In this thesis, the effect of mechanical deformation on structure, thermal stability and hardness of a single-phase spray-deposited quasicrystalline alloy with composition Al62.5Cu25Fe12.5 has been investigated in detail. The purpose of the investigation was to study the effect of mechanical milling at different milling speeds (which approximately scale with the milling intensity) on mechanically-induced phase transformations during milling and on the phase evolution during subsequent heating. The results of the milling experiments indicate that, irrespective of the milling speeds used, mechanical milling of Al62.5Cu25Fe12.5 quasicrystals leads to the formation of a disordered CsCl-type ß phase with grain size of about 10 – 20 nm. The analysis of the kinetics of the QC–to–ß phase transformation reveals that the milling intensity has a considerable effect on the characteristics of the transformation. The increase of the milling speed considerably shortens the incubation time needed to start the QC–to–ß phase transformation. Also, the overall transformation is much faster for milling at high speeds. The QC–to–ß phase transformation starts when the grain size of the quasicrystals is reduced to about 10 nm irrespective of the milling speed used and clearly indicates that a critical grain size of the quasicrystals for initiating the transformation exists. On the other hand, no critical value of lattice strain was found for the QC–to–ß transformation. This indicates that the phase transformation is controlled by the local length scale (i.e. the grain size) and by the corresponding grain boundaries rather than by the energy stored in the lattice. Energetic considerations obtained through a simple model based on the mass and velocity of the milling balls reveal that the energy needed for the QC–to–ß transformation increases with increasing the milling speed, that is, the energetic efficiency of the process decreases with increasing the milling intensity. This indicates that part the extra energy supplied during milling at high intensities is not used to induce the phase transformation but it is dissipated by heat. During heating, the milled powder displays a multi-step thermal behavior characterized by the grain growth of the disordered ß phase at low temperatures, followed, at higher temperatures, by its transformation into the original icosahedral quasicrystalline phase. The transformation is gradual and the quasicrystals and the disordered ß phase coexist over a temperature interval of more than 250 K. The phase transformations occurring during milling and subsequent annealing have a remarkable effect on the hardness, which can be tuned within a wide range of values (7–9.6 GPa) as a function of the volume fraction of the different phases. This suggests that a composite material with optimized mechanical properties can be produced by an appropriate thermo-mechanical treatment. The quasicrystals milled at a very low speed show a transition between Hall-Petch to inverse Hall-Petch behavior at a grain size of about 40 nm, which represents the critical value for grain size softening of the present Al62.5Cu25Fe12.5 quasicrystals. This behavior may be attributed to the complexity of the quasicrystalline structure and to its peculiar deformation mechanism at room temperature (i.e. shear banding), where meta-dislocation-assisted deformation is almost absent. In order to analyze the effectiveness of the Al62.5Cu25Fe12.5 quasicrystals as reinforcing agent in metal matrix composites, Al-based composites were synthesized by hot extrusion of elemental Al blended with different amounts of Al62.5Cu25Fe12.5 quasicrystalline particles. The work was focused on two specific aspects: evaluation of the mechanical properties through room temperature compression tests and modeling of the resulting properties. The addition of the quasicrystalline reinforcement is very effective for improving the room temperature mechanical properties of pure Al. The compressive strength increases from 155 MPa for pure Al to 330 and 407 MPa for the composites with 20 and 40 vol.% of reinforcement, respectively, reaching an ultimate strain of 55 % and 20 % before fracture occurs. These results indicate that the addition of the QC reinforcement leads to composite materials with compressive strengths exceeding that of pure Al by a factor of 2 – 2.5, while retaining appreciable plastic deformation. The mechanical properties of the composites have been modeled by taking into account the combined effect of load bearing, dislocation strengthening and matrix ligament size effects. The calculations are in very good agreement with the experimental results and reveal that the reduction of the matrix ligament size, which results in a similar strengthening effect as that observed for grain refinement, is the main strengthening mechanism in the current composites. Finally, the interfacial reaction between the Al matrix and the QC reinforcement has been used to further enhance the strength of the composites through the formation of a new microstructure consisting of the Al matrix reinforced with Al7Cu2Fe w-phase particles. The optimization of the structure-property relationship was done through the systematic variation of the processing temperature during consolidation. The mechanical behavior of these transformation-strengthened composites is remarkably improved compared to the parent material. The yield strength of the composites significantly increases as the Al + QC -> ω transformation progresses from 195 MPa for the sample reinforced only with QC particles to 400 MPa for the material where the Al + QC -> ω reaction is complete. These results clearly demonstrate that powder metallurgy, i.e. powder synthesis by ball milling followed by consolidation into bulk specimens, is an attractive processing route for the production of novel and innovative lightweight composites characterized by high strength combined with considerable plastic deformation. In addition, these findings indicate that the mechanical behavior of Al-based composites reinforced with Al62.5Cu25Fe12.5 quasicrystalline particles can be tuned within a wide range of strength and plasticity depending on the volume fraction of the reinforcement as well as on the extent of the interfacial reaction between Al matrix and QC reinforcing particles.

