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

The experimental analysis of near-surface residual stresses by X-ray diffraction methods is based on measuring the spacings of lattice planes while the inclination ψ with respect to the surface plane is changed stepwise A characteristic feature of conventional techniques is that the penetration depth of the X-rays is altered as inclination is varied By simultaneously varying three different goniometer angles in a particular fashion, both the penetration depth and the measuring direction can be held constant while ψ is varied Thus the normal and shear stresses can be derived from the sin2ψ plots by means of standard evaluation procedures developed for gradient-free stress states The depth profile of residual stress is then obtained via Laplace transformation of the results from several stress measurements carried out at different penetration depths In the present paper, the feasibility of this experimental approach for characterizing the strongly graded, non-equiaxed stress state existing at a machined surface is demonstrated The results from constant-penetration-depth measurements on the ground surface of an engineering ceramic are compared with those from conventional sin2ψ measurements

X-ray diffraction at constant penetration depth – a viable approach for characterizing steep residual stress gradients

Increasing the fatigue limit of a bearing steel by dynamic strain ageing

Der Portevin-Le Chatelier-effekt bei α-kupfer-zinn-legierungen

During surface hardening of steels like laser hardening, rapid thermal changes are imposed to the material. The modelling of these hardening processes allows the determination of time-dependent temperature fields and phase transformations within the affected zones. While there are many investigations on the transformation behaviour during cooling, there is a lack of data concerning the transformation during heating at very high heating rates. Therefore, experiments simulating the fast temperature changes are necessary to implement the effects of short time phase transformation during hardening into simulation. In this paper the effect of heating rates up to 10000 K/s on the austenitizing behaviour of the steel AISI 4140 (German grade 42CrMo4) are presented and described using an Avrami-function. The results obtained are summarized in time-temperature-transformation diagrams for continuous heating and isothermal time-temperature-transformation diagrams which may be used as input data for the simulation.

Description of short time phase transformations during the heating of steels based on high-rate experimental data

In a three-dimensional Finite-Element-Simulation of shot peening, a combined isotropickinematic viscoplastic material description was introduced in order to describe the cyclic softening effects during peening. After verifying the model in the simulation of push-pull tests at different strain amplitudes it could be used for the shot peening simulation. The simulated residual stress profile is compared with experimental results determined by X-ray diffraction and with simulated results of a simpler isotropic viscoplastic material model.

Otmar Vöhringer

Papers

X-ray diffraction at constant penetration depth – a viable approach for characterizing steep residual stress gradients

Increasing the fatigue limit of a bearing steel by dynamic strain ageing

Der Portevin-Le Chatelier-effekt bei α-kupfer-zinn-legierungen

Description of short time phase transformations during the heating of steels based on high-rate experimental data

Finite Element Simulation of the Residual Stress States after Shot Peening