förderung Förderung von 2008 bis 2009
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 55968777
 
Erstellungsjahr 2010

Zusammenfassung der Projektergebnisse

Innovative metal/ceramic composites produced by melt infiltration of ceramic performs prepared by freeze-casting technique have been examined for the first time in this study. These composites exhibit a characteristic hierarchical structure: on a mesoscopic length scale, lamellar domains with sizes up to several millimetres are observed. The individual domains are composed of alternating ceramic and metallic lamellae with thickness in the range from 20 to 100 µm. The structure of the composite under study has striking similarity with that of lamellar two phase alloys (e.g. γ–TiAl based alloys). In these two phase alloys it is observed that the size and distribution of the individual domains control the mechanical properties of the overall material. Hence, the aim of the present study is to characterize the mechanical properties of the composite at different size scales. Elastic properties of samples having multiple and single domains were investigated nondestructively using ultrasonic spectroscopic techniques. The elastic-plastic flow behaviour of individual domains was studied under compression on miniature samples prepared from polydomain material. The damage evolution during compressive loading was observed by in-situ and ex-situ microscopy techniques. Studies of processing-induced thermal residual stress and strain distributions as well as the internal load transfer under external compressive loading were carried out using synchrotron X-ray energy-dispersive diffraction. The results were compared with micromechanical models for fiber-reinforced composites or laminates. The elastic analysis showed that highest stiffness is observed along the direction parallel to the preform freezing direction, the lowest along the direction perpendicular to it. At length scales significantly larger than the domain size, elastic constants of poly-domain samples show transverse isotropy with respect to the freezing direction. Furthermore, the longitudinal elastic constants of poly-domain samples lie close to or within the bounds predicted by the unidirectional fiber model. Individual domains with different orientations show pronounced elastic anisotropy. They were discussed in the light of a model based on 3D laminate structures with alternating layers of random thickness. Compressive mechanical tests of single-domain samples showed that the composite is strong and brittle when loaded along directions parallel to the freezing direction. When loaded along other directions, the behavior is controlled by the soft metallic alloy. The plastic anisotropy is less pronounced than theoretical predictions for simple laminates, which is explained by the presence of bridges between the ceramic lamellae. Compressive behavior of samples with non-coated and Cu- and Cu2O-coated preforms were studied. Detail study of damage mechanism showed that coating of the preform prior to the melt infiltration weakens the interface between the metallic and the ceramic lamellae. This in turn results into a reduced compressive strength along the freezing direction for the samples with coated preform. Study of thermal residual stress and strain distributions showed that strongly fluctuating local phase-specific residual stresses are present in the as-produced state which can be explained taking into account the thermal expansion mismatch of the alloy and the ceramic preform. Studies of internal load transfer under externally applied stresses show that the load transfer mechanism for loading along the freezing direction and along approximately 0° to domain orientation are essentially similar. In the macroscopic elastic regime, the metallic and the ceramic phases share the load. However, once the metallic phase starts to deform plastically, it transfers the load to the ceramic phase and acts only as a supporting material. Load transfer from the metallic phase to the ceramic phase is significant but not complete. This may be attributed to the localized damage taking place within the ceramic lamellae and along the interface. Domain-level mechanical properties of innovative metal/ceramic composites based on freezecast ceramic preforms have been studied for the first time in this work. So far, research has mostly been concentrated on the preparation of samples with one single domain and analysis of their mechanical properties. However, from a practical application point of view, bulk samples having multiple domains are of importance. Elastic analysis of poly-domain samples have been carried out in this study, however, detail experimental study of elastic-plastic flow behavior and damage evolution is required to thoroughly understand the behavior of the composite material. From a methodology point of view, several relatively new and innovative techniques have been used in this project. Energy-dispersive synchrotron X-ray diffraction is not yet a very well known technique to carry out phase specific stress analysis in multi phase materials. This technique was used very successfully in this work to obtain information about complex stress states in individual phases within the composite. Based on the knowledge and expertise gained during this work, energy dispersive synchrotron X-ray diffraction might be used in future to carry out in-situ stress analysis in multiphase materials having complex structures (e.g. in metal/ceramic composites with interpenetrating structure). One drawback of energydispersive synchrotron X-ray diffraction though is the small gauge volume, resulting in relative poor grain statistics and significant measurement errors in coarse-grained phases. A possible solution is to use neutron diffraction as a complementary method for stress analysis in continuative studies. The stiffness of the composite under study is comparable to known metal/ceramic composites made from Al-alloys and alumina. The compressive strengths of single domain samples along freezing direction lie in the range of 700-800 MPa. These along with the facts that the studied composite material is relatively cheap (due to inexpensive processing route of the preform) and versatile in terms of reinforcement content (it is possible to fabricate preforms with wide range of ceramic content, without sacrificing the melt infiltration characteristics), make them potentially attractive for applications where high specific stiffness and strength is necessary and high contact loads prevail.

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