No description available.

Aluminium matrix composites are among the most promising candidate materials for light weight and high strength applications such as transportation and armour. In a previous study 6061 aluminum matrix composites reinforced with plain weave carbon fiber preform (AS4 Hexcel) were successfully fabricated by squeeze casting using the laminate fabrication technique. This research aims at optimizing the fabrication process in order to achieve improved strength and mechanical properties. It focuses on the liquid infiltration squeeze casting method. Good mechanical bonding between fiber and aluminium is achieved thanks to improved infiltration and impregnation of the fabric by liquid aluminium. Oxidation products at fiber/aluminium interface and porosity are reduced. As a result, composites are produced with overall improved mechanical properties. The flexural strength is increased by up to 19.9% and 15.4% compared to the laminate approach and the reference 6061 aluminium alloy squeeze cast under identical conditions, respectively. Similarly, overall hardness is improved. However, the impact strength is reduced by 7.76% and 25.78% when compared to casts fabricated by the laminate method and the reference aluminium alloy, respectively. The thesis constitutes a good basis for further research on fiber and particle reinforced aluminium matrix composites with the goal of further improving fracture toughness, particularly for gradient materials used in armour applications.

In Kenntnis der beschriebenen verfahrenstechnischen Einflussfaktoren auf die Struktur und die Eigenschaften der faserverstärkten MMC geht es darum, ein prepregtechnologisches Verfahren für das Herstellen lang- bzw. endlosfaserverstärkter MMC mit Aluminiummatrix zu erarbeiten und die Machbarkeit im Hinblick auf die Reproduzierbarkeit und das Erreichen der prinzipiell möglichen mechanischen Eigenschaftskennwerte nachzuweisen. Es geht darum, den Einfluss der Prozessparameter auf die Struktur der Prepregs und der verdichteten MMC`s zu erfassen und zu bewerten. In Kenntnis dieser Zusammenhänge sind die qualitätsrelevanten Kennwerte der Werkstoffstruktur denen der mechanischen Eigenschaften gegenüberzustellen. In Weiterführung der o.g. Darstellungen zwischen Struktur- und Eigenschaften der Verbundwerkstoffe geht es darum, die experimentell bestimmten Kennwerte der Festigkeit und des E-Moduls auf Übereinstimmung mit den entsprechenden Korrelationsmodellen (Shear Lag Modell, Grenzwert- und Modellkonzept, EIAS-Methode) zu überprüfen. Ferner geht es darum, die Zusammenhänge zwischen den im Modell definierten idealen Gefügebedingungen mit den realen im Hinblick auf deren Einfluss auf die Eigenschaftskennwerte kritisch zu diskutieren. Aufbauend auf den definierten Zielstellungen kann nach der erfolgten Bewertung aller Untersuchungsergebnisse gezeigt werden, dass die entworfene Technologie zur Herstellung langfaserverstärkter Metallmatrix-Verbundwerkstoffe dazu geeignet ist, Fasern mit geringem Durchmesser und hoher Flexibilität (am Beispiel der hochfesten Kohlenstofffasern vom Typ HTA 5131) mit dem ausgewählten Matrixmetall (am Beispiel der AlSi5-Legierung) zunächst zu Prepregs und weiterhin in verdichtete Verbundstrukturen zusammenzuführen. Das vergleichende Gegenüberstellen von Simulation und Experiment dienen einerseits dem Verifizieren der Prozessmodelle. Die Prozessmodelle stellen andererseits die wertvolle versuchstechnische Grundlage zur Definition und auch der Einengung des Arbeitsfensters für die Prozessparameter und der Anzahl erforderlicher Verifikationsexperimente im Hinblick auf die Probebeschichtungen und deren Auswertung dar. Derartige Modelle leisten eine wichtige Hilfestellung zum weiteren Erhöhen der Prozessstabilität und damit auch dem Nachweis der Reproduzierbarkeit. / The aim of this work is a described prepreg-technological method to create aluminium based MMC reinforced with continuous fibres and