Benjamin Milkereit - Academia.edu (original) (raw)

Papers by Benjamin Milkereit

For aluminium alloys, precipitation strengthening is controlled by age-hardening heat treatments,... 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.

Springer eBooks, 2015

The undercooled supersaturated aluminium alloy EN AW-6082 has been investigated during in-situ qu... more The undercooled supersaturated aluminium alloy EN AW-6082 has been investigated during in-situ quenching experiments by means of differential scanning calorimetry and thermomechanical analysis. Calorimetry resulted in continuous time-temperature-precipitation diagrams and thermomechanical analysis in stress/strain curves. In this work, the above results have been correlated. Supersaturation of the undercooled aluminium alloy has been determined from calorimetric data. Next, a solid solution strengthening model has been used to calculate yield strength. The model has further been extended by a temperature dependent term considering other strengthening mechanisms. Yield strength has been computed for a broad range of quenching rates and quenching temperatures. Results have been compared with experimental data from thermomechanical analysis and found to correlate well.

Der Einfluss der Abschreckgeschwindigkeit auf das Ausscheidungsverhalten von Aluminiumlegierungen... more 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 für Aluminiumlegierungen kaum verfügbar. Daher wurde in dieser Arbeit ein Verfahren zur Aufnahme von kontinuierlichen Zeit-Temperatur-Ausscheidungs-Diagrammen für Aluminiumlegierungen mittels Differential Scanning Calorimetry (DSC) entwickelt. Werden Al-Mg-Si-Legierungen in legierungsspezifisch langsamen Kühlgeschwindigkeitsbereichen von Lösungsglühbedingungen in einem DSC abgekühlt, sind in den Abkühlkurven mindestens zwei exotherme Ausscheidungsreaktionsbereiche zu erkennen, die Hoch-sowie die Niedertemperaturreaktionen. Die

Materials Science Forum, Jun 1, 2014

Isothermal time-temperature-precipitation (TTP) diagrams deliver important material data like tem... more Isothermal time-temperature-precipitation (TTP) diagrams deliver important material data like temperature and time ranges critical for precipitation. During the last years an in-situ calorimetric method to record continuous cooling precipitation diagrams has been developed to application level by our group. However, isothermal TTP-diagrams were still determined by ex-situ analyses only. In this work in-situ measurements of precipitation reactions were carried out during isothermal soaking. Therefore the whole heat treatment cycle was performed in a differential scanning calorimeter (DSC). Al-Mg-Si-alloys 6063 and 6005A were analysed. Solution annealing and overcritical quenching to several temperatures between 450 °C and 250 °C was followed by isothermal soaking. Based on the heat flow curves during isothermal soaking TTP-diagrams were determined. Further microstructure investigations by scanning electron microscopy and hardness tests after artificial ageing were performed. Both alloys show similar results. In the TTP-diagramms three so-called ”C-curves” could be observed. Every C-curve is expected to represent precipitation of a different phase. Hardness and microstructure investigations correspond with the data of the TTP-diagramms.

Materials Science Forum, Nov 1, 2016

Quenching is a critical step during the strengthening age hardening of Aluminium alloys. To obtai... more Quenching is a critical step during the strengthening age hardening of Aluminium alloys. To obtain optimal technological results, parts should be quenched with the upper critical cooling rate. The precipitation behaviour of Al alloys during cooling from solution annealing and thereby the critical cooling rates are typically investigated by in-situ measurements with differential scanning calorimetry (DSC). Conventional DSCs are limited at cooling rates below 10 Ks-1. Unfortunately, medium to high strength Al alloys typically have critical cooling rates between 10 and some 100 Ks-1. Recently it was shown that dilatometry is generally able for in-situ detection of precipitation in Al alloys. Dilatometry allows controlled cooling up to some 100 Ks-1 and therefore covers the cooling rate range relevant. In this work, we aim to show up and discuss possibilities and limitations of dilatometric detection of quench induced precipitates in 2xxx, and 7xxx Al alloys. The basic method will be presented and results will be compared with DSC work.

