Stefanie Sandlöbes

RWTH Aachen University · Institut für Metallkunde und Metallphysik

Phase transformations and crystallographic defects are two essential tools to drive innovations in materials. Bulk materials design via tuning chemical compositions is systematized using phase diagrams. It is shown here that the same thermodynamic concept can be applied to manipulate the chemistry at defects. Grain boundaries in Mg–Ga system are ch...

The structure and composition of the native oxide forming on the basal plane (0001) of the magnesium-based alloy Mg-2Al-0.1Ca was investigated by combining scanning transmission electron microscopy (STEM) and atom probe tomography (APT). While STEM measurements demonstrated the growth of a (111) MgO oxide layer with 3-4 nm thickness on the basal (0...

Magnesium and its alloys offer huge potential for lightweight applications. However, many Mg alloys suffer from limited room‐temperature formability. It has previously been shown that the addition of aluminium and calcium to Mg can improve ductility. Therefore, in the present work, it is aimed to systematically vary the alloying content of Al and C...

A cast Mg‐4.65Al‐2.82Ca alloy with a microstructure containing an α‐Mg matrix is studied, which is reinforced with a C36 Laves phase skeleton. Such ternary alloys are targeted for elevated temperature applications in automotive engines, since they possess excellent creep properties. However, in application, the alloy may be subjected to a wide rang...

Laves phases such as CaMg 2 (C14), Ca(Al,Mg) 2 (C36), and CaAl 2 (C15) have high melting points, high hardness at room and elevated temperatures, but are inherently brittle. Cast Mg–Al–Ca alloys having Laves phase skeleton thus have good creep properties but limited ductility. An understanding of the co-deformation behaviour of α-Mg and Laves phase...

We study a cast Mg-4.65Al-2.82Ca alloy with a microstructure containing $\alpha$-Mg matrix reinforced with a C36 Laves phase skeleton. Such ternary alloys are targeted for elevated temperature applications in automotive engines since they possess excellent creep properties. However, in application, the alloy may be subjected to a wide range of stra...

The Laves phase skeleton in cast Mg-Al-Ca alloys is known to provide considerable strengthening. Laves phases such as CaMg$_2$ (C14), Ca(Al,Mg)$_2$ (C36), and CaAl2 (C15) have high melting points, high hardness at room and elevated temperatures, but unfortunately are inherently brittle. Mg-Al-Ca alloys thus have good creep properties but limited du...

The room temperature plasticity of the cubic C15 CaAl2 Laves phase was investigated using nanomechanical testing and electron microscopy. The correlation between slip traces in the vicinity of nanoindents and crystallographic orientation data allowed us to gain statistical data on the activated slip and crack planes for 10 different crystallographi...

The micro-galvanic coupling between ferrite and austenite in X6MnAl12-3 was investigated depending on intercritical annealing (ICA) temperature and consequently different Mn contents in reverted austenite, phase fractions, and interface area combining microstructural changes, electrochemical testing and the solution’s element concentration during i...

The Mg-Al-Ca system has been shown to be a promising alloy system since it combines the low density of Mg with a high creep resistance at high alloying contents and a high ductility for dilute alloys, while simultaneously avoiding the requirement of alloying with costly rare earth elements. Nevertheless, the adequate preparation of the alloy system...

The investigation of the deformation behaviour of intermetallic phases is mostly limited to high temperatures due to their low ductility at ambient temperature. Therefore, within this study, nanoindentation experiments on the Al$_{2}$Cu phase were performed from ambient temperature up to 300{\deg}C in conjunction with TEM investigations of the defo...

Medium-Mn steels exhibit excellent mechanical properties and lower production costs compared to high-Mn steels, which makes them a potential material for future application in the automotive industry. Intercritical annealing (ICA) after cold rolling allows to control the stacking fault energy (SFE) of austenite, the fraction of ferrite and reverted...

Characteristic densification in cold sintered microstructures could also have a strong influence in defining their mechanical response. For the first time, nanoindentation and micro-pillar testing is used to study these details. Based on our recent work, we selected cold sintered (250 °C, ∼ 99 % dense) and conventionally sintered (900 °C, ∼ 98 % de...

Intermetallic phases can significantly improve the creep resistance of magnesium alloys, extending their use to higher temperatures. However, little is known about the deformation behaviour of these phases at application temperatures, which are commonly below their macroscopic brittle-to-ductile-transition temperature. In this study, we therefore i...