the verification of reproducibility to achieve relevant mechanical properties. This aim is based on the knowledge of technological influences on structure and properties of fibre reinforced MMC. And so activities are focussed on the evaluation of the influence of process functions on structure and mechanical properties of prepregs and compacted MMC. The comparison between structure and properties is necessary to describe the correlation function of composite material. Furthermore the application of different correlation models (“Shear Lag”, “Grenzwert- Modellkonzept“, “EIAS”) to describe the influence of composite structures on strength and Young`s modulus is necessary to compare theoretical results with those of relevant experiments. The object is a critical quantification of the influence of real structure parameters compared with those of a model defined structure. In view of the described aim it is shown that the described technology to manufacture long fibre reinforced MMC is applicable for preparation of carbon fibres (HTA 5131) with low diameter and high flexibility and metal matrix (AlSi5) to prepregs and compact composites. The comparison of simulated and experimental results is the base for verification of different process models. So it is possible to describe and optimize the process function and moreover to minimize the number of technological experiments. After optimization specified models are a good base to achieve a high level of stability and reproducibility for all steps in prepreg technology.

In Kenntnis der beschriebenen verfahrenstechnischen Einflussfaktoren auf die Struktur und die Eigenschaften der faserverstärkten MMC geht es darum, ein prepregtechnologisches Verfahren für das Herstellen lang- bzw. endlosfaserverstärkter MMC mit Aluminiummatrix zu erarbeiten und die Machbarkeit im Hinblick auf die Reproduzierbarkeit und das Erreichen der prinzipiell möglichen mechanischen Eigenschaftskennwerte nachzuweisen. Es geht darum, den Einfluss der Prozessparameter auf die Struktur der Prepregs und der verdichteten MMC`s zu erfassen und zu bewerten. In Kenntnis dieser Zusammenhänge sind die qualitätsrelevanten Kennwerte der Werkstoffstruktur denen der mechanischen Eigenschaften gegenüberzustellen. In Weiterführung der o.g. Darstellungen zwischen Struktur- und Eigenschaften der Verbundwerkstoffe geht es darum, die experimentell bestimmten Kennwerte der Festigkeit und des E-Moduls auf Übereinstimmung mit den entsprechenden Korrelationsmodellen (Shear Lag Modell, Grenzwert- und Modellkonzept, EIAS-Methode) zu überprüfen. Ferner geht es darum, die Zusammenhänge zwischen den im Modell definierten idealen Gefügebedingungen mit den realen im Hinblick auf deren Einfluss auf die Eigenschaftskennwerte kritisch zu diskutieren. Aufbauend auf den definierten Zielstellungen kann nach der erfolgten Bewertung aller Untersuchungsergebnisse gezeigt werden, dass die entworfene Technologie zur Herstellung langfaserverstärkter Metallmatrix-Verbundwerkstoffe dazu geeignet ist, Fasern mit geringem Durchmesser und hoher Flexibilität (am Beispiel der hochfesten Kohlenstofffasern vom Typ HTA 5131) mit dem ausgewählten Matrixmetall (am Beispiel der AlSi5-Legierung) zunächst zu Prepregs und weiterhin in verdichtete Verbundstrukturen zusammenzuführen. Das vergleichende Gegenüberstellen von Simulation und Experiment dienen einerseits dem Verifizieren der Prozessmodelle. Die Prozessmodelle stellen andererseits die wertvolle versuchstechnische Grundlage zur Definition und auch der Einengung des Arbeitsfensters für die Prozessparameter und der Anzahl erforderlicher Verifikationsexperimente im Hinblick auf die Probebeschichtungen und deren Auswertung dar. Derartige Modelle leisten eine wichtige Hilfestellung zum weiteren Erhöhen der Prozessstabilität und damit auch dem Nachweis der Reproduzierbarkeit. / The aim of this work is a described prepreg-technological method to create aluminium based MMC reinforced