Social Science Research Network, 2022

Social Science Research Network, 2021

The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently... more The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3^rd 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.

Springer eBooks, 2020

When simulating the material behavior during thermo-mechanical processes, the understanding of th... 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.

HTM Journal of Heat Treatment and Materials, Dec 10, 2013

Kurzfassung Der Werkstoff 7CrMoVTiB10-10 (T24) wurde als warmfester Stahl für den Einsatz in 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.

Materials, Apr 2, 2019

In this work, a method is presented which allows the determination of calorimetric information, a... more In this work, a method is presented which allows the determination of calorimetric information, and thus, information about the precipitation and dissolution behavior of aluminum alloys during heating rates that could not be previously measured. Differential scanning calorimetry (DSC) is an established method for in-situ recording of dissolution and precipitation reactions in various aluminum alloys. Diverse types of DSC devices are suitable for different ranges of scanning rates. A combination of the various available commercial devices enables heating and cooling rates from 10 −4 to 5 Ks −1 to be covered. However, in some manufacturing steps of aluminum alloys, heating rates up to several 100 Ks −1 are important. Currently, conventional DSC cannot achieve these high heating rates and they are still too slow for the chip-sensor based fast scanning calorimetry. In order to fill the gap, an indirect measurement method has been developed, which allows the determination of qualitative information, regarding the precipitation state, at various points of any heat treatment. Different rapid heat treatments were carried out on samples of an alloy EN AW-6082 in a quenching dilatometer and terminated at defined temperatures. Subsequent reheating of the samples in the DSC enables analysis of the precipitation state of the heat-treated samples. This method allows for previously un-measurable heat treatments to get information about the occurring precipitation and dissolution reactions during short-term heat treatments.

Materials & Design, 2020

• Air cooling results in heterogenous nucleation of b-Mg 2 Si and g-MgZn 2 on grain boundaries an... more • Air cooling results in heterogenous nucleation of b-Mg 2 Si and g-MgZn 2 on grain boundaries and dispersoids. • Artificial ageing temperature influences the Mg-Zn (g) hardening phases orientation relationship with the aluminium matrix. • 6xxx type hardening phases are found to coexist with the 7xxx type hardening in the T7 temper if water quenched.

Journal of Materials Science, Oct 1, 2021

During heating of Al alloys, typically a sequence of precipitation and dissolution reactions occu... more During heating of Al alloys, typically a sequence of precipitation and dissolution reactions occurs and the single (partly opposing) reactions superimpose. Differential scanning calorimetry (DSC) is one common technique to analyse the kinetic development of precipitation and dissolution in Al alloys, but the superposition of the exothermic precipitation and endothermic dissolution reactions complicates the DSC signal interpretation, as DSC measures the sum of any heat effect. Synchrotron high-energy X-ray diffraction (HEXRD) allows the kinetic development of phase transformations to be obtained and can support the separation of superimposed DSC signals. HEXRD results from this work offer a new approach to separate part of the superimposed reactions and their kinetic development for the equilibrium phases b-Mg 2 Si in EN AW-6082 and g-Mg(Zn,Cu,Al) 2 in EN AW-7150. Comparing DSC and HEXRD results confirms serious overlap issues. Common DSC evaluation methods alone, using zero crossing between endothermic and exothermic heat flow or peak positions can be misleading regarding individual reaction start and finish temperatures as well as regarding reaction intensities, which can be unambiguously determined by in situ HEXRD.