The microstructure of Mg-Al-Ca alloys consists of a hard intra- and intergranular eutectic Laves phase network embedded in a soft α-Mg matrix. For such heterogeneous microstructures, the mechanical response and co-deformation of both phases under external load are not yet fully understood. We therefore used nano- and microindentation in combination...

Intermetallic phases can significantly improve the creep resistance of magnesium alloys, extending their use to higher temperatures. However, little is known about the deformation behaviour of these phases at application temperatures, which are commonly below their macroscopic brittle-to-ductile-transition temperature. In this study, we therefore i...

The microstructure of Mg-Al-Ca alloys consists of a hard intra- and intergranular eutectic Laves phase network embedded in a soft $\alpha$-Mg matrix. For such heterogeneous microstructures, the mechanical response and co-deformation of both phases under external load are not yet fully understood. We therefore used nano- and microindentation in comb...

Two approaches in materials physics have proven immensely successful in alloy design: First, thermodynamic and kinetic descriptions for tailoring and processing alloys to achieve a desired microstructure. Second, crystal defect manipulation to control strength, formability and corrosion resistance. However, to date, the two concepts remain essentia...

The deformation behaviour of the intermetallic Al$_{2}$Cu-phase was investigated using atomistic simulations and micropillar compression, where slip on the unexpected {211} and {022} slip planes was revealed. Additionally, all possible slip systems for the intermetallic phases were further evaluated and a preference for the activation of slip syste...

The mechanical behaviour of MgAl alloys can be largely improved by the formation of an intermetallic Laves phase skeleton, in particular the creep strength. Recent nanomechanical studies revealed plasticity by dislocation glide in the (Mg,Al)2Ca Laves phase, even at room temperature. As strengthening skeleton, this phase remains, however, brittle a...

In this study, we reveal that strain partitioning control is an essential technique to improve the mechanical response of medium-Mn steels by the micro digital image correlation analysis. An Fe-12Mn-0.06C-3Al (wt%) steel, which shows transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP), was used as a model alloy. Two alte...

The deformation behaviour of the intermetallic Al2Cu θ-phase was investigated using atomistic simulations and micropillar compression, where slip on the unexpected {211} and {022} slip planes was revealed. Additionally, all possible slip systems for the intermetallic phases were further evaluated and a preference for the activation of slip systems...

The mechanical behaviour of Mg-Al alloys can be largely improved by the formation of an intermetallic Laves phase skeleton, in particular the creep strength. Recent nanomechanical studies revealed plasticity by dislocation glide in the (Mg,Al)$_2$Ca Laves phase, even at room temperature. As strengthening skeleton, this phase remains, however, britt...

Strain heterogeneity at the microstructural level plays a vital role in the deformation and fracture behaviour of dual or multi-phase materials. In the present work, the strain heterogeneity, localisation and partitioning arising at the sub-micron scale during elevated temperature (170 °C) tensile deformation of an Mg–5Al–3Ca alloy was investigated...

The tensile behavior at elevated temperature of heavily plastically deformed Al-Mg alloys with an Mg content of 1 to 5% was investigated. Large plastic deformation was imposed by confined channel die pressing at room temperature up to 18 passes. During heating to test temperature the specimens were observed to undergo recovery and partly recrystall...

Strain heterogeneity at the microstructural level plays a vital role in the deformation and fracture behaviour of dual or multi-phase materials. In the present work, the strain heterogeneity, localization and partitioning arising at the sub-micron scale during elevated temperature (170 {\deg}C) tensile deformation of an Mg-5Al-3Ca alloy was investi...

A promising approach to deform metallic-intermetallic composite materials is the application of electric current pulses during the deformation process to achieve a lower yield strength and enhanced elongation to fracture. This is known as the electroplastic effect. In this work, a novel setup to study the electroplastic effect during nanoindentatio...

A self-consistent thermodynamic description of the Zn–Al–Cu–Mg quaternary alloy system is developed. The as-cast microstructures of key Zn–Al–Cu–Mg samples in the low-alloyed range are studied experimentally. These are interpreted by computational thermodynamic analysis applying both Scheil and equilibrium simulations. Contradictions concerning the...

A promising approach to deform metallic-intermetallic composite materials is the application of electric current pulses during the deformation process to achieve a lower yield strength and enhanced elongation to fracture. This is known as the electroplastic effect. We developed a novel setup to study the electroplastic effect during nanoindentation...