with continuous fibres and the verification of reproducibility to achieve relevant mechanical properties. This aim is based on the knowledge of technological influences on structure and properties of fibre reinforced MMC. And so activities are focussed on the evaluation of the influence of process functions on structure and mechanical properties of prepregs and compacted MMC. The comparison between structure and properties is necessary to describe the correlation function of composite material. Furthermore the application of different correlation models (“Shear Lag”, “Grenzwert- Modellkonzept“, “EIAS”) to describe the influence of composite structures on strength and Young`s modulus is necessary to compare theoretical results with those of relevant experiments. The object is a critical quantification of the influence of real structure parameters compared with those of a model defined structure. In view of the described aim it is shown that the described technology to manufacture long fibre reinforced MMC is applicable for preparation of carbon fibres (HTA 5131) with low diameter and high flexibility and metal matrix (AlSi5) to prepregs and compact composites. The comparison of simulated and experimental results is the base for verification of different process models. So it is possible to describe and optimize the process function and moreover to minimize the number of technological experiments. After optimization specified models are a good base to achieve a high level of stability and reproducibility for all steps in prepreg technology.

In this thesis, the effect of mechanical deformation on structure, thermal stability and hardness of a single-phase spray-deposited quasicrystalline alloy with composition Al62.5Cu25Fe12.5 has been investigated in detail. The purpose of the investigation was to study the effect of mechanical milling at different milling speeds (which approximately scale with the milling intensity) on mechanically-induced phase transformations during milling and on the phase evolution during subsequent heating. The results of the milling experiments indicate that, irrespective of the milling speeds used, mechanical milling of Al62.5Cu25Fe12.5 quasicrystals leads to the formation of a disordered CsCl-type ß phase with grain size of about 10 – 20 nm. The analysis of the kinetics of the QC–to–ß phase transformation reveals that the milling intensity has a considerable effect on the characteristics of the transformation. The increase of the milling speed considerably shortens the incubation time needed to start the QC–to–ß phase transformation. Also, the overall transformation is much faster for milling at high speeds. The QC–to–ß phase transformation starts when the grain size of the quasicrystals is reduced to about 10 nm irrespective of the milling speed used and clearly indicates that a critical grain size of the quasicrystals for initiating the transformation exists. On the other hand, no critical value of lattice strain was found for the QC–to–ß transformation. This indicates that the phase transformation is controlled by the local length scale (i.e. the grain size) and by the corresponding grain boundaries rather than by the energy stored in the lattice. Energetic considerations obtained through a simple model based on the mass and velocity of the milling balls reveal that the energy needed for the QC–to–ß transformation increases with increasing the milling speed, that is, the energetic efficiency of the process decreases with increasing the milling intensity. This indicates that part the extra energy supplied during milling at high intensities is not used to induce the phase transformation but it is dissipated by heat. During heating, the milled powder displays a multi-step thermal behavior characterized by the grain growth of the disordered ß phase at low temperatures, followed, at higher temperatures, by its transformation into the original icosahedral quasicrystalline