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

Simulation of laser hardening of steels was carried out, involving the finite element system ABAQUS and user defined material laws, which led to a coupled calculation of temperature, phase and stress development during heating and cooling Additionally, the simulation allowed the phase distributions within the affected zone to be determined Good correspondence between the results of the simulated and experimental data was obtained in respect of the influence of irradiation time and base material on the size of the hardened zone

Simulation of Phase Transformations and Temperature Profiles by Temperature Controlled Laser Hardening: Influence of Properties of Base Material

The influence of graphite shape on mechanical properties of ferritic cast iron

The influence of dynamic strain aging (DSA) on mechanical properties of cast iron was evaluated by means of tensile and compression tests performed in the range 400 K ≲T≲ 700 K. In the range 400 K ≲ T ≲ 500 K, type I-DSA is caused by elastic interaction of mobile dislocations with diffusing carbon and/or nitrogen atoms. The deformation-induced growth of mobile dislocation density increases if graphite is changing from nodular graphite to vermicular and flake graphite. In the case of type II-DSA in the range 500 ≲ T ≲ 700 K, activation energy increases from 1.1 to 1.9 eV with increasing strain. Type II-DSA considerably reduces the fracture strain of nodular cast iron and in some cases a sharp drop in the ultimate tensile strength/temperature curve is observed.

Dynamic Strain Aging of Cast Iron

In the presented research work Al2O3 ceramic plates were subjected to frictional and thermal loadings with the main purpose to investigate typical failure mechanisms of ceramic friction components under such loading conditions. This was done by calculating the local temperatures and stresses using the finite element method and subsequently verifying the model developed using experimental results. By conducting residual stress analyses, 4-point bending tests, ceramographic analyses and fractographic observations on the ceramic components, a detailed interpretation of the crack initiation and propagation was possible.



Eigenspannungen und Schadigungen in Al2O3 – Keramik nach trockener Friktionsbeanspruchung



Keramikscheiben aus Al2O3 wurden an einem Laserprufstand thermisch und an einem Friktionsprufstand thermisch-mechanisch belastet, um gezielt Schadigungen in das Material einzubringen. Anschliesend wurde der Schadigungszustand durch Analysen des Eigenspannungszustandes, 4-Punkt Biegeversuche sowie keramographische und fraktographische Untersuchungen charakterisiert. Parallel wurde eine Finite-Elemente-Simulation durchgefuhrt, um die auftretenden Temperaturen und Spannungen zu berechnen und eine Vorhersage des Schadigungsablaufes zu ermoglichen.

Otmar Vöhringer

Papers

Simulation of Phase Transformations and Temperature Profiles by Temperature Controlled Laser Hardening: Influence of Properties of Base Material

The influence of graphite shape on mechanical properties of ferritic cast iron

Dynamic Strain Aging of Cast Iron

Interaction between residual stresses and surface defects in alumina ceramics under tribological loading

Einsatzspannung für mechanische Zwillingsbildung bei α-Kupferlegierungen