Matthias Kolbe

The high temperature decrease of the critical resolved shear stress in nickel-base superalloys

Abstract Nickel-base superalloys show a typical decrease in the critical resolved shear stress (CRSS) around 750–800°C. In situ tensile tests of the alloys NIMONIC 105 and NIMONIC PE 16 carried out in a high voltage transmission electron microscope revealed the onset of a new deformation mechanism at 780°C. Above 780°C the deformation is achieved by the viscous motion of pairs of identical γ- matrix dislocations with Burgers vector of type a/6〈112〉 (a: lattice constant). The dislocations are dragging behind long intrinsic stacking faults and thus establish microtwins. Along the trace of the moving dislocations the γ ’-particles are sheared. A crystallography/diffusion-based model is presented which explains the (i) unusual pairing of a/6〈112〉 dislocations and (ii) the restriction of the viscous dislocation movement to high temperatures.

Rapid solidification of Cu84Co16 alloy undercooled into the metastable miscibility gap under different conditions

Abstract The Cu 84 Co 16 alloy melt processed by differential thermal analysis, electromagnetic levitation and drop tube experiences a liquid phase separation when it is undercooled into the metastable miscibility gap. The phase separation and coagulation processes are mainly controlled by the degree of undercooling, cooling rate and convection level in the containerless states. Disperse structures have been formed in droplets solidified during free fall in the drop tube.

Magnetohydrodynamic Modeling and Experimental Validation of Convection Inside Electromagnetically Levitated Co-Cu Droplets

A magnetohydrodynamic model of internal convection of a molten Co-Cu droplet processed by the ground-based electromagnetic levitation (EML) was developed. For the calculation of the electromagnetic field generated by the copper coils, the simplified Maxwell’s equations were solved. The calculated Lorentz force per volume was used as a momentum source in the Navier–Stokes equations, which were solved by using a commercial computational fluid dynamics package. The RNG k-ε model was adopted for the prediction of turbulent flow. For the validation of the developed model, a Co16Cu84 sample was tested using the EML facility in the German Aerospace Center, Cologne, Germany. The sample was subjected to a full melt cycle, during which the surface of the sample was captured by a high-speed camera. With a sufficient undercooling, the liquid phase separation occurred and the Co-rich liquid phase particles could be observed as they were floating on the surface along streamlines. The convection velocity was estimated by the combination of the displacement of the Co-rich particles and the temporal resolution of the high-speed camera. Both the numerical and experimental results showed an excellent agreement in the convection velocity on the surface.

Nucleation transitions in undercooled Cu70Co30 immiscible alloy

High temperature differential scanning calorimetry (DSC) is applied to undercool and crystallize melts of a Cu70Co30 alloy into the metastable miscibility gap. The kinetic prefactor Γ and the activation energy ΔG* of the nucleation rate are determined based on the statistical analysis within classical nucleation theory. The value of Γ reaches 2.64 (0.21) × 1037 m−3 s−1, which is close to that of the value for homogenous nucleation and much larger than that of undercooled pure Co melts. The value of ΔG* is estimated to be 67 (2.5) kBT which is also higher than that of undercooled pure Co melts. The nucleation of the crystallization of the Co-rich phase is governed by homogeneous nucleation or conditions that are indistinguishable from homogeneous nucleation and the Cu-rich liquid phase effectively prevents the occurrence of heterogeneous nucleation for the nucleation of the Co-rich phase in the liquid-phase separated Cu70Co30 alloy. The results indicate that nucleation of the crystalline phase is sensitive...

Undercooling and demixing of copper-based alloys

Since the beginning of materials science research under microgravity conditions immiscible alloys have been an interesting subject. New possibilities to investigate such systems are offered by containerless processing techniques. Of particular interest is the ternary system Cu-Fe-Co, and its limiting binaries, Cu-Co and Cu-Fe. They all show a metastable miscibility gap in the regime of the undercooled melt. Within the ESA-MAP project “CoolCop”, different aspects of this alloy have been investigated; results obtained so far are reported here.

Tensile tests of Fe70Al30 in a TEM in the temperature range of the yield stress anomaly

Abstract Fe 70 Al 30 is a long-range ordered material, which exhibits a yield stress anomaly. Thin single-crystal foils of Fe 70 Al 30 have been strained inside a transmission electron microscope in the temperature range of the yield stress anomaly. The structure in this range is DO 3 . The dislocations had Burgers vectors parallel to the 〈111〉 directions and were predominantly of screw character. All dislocations were two-fold dissociated; no four-fold dissociations