TY - BOOK

T1 - Exploration of phase transformations in lightweight structural materials by means of in situ high-energy X-ray methods

AU - Graf, Gloria

N1 - no embargo

PY - 2023

Y1 - 2023

N2 - Lightweight structural metallic materials are characterized by their low density and high specific strength. These properties help to reduce the weight in automotive and aircraft applications by substituting heavier materials, such as steels or Ni-base alloys. Consequently, the fuel consumption as well as the accompanying emission of environmentally harmful greenhouse gases can be significantly decreased. In this thesis new trends arising in the production and design of three different classes of lightweight alloys, i.e. Mg alloys, Al alloys and intermetallic ¿-TiAl based alloys, are covered. In this regard, the investigation of phase transformations plays a key role, as the properties of a material are governed by its phase constitution and microstructure. An ideal tool for the in situ investigation of the phase evolution at conditions comparable to those during processing, post-processing and service is the use of synchrotron radiation due to the high temporal resolution achievable. Thus, the studies performed in this thesis are based on synchrotron scattering and diffraction methods. The covered topics are briefly addressed in the following: 1) Regarding the development of Al alloys, many studies have recently focused on the novel class of Al-Mg-Zn-Cu crossover alloys. Apart from their excellent formability, crossover alloys can be age-hardened and show improved recyclability. Due to their decent weldability they could also be used for additive manufacturing. In this thesis the low quench rate sensitivity of a novel Al crossover alloy adapted specifically for additive manufacturing was demonstrated. 2) Additive manufacturing, especially wire-arc additive manufacturing, demonstrates a high potential as an alternative to conventional production methods for Mg alloys. With wire-arc additive manufacturing it was shown that strengths comparable to those of wrought alloys and even higher elongations can be achieved. This thesis deals with the age-hardening behavior of a wire-arc additively manufactured AZ91 Mg alloy. Enhanced kinetics during aging were found owed to a higher amount of nucleation sites compared to cast alloys. 3) Due to the extreme cooling rates encountered during laser powder bed fusion non-equilibrium phase transformations can occur. In intermetallic ¿-TiAl based alloys they are difficult to distinguish using microscopy owed to the multitude of phase transformations that generally occur during cooling. Hence, a new setup was applied to study the solidification and solid-state transformations in a Ti-48Al-2Cr-2Nb alloy in an in situ manner. During laser fusion the peritectic solidification was detected. Further, the formation of ¿ was strongly suppressed. 4) The introduction of cost-efficient Mn-containing ¿-TiAl based alloys could lead to a broader application of this class of materials. However, for the alloy design the knowledge about the thermal and chemical stability range of the brittle Ti(Mn,Al)2 phase, which can be formed at service conditions, is crucial. This work shows that this phase is stable below 900 °C in the Ti-42Al-5Mn alloy and is formed if the local Mn concentration exceeds roughly 16 at.%. In sum, these investigations further the knowledge about lightweight alloys and can be used as a basis for the optimization of processes and alloys.

AB - Lightweight structural metallic materials are characterized by their low density and high specific strength. These properties help to reduce the weight in automotive and aircraft applications by substituting heavier materials, such as steels or Ni-base alloys. Consequently, the fuel consumption as well as the accompanying emission of environmentally harmful greenhouse gases can be significantly decreased. In this thesis new trends arising in the production and design of three different classes of lightweight alloys, i.e. Mg alloys, Al alloys and intermetallic ¿-TiAl based alloys, are covered. In this regard, the investigation of phase transformations plays a key role, as the properties of a material are governed by its phase constitution and microstructure. An ideal tool for the in situ investigation of the phase evolution at conditions comparable to those during processing, post-processing and service is the use of synchrotron radiation due to the high temporal resolution achievable. Thus, the studies performed in this thesis are based on synchrotron scattering and diffraction methods. The covered topics are briefly addressed in the following: 1) Regarding the development of Al alloys, many studies have recently focused on the novel class of Al-Mg-Zn-Cu crossover alloys. Apart from their excellent formability, crossover alloys can be age-hardened and show improved recyclability. Due to their decent weldability they could also be used for additive manufacturing. In this thesis the low quench rate sensitivity of a novel Al crossover alloy adapted specifically for additive manufacturing was demonstrated. 2) Additive manufacturing, especially wire-arc additive manufacturing, demonstrates a high potential as an alternative to conventional production methods for Mg alloys. With wire-arc additive manufacturing it was shown that strengths comparable to those of wrought alloys and even higher elongations can be achieved. This thesis deals with the age-hardening behavior of a wire-arc additively manufactured AZ91 Mg alloy. Enhanced kinetics during aging were found owed to a higher amount of nucleation sites compared to cast alloys. 3) Due to the extreme cooling rates encountered during laser powder bed fusion non-equilibrium phase transformations can occur. In intermetallic ¿-TiAl based alloys they are difficult to distinguish using microscopy owed to the multitude of phase transformations that generally occur during cooling. Hence, a new setup was applied to study the solidification and solid-state transformations in a Ti-48Al-2Cr-2Nb alloy in an in situ manner. During laser fusion the peritectic solidification was detected. Further, the formation of ¿ was strongly suppressed. 4) The introduction of cost-efficient Mn-containing ¿-TiAl based alloys could lead to a broader application of this class of materials. However, for the alloy design the knowledge about the thermal and chemical stability range of the brittle Ti(Mn,Al)2 phase, which can be formed at service conditions, is crucial. This work shows that this phase is stable below 900 °C in the Ti-42Al-5Mn alloy and is formed if the local Mn concentration exceeds roughly 16 at.%. In sum, these investigations further the knowledge about lightweight alloys and can be used as a basis for the optimization of processes and alloys.

KW - Strukturwerkstoffe für Leichtbau

KW - Phasenumwandlungen

KW - in situ Synchrotron Untersuchungen

KW - Additive Fertigung

KW - lightweight structural materials

KW - in situ synchrotron studies

KW - phase transformations

KW - additive manufacturing

M3 - Doctoral Thesis

ER -