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The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3<sup>rd</sup> generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room... more
The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3<sup>rd</sup> generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room temperature, followed by low temperature artificial ageing (i.e. T4-HPT-AA). A micro-hardness of ~240 Hv was achieved on ageing at 110°C/60h after HPT. A further improvement in the hardness to ~260 Hv was accomplished by a pre-ageing 110°C/24h before HPT in combination with a post-HPT ageing process at 110°C for 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (~120 Hv). After HPT the grain size was dramatically refined to the ultrafine-grained (UFG) structure, accompanied by a large amount of dislocations. No long-range ordered precipitates were observed after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters were homogeneously distributed in the matrix of T4 and T6 processed samples and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters to the dislocations. A strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order strengthening mechanisms was used to predict the yield strength of the alloy. This model indicates that the combined effect due to all three types of Cu-Mg clusters (clustering in matrix, clustering at GBs and at dislocations) is dominant for the strength in all conditions.
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When simulating the material behavior during thermo-mechanical processes, the understanding of the microstructure evolution is fundamental. Therefore, state parameter-based models are utilized to describe physical effects such as work... more
When simulating the material behavior during thermo-mechanical processes, the understanding of the microstructure evolution is fundamental. Therefore, state parameter-based models are utilized to describe physical effects such as work hardening, precipitation hardening, solid solution hardening and cross core diffusion. Using the thermo-kinetic software package MatCalc, temperature- and strain rate dependent flow curves of compression tests are successfully simulated. The theoretical background of the underlying physical models and the influence of alloying elements on the cross core diffusion behavior are discussed. Various Al-alloys are investigated and the experimentally obtained flow curves are evaluated in terms of initial strain hardening rate, initial yield stress and saturation stress. In Al-alloys, especially the effect of Mg is dominant due to its ability to diffuse from the compression side to the tension side of the dislocations core, leading to additional barriers for the dislocation movement.
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Kurzfassung Der Werkstoff 7CrMoVTiB10-10 (T24) wurde als warmfester Stahl für den Einsatz in der Membranwand neuer Kohlekraftwerke der 600/620°C-Klasse entwickelt. Trotz seiner umfangreichen Qualifizierung kam es zu Schäden bei der... more
Kurzfassung Der Werkstoff 7CrMoVTiB10-10 (T24) wurde als warmfester Stahl für den Einsatz in der Membranwand neuer Kohlekraftwerke der 600/620°C-Klasse entwickelt. Trotz seiner umfangreichen Qualifizierung kam es zu Schäden bei der Inbetriebnahme durch Spannungsrisskorrosion. Zur Reduzierung der Spannungsrisskorrosionsempfindlichkeit wurden verschiedene Anlassbehandlungen durchgeführt. Grundlegende Untersuchungen sollen nun die Prozesse beleuchten, die bei diesen Glühbehandlungen ablaufen. Für die geplanten Untersuchungen wurde Probenmaterial aus zwei nahtlosen Rohren (aus unterschiedlichen Schmelzen) des Stahls T24 entnommen. In den Proben wurde anhand von Wärmebehandlungen in einem Dilatometer ein Schweißgefüge eingestellt, wie es in der Wärmeeinflusszone (WEZ) der Schweißverbindungen auftritt. An diesem Probenmaterial erfolgten anschließend unterschiedliche Anlassbehandlungen. Die sich einstellenden Gefügeveränderungen wurden in Abhängigkeit von der Erwärmrate bzw. Anlasstemperatur und -dauer durch kalorimetrische und dilatometrische Analysen charakterisiert. Hierzu wurden Proben kontinuierlich in verschiedenen DSC-Geräten und in einem Dilatometer mit Heizraten zwischen 0,02 K/s und 100 K/s erwärmt. Zusätzlich erfolgten isotherme Glühbehandlungen dieser Proben mit Temperaturen zwischen 450 °C und 700 °C im Dilatometer. Die Ergebnisse der Dilatometrie und Kalorimetrie lassen auf mehrere Anlassreaktionen schließen. Die Ergebnisse erlauben eine Zuordnung der beobachteten Reaktionen zu den in der Literatur beschriebenen Anlassstufen und somit eine gezielte Gestaltung der Anlassbehandlung der geschweißten T24-Verbindungen zur Reduzierung ihrer Rissanfälligkeit im laufenden Betrieb.