Intermetallic phases, such as Mg2Ca, have been shown to significantly improve the creep strength of magnesium alloys. However, the relevant deformation mechanisms of the intermetallics for further alloy development are largely unknown as the application temperature of the intermetallic-metallic composites lies in their brittle low temperature regim...

As-cast Mg-Al-Ca alloys are among the most promising alloys for elevated temperature applications (≤200 °C) due to their superior creep properties when compared to conventional AZ or AM series Mg alloys. The microstructures of Mg-Al-Ca alloys consist of a soft α-Mg phase reinforced with hard interconnected Laves phases. These interconnected Laves p...

Topologically close packed phases, among them the $\mu$-phase studied here, are commonly considered as being hard and brittle due to their close packed and complex structure. Nanoindentation enables plastic deformation and therefore investigation of the structure of mobile defects in the $\mu$-phase, which, in contrast to grown-in defects, has not...

In situ microscopic-digital image correlation (µ-DIC) is used to investigate the strain partitioning and strain localization behavior in a medium manganese steel. Continuous yielding results from strain partitioning with higher strain in the reverted austenite (γR) islands and less strain in the tempered martensite matrix, both in hot and cold roll...

We address the differences in yield stresses between hot and cold rolled medium manganese steel showing continuous yielding. Continuous yielding in both, the hot and cold rolled samples were resulting from reverted austenite islands plastically deforming first and less strain in the tempered martensite matrix. At higher global strains, strain was t...

We study the microstructure formation and mechanical properties of Ti-1Fe (wt%) and Ti-3Fe (wt%) alloys for different heat treatments in the β-phase and α + β-phase regions. By applying different heat treatment routes, we observe different microstructure formation mechanisms causing a wide range of mechanical properties from high strength (1.3 GPa)...

We unravel the nature of twin boundary-associated strengthening in Fe-Mn-C twinning-induced plasticity steel (TWIPs) by micro-pillar compression tests. Dislocation interactions with a coherent twin boundary and their role on strain hardening were investigated. The results indicate that twin-matrix bundles dynamically introduced by deformation twinn...

This paper presents original data related to the research article "Local mechanical properties and plasticity mechanisms in a Zn-Al eutectic alloy" (Wu et al., 2018). The raw data provided here was used for in-situ digital image correlation on the microstructural level using a new method described in the related study. The data includes sample prep...

In multiphase alloys, the mechanical properties are controlled by both the local properties of individual microstructural constituents, as well as the mutual effect of these constituents as an aggregate. To this end, we systematically studied the local mechanical properties and deformation mechanisms of the microstructural constituents in a ZnAl4Cu...

The role of TCP phases in deformation of superalloys and steels is still not fully resolved. In particular, the intrinsic deformation mechanisms of these phases are largely unknown including the active slip systems in most of these complex crystal structures. Here, we present a first detailed investigation of the mechanical properties of the Fe7Mo6...

The role of TCP phases in deformation of superalloys and steels is still not fully resolved. In particular, the intrinsic deformation mechanisms of these phases are largely unknown including the active slip systems in most of these complex crystal structures. Here, we present a first detailed investigation of the mechanical properties of the Fe7Mo6...

The role of topologically close packed (TCP) phases in deformation of superalloys and steels is still not fully resolved. In particular, the intrinsic deformation mechanisms of these phases are largely unknown including the active slip systems in most of these complex crystal structures. Here, we present a first detailed investigation of the mechan...

We study the micro-mechanical behaviour of single-crystalline long-period-stacking ordered (LPSO) structures, α-Mg and bi-crystalline Mg/LPSO micro-pillars, all cut from the same Mg97Y2Zn1 (at.%) alloy. To investigate the deformation and co-deformation mechanisms of Mg-LPSO alloys we performed micro-pillar compression experiments with micro-pillars...

In this study we investigated the creep behaviour of three eutectic ZnAl4Cu1Mg alloys with different Mg contents, namely ZnAl4Cu1Mg0.04, ZnAl4Cu1Mg0.21 and ZnAl4Cu1Mg0.31 (in wt%), using uniaxial tensile creep tests at temperatures between 25 °C and 105 °C. Furthermore, we studied the creep properties of the individual microstructural constituents...

A framework for more ductile magnesium Development of ductile magnesium alloys is key to their use in reducing the weight of vehicles and other applications. Wu et al. tackle this issue by determining the underlying mechanisms in unprocessed magnesium alloys. Dilute amounts of solutes enhanced certain ductility-improving mechanisms over ones that c...

