Nanoindentation study of the temperature dependence of plastic instability in Al alloys.

Plastic instability, i.e. repetitive yielding that occurs during plastic deformation at low strain rates and moderately high temperatures, results in severe strain localization, reduction in ductility and formation of surface striations during forming processes. In order to gain insights into the different rate controlling mechanisms that govern PLC type plastic instability, it is useful to probe its thermal dependence. Such investigations facilitate the estimation of thermally activated parameters associated with the phenomenon and provide insights into the underlying microscopic mechanisms. In this work, we present an elevated temperature nanoindentation test method for characterizing the thermal dependence of plastic instability and assessing the activation energy associated with the phenomenon in Al–Mg and Al–Li based alloys. The method exploits the nanoscale force–displacement resolution capabilities of the nanoindenter, precludes the ambiguities inherent in the uniaxial testing based methods and offers increased reliability because of the statistical significance of the data achieved. Results show that the activation energies established by this method for these two alloys are consistent with values derived with other methods, and reflect the different rate controlling mechanisms associated with plastic instability in these alloy systems. Please click Additional Files below to see the full abstract

Grain-scale investigation of the anisotropy of PLC-type plastic instability

Various aspects of Portevin Le-Chatelier (PLC) type plastic instability, particularly the influence of strain rate, temperature and precipitation on the phenomenon have been investigated. Such investigations give insights into the underlying governing mechanisms and provide the basis for developing mechanistic and numerical models for these mechanisms. One aspect that is yet to be understood is the influence of anisotropy on plastic instability. So far, experimental efforts aimed at understanding this influence have been focussed on the influence of sample orientation and texture. However, direct measurement of the response of single crystals during uniaxial testing is essential for accurate characterization of the influence of anisotropy. Yet, such an endeavour is largely limited by the difficulty of producing single crystals of technical alloys. Insight into the orientation dependence of plastic instability is thus achieved in this work with a combination of spherical nanoindentation of single grains of Mg AZ91 and local orientation image analysis of cross sections of the nanoindents. Our results indicate that the local stresses arising from the underlying mechanisms that govern plastic instability in this alloy are strongly orientation dependent. In this talk, we will highlight the origin of the orientation dependence and the influence of twinning, and discuss the implications for macroscopic deformatio

On the mechanistic origin of the enhanced strength and ductility in rare earth-based Mg alloys

The applicability of classical wrought Mg alloys is limited by their comparatively poor room temperature ductility and low yield strength. Conversely, various experimental and computational efforts do confirm that low concentrations of rare earth (RE) in Mg significantly improves these properties. However, the mechanistic origin of these improvements are still been debated. In order to contribute to the discourse, we carried out in-depth comparison of deformation modes in single crystals of pure Mg and a homogenized Mg–0.75 at.% Gd alloy oriented for twinning, pyramidal- and basal-slip using a combination of microcompression testing, scanning transmission electron microscopy and small angle x-ray scattering technique. We observed a fivefold increase in basal CRSS and a fourfold decrease of the pyramidal/basal CRSS (P/B) ratio as a result of Gd addition. We also observed that slip was planar in the basal orientation of the alloy but wavy in pure Mg. Pyramidal slip and twinning activity in the two systems were however similar; an indication that the same mechanisms underlie deformation in these orientation. We show that the observed planar slip, increase in basal CRSS and decrease in P/B ratio are consequence of Gd-rich short-range ordered (SRO) clusters in the alloy. Our analysis show that these SRO clusters lead to significantly high strengths in the basal orientation since additional stress is required to destroy the ordering therein. This not only leads to a dramatic increase in yield strength, given the drastic reduction in P/B CRSS ratio, it should also significantly promote pyramidal slip activities in polycrystals and by extension ductility improvements

An effective activation method for industrially produced TiFeMn powder for hydrogen storage

This work proposes an effective thermal activation method with low technical effort for industrially produced titanium-iron-manganese powders (TiFeMn) for hydrogen storage. In this context, the influence of temperature and particle size of TiFeMn on the activation process is systematically studied. The results obtained from this investigation suggest that the activation of the TiFeMn material at temperatures as low as 50 °C is already possible, with a combination of “Dynamic” and “Static” routines, and that an increase to 90 °C strongly reduces the incubation time for activation, i.e. the incubation time of the sample with the two routines at 90 °C is about 0.84 h, while ∼ 277 h is required for the sample treated at 50 °C in both “Dynamic” and “Static” sequences. Selecting TiFeMn particles of larger size also leads to significant improvements in the activation performance of the investigated material. The proposed activation routine makes it possible to overcome the oxide layer existing on the compound surface, which acts as a diffusion barrier for the hydrogen atoms. This activation method induces further cracks and defects in the powder granules, generating new surfaces for hydrogen absorption with greater frequency, and thus leading to faster sorption kinetics in the subsequent absorption-desorption cycles