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Research Interests: Materials Engineering, Mechanical Engineering, Materials Science, Metallurgy, Scanning Transmission Electron Microscopy, and 10 moreDifferential scanning calorimetry, Aluminium Alloys, Materials Design, Alloy, Precipitation, Heterogeneous Nucleation, Aluminium, Precipitation Hardening, Extrusion, and Aluminium Alloy
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Although metallic materials processed by laser beam melting have significantly different microstructures compared to their conventional counterparts no adjusted heat treatment parameters have been published to date. Conventional heat... more
Although metallic materials processed by laser beam melting have significantly different microstructures compared to their conventional counterparts no adjusted heat treatment parameters have been published to date. Conventional heat treatment will therefore not always result in the desired mechanical properties. This project examines the effect of heat treatment on the properties of laser beam melted components produced with AlSi10Mg (EN AC-43000) precipitation hardening cast aluminium alloy and X5CrNiCuNb16-4 (1.4542 or 17-4 PH) martensitic precipitation hardening steel. Comparisons to conventionally manufactured material are made in parallel. The kinetics of phase transformation during heat treatment is analysed in-situ by means of differential scanning calorimetry (AlSi10Mg and X5CrNiCuNb16-4) and dilatometry (X5CrNiCuNb16-4). The results show considerable differences in phase transformation kinetics between laser beam melted and conventionally processed materials.
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The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3&amp;lt;sup&amp;gt;rd&amp;lt;/sup&amp;gt; generation Al-Cu-Li alloy was investigated. Solution treated samples... more
The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3&amp;lt;sup&amp;gt;rd&amp;lt;/sup&amp;gt; generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room temperature, followed by low temperature artificial ageing (i.e. T4-HPT-AA). A micro-hardness of ~240 Hv was achieved on ageing at 110°C/60h after HPT. A further improvement in the hardness to ~260 Hv was accomplished by a pre-ageing 110°C/24h before HPT in combination with a post-HPT ageing process at 110°C for 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (~120 Hv). After HPT the grain size was dramatically refined to the ultrafine-grained (UFG) structure, accompanied by a large amount of dislocations. No long-range ordered precipitates were observed after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters were homogeneously distributed in the matrix of T4 and T6 processed samples and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters to the dislocations. A strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order strengthening mechanisms was used to predict the yield strength of the alloy. This model indicates that the combined effect due to all three types of Cu-Mg clusters (clustering in matrix, clustering at GBs and at dislocations) is dominant for the strength in all conditions.
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Quenching is an important step during age hardening of aluminium alloys. It significantly influences on microstructures, properties, residual stresses and component distortion. Due to high heat transfer, thermal stresses occur in quenched... more
Quenching is an important step during age hardening of aluminium alloys. It significantly influences on microstructures, properties, residual stresses and component distortion. Due to high heat transfer, thermal stresses occur in quenched components. I.e., all premature precipitates during undercritical quenching form on condition of mechanical stresses. Opposite, quench sensitivity investigations, e.g. recording of continuous cooling precipitation diagrams, are usually performed in stress-free conditions and may therefore be incomplete. We have developed a new method of thermomechanical analysis and calorimetric reheating to investigate stress induced precipitation during quenching of aluminium alloys. For aluminium alloy 2024, it has been shown for the very first time that mechanical stresses during quenching also influence on quench-induced precipitation reactions.
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Precipitation in Al-Si alloys during cooling from solution annealing was investigated with advanced DSC techniques in a wide cooling rate range (2 K/s-0.0001 K/s). Microstructural analyses show that Si particles of different shape and... more
Precipitation in Al-Si alloys during cooling from solution annealing was investigated with advanced DSC techniques in a wide cooling rate range (2 K/s-0.0001 K/s). Microstructural analyses show that Si particles of different shape and size can precipitate during cooling. The dependence of precipitation enthalpy on cooling rate and temperature is modelled, providing a consistent physical description of precipitate volume fraction and solute Si amount during cooling. This allows control of precipitation via precise heat treatments and thus allows generation of well-defined microstructural states. Thereby, samples having an equal amount of solute Si but different precipitation states could be tested in order to investigate their influence on the mechanical behaviour. A big advantage of this method is that the strengthening contribution of precipitates can be determined without the need to assume any - potentially inaccurate - superposition law between particle and solute strengthening.
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This work developed a systematic method for a metallographic preparation of single powder particles with diameters of approx. 20 to 40 μm. It was motivated by the objective of understanding additive manufacturing processes such as Laser... more
This work developed a systematic method for a metallographic preparation of single powder particles with diameters of approx. 20 to 40 μm. It was motivated by the objective of understanding additive manufacturing processes such as Laser Powder Bed Fusion. A fundamental aspect of the relationship between manufacturing, structure, and properties is the correlation of rapid solidification and resulting microstructure. During powder-based additive manufacturing processes, cooling rates up to 1 MK/s are attained. A thermal analysis determining the characteristics of solidification at such rapid cooling rates can be performed with the aid of chip sensor-based, dynamic Differential Fast Scanning Calorimetry, DFSC. For this purpose, the heat flow during the solidification of single powder particles is measured and, for instance, the solidification onset temperature is evaluated as a function of cooling rate. It is thus possible to estimate the undercooling which has a significant impact on the resulting structure. Subsequently, cross sections of single powder particles must be prepared for the analysis of the resulting structure.
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Kurzfassung Der Einfluss der Abschreckgeschwindigkeit auf das Ausscheidungsverhalten von Aluminiumlegierungen wird, in Anlehnung an ZTU-Diagramme von Stählen, in kontinuierlichen Zeit-Temperatur-Ausscheidungs-Diagrammen dargestellt.... more
Kurzfassung Der Einfluss der Abschreckgeschwindigkeit auf das Ausscheidungsverhalten von Aluminiumlegierungen wird, in Anlehnung an ZTU-Diagramme von Stählen, in kontinuierlichen Zeit-Temperatur-Ausscheidungs-Diagrammen dargestellt. Solche Diagramme sind bisher kaum verfügbar, können jetzt aber mit Hilfe der Differential Scanning Calorimetry (DSC) erfasst werden. Wird eine Charge der Aluminiumlegierung EN AW-6005A in einem Kühlgeschwindigkeitsbereich von 0,1–30 K/min von Lösungsglühbedingungen in einem DSC abgekühlt, sind in den Abkühlkurven zwei exotherme Ausscheidungsreaktionen zu erkennen, eine Hoch- sowie eine Niedertemperaturreaktion. Um Aufschluss darüber zu erhalten, welche Phasen dabei ausgeschieden werden, wurden umfangreiche Gefügeanalysen mittels Licht- und Rasterelektronenmikroskopie, energiedispersiver Röntgen-Mikroanalyse (EDX), Röntgendiffraktometrie (XRD) und Elektronen-Rückstreubeugung (EBSD) sowie Härteprüfungen an unterschiedlich abgekühlten Proben durchgeführt. Es konnte gezeigt werden, dass bei der Hochtemperaturreaktion Mg2Si ausgeschieden wird. Die Keimbildung der Mg2Si-Partikel erfolgt überwiegend heterogen an Primärausscheidungen.
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The application properties of metallic materials are frequently adjusted by heat treatments utilizing controlled microstructural changes—i.e., solid–solid phase transformations like nondiffusional martensitic transformation or diffusional... more
The application properties of metallic materials are frequently adjusted by heat treatments utilizing controlled microstructural changes—i.e., solid–solid phase transformations like nondiffusional martensitic transformation or diffusional secondary phase precipitation and/or dissolution. For technical application, knowledge about the characteristic temperatures and times but moreover about their time dependence (kinetics) is required. As the relevant solid–solid phase transformations all show a heat effect (e.g., precipitation → exothermic; dissolution → endothermic), one outstanding measurement technique to follow these phase transformations is calorimetry, particularly differential scanning calorimetry (DSC). Appropriate combinations of DSC methods and devices to cover nine orders of magnitude in heating and cooling rates (10−4–105 K/s) will be introduced, using dissolution and precipitation reactions in aluminum alloys as examples. Basically, these techniques allow one to record time–temperature transformation (or precipitation/dissolution) diagrams for various materials during heating, isothermal annealing, and even during continuous cooling, making DSC a very powerful tool for the investigation of solid–solid phase transformations. Nowadays, physically based models verified with DSC results moreover allow one to predict precipitation volume fractions and solute mass fractions.
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Different quench sensitivity behaviours for aluminium alloys 7150, 7055, 7085 and 7037 have been systematically studied by using small angle X-ray scattering (SAXS) at the Australian synchrotron. The results have shown that due to... more
Different quench sensitivity behaviours for aluminium alloys 7150, 7055, 7085 and 7037 have been systematically studied by using small angle X-ray scattering (SAXS) at the Australian synchrotron. The results have shown that due to different chemical compositions, the ageing behaviour is different after different cooling paths from the solution treatment temperature. The size of ?' precipitates formed during ageing generally decreases with increasing cooling rates. The volume fraction of ?' precipitates formed during ageing increase with increasing cooling rate, indicating that more solute atoms has been retained in solution prior to ageing. For all the studied alloys, no ?' precipitates can be detected below a certain cooling rate, demonstrating that the matrix concentration is not sufficiently high for ?' precipitation during subsequent ageing. The current paper demonstrates that the SAXS technique can be used to quantitatively analyse the effect of quench rate on the size and volume fraction of ?' precipitates in 7xxx alloys in the T6 condition. Moreover, the matrix concentration and/or the chemical composition of ?' precipitates can also be characterised.
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During quenching of medium to high strength heat-treatable Al based alloys precipitation on defects such as grain boundaries is in practice nearly unavoidable, and the quench sensitivity generally increases with increasing content of main... more
During quenching of medium to high strength heat-treatable Al based alloys precipitation on defects such as grain boundaries is in practice nearly unavoidable, and the quench sensitivity generally increases with increasing content of main alloying elements. Also the minor alloying elements Zr, Mn and Cr, which form intermetallic particles of sizes typically in the range of 10-100 nm (termed ‘dispersoids’), strongly influence the quench sensitivity as those particles act as nucleation sites for the quench-induced phase. The latter particularly holds for incoherent dispersoids, whereby the dispersoids typically lose their coherence due to (local) recrystallization. A new model for quench-induced precipitation and the resulting yield strength in age hardened condition for Al-Mg-Si (6xxx) and Al-Zn-Mg (7xxx) alloys has been derived. The new model particularly incorporates recent improved models for diffusion-controlled reactions, the advances in modelling of the thermodynamics of complex alloy systems, as well as computationally efficient schemes for integrating these components. For the validation of the model, a range of experimental data is used which covers the microstructure on a range of length scales, the thermodynamics of the reactions and the resulting mechanical properties. This includes transmission electron microscopy (TEM), high resolution TEM (HRTEM), scanning electron microscopy, and high resolution fast and slow differential scanning calorimetry. The model shows a near perfect correspondence with data on all (>10) alloys studied extensively, and allows prediction of the influence of the major alloying elements and 3 dispersoid forming elements on quench sensitivity.
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Several age hardening aluminium alloys, like high alloyed 2XXX, 6XXX and 7XXX alloys require high critical quenching rates of some 100 K/s from solution annealing to suppress premature precipitation and achieve maximum strength after... more
Several age hardening aluminium alloys, like high alloyed 2XXX, 6XXX and 7XXX alloys require high critical quenching rates of some 100 K/s from solution annealing to suppress premature precipitation and achieve maximum strength after aging. Knowledge of the precipitation behaviour during quenching is crucial for the design of quenching processes of aluminium alloys. For monitoring the precipitation behaviour during moderate quenching, a calorimetric method (0.01 to 5 K/s) has already been successfully developed. New Differential Fast Scanning Calorimeters (DFSC, up to some 106 K/s) allow rapid quenching of aluminium alloys, but due to weak precipitation reactions the quenching results can hardly be evaluated. Hence, a new method has been developed, to monitor precipitation during rapid quenching of aluminium alloys by calorimetric reheating experiments. Quenching and reheating experiments of high alloyed, quench sensitive aluminium alloys, like 7049A will be presented.
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Solid-solid phase transformations play an important role for the adjustment of machining- and application-properties of metallic materials. During recent years, we developed advanced techniques to analyze solid-solid phase transformation... more
Solid-solid phase transformations play an important role for the adjustment of machining- and application-properties of metallic materials. During recent years, we developed advanced techniques to analyze solid-solid phase transformation by DSC in a very wide dynamic range. This talk will introduce these techniques, using reactions in aluminium alloys as examples. To achieve the required high dynamic range during cooling we combine three different indirect and direct measuring techniques and five types of DSC devices. Nowadays physically-based models verified with our DSC results moreover allow to predict precipitation volume fractions, solute mass fractions. Latest results on property predictions as function of cooling rate will be presented.
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Dilatometry and calorimetry are in situ methods to detect changes microstructure during heating or cooling of materials. They have been used for the analysis design of a short time heat treatment of the martensitic, press hardening,... more
Dilatometry and calorimetry are in situ methods to detect changes microstructure during heating or cooling of materials. They have been used for the analysis design of a short time heat treatment of the martensitic, press hardening, automobile body steel 22MnB5. The use of these both complementary experimental methods allows a critical consideration of the established results.
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The application properties of metallic materials are frequently adjusted by heat treatments utilising controlled microstructural changes—i.e. solid–solid phase transformations like non-diffusional martensitic transformation or diffusional... more
The application properties of metallic materials are frequently adjusted by heat treatments utilising controlled microstructural changes—i.e. solid–solid phase transformations like non-diffusional martensitic transformation or diffusional secondary-phase precipitation and/or dissolution. For technical application, knowledge about the characteristic temperatures and times but moreover about their time dependence (kinetics) is required. As the relevant solid–solid phase transformations all show a heat effect (e.g. precipitation → exothermic; dissolution → endothermic), one outstanding measurement technique to follow these phase transformations is calorimetry, particularly differential scanning calorimetry (DSC). Appropriate combinations of DSC methods and devices to cover nine orders of magnitude in heating and cooling rates (10−4–105 K/s) will be introduced, using dissolution and precipitation reactions in aluminium alloys as examples. Basically, these techniques allow one to record time–temperature transformation (or precipitation/dissolution) diagrams for various materials during heating, isothermal annealing and even during continuous cooling, making DSC a very powerful tool for the investigation of solid–solid phase transformations. Nowadays, physically based models verified with DSC results moreover allow one to predict precipitation volume fractions and solute mass fractions.
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Abstract Additive manufacturing, e.g. by laser powder bed fusion (LPBF), is very attractive for lightweight constructions, as complex and stress-optimised structures integrating multiple functions can be produced within one process.... more
Abstract Additive manufacturing, e.g. by laser powder bed fusion (LPBF), is very attractive for lightweight constructions, as complex and stress-optimised structures integrating multiple functions can be produced within one process. Unfortunately, high strength AlZnMgCu alloys tend to hot cracking during LPBF and thus have not so far been applicable. In this work the melting and solidification behaviour of AlZnMgCu alloy powder variants with particle surface inoculation was analysed by Differential Fast Scanning Calorimetry. The aim is to establish a method that makes it possible to assess powder modifications in terms of their suitability for LPBF on a laboratory scale requiring only small amounts of powder. Therefore, solidification undercooling is evaluated at cooling rates relevant for LPBF. A method for the temperature correction and normalisation of the DFSC results is proposed. Two ways of powder modification were tested for the powder particles surface inoculation by titanium carbide (TiC) nanoparticles: via wet-chemical deposition and via mechanical mixing. A low undercooling from DFSC correlates with a low number of cracks of LPBF-manufactured cubes. It appears that a reduced undercooling combined with reduced solidification onset scatter indicates the possibility of crack-free LPBF of alloys that otherwise tend to hot cracking.
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For aluminium alloys, precipitation strengthening is controlled by age-hardening heat treatments, including solution treatment, quenching, and ageing. Quenching is considered a critical step, because detrimental quench-induced... more
For aluminium alloys, precipitation strengthening is controlled by age-hardening heat treatments, including solution treatment, quenching, and ageing. Quenching is considered a critical step, because detrimental quench-induced precipitation must be avoided to exploit the full age-hardening potential of the alloy. This work presents a comprehensive report on the solid-solid phase transformation kinetics in Al alloys covering to a vast extent quench-induced precipitation during continuous cooling over a dynamic cooling rate range of ten orders of magnitude.
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The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3<sup>rd</sup> generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room... more
The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3<sup>rd</sup> generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room temperature, followed by low temperature artificial ageing (i.e. T4-HPT-AA). A micro-hardness of ~240 Hv was achieved on ageing at 110°C/60h after HPT. A further improvement in the hardness to ~260 Hv was accomplished by a pre-ageing 110°C/24h before HPT in combination with a post-HPT ageing process at 110°C for 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (~120 Hv). After HPT the grain size was dramatically refined to the ultrafine-grained (UFG) structure, accompanied by a large amount of dislocations. No long-range ordered precipitates were observed after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters were homogeneously distributed in the matrix of T4 and T6 processed samples and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters to the dislocations. A strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order strengthening mechanisms was used to predict the yield strength of the alloy. This model indicates that the combined effect due to all three types of Cu-Mg clusters (clustering in matrix, clustering at GBs and at dislocations) is dominant for the strength in all conditions.
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Research Interests:
During quenching of medium to high strength heat-treatable Al based alloys precipitation on defects such as grain boundaries is in practice nearly unavoidable, and the quench sensitivity generally increases with increasing content of main... more
During quenching of medium to high strength heat-treatable Al based alloys precipitation on defects such as grain boundaries is in practice nearly unavoidable, and the quench sensitivity generally increases with increasing content of main alloying elements. Also the minor alloying elements Zr, Mn and Cr, which form intermetallic particles of sizes typically in the range of 10-100 nm (termed ‘dispersoids’), strongly influence the quench sensitivity as those particles act as nucleation sites for the quench-induced phase. The latter particularly holds for incoherent dispersoids, whereby the dispersoids typically lose their coherence due to (local) recrystallization. A new model for quench-induced precipitation and the resulting yield strength in age hardened condition for Al-Mg-Si (6xxx) and Al-Zn-Mg (7xxx) alloys has been derived. The new model particularly incorporates recent improved models for diffusion-controlled reactions, the advances in modelling of the thermodynamics of comple...
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Precipitation in Al-Si alloys during cooling from solution annealing was investigated with advanced DSC techniques in a wide cooling rate range (2 K/s-0.0001 K/s). Microstructural analyses show that Si particles of different shape and... more
Precipitation in Al-Si alloys during cooling from solution annealing was investigated with advanced DSC techniques in a wide cooling rate range (2 K/s-0.0001 K/s). Microstructural analyses show that Si particles of different shape and size can precipitate during cooling. The dependence of precipitation enthalpy on cooling rate and temperature is modelled, providing a consistent physical description of precipitate volume fraction and solute Si amount during cooling. This allows control of precipitation via precise heat treatments and thus allows generation of well-defined microstructural states. Thereby, samples having an equal amount of solute Si but different precipitation states could be tested in order to investigate their influence on the mechanical behaviour. A big advantage of this method is that the strengthening contribution of precipitates can be determined without the need to assume any - potentially inaccurate - superposition law between particle and solute strengthening.
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Binary NiTi alloys are the most common shape memory alloys in medical applications, combining good mechanical properties and high biocompatibility. In NiTi alloys, the shape memory effect is caused by the transformation of an austenite... more
Binary NiTi alloys are the most common shape memory alloys in medical applications, combining good mechanical properties and high biocompatibility. In NiTi alloys, the shape memory effect is caused by the transformation of an austenite phase to a martensite phase and the reverse process. Transformation temperatures are strongly influenced by the exact chemical composition of the NiTi phase and the presence of precipitates in the microstructure induced by thermo-mechanical treatment, especially solution annealing and ageing. Isothermal time–temperature precipitation diagrams can be found in the literature. Cooling is frequently not considered, as water quenching is typically assumed to be sufficient. To the best of our knowledge, continuous heating dissolution (CHD) and continuous cooling precipitation (CCP) diagrams do not exist. Differential scanning calorimetry (DSC) is a common method to analyse the austenite/martensite transformation in shape memory alloys, but it has not yet be...