We have investigated twin boundaries in double-lattice hexagonal close-packed metallic materials, focusing on their atomic geometry. Combining accurate ab-initio methods and large-scale atomistic simulations we address the following two fundamental questions: (i) What are the possible intrinsic twin boundary structures in hcp crystals? (ii) Are the...

Medium manganese steels which belong to the 3rd generation advanced high strength steels are a promising material for the automotive industry due to their remarkable strength-ductility-combination enabling to reduce material and production costs. Today it is well established that the mechanical properties are strongly dependent on the intercritical...

Using metastability to engineer the microstructure of Ti-6V-4Al produced by selective laser melting

Dislocation structures forming during cyclic loading of fcc metals are fatigue damage precursors. Their specific structures are caused by the motion and interactions of dislocations. Depending on the load conditions, the grain orientation, the stacking fault energy, a variety of different dislocation structures appear in the material such as labyri...

Fatigue is one of the major failure modes of structural materials. While the effects of strengthening precipitates on the mechanical properties of heat treatable aluminum alloys during forming operations are well-studied, only little is known about the related mechanisms during fatigue. We study the influence of precipitates during low cycle fatigu...

Metals are the backbone of manufacturing owing to their strength and formability. Compared to polymers they have high mass density. There is, however, one exception: magnesium. It has a density of only 1.7 g/cm 3 , making it the lightest structural material, 4.5 times lighter than steels, 1.7 times lighter than aluminum, and even slightly lighter t...

We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 C, nano-sized k-carbides form, as charact...

Mg is the most important lightweight engineering alloy enabling future weight-reduced and fuel-saving engineering solutions. Yet, Mg is soft. Long-period stacking ordered (LPSO) structures in Mg alloys have unique crystal structures, characterized by both complex chemical and stacking order. They are essential for strengthening of Mg alloys. The fo...

Selective laser melting is a promising powder-bed-based additive manufacturing technique for titanium alloys: near net-shaped metallic components can be produced with high resource-efficiency and cost savings. For the most commercialized titanium alloy, namely Ti-6Al-4V, the complicated thermal profile of selective laser melting manufacturing (shar...

The effect of Si additions on the oxidation behavior of Cr2AlC based coatings is investigated. Oxidation experiment was performed at 1120 °C in air for 4 h for Cr2AlC and Cr2Al1−xSixC (0 < x ≤ 0.06) coatings. The crystal structure, microstructure and chemical composition of the as-deposited as well as oxidized coatings have been investigated. Alloy...

The phase formation of Nb2AlC was studied by combinatorial thin film synthesis and ab initio calculations. Thin films with lateral chemical composition gradients were synthesized by DC magnetron sputtering at substrate temperatures of 710–870 °C. The lowest formation temperature for Nb2AlC is between 710 and 750 °C. A predominantly single phase Nb2...

We present a systematic atomic scale analysis of the structural evolution of long-period-stacking-ordered (LPSO) structures in the (i) α-Mg matrix and in the (ii) interdendritic LPSO phase of an Mg97Y2Zn1 (at. %) alloy annealed at 500°C, using high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Vari...

We have computed formation energies for all technologically relevant transition metal solutes in the α, β, and ω phases of Ti, employing ab initio simulations. We analyze and explain their periodic-table trends, and from their differences we derive stabilization energies which provide direct insight into phase stabilization effects of the various s...

Aluminum single crystals with three different double-slip orientations ( , , ) and two aluminum bi-crystals – one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured...

We investigate the precipitation and decomposition phenomena in a Zn-Al-Cu-Mg alloy using transmission electron microscopy and atom probe tomography. Uniaxial tensile tests reveal brittle fracture at room temperature and ductile failure after work hardening at 85 °C. For the first time we identify and structurally and chemically characterise precip...

We study the effects of dilute Mg addition on the microstructure formation and mechanical properties of a ZnAl4Cu1 alloy. On the basis of the composition of the commercial alloy Z410 (4wt% Al, 1wt% Cu, and 0.04wt% Mg), three laboratory alloys with different Mg contents (0.04wt%, 0.21wt% and 0.31wt%) are characterised in terms of their mechanical pr...

We study the effects of different heat treatment routes on microstructure engineering and the resulting mechanical response in a plain binary Ti–4Mo (wt%) model alloy. We observe a broad variety of microstructure formation mechanisms including diffusion driven allotropic phase transformations as well as shear and/or diffusion dominated modes of mar...