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

Creep tests were performed to determine the microstructural features governing the macroscopic thermo-mechanical behaviour of combustion chamber alloys (NiCr22Co12Mo9 and CoCr22Ni22W14) and to understand the influences of technological treatments that are performed in manufacturing and refurbishment processes. Depending on temperature, different modes of dislocation/solute atom interaction lead to stress–strain interdependencies with work hardening, serrated flow or no work hardening, respectively. Plastic deformation behaviour at a given temperature is governed by the precipitation structure, which can be severely influenced by mechanical and thermal treatments.

High-temperature deformation of combustion chamber materials in different component conditions

The stress relaxation behaviour of the high temperature cobalt base superalloy Co Ni 23 Cr 22 W14 (Haynes alloy 188) was investigated in the temperature range 293 K

Stress Relaxation Behaviour of the Cobalt Base Superalloy Co Ni 23 Cr 22 W 14

An vier ferritischen Guseisen mit praktisch gleicher chemischer Zusammensetzung, aber unterschiedlicher Graphitausbildung und an der graphitfreien Basislegierung wurde das mikroplastische Verformungsverhalten unter Zugbeanspruchung untersucht. Beim Ubergang von der globularen zur vermicularen und lamellaren Graphitform werden die Feinstdehngrenzen wesentlich starker als die 0,2%-Dehngrenzen abgesenkt. Die Zusammenhange zwischen Querkontraktion und Langsdehnung werden dabei stark verandert. Die im Vergleich zur graphitfreien Basislegierung bei gegebener Nennspannung groseren plastischen Dehnungen der Guseisen beruhen auf plastischen Matrixverformungen aufgrund der inneren Kerbwirkung des Graphits und der integralen Spannungsanhebung wegen der geringen Spannungsaufnahme des Graphits sowie auf Volumenvergroserungen aufgrund von Risbildung im Graphit und Ablosungen an den Grenzflachen Graphit/Matrix. Die Volumenvergroserungen sind bei Guseisen mit groblamellarem Graphit besonders stark und kontrollieren das mikroplastische Verformungsgeschehen in entscheidender Weise.



Systematic Analysis of Microplastic Strain Components in Uniaxial Tensile Deformation of Ferritic Cast Irons



In order to analyse the microplastical strain components of cast iron, four ferritic cast irons with similar chemical compositions but different graphite shapes as well as the graphite-free base alloy have been tested in uniaxial tension. Graphite shape changings from nodular over vermicular to lamellar shape reduce the proof stresses at very small strains much more than the 0,2% proof stresses. Also, the relationships between lateral contractions and longitudinal strains are strongly changed. As a consequence of three graphite-induced strain components at any given stress, the plastic strains of the cast irons are greater than the plastic strain of the graphite-free base alloy: i( plastic matrix-deformation, caused by the notch effect of graphite particles; ii) plastic matrix-deformation, caused by integral stress rise in the matrix due to the presence of graphite, and (iii) volume changes, caused by cracking in the graphite particles as well as decohesion at the graphite/matrix interface. In the case of coarse flake graphite cast irons, very pronounced volume changes occur, which strongly influence the stress, strain behaviour.

Systematische Analyse der mikroplastischen Verformungsanteile bei einachsiger Zugbeanspruchung von ferritischen Gußeisen

The deformation behaviour of two thin sheet nickel-base superalloys was studied in tensile tests, short-time creep tests and relaxation tests up to temperatures of 1473 K. It will be shown that the deformation behaviour can be separated into four strain rate dependent temperature ranges in which different deformation processes occur.

Otmar Vöhringer

Papers

High-temperature deformation of combustion chamber materials in different component conditions

Stress Relaxation Behaviour of the Cobalt Base Superalloy Co Ni 23 Cr 22 W 14

Systematische Analyse der mikroplastischen Verformungsanteile bei einachsiger Zugbeanspruchung von ferritischen Gußeisen

Plastic Deformation Behaviour of Thin Sheet Nickel-base Materials at Temperatures up to 1473K

Thermisch aktivierte verformung von α-Kupfer-Zinn-Legierungen