Journal of Materials Science, Jan 11, 2021

The aluminum alloy 2618A is applied for engine components such as radial compressor wheels which ... more The aluminum alloy 2618A is applied for engine components such as radial compressor wheels which operate for long time at elevated temperatures. This results in coarsening of the hardening precipitates and degradation in mechanical properties during the long-term operation, which is not taken into account in the current lifetime prediction models due to the lack of quantitative microstructural and mechanical data. To address this issue, a quantitative investigation on the evolution of precipitates during long-term aging at 190°C for up to 25,000 h was conducted. Detailed transmission electron microscopy (TEM) was combined with Brinell hardness measurements and thorough differential scanning calorimetry (DSC) experiments. The results show that GPB zones and S-phase Al 2 CuMg grow up to \ 1,000 h during which the GPB zones dissolve and S-phase precipitates form. For longer aging times, only S-phase precipitates coarsen, which can be well described using the Lifshitz-Slyozov-Wagner theory of ripening. A thorough understanding of the underlying microstructural processes is a prerequisite to enable the integration of aging behavior into the established lifetime models for components manufactured from alloy 2618A. aerospace and transportation industry due to its favorable material properties (e.g., slow long-term degradation) [1]. It contains Fe and Ni in the form of intermetallic compounds of lm size, which retain microstructural stability and provide dispersion

Materials Science Forum, Nov 1, 2016

The precipitation behaviour in a novel Al 0.5 MoTaTi complex concentrated alloy (CCA) is describe... more The precipitation behaviour in a novel Al 0.5 MoTaTi complex concentrated alloy (CCA) is described in this study. The alloy was fabricated through an arc-melting process. Solution heat treatment and additional aging was performed at 1673 K and 1273 K, respectively. It was found, that a disordered body-centred cubic phase (bcc) precipitates in cuboidal-and loop-shaped form within an ordered B2 phase. The mechanism of the formation of the loops is believed to be related to the transformation of anti-phase boundaries (APB) in the ordered B2 phase at high temperatures, which act as nucleation sites for the disordered bcc phase during cooling and aging.

Materials & Design, May 1, 2018

Dispersoid strengthening in Al-Mn-Fe-Si(-Mg) aluminum wrought alloys can play an important role i... more Dispersoid strengthening in Al-Mn-Fe-Si(-Mg) aluminum wrought alloys can play an important role in applications at elevated temperatures. Fine and evenly distributed dispersoids are needed to increase alloy strength, which can be achieved by conducting an adapted homogenization heat treatment. In this work for the first time, the precipitation behavior of dispersoids in direct chill casted Al-Mn-Fe-Si(-Mg) alloys was systematically investigated in situ during heating by means of differential scanning calorimetry. In situ analysis of the phase transformation kinetics during heating and homogenization was complemented by ex situ testing of the electrical conductivity and Vickers hardness, as well as micro-and nano-structural analyses by optical light and transmission electron microscopy. The influence of the heating rate on precipitation behavior was determined by calorimetric in situ experiments, covering a wide heating rate, ranging from 0.003 to 2 K/s. The influence of 400 and 550 °C homogenization temperatures and soaking durations on hardness and electrical conductivity was also investigated. The highest dispersoid strengthening was obtained after heat treating the as-cast Mg-containing alloy at 400 °C for 3 to 10 h. Furthermore, the Mg content heavily influenced the precipitation behavior by forming β-Mg2Si precursor phases, which can act as nucleation sites for dispersoid precipitation.

HTM Journal of Heat Treatment and Materials, Dec 11, 2018

Although metallic materials processed by laser beam melting have significantly different microstr... 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.

Metals, Apr 13, 2018

We use a systematic approach to investigate the influence of the specific solution condition on q... more We use a systematic approach to investigate the influence of the specific solution condition on quench-induced precipitation of coarse secondary phase particles during subsequent cooling for a wide range of cooling rates. Commercially produced plate material of aluminum alloy EN AW-6082 was investigated and the applied solution treatment conditions were chosen based on heating differential scanning calorimetry experiments of the initial T651 condition. The kinetics of the quench-induced precipitation were investigated by in situ cooling differential scanning calorimetry for a wide range of cooling rates. The nature of those quench-induced precipitates was analyzed by electron microscopy. The experimental data was evaluated with respect to the detrimental effect of incomplete dissolution on the age-hardening potential. We show that if the chosen solution temperature and soaking duration are too low or short, the solution treatment results in an incomplete dissolution of secondary phase particles. This involves precipitation during subsequent cooling to start concurrently with the onset of cooling, which increases the quench sensitivity. However, if the solution conditions allow the formation of a complete solid solution, precipitation will start after a certain degree of undercooling, thus keeping the upper critical cooling rate at the usual alloy-specific level.

