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

Surface hardening treatments like inductive hardening or laser beam hardening of steels lead to extremely rapid heating and cooling processes. Up to now, their kinetic is almost not available to experimental studies, but necessary for a deeper understanding of the processes and for process simulation. In this paper experimental work concerning the transformation behaviour during short time heating and during self quenching after short time austenitization will be presented at the quenching and tempering steel AISI 4140 (German grade 42CrMo4) and described mathematically using adequately improved models.

Experimentelle Analyse und Modellierung des Kurzzeitum-wandlunasverhaltens von Stählen am Beispiel von 42CrMo4

During creep loading metallic substrates impose deformation on deposited ceramic thermal barrier coatings (TBC). Strain accommodation of the TBC is not attained by plastic deformation, but by means of crack initiation, crack opening, crack propagation or sliding of adjacent crack faces. In technical applications a distinction is made between tolerated or desired cracks perpendicular to the surface, and detrimental cracks parallel to the substrate-coating interface. Thus, TBC can respond to creep deformation by segmentation or spallation, the latter being referred to as failure. The parameters influencing the probability of either segmentation or spallation are temperature, creep rate, magnitude of creep deformation, layer thickness, and microstructure of the TBC. It can be stated that spallation failure probability increases with increasing creep rate, creep deformation, and layer thickness. The presence of pores between single spraying layers also strongly augments the likelihood of spallation. No significant influence of temperature on spallation failure probability can be found in the range from 850 C to 1,050 C. Light microscopy and scanning electron microscopy investigation show that the microstructure of the ceramic TBC changes during creep, and that the density of cracks detected on micrographs with low magnification (x50) increases with increasing creep deformation. On themore » other hand, the density of microcracks visible with high magnification (x500) is constant, or even decreases with increasing creep deformation. These findings are explained by sintering processes enabled by stress relaxation due to formation of macroscopic cracks perpendicular to the surface as a response to creep deformation. A relationship between microstructural changes and the emission of signals recorded during creep is presented.« less

The creep damage behavior of the plasma-sprayed thermal barrier coating system NiCr22Co12Mo9-NiCoCrAlY-ZrO2/7%Y2O3

The microstructure of the surface layer of normalized, martensitic hardened and quenched and tempered 42 CrMo 4 steel specimens was investigated after equal shot-peening treatments. To characterize the micro-residual-stress states, the mean micro-strains, the domain size and the dislocation densities were determined with an X-ray one-line profile analysis. For microhardness values 460 HV 0.3 reveal shot-peening-induced work-softening mainly caused by rearrangements of dislocations

Mikroeigenspannungszustände kugelgestrahlter Randschichten von 42 CrMO 4

Using a modified air blasting machine warm peening at 20 O C < T I 410 "C was feasible. An optimized peening temperature of about 310 "C was identified for a 450 "C quenched and ternpered steel AISI 4140. Warm peening was also investigated for a normalized, a 650 "C quenched and tempered, and a martensitically hardened material state. The quasi static surface compressive yield strengths as well as the cyclic surface yield strengths were determined from residual stress relaxation tests conducted at different stress amplitudes and numbers of loading cycles. Dynamic and static strain aging effects acting during and after warm peening clearly increased the residual stress stability and the alternating bending strength for all material states.

Otmar Vöhringer

Papers

Experimentelle Analyse und Modellierung des Kurzzeitum-wandlunasverhaltens von Stählen am Beispiel von 42CrMo4

The creep damage behavior of the plasma-sprayed thermal barrier coating system NiCr22Co12Mo9-NiCoCrAlY-ZrO2/7%Y2O3

Mikroeigenspannungszustände kugelgestrahlter Randschichten von 42 CrMO 4

Influence of Optimized Warm Peening on Residual Stress Stability and Fatigue Strength of AISI 4140 in Different Material States

Shot Peening Plus Subsequent Short-Time Annealing - A Way to Increase the Residual Stress Stability and Alternating Bending Strength of AISI 4140