Micromechanisms governing plastic instability in Al–Li based alloys

Die Erforschung der zugrundeliegenden mikroskopischen Mechanismen, die plasti- sche Instabilität in mischkristall– und ausscheidungsverfestigten Aluminium–Legierungen bestimmen, ist Gegenstand verschiedener Untersuchungen gewesen. Diese Studien sind weitgehend motiviert durch die Notwendigkeit, Strategien zur Abschwächung uner- wünschter Effekte wie Duktilitätsminderung und Ausbildung von Oberflächenriefen, wel- che in eben solchen Legierungen auftreten, zu entwerfen. Während der mikroskopische Ursprung von plastischer Instabilität in mischkristallverfestigten Al–Legierungen ziemlich gut bekannt ist, gibt es immer noch kein überzeugendes Modell welches eine klare mechanistische Beschreibung im Einklang mit experimentellen Beobachtungen für plastische Instabilität in ausscheidungsverfestigten Al–Legierungen anbietet. In der vorliegenden Arbeit wurden detaillierte experimentelle Untersuchungen un- terschiedlicher Härtestufen der Mehrkomponenten–Aluminium–Lithium (Al–Li) Legie- rung AA2198 durchgeführt. Sowohl mechanische als auch mikrostrukturelle Charakteri- sierungstechniken wurden verwendet, um mikrostrukturelle Eigenschaften globalem und lokalem mechanischem Verhalten zuzuordnen. Insbesondere wurden hochauflösende Nanoindentations– und Mikrozugverfahren zur mechanischen Untersuchung verwendet; die relevanten mikrostrukturellen Eigenschaften dagegen wurden mit Methoden basierend auf Transmissionselektronenmikroskopie (TEM) untersucht, einschließlich in–situ TEM Zugbelastung zusammen mit Hochenergie–Röntgenbeugung und Atomsonden– Tomographie. Die experimentellen Ergebnisse zeigten eindeutig, dass das Auftreten von plastischer Instabilität in AA2198 nicht ausreichend erklärt werden kann durch dynamische Reckalte- rung von temporär durch Li Atome blockierte mobile Versetzungen, welches der weitge- hend akzeptierte zugrundeliegende Mechanismus in Al–Li basierten Legierungen ist. Dar- über hinaus zeigten theoretische Analysen von Verfestigungsmechanismen in den unter- suchten Wärmebehandlungszuständen, dass die Verformung nur im überalterten Zustand durch Ordnungshärtung bestimmt ist – auch plastische Instabilität tritt lediglich in diesem Wärmebehandlungszustand auf. Mit Hilfe der Fülle der experimentell erzielten Resultate wurde ein mechanistisches Modell entwickelt, welches die der plastischen Instabilität zugrundeliegenden mikroskopi- schen Mechanismen in ausscheidungsverfestigten Al–Li basierten Legierungen beschreibt. In diesem wird beschrieben wie, entgegen bisheriger Annahmen, plastischen Instabilität durch einen diffusionskontrollierten Pseudo–Sperrmechanismus bestimmt wird, welcher die Ordnungshärtung bei niedrigen Verformungsraten begleitet. Die Anwendbarkeit des Modells auf andere AL–Li basierte Legierungssysteme wurde untersucht. Durch kritische Untersuchung des Instabilitätsverhaltens einer Anzahl von in der Literatur beschriebenen binären und mehrkomponentigen Al–Li basierten Legierungen wurde demonstriert, dass plastische Instabilität nur dann in diesen Systemen auftritt wenn die Festigkeit durch Ord- nungshärtung bestimmt wird.The investigation of the underlying microscopic mechanisms that govern plastic in- stability in solution strengthened and precipitation strengthened Al alloys has been the subject of several studies. These studies are largely motivated by the need to devise strategies to mitigate the undesirable effects, such as reduction in ductility and formation of surface striations, which occur in alloys that exhibit this phenomenon. While the microscopic origin of plastic instability in solution strengthened alloys Al alloys is fairly well established, there is yet no convincing model that is consistent with experimental observations and gives a clear mechanistic description of the origin of the phenomenon in precipitation strengthened Al alloys. In this work, detailed experimental investigations of several tempers of a multi– component Al–Li based alloy, AA2198, have been carried out. Both mechanical and mi- crostructural characterization techniques were employed in order to correlate microstruc- tural characteristics to global and local mechanical behaviour. Specifically, high resolution nanoindentation and micro–tensile testing were used for mechanical testing, while trans- mission electron microscopy based methods – including in situ TEM tensile straining, along with high energy x–ray diffraction and atom probe tomography were used to inves- tigate the relevant microstructural characteristics. The experimental results clearly showed that dynamic strain aging of temporarily trapped mobile dislocations by Li atoms, widely accepted as the underlying mechanism for plastic instability in Al–Li based alloys, cannot sufficiently account for the occurrence of plastic instability in AA2198. Moreover, theoretical analyses of strengthening mecha- nisms in the investigated tempers showed that only the overaged temper, which is also the only temper that displayed plastic instability, is governed by order hardening. In light of the wealth of experimental results, a mechanistic model describing the microscopic mechanisms underlying plastic instability in precipitation strengthened Al–Li based alloy systems was developed. It is proposed that the phenomenon is governed by an altogether different mechanism than what has so far been considered, namely a diffusion controlled pseudo–locking mechanism that accompanies order hardening at low strain rates. The applicability of the model to other Al–Li alloy based systems was also examined. It was demonstrated, by critical examination of the instability behaviour of a number of binary and multi–component Al–Li based alloy systems reported in literature, that plastic instability only occurs in these alloy systems when strength is governed by order hardening

Evaluation of the transformation mechanisms and mechanical properties of ferrite: martensite microalloyed steels

The influence of starting point microstructures on the transformation mechanisms and mechanical properties of a micro alloyed steel after annealing in the alpha + gamma region have been investigated. Three different microstructures: austenite, pearlite in a ferrite matrix and martensite were used as starting point microstructures for the production of dual (alpha + ) phase structures in the test steel. Photomicrographs obtained from metallographic examination of the heat treated samples were used as criteria for the assessment of results obtained from impact toughness and hardness testing. The results obtained showed that the transformation mechanisms and hence the morphology of ferrite - martensite microalloyed steels are strongly influenced by their initial microstructural details. Ferrite - martensite structures produced via the intercritical quench (IQ) treatment, with martensite as the starting point microstructure, have the best combination of hardness and impact energy