For aluminium alloys, precipitation strengthening is controlled by age-hardening heat treatments,... 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.

Springer eBooks, 2015

The undercooled supersaturated aluminium alloy EN AW-6082 has been investigated during in-situ qu... more The undercooled supersaturated aluminium alloy EN AW-6082 has been investigated during in-situ quenching experiments by means of differential scanning calorimetry and thermomechanical analysis. Calorimetry resulted in continuous time-temperature-precipitation diagrams and thermomechanical analysis in stress/strain curves. In this work, the above results have been correlated. Supersaturation of the undercooled aluminium alloy has been determined from calorimetric data. Next, a solid solution strengthening model has been used to calculate yield strength. The model has further been extended by a temperature dependent term considering other strengthening mechanisms. Yield strength has been computed for a broad range of quenching rates and quenching temperatures. Results have been compared with experimental data from thermomechanical analysis and found to correlate well.

Der Einfluss der Abschreckgeschwindigkeit auf das Ausscheidungsverhalten von Aluminiumlegierungen... more 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 für Aluminiumlegierungen kaum verfügbar. Daher wurde in dieser Arbeit ein Verfahren zur Aufnahme von kontinuierlichen Zeit-Temperatur-Ausscheidungs-Diagrammen für Aluminiumlegierungen mittels Differential Scanning Calorimetry (DSC) entwickelt. Werden Al-Mg-Si-Legierungen in legierungsspezifisch langsamen Kühlgeschwindigkeitsbereichen von Lösungsglühbedingungen in einem DSC abgekühlt, sind in den Abkühlkurven mindestens zwei exotherme Ausscheidungsreaktionsbereiche zu erkennen, die Hoch-sowie die Niedertemperaturreaktionen. Die

Materials Science Forum, Jun 1, 2014

Isothermal time-temperature-precipitation (TTP) diagrams deliver important material data like tem... more Isothermal time-temperature-precipitation (TTP) diagrams deliver important material data like temperature and time ranges critical for precipitation. During the last years an in-situ calorimetric method to record continuous cooling precipitation diagrams has been developed to application level by our group. However, isothermal TTP-diagrams were still determined by ex-situ analyses only. In this work in-situ measurements of precipitation reactions were carried out during isothermal soaking. Therefore the whole heat treatment cycle was performed in a differential scanning calorimeter (DSC). Al-Mg-Si-alloys 6063 and 6005A were analysed. Solution annealing and overcritical quenching to several temperatures between 450 °C and 250 °C was followed by isothermal soaking. Based on the heat flow curves during isothermal soaking TTP-diagrams were determined. Further microstructure investigations by scanning electron microscopy and hardness tests after artificial ageing were performed. Both alloys show similar results. In the TTP-diagramms three so-called ”C-curves” could be observed. Every C-curve is expected to represent precipitation of a different phase. Hardness and microstructure investigations correspond with the data of the TTP-diagramms.

Materials Science Forum, Nov 1, 2016

Quenching is a critical step during the strengthening age hardening of Aluminium alloys. To obtai... more Quenching is a critical step during the strengthening age hardening of Aluminium alloys. To obtain optimal technological results, parts should be quenched with the upper critical cooling rate. The precipitation behaviour of Al alloys during cooling from solution annealing and thereby the critical cooling rates are typically investigated by in-situ measurements with differential scanning calorimetry (DSC). Conventional DSCs are limited at cooling rates below 10 Ks-1. Unfortunately, medium to high strength Al alloys typically have critical cooling rates between 10 and some 100 Ks-1. Recently it was shown that dilatometry is generally able for in-situ detection of precipitation in Al alloys. Dilatometry allows controlled cooling up to some 100 Ks-1 and therefore covers the cooling rate range relevant. In this work, we aim to show up and discuss possibilities and limitations of dilatometric detection of quench induced precipitates in 2xxx, and 7xxx Al alloys. The basic method will be presented and results will be compared with DSC work.

Social Science Research Network, 2022

Social Science Research Network, 2021

The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently... more The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3^rd 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.

