Peter Binkele

Experimental and numerical characterisation of in-plane deformation in two-phase materials

The aim of the present work consists in the comparison of in-plane strain fields with out-of-plane displacements in micro-areas of an Ag/Ni-composite after a macroscopic compressive deformation of 8.6%. The in-plane deformations in an Ag/Ni-composite have been analysed experimentally with a high resolution object grating technique and numerically using the finite element method. The out-of-plane displacements were measured with an atomic force microscope (AFM). The development of local strain fields in micro-areas at the surface of an Ag/Ni-composite was simulated numerically using the FE-method in plane strain condition. A real cut-out of the microstructure served as input for the calculation. The out-of-plane displacements determined by AFM measurements were used further to correct the in-plane values of strains evaluated by the object grating technique. The roughness on the surface of the sample was characterised by fractal dimensions and compared with the in-plane strains in the same micro-region.

An atomistic Monte Carlo simulation of precipitation in a binary system

Abstract An atomistic Monte Carlo simulation of coherent precipitation on a body-centred cubic crystal lattice is presented. A binary system with atom types A and B is considered, while the ‘diffusion’ of the atoms is realized via a vacancy mechanism. Starting with a random distribution of 91 % A atoms and 9 % B atoms, the formation and growth of precipitates is simulated at a constant temperature of 773 K. As a result of the simulation, the precipitate radii distributions at different states of the system and the time evolution of the precipitate mean radii have been calculated. The results of the simulation are compared with the predictions of the classical Lifshitz–Slyozov–Wagner (LSW) theory. At the beginning of precipitation the mean radius R¯(t) grows proportional to t0.180, while at later states the growth exponent approaches the classical value of 1/3. In order to reveal the growth exponents exact value, an even larger simulation is required, which remains as an interesting task for the future.

Copper precipitates in 15 NiCuMoNb 5 (WB 36) steel : material properties and microstructure, atomistic simulation, and micromagnetic NDE techniques

The material investigations presented confirm the results of earlier MPA investigations that the service-induced hardening and decrease in toughness in WB 36 materials are caused by the precipitation of copper. In the initial state of the material, generally only a part of the alloyed copper is precipitated. The other part is still in solution and can be precipitated during long-term operation at temperatures above 320‐350°C. The copper precipitation leads to a distortion of the crystal lattice surrounding the copper precipitates and yields internal micro-stresses. If the number and size of the copper precipitates change during operation of a component, a change of the residual-stress level occurs. Formation and growth of copper precipitates was simulated using atomistic calculations. In addition, it was possible to mathematically follow the movement of dislocations and their attachment to precipitates. In this way the nano-simulation was established as a scientific method for the numerically based understanding of precipitation hardening. The results obtained from load stress-related Barkhausen noise measurements demonstrated that these micro-magnetic procedures are generally suitable for the verification of copper precipitation. The goal of current research is to establish these findings statistically through further experimental measurements. In addition, the influence of different deformation states, macro residual stress, and thermal-induced residual stress have to be researched. This is important for future developments of non-destructive inspection techniques applied to inservice components.

Atomistic multiscale simulations on the anisotropic tensile behaviour of copper-alloyed alpha-iron at different states of thermal ageing

The mechanical behaviour of steels is strongly related to their underlying atomistic structures which evolve during thermal treatment. Cu-alloyed α-Fe undergoes a change in material behaviour during the ageing process, especially at temperatures of above 300°C, where precipitates form on a large time-scale within the α-Fe matrix, yielding first a precipitation strengthening of the material. As the precipitates grow further in time, the material strength decreases again. This complex process is modelled with a multiscale approach, combining Kinetic Monte Carlo (KMC) with Molecular Dynamics (MD) simulations in a sequential way and exploiting the advantages of both methods while simultaneously circumventing their particular disadvantages. The formation of precipitates is modelled on a single-crystal lattice with a diffusion based KMC approach. Transferring selected precipitation states at different ageing times to MD simulations allows the performance of nano tensile tests and the analysis of failure initiation. The anisotropic tensile behaviour is investigated in the [100], [110] and [111] directions, showing monotonically decreasing tensile strengths and deformation strains. Hence precipitation strengthening is mainly due to dislocation–precipitate interactions which are non-existent at small tensile loadings in this scenario. At the point of ductile failure, dislocations are generated at the interfaces between precipitates and the Fe matrix. Straining in the [100] direction, they lie on {110} and {112} glide planes, as expected. With the method presented here, the changes of the anisotropic tensile moduli are related to different states of thermal ageing, i.e., to nucleation, growth and Ostwald ripening of Cu precipitates.

Molecular dynamics investigations of the strengthening of Al-Cu alloys during thermal ageing

Classical molecular dynamics simulations of the interaction of edge dislocations with solid soluted copper atoms and Guinier-Preston zones (I and II) in aluminium are performed using embedded atom method potentials. Hereby, the strengthening mechanism and its modulus are identified for different stages of thermally aged Al-Cu alloys. Critical resolved shear stresses are calculated for different concentrations of solid soluted copper. In case of precipitate strengthening, the Guinier-Preston zone size, its orientation and offset from the dislocation plane are taken as simulation parameters. It is found that in case of solid soluted copper, the critical resolved shear stress is proportional to the copper concentration. In case of the two subsequent aging stages both the dislocation depinning mechanism as well as the depinning stress are highly dependent on the Guinier-Preston zone orientation and to a lesser degree to its size.

MD-Simulations on Metallic Alloys

Strengthening effects in solid materials depend on different mechanisms, which have been analyzed empirically since very long time. The here presented work shows the investigation on atomistic length scale of precipitate hardening, with detailed look on dislocation obstacle interactions, and grain-boundary-strengthening using Molecular Dynamics to perform numerical simulations on polycrystalline aluminum systems including copper as alloying element. Work hardening effects has been simulated for a notched iron specimen, where periodic loading and compression has been applied. Temperature and density influences are shown for nanoporose iron models.