Plasticity of topologically close-packed phases in the Fe-Ta(-Al) system

Understanding the structure-property relationships of materials plays a significant role in the development of materials for technical applications. Due to the many possible combinations of two or more elements, intermetallic phases can be very interesting for these developments. High strength up to high temperatures makes intermetallics promising materials for high-temperature applications. However, their complex structure, resulting in a pronounced brittleness, has so far limited their applicability. We focus on the understanding of plastic deformation in topologically close-packed (TCP) phases, which form one of the largest groups of intermetallics. To do this, we use nanomechanical tests that allow us to study plasticity even in the most brittle materials. Here, we consider the Fe-Ta(-Al) system that contains two closely related TCP phases, a C14 Laves phase and a µ-phase. The building block-like structure of these phases enables a systematic investigation as well as a transfer of the findings to other complex crystals. The mechanical properties of the two TCP phases in the Fe-Ta(-Al) system, investigated by state-of-the-art micromechanical testing, are introduced in this work. The influence of the crystal structure and chemical composition on the mechanical properties and the deformation mechanisms of the TCP phases are discussed

Chemical and structural characterization of the native oxide scale on a Mg-based alloy

In this study, the structure and composition of the native oxide forming on the basal plane (0001) of Mg-2Al-0.1Ca is investigated by a correlative approach, combining scanning transmission electron microscopy (STEM) and atom probe tomography (APT). Atom probe specimens were prepared conventionally in a Ga focused ion beam (FIB) as well as a Xe plasma FIB in a cryogenic setup and subsequently cleaned in the atom probe to remove surface contamination before oxidation. While thermal energy input from the laser and longer atmospheric exposure time increased the measured hydrogen content in the specimen's apex region, cryo preparation revealed, that the hydrogen uptake in magnesium is independent of the employment of conventional or cryogenic FIB preparation. TEM measurements demonstrated the growth of a (111) MgO oxide layer with 3-4 nm thickness on the basal (0001) plane of the Mg atom probe specimen. APT data further revealed the formation of an aluminum-rich region between bulk Mg and the native oxide. The aluminum enrichment of up to ~20 at.% at the interface is consistent with an inward growth of the oxide scale

Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation

Cost Effectiveness in River Management: Evaluation of Integrated River Policy System in Tidal Ouse

The River Ouse forms a significant part of Humber river system, which drains about one fifth the land area of England and provides the largest fresh water source to the North Sea from UK. The river quality in the tidal river suffered from sag of dissolved oxygen (DO) during last few decades, deteriorated by the effluent discharges. The Environment Agency (EA) proposed to increase the water quality of Ouse by implementing more potent environmental policies. This paper explores the cost effectiveness of water management in the Tidal Ouse through various options by taking into account the variation of assimilative capacity of river water, both in static and dynamic scope of time. Reduction in both effluent discharges and water abstraction were considered along side with choice of effluent discharge location. Different instruments of environmental policy, the emission tax-subsidy (ETS) scheme and tradable pollution permits (TPP) systems were compared with the direct quantitative control approach. This paper at the last illustrated an empirical example to reach a particular water quality target in the tidal Ouse at the least cost, through a solution of constrained optimisation problem. The results suggested significant improvement in the water quality with less cost than current that will fail the target in low flow year

Fundamental research into the role of intermetallic phases in joining of aluminium alloys to steel

Bildung und Wachstum intermetallischer Phasensäume am Übergang zwischen Fe bzw. Stahl und sowohl reinem als auch Si-Legiertem Al wurden bei verschiedenen Werkstoffzuständen untersucht, ebenso wie der Einfluss von Phasensaumdicke und -Aufbau auf die mechanischen Verbindungseigenschaften. In elementaren Interdiffusionsversuchen ist die Saumdicke wesentlich durch das parabolische Wachstum der $\eta$ Phase (Al$_{5}$Fe$_{2}$) bestimmt. Die Zugabe von Si in Al, welche bekanntermaßen das Phasenwachstum bei der Reaktion mit Al Schmelzen hemmt, bewirkte ein beschleunigtes Wachstum bei 600°C Reaktionstemperatur. Untersuchungen der Verformungs-Mikrostruktur unter Härteeindrücken offenbarten einen deutlichen Einfluss der äußeren und inneren Phasengrenzen auf die Härtewerte. Wärmebehandlung von Stahl/Al Rührreibschweißungen zeigte, dass die Verbindungsfestigkeit mit reinem Al durch die Bildung von Kirkendall-Poren an der Grenzfläche Phasensaum/Al bestimmt wird, mit Al-Si hingegen durch die Dicke der $\eta$ Phase.Formation and evolution of intermetallic reaction layers at the interface between Fe or low C steel and both pure and Si containing Al was studied in different material states, as well as the impact of layer thickness and build-up on mechanical properties of joints. In basic interdiffusion experiments, width of reaction layers is governed mainly by the parabolic growth of the $\eta$ phase (Al$_{5}$Fe$_{2}$). The addition of Si to Al, which is known to decelerate layer growth during interdiffusion with Al melts, was found to accelerate the growth at 600°C. Investigations of the deformation microstructure beneath hardness indentations elicited a pronounced influence of the reaction layers outer and inner interfaces on the observed hardness. Heat treating of dissimilar friction stir welds showed that with pure Al, the joint strength is governed by the formation of Kirkendall-porosity at the reaction layer/Al interface; in joints with Al-Si it is controlled by the thickness of the $\eta$ phase layer