Springer eBooks, 2020

When simulating the material behavior during thermo-mechanical processes, the understanding of th... 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.

HTM Journal of Heat Treatment and Materials, Dec 10, 2013

Kurzfassung Der Werkstoff 7CrMoVTiB10-10 (T24) wurde als warmfester Stahl für den Einsatz in 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.

Materials, Apr 2, 2019

In this work, a method is presented which allows the determination of calorimetric information, a... more In this work, a method is presented which allows the determination of calorimetric information, and thus, information about the precipitation and dissolution behavior of aluminum alloys during heating rates that could not be previously measured. Differential scanning calorimetry (DSC) is an established method for in-situ recording of dissolution and precipitation reactions in various aluminum alloys. Diverse types of DSC devices are suitable for different ranges of scanning rates. A combination of the various available commercial devices enables heating and cooling rates from 10 −4 to 5 Ks −1 to be covered. However, in some manufacturing steps of aluminum alloys, heating rates up to several 100 Ks −1 are important. Currently, conventional DSC cannot achieve these high heating rates and they are still too slow for the chip-sensor based fast scanning calorimetry. In order to fill the gap, an indirect measurement method has been developed, which allows the determination of qualitative information, regarding the precipitation state, at various points of any heat treatment. Different rapid heat treatments were carried out on samples of an alloy EN AW-6082 in a quenching dilatometer and terminated at defined temperatures. Subsequent reheating of the samples in the DSC enables analysis of the precipitation state of the heat-treated samples. This method allows for previously un-measurable heat treatments to get information about the occurring precipitation and dissolution reactions during short-term heat treatments.

Materials & Design, 2020

• Air cooling results in heterogenous nucleation of b-Mg 2 Si and g-MgZn 2 on grain boundaries an... more • Air cooling results in heterogenous nucleation of b-Mg 2 Si and g-MgZn 2 on grain boundaries and dispersoids. • Artificial ageing temperature influences the Mg-Zn (g) hardening phases orientation relationship with the aluminium matrix. • 6xxx type hardening phases are found to coexist with the 7xxx type hardening in the T7 temper if water quenched.

Journal of Materials Science, Oct 1, 2021

During heating of Al alloys, typically a sequence of precipitation and dissolution reactions occu... more During heating of Al alloys, typically a sequence of precipitation and dissolution reactions occurs and the single (partly opposing) reactions superimpose. Differential scanning calorimetry (DSC) is one common technique to analyse the kinetic development of precipitation and dissolution in Al alloys, but the superposition of the exothermic precipitation and endothermic dissolution reactions complicates the DSC signal interpretation, as DSC measures the sum of any heat effect. Synchrotron high-energy X-ray diffraction (HEXRD) allows the kinetic development of phase transformations to be obtained and can support the separation of superimposed DSC signals. HEXRD results from this work offer a new approach to separate part of the superimposed reactions and their kinetic development for the equilibrium phases b-Mg 2 Si in EN AW-6082 and g-Mg(Zn,Cu,Al) 2 in EN AW-7150. Comparing DSC and HEXRD results confirms serious overlap issues. Common DSC evaluation methods alone, using zero crossing between endothermic and exothermic heat flow or peak positions can be misleading regarding individual reaction start and finish temperatures as well as regarding reaction intensities, which can be unambiguously determined by in situ HEXRD.

Journal of Materials Science, Jan 11, 2021

The aluminum alloy 2618A is applied for engine components such as radial compressor wheels which ... more The aluminum alloy 2618A is applied for engine components such as radial compressor wheels which operate for long time at elevated temperatures. This results in coarsening of the hardening precipitates and degradation in mechanical properties during the long-term operation, which is not taken into account in the current lifetime prediction models due to the lack of quantitative microstructural and mechanical data. To address this issue, a quantitative investigation on the evolution of precipitates during long-term aging at 190°C for up to 25,000 h was conducted. Detailed transmission electron microscopy (TEM) was combined with Brinell hardness measurements and thorough differential scanning calorimetry (DSC) experiments. The results show that GPB zones and S-phase Al 2 CuMg grow up to \ 1,000 h during which the GPB zones dissolve and S-phase precipitates form. For longer aging times, only S-phase precipitates coarsen, which can be well described using the Lifshitz-Slyozov-Wagner theory of ripening. A thorough understanding of the underlying microstructural processes is a prerequisite to enable the integration of aging behavior into the established lifetime models for components manufactured from alloy 2618A. aerospace and transportation industry due to its favorable material properties (e.g., slow long-term degradation) [1]. It contains Fe and Ni in the form of intermetallic compounds of lm size, which retain microstructural stability and provide dispersion

Materials Science Forum, Nov 1, 2016

The precipitation behaviour in a novel Al 0.5 MoTaTi complex concentrated alloy (CCA) is describe... more The precipitation behaviour in a novel Al 0.5 MoTaTi complex concentrated alloy (CCA) is described in this study. The alloy was fabricated through an arc-melting process. Solution heat treatment and additional aging was performed at 1673 K and 1273 K, respectively. It was found, that a disordered body-centred cubic phase (bcc) precipitates in cuboidal-and loop-shaped form within an ordered B2 phase. The mechanism of the formation of the loops is believed to be related to the transformation of anti-phase boundaries (APB) in the ordered B2 phase at high temperatures, which act as nucleation sites for the disordered bcc phase during cooling and aging.

Materials & Design, May 1, 2018

Dispersoid strengthening in Al-Mn-Fe-Si(-Mg) aluminum wrought alloys can play an important role i... more Dispersoid strengthening in Al-Mn-Fe-Si(-Mg) aluminum wrought alloys can play an important role in applications at elevated temperatures. Fine and evenly distributed dispersoids are needed to increase alloy strength, which can be achieved by conducting an adapted homogenization heat treatment. In this work for the first time, the precipitation behavior of dispersoids in direct chill casted Al-Mn-Fe-Si(-Mg) alloys was systematically investigated in situ during heating by means of differential scanning calorimetry. In situ analysis of the phase transformation kinetics during heating and homogenization was complemented by ex situ testing of the electrical conductivity and Vickers hardness, as well as micro-and nano-structural analyses by optical light and transmission electron microscopy. The influence of the heating rate on precipitation behavior was determined by calorimetric in situ experiments, covering a wide heating rate, ranging from 0.003 to 2 K/s. The influence of 400 and 550 °C homogenization temperatures and soaking durations on hardness and electrical conductivity was also investigated. The highest dispersoid strengthening was obtained after heat treating the as-cast Mg-containing alloy at 400 °C for 3 to 10 h. Furthermore, the Mg content heavily influenced the precipitation behavior by forming β-Mg2Si precursor phases, which can act as nucleation sites for dispersoid precipitation.

HTM Journal of Heat Treatment and Materials, Dec 11, 2018

Although metallic materials processed by laser beam melting have significantly different microstr... 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.

Metals, Apr 13, 2018

We use a systematic approach to investigate the influence of the specific solution condition on q... more We use a systematic approach to investigate the influence of the specific solution condition on quench-induced precipitation of coarse secondary phase particles during subsequent cooling for a wide range of cooling rates. Commercially produced plate material of aluminum alloy EN AW-6082 was investigated and the applied solution treatment conditions were chosen based on heating differential scanning calorimetry experiments of the initial T651 condition. The kinetics of the quench-induced precipitation were investigated by in situ cooling differential scanning calorimetry for a wide range of cooling rates. The nature of those quench-induced precipitates was analyzed by electron microscopy. The experimental data was evaluated with respect to the detrimental effect of incomplete dissolution on the age-hardening potential. We show that if the chosen solution temperature and soaking duration are too low or short, the solution treatment results in an incomplete dissolution of secondary phase particles. This involves precipitation during subsequent cooling to start concurrently with the onset of cooling, which increases the quench sensitivity. However, if the solution conditions allow the formation of a complete solid solution, precipitation will start after a certain degree of undercooling, thus keeping the upper critical cooling rate at the usual alloy-specific level.