Otmar Vöhringer

Bio: Otmar Vöhringer is an academic researcher from Karlsruhe Institute of Technology. The author has contributed to research in topics: Residual stress & Shot peening. The author has an hindex of 22, co-authored 96 publications receiving 2655 citations.

Constitutive description of dynamic deformation: physically-based mechanisms

Abstract: The response of metals to high-strain-rate deformation is successfully described by physically-based mechanisms which incorporate dislocation dynamics, twinning, displacive (martensitic) phase transformations, grain-size, stacking fault, and solution hardening effects. Several constitutive equations for slip have emerged, the most notable being the Zerilli–Armstrong and MTS. They are based on Becker’s and Seeger’s concepts of dislocations overcoming obstacles through thermal activation. This approach is illustrated for tantalum and it is shown that this highly ductile metal can exhibit shear localization under low temperature and high-strain-rate deformation, as predicted from the Zerilli–Armstrong equation. A constitutive equation is also developed for deformation twinning. The temperature and strain-rate sensitivity for twinning are lower than for slip; on the other hand, its Hall–Petch slope is higher. Thus, the strain rate affects the dominating deformation mechanisms in a significant manner, which can be quantitatively described. Through this constitutive equation it is possible to define a twinning domain in the Weertman– Ashby plot; this is illustrated for titanium. A constitutive description developed earlier and incorporating the grain-size dependence of yield stress is summarized and its extension to the nanocrystalline range is implemented. Computational simulations enable the prediction of work hardening as a function of grain size; the response of polycrystals is successfully modeled for the 50 nm–100 m range. The results of shock compression experiments at pulse durations of 3–10 ns (this is two–three orders less than gas-gun experiments) are presented. They prove that the defect structure is generated at the shock front; the substructures observed are similar to the ones at much larger durations. A mechanism for dislocation generation is presented, providing a constitutive description of plastic deformation. The dislocation densities are calculated which are in agreement with observations. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle–Grady relationship. © 2002 Elsevier Science B.V. All rights reserved.

Residual stress relaxation in an AISI 4140 steel due to quasistatic and cyclic loading at higher temperatures

Abstract: Residual stresses can be relaxed by supplying sufficiently high amounts of thermal and/or mechanical energy, which converts the residual elastic strains to microplastic strains. In order to better understand this relaxation behavior, shot peening induced residual stresses in normalized condition and in quenched and tempered condition of the steel AISI 4140 (German grade 42 CrMo 4) were investigated in annealing experiments, quasistatic loading experiments and bending fatigue experiments at 25, 250 and 400°C. The residual stress relaxation during alternating bending occurs in different regimes. First, thermal relaxation reduces the residual stresses during specimen heating. The relaxation during the first cycle can be discussed on the basis of the effects due to quasistatic loading, if the inhomogeneous distribution of the loading stress is taken into account. Differences in the behavior after the two heat treatments result from the Bauschinger-effect and effects of dynamic strain ageing. Owing to cyclic creep effects, the interval between the first cycle (N=1) and the number of cycles to crack initiation Ni is characterized by residual stresses which decrease linearly with the logarithm of N. Finally for N>Ni the reduction of residual stresses with the logarithm of N is stronger than linear.

Effects of warm peening on fatigue life and relaxation behaviour of residual stresses in AISI 4140 steel

Abstract: A new device has been built which allows shot peening in an air blast machine at elevated temperatures. The effects of conventional shot peening and peening at elevated temperatures on the characteristics of regions close to the surface, on the stability of residual stresses and half widths of X-ray interference lines and on the fatigue strength are presented for a quenched and tempered AISI 4140 steel (German grade 42CrMo4). The alternating bending strength is increased by warm peening compared with conventional shot peening. Additional investigations of samples conventionally peened and then annealed confirm that these effects are due to the stability of the dislocation structure, which is highly affected by strain ageing effects. This causes an additional benefit owing to higher stability of the residual stresses induced.

High-entropy alloys

Abstract: 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.