Heinrich Kestler

Phase stability of a γ-TiAl based alloy upon annealing: comparison between experiment and thermodynamic calculations

Abstract Experimental results focusing on changes in type and composition of constituting phases of a γ-TiAl sheet material exhibiting a fine-grained microstructure which has been has been exposed to 700 and 800 °C for up to 10,000 h in air furnaces have been successfully verified by thermodynamic equilibrium calculations.

Molybdenum alloys for high temperature applications in air

Abstract Molybdenum base silicide alloys exhibit promising oxidation resistance in addition to the inherent high temperature strength of refractory metals. However, alloys with sufficient oxidation resistance are effectively brittle up to temperatures above 816°C (1500°F). Recent progress in alloy and process development, utilising a PM manufacturing route with mechanical alloying as a crucial step, has allowed significant improvement of both oxidation resistance and mechanical properties via micro-alloying additions including nano-dispersed second phase oxide particles.

A unified description of creep in pure and dispersion-strengthened copper

Abstract A constitutive model for creep deformation in pure and dispersion-strengthened copper is presented. This description reproduces the characteristic features of steady-state and constant-str...

OpenFOAM Modeling of Particle Heating and Acceleration in Cold Spraying

In cold spraying, a powder material is accelerated and heated in the gas flow of a supersonic nozzle to velocities and temperatures that are sufficient to obtain cohesion of the particles to a substrate. The deposition efficiency of the particles is significantly determined by their velocity and temperature. Particle velocity correlates with the amount of kinetic energy that is converted to plastic deformation and thermal heating. The initial particle temperature significantly influences the mechanical properties of the particle. Velocity and temperature of the particles have nonlinear dependence on the pressure and temperature of the gas at the nozzle entrance. In this contribution, a simulation model based on the reactingParcelFoam solver of OpenFOAM is presented and applied for an analysis of particle velocity and temperature in the cold spray nozzle. The model combines a compressible description of the gas flow in the nozzle with a Lagrangian particle tracking. The predictions of the simulation model are verified based on an analytical description of the gas flow, the particle acceleration and heating in the nozzle. Based on experimental data, the drag model according to Plessis and Masliyah is identified to be best suited for OpenFOAM modeling particle heating and acceleration in cold spraying.

Electron Beam Welding of TZM Sheets

In the scope of this work, 2 mm thick TZM sheet metal is butt welded by electron beam welding (EBW) without filler material and a systematic investigation of the most relevant welding parameters to improve the weld quality is conducted. With the aid of design of experiment (DoE), it is shown that with careful selection of the welding parameters it is possible to considerably reduce the size of the fusion zone and the heat affected zone and the grain size of both. Furthermore, the influence of the parameters on the quality of the weld and the characterizing values ultimate tensile strength and hardness of fusion zone is presented. It is concluded, which parameters influence the quality of the weld and suppress pores and cracks.

Influence of processing on the microstructure and mechanical behaviour of Mo-Si-B alloys

Mo-Si-B materials consisting of a Mo(Si) solid solution and the intermetallic phases Mo3Si and Mo5SiB2 (T2) were prepared by mechanical alloying (MA) as the crucial step of a powdermetallurgical process. After consolidation via an industrial processing route (cold isostatic pressing, sintering, hot isostatic pressing) the resulting microstructures of Mo-Si-B alloys up to 45% of intermetallic phases reveal a continuous α-Mo matrix with embedded, homogeneously distributed intermetallic particles. Clearly, increasing the amount of Mo solid solution reduces the BDTT (demonstrated by three point bending tests between room temperature and 1200°C), however, values below 900°C could not be obtained due to grain boundary embrittlement caused by Si segregation. Alloying with Zr was proven by Auger analysis in Mo-Si solid solutions to reduce this segregation. Therefore, in a second trial Zr as a (micro-) alloying element was added. The influence of microalloying on ductility and strength is comparatively discussed with reference compositions Mo-6Si-5B and Mo-9Si-8B.

Sparking new design ideas for electricity generation

The traditional economic advantages of near net shape powder metallurgy technology can be combined with mathematical and finite element modelling and deployed to produce satisfactory results in the gritty and down to earth industrial environment of day-to-day power generation, where breakdowns can cost both money and reputation…