Mechanical properties of nanocrystalline materials

Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

High-entropy alloys

Overview of constitutive laws, kinematics, homogenization and multiscale methods in crystal plasticity finite-element modeling: Theory, experiments, applications

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

Influence of grain size on the compressive deformation of wrought Mg-3Al-1Zn

As a part of the DFG collaborative research center 483 'High performance sliding and friction systems based on advanced ceramics' the high potential of engineering ceramics shall be demonstrated on four exemplary choosen systems. The results that are given in the present study were gained whithin the research projekt C2 which works on the modelling of the damage behaviour occurring in clutches. For the thorough scientific examination of the degradation processes of ceramic materials two methods are applied. Experiments under high frictional loadings that are applied by a dry friction testing system are combined with the mathematical analysis using the Finite-Element-Method (FEM). For a deeper understanding of the origin and development of the damages resulting from the friction experiments 4-point bending tests and microfractography were carried out. The present paper deals with the results of these fractographic examinations as well as the determination of residual strength which depends on the stress history of the ceramic material.

Verhalten von Al2O3-Keramik unter Friktionsbeanspruchung - Untersuchungen des Schädigungsverhaltens keramischer Werkstoffe an einem Trockenreibungsprüfstand

Der Einfluß der Graphitform auf den Elastizitätsmodul von ferritischen Gußeisenwerkstoffen

During surface hardening of steels using pulsed laser sources, rapid thermal changes are imposed on the material. These changes yield locally different austenitizing and hardening conditions in the laser-affected zone driven by the thermal conductivity and transformation behaviour of the material. The hardened surfaces are characterized systematically by a depth-resolved analysis of X-ray interference lines. A diffractometer with CoKa-radiation is used to determine the half width, the state of residual micro stress and the dislocation density in the laser-hardened zone. The X-ray characterization is accompanied by investigations in micro hardness, light and electron microscopy. Characteristic profiles of different microstructural parameters are obtained along the laser hardened zone. Specimens made of two different steels with similar contents of carbon and different contents of carbide forming elements are laser treated by a temperature controlled pulsed solid state laser. Especially designed specimens are used to reproduce the effects of the pulsation without the influence of the feed. The influence of frequency and duration of the laser hardening process are investigated separately. Additionally, investigations on specimens with different preceding heat treatments are carried out.

Microstructural Characterization of Laser Hardened Steel Surfaces

The Ni-base solid solution- and carbide hardening alloy NiCr22Co12Mo9 is used preferentially for cladding the combustion chambers of stationary gas- and aircraft turbines. In order to increase combustion temperatures, the combustion chamber is protected against excessive temperatures by the application of a multi-layered coating consisting of a thermally insulating ceramic layer and a corrosion resistant layer. By considering the mechanical behaviour separately from the corrosion behaviour, it becomes clear that this type of coating detrimentally affects the creep properties of the substrate metal. The purpose of the investigation was to determine the microstructural cause of this behaviour. The deterioration in the creep properties of the alloy NiCr22Co12Mo9 after coating was found to be caused by the degradation of the M 23 C 6 -carbide precipitate in the substrate material and by a weakening in the mechanisms of solid solution hardening and precipitation hardening taking place at the interface, on which the creep strength of the material relies.

Mikrostruktur und Ausscheidungsverhalten von Superlegierungen mit Wärmedämmschichtsystemen

Reine Sintereisenwerkstoffe mit Dichten zwischen 6,88 und 7,57 g/cm3 wurden im Temperaturbereich von –184 bis 600°C quasistatisch zugverformt Erganzende Druckverformungen erfolgten bei 20°C Anwachsende Dichte bewirkt ansteigende Werkstoffwiderstands- und Duktilitatskenngrosen aufgrund zunehmender tragender Probenquerschnitte, geringerer Porenzahl, runderer Poren mit geringerer Kerbwirkung sowie einer geringeren Zahl der von Poren ausgehenden Rissen Bei Druckverformung entstehen nach gleichen Deformationen groosere Fliesspannungen als bei Zugverformung, da Porenschlieseffekte und die Behinderung von Risbildungen die tragenden Probenquerschnitte vergrosern Das Verformungsverhalten wird bei tiefen Temperaturen durch thermisch aktivierte Prozesse mit kurzreichenden Wechselwirkungen von Versetzungen mit Gleithindernissen, bei mittleren Temperaturen durch dynamische Reckalterungsvorgange aufgrund von elastischen Wechselwirkungen bewegter Versetzungen mit diffundierenden Kohlenstoffatomen und bei hohen Temperaturen durch erholungskontrollierte Versetzungskriechprozesse bestimmt



Quasistatic deformation behaviour of pure sintered iron in the temperature range between –184 and 600°C



The deformation behaviour of pure sintered iron materials with densities between 6,88 und 7,57 g/cm3 was investigated in tension tests in the temperature range of –184 and 600°C Supplementary compression tests were carried out at 20°C Increasing density leads to increasing material resistances and ductility properties due to the increase of the bearing specimen cross sections as well as due to smaller numbers of pores, more spherical pores with smaller notch effects and smaller numbers of mircocracks, which are initiated at pores After equal deformations, due to pore closing effects and the impediment of crack initation, the flow stresses of compressively deformed specimens are larger than those of tensily deformed The deformation behaviour is dominated at low temperatures by thermal activated glide processes of dislocations and their interactions with short range obstacles, at middle temperatures by dynamic strain ageing due to elastic interactions of glide dislocations and diffusing carbon atoms and at high temperatures by recovery controlled dislocation creep processes

Otmar Vöhringer

Papers

Verhalten von Al2O3-Keramik unter Friktionsbeanspruchung - Untersuchungen des Schädigungsverhaltens keramischer Werkstoffe an einem Trockenreibungsprüfstand

Der Einfluß der Graphitform auf den Elastizitätsmodul von ferritischen Gußeisenwerkstoffen

Microstructural Characterization of Laser Hardened Steel Surfaces

Mikrostruktur und Ausscheidungsverhalten von Superlegierungen mit Wärmedämmschichtsystemen

Quasistatisches Verformungsverhalten reiner Sintereisen im Temperaturbereich von -184 bis 600°c