phase. The transformation is gradual and the quasicrystals and the disordered ß phase coexist over a temperature interval of more than 250 K. The phase transformations occurring during milling and subsequent annealing have a remarkable effect on the hardness, which can be tuned within a wide range of values (7–9.6 GPa) as a function of the volume fraction of the different phases. This suggests that a composite material with optimized mechanical properties can be produced by an appropriate thermo-mechanical treatment. The quasicrystals milled at a very low speed show a transition between Hall-Petch to inverse Hall-Petch behavior at a grain size of about 40 nm, which represents the critical value for grain size softening of the present Al62.5Cu25Fe12.5 quasicrystals. This behavior may be attributed to the complexity of the quasicrystalline structure and to its peculiar deformation mechanism at room temperature (i.e. shear banding), where meta-dislocation-assisted deformation is almost absent. In order to analyze the effectiveness of the Al62.5Cu25Fe12.5 quasicrystals as reinforcing agent in metal matrix composites, Al-based composites were synthesized by hot extrusion of elemental Al blended with different amounts of Al62.5Cu25Fe12.5 quasicrystalline particles. The work was focused on two specific aspects: evaluation of the mechanical properties through room temperature compression tests and modeling of the resulting properties. The addition of the quasicrystalline reinforcement is very effective for improving the room temperature mechanical properties of pure Al. The compressive strength increases from 155 MPa for pure Al to 330 and 407 MPa for the composites with 20 and 40 vol.% of reinforcement, respectively, reaching an ultimate strain of 55 % and 20 % before fracture occurs. These results indicate that the addition of the QC reinforcement leads to composite materials with compressive strengths exceeding that of pure Al by a factor of 2 – 2.5, while retaining appreciable plastic deformation. The mechanical properties of the composites have been modeled by taking into account the combined effect of load bearing, dislocation strengthening and matrix ligament size effects. The calculations are in very good agreement with the experimental results and reveal that the reduction of the matrix ligament size, which results in a similar strengthening effect as that observed for grain refinement, is the main strengthening mechanism in the current composites. Finally, the interfacial reaction between the Al matrix and the QC reinforcement has been used to further enhance the strength of the composites through the formation of a new microstructure consisting of the Al matrix reinforced with Al7Cu2Fe w-phase particles. The optimization of the structure-property relationship was done through the systematic variation of the processing temperature during consolidation. The mechanical behavior of these transformation-strengthened composites is remarkably improved compared to the parent material. The yield strength of the composites significantly increases as the Al + QC -> ω transformation progresses from 195 MPa for the sample reinforced only with QC particles to 400 MPa for the material where the Al + QC -> ω reaction is complete. These results clearly demonstrate that powder metallurgy, i.e. powder synthesis by ball milling followed by consolidation into bulk specimens, is an attractive processing route for the production of novel and innovative lightweight composites characterized by high strength combined with considerable plastic deformation. In addition, these findings indicate that the mechanical behavior of Al-based composites reinforced with Al62.5Cu25Fe12.5 quasicrystalline particles can be tuned within a wide range of strength and plasticity depending on the volume fraction of the reinforcement as well as on the extent of the interfacial reaction between Al matrix and QC reinforcing particles.

