Joachim Rösler

A finite element model of high speed metal cutting with adiabatic shearing

A finite element model of a two-dimensional orthogonal cutting process is developed. The simulation uses standard finite element software together with a special mesh generator that is able to mesh the chip completely with regular quadrilateral elements and a strong mesh refinement in the shear zone for continuous and segmented chips. The techniques of remeshing and to ensure convergence of the implicit calculation is described. Results for the formation of segmented chips are presented and the segmentation process is studied. Of special interest is the occurrence of split shear bands. The influence of the elastic properties and of the cutting speed is also discussed.

Stress state and failure mechanisms of thermal barrier coatings: role of creep in thermally grown oxide

Abstract The mechanical loading of the thermal barrier coating (TBC)/thermally grown oxide (TGO)/bond coat interface region is calculated for a TBC coated superalloy specimen using a finite element model. It is shown that the evolving stress state depends crucially on the ratio of the loading rate caused by growth and swelling of the oxide layer and the unloading rate by creep relaxation. For grain sizes below 1 μm, creep of the oxide layer cannot be neglected. From this calculations, possible damage scenarios are inferred and implications for future design of TBC layers are considered. Tailoring the TGO creep properties, for example by reducing its grain size or designing a multilayer TGO scale, is identified as the most relevant aspect.

A theoretical concept for the design of high-temperature materials by dual-scale particle strengthening

The creep behavior of dual scale particle strengthened (DSPS) metals containing particles of two different size scales, namely nanometer size dispersoids and reinforcements with typical dimensions in the micrometer to millimeter range, is analyzed theoretically. Based on the concept of thermally activated dislocation detachment from dispersoid particles as rate-controlling mechanism in dispersion hardened matrices, a new creep equation for this advanced material class is developed. Analysis of the model leads to the prediction that creep strength levels far superior to todays best dispersion or reinforcement strengthened high temperature materials can be achieved by using dispersion and reinforcement hardening in combination and following certain design guidelines, related to the selected particle parameters. In particular, it is shown that a volume fraction mix of about 3/4 reinforcements with about 1/4 dispersoids is ideal in many cases provided reinforcements with sufficient aspect ratio and size are selected.

Nano-structured materials produced from simple metallic alloys by phase separation.

A method which is able to produce different types of nano-structured materials, namely nano-particles, nano-structured surfaces and nano-porous membranes, from two-phase metallic alloys is reviewed. The new process first establishes nano-structures in the bulk alloy and then separates them by selective phase dissolution. Variation in processing makes it possible to produce different types of nano-structure even from the same alloy. The process can be applied to many different alloy systems. An overview is presented emphasizing the versatility of the process with examples of different nano-structure types that can be produced. Further, the new method is discussed in relation to similar processes (specifically, electrochemical processes) which have been used for nano-structure synthesis.

The influence of thermal conductivity on segmented chip formation

A two-dimensional finite element model of the machining process is presented. After a short discussion of the modelling technique and the remeshing algorithm used, the influence of thermal conductivity on the chip segmentation process is studied. Increasing thermal conductivity leads to a decreasing degree of segmentation and to an increase in the cutting force. The influence of the thermal conductivity on the width of the shear bands and on maximum temperatures is also discussed.

Lattice misfit measurement in Inconel 706 containing coherent γ′ and γ″ precipitates

Abstract Lattice misfit is an important parameter in Ni–Fe based alloy IN706 as coherency strain influences the strength of the alloy and the stability of the microstructure. X-ray and synchrotron measurements were performed on bulk samples with precipitates in constrained lattice. Lattice parameters of different phases and the lattice misfit could be determined even though the peaks were completely overlapped.

Cytocompatibility of a free machining titanium alloy containing lanthanum

Titanium alloys like Ti6Al4V are widely used in medical engineering. However, the mechanical and chemical properties of titanium alloys lead to poor machinability, resulting in high production costs of medical products. To improve the machinability of Ti6Al4V, 0.9% of the rare earth element lanthanum (La) was added. The microstructure, the mechanical, and the corrosion properties were determined. Lanthanum containing alloys exhibited discrete particles of cubic lanthanum. The mechanical properties and corrosion resistance were slightly decreased but are still sufficient for many applications in the field of medical engineering. In vitro experiments with mouse macrophages (RAW 264.7) and human bone-derived cells (MG-63, HBDC) were performed and revealed that macrophages showed a dose response below and above a LaCl3 concentration of 200 microM, while MG-63 and HBDC tolerated three times higher concentrations without reduction of viability. The viability of cells cultured on disks of the materials showed no differences between the reference and the lanthanum containing alloy. We therefore propose that lanthanum containing alloy appears to be a good alternative for biomedical applications, where machining of parts is necessary.

Deformation behaviour of freestanding single-crystalline Ni3Al-based nanoparticles

Abstract Deformation on a single freestanding metallic object of submicron size is usually not performed. That is primarily because it is not easy to handle isolated single objects in this size scale. Here we demonstrate a method of performing compression testing on a freestanding cubic shaped single crystalline Ni3Al-type nanoparticle (∼300 nm). The particles were deformed with the help of a nano-manipulation system inside a scanning electron microscope. The nanoscale test specimens were obtained from a nickel-based superalloy by electrochemically extracting the Ni3Al-γ′ precipitates. Stress-strain curves are generated when a cubic particle is deformed under compression between a tungsten “micro-hammer” and a silicon “micro-anvil”. Deformations conducted on particles in two different states, undeformed and predeformed, show distinctly different deformation behaviour. Sudden strain “bursts” are observed during tests on undeformed specimens indicating that deformation is possibly governed by dislocation nu...

High temperature stability of Cr-carbides in an experimental Co–Re-based alloy

Abstract The stability of the microstructure at high temperatures was studied in an experimental Co–Re-based alloy. The experimental alloy is mainly strengthened by Cr-carbides, particularly by those in the form of thin lamellar plates. Electron microscopic investigation on samples exposed for up to 1 000 h to temperatures of 1 000 and 1 200 °C showed that Cr23C6 type carbides present in the alloy in different morphologies are unstable at these temperatures. It was also observed that the alloy hardness dropped after exposing the samples to elevated temperatures and much of this loss occurred within the first 100 h. In-situ diffraction measurements with synchrotron radiation showed that carbide dissolution started as early as 3 h of holding at 1 000 °C. Moreover, in-situ small angle neutron scattering results indicated that the carbides at the grain boundaries and the blocky carbides dissolve first and then the thin lamellar carbides. Further, the enrichment of Cr in the Co-matrix phase, which took place due to the dissolution of Cr-carbides, stabilized a Cr–Re-rich σ phase. Although the dissolution of lamellar carbides results in a significant loss of strength, the formation of σ phase with extremely high hardness partly compensated the for loss. The σ phase is stable even at 1 200 °C.

Oxidation behaviour of experimental Co–Re-base alloys in laboratory air at 1000°C

Abstract The oxidation behaviour of experimental Co – Re-based alloy at 1000 °C was studied. A set of binary, ternary and quaternary alloys from the Co – Re – Cr – C system was used as model alloys to understand the role each alloying element plays on oxidation. The morphology and composition of the oxide scale that formed was analysed by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. It was found that the present Co – Re alloys with 23 at.% and 30 at.% Cr additions behaved very similarly to Co – Cr binary alloys with equivalent Cr content. The oxide scale was multilayered, consisting of a dense CoO outer layer, a porous mixed oxide layer containing Co-oxide and Co – Cr spinel, and a discontinuous and non-protective Cr3O2 layer. The binary Co – Re alloy behaved differently in oxidation, and it formed only a monolithic CoO scale. However, Re in combination with Cr promotes Cr – Re-rich phase formation, which oxidises preferentially compared to the Co matrix. Carbon ties up part of the Cr to form Cr23C6 type carbides. However, these carbides are not stable at 1000 °C and dissolved with time, therefore C had only a minor role in the oxidation behaviour. In general, increasing Cr content in the alloy improved oxidation resistance.