Les verres métalliques ont commencé à être produit dans les années 1960 et sous forme massive dans les années 1980. De nombreuses études se sont intéressées à ces matériaux sous leur forme amorphe et ont conclu qu’ils avaient une forte résistance mécanique mais présentaient un comportement très fragile. Dans le cadre du projet EDDAM débuté en 2011, ces matériaux ont été introduits sous forme de petites sphères dans une matrice d’aluminium. Le premier objectif de notre étude est de voir si le verre métallique sous cette forme permet de le rendre peu fragile. Le second objectif est de trouver une alternative aux renforts céramique dans les composites à matrice métallique qui présentent une faible cohésion à l’interface matrice/inclusion. Dans le but de caractériser l’endommagement dans des nouveaux composites amorphe-cristallins métalliques, la tomographie aux rayons X a été utilisée. Cette technique permet de caractériser de manière non destructive l’endommagement des matériaux et de le visualiser en 3D. Cela apporte une contribution à l’étude des matériaux composites par rapport aux techniques classiques utilisées. L’objectif général de cette thèse a été d’étudier l’endommagement en termes d’amorçage, de croissance et de coales- cence des matériaux composites amorphe-cristallins métallique par tomographie aux rayons X lors d’essais de traction monotone in situ. Les matériaux sélectionnés sont constitués d’une matrice aluminium ("molle" de type 1070A ou "dure" de type 5083) et de renforts en verre métallique Zr57Cu20Al10Ni8Ti5 de taille peu dispersée et répartis de manière homogène, avec différentes fractions volumiques (1%, 4% et 10%). Les matériaux composites ont été élaborés par la voie de la métallurgie des poudres au Spark Plasma Sintering (SPS) suivi d’une étape d’extrusion à chaud. Une attention particulière a été portée sur la caractérisation microstructurale des constituants de base. L’analyse qualitative a permis de comparer l’ensemble des composites fabriqués au SPS et ceux extrudés à chaud après SPS. Les différents modes d’amorçage de l’endommagement ont été observés ainsi que la croissance et la coa- lescence amenant la rupture des composites. L’analyse quantitative a été essentiellement consacrée au premier stade de l’endommagement. La croissance et la coalescence étant très rapide, il a été difficile de les suivre lors des essais interrompus. La modélisation d’un composite amorphe-cristallin métallique à matrice molle a été introduite dans le but de reproduire l’endommagement observé lors des analyses expérimentales. Cette première approche nécessite d’être approfondie dans le but de prédire, compte tenu des propriétés mécaniques des différentes phases et de la fraction volumique des renforts, le mode d’endommagement préférentiel apparaissant dans les composites étudiés. Elle montre cependant les prémices d’une modélisation innovante basée sur la microstructure expérimentale. / Metallic glasses have been produced in the 1960s and bulk metallic glasses in the 1980s. Many studies, focused on these materials in their amorphous state, concluded that they had high mechanical strength but shown low ductility. As part of EDDAM project that started in 2011, these materials were introduced as small particles in an aluminum matrix. The first objective of this study is to see if the metallic glass is less brittle in this form. The second objective is to find an alternative of ceramic reinforcements in metal matrix composites. These materials have low cohesion at the matrix/inclusion interface. In order to characterize the damage in new amorphous-crystalline composite, X-ray tomography was used. This allows to characterize damage in materials and to obtain a 3D viewing. The main objective of this thesis was to study damage (nucleation, growth and coalescence) in composite materials using X-ray tomography during tensile tests. Selected materials are constituted of an aluminum matrix and small metallic glass reinforcements (Zr57Cu20Al_10Ni8Ti5). Composites with different volume fractions (from 1vol.% to 10vol.%) were prepared by Spark Plasma Sintering (SPS) and hot extrusion. A particular attention was paid to the microstructural characterization of the basic constituents. Qualitative analysis was used to compare SPS composites with SPS plus hot extrusion composites. Damage nucleation, growth and coalescence were observed. Quantitative analysis was mainly devoted to the first damage step. Growth and coalescence were difficult to follow due to fast rupture and interrupted tensile tests. The modeling of an amorphous-crystalline composite has been introduced in order to reproduce experimental damage analyses. The first approach requires further investigation to predict damage with different volume fractions. However, this part shows the beginning of an innovative model based on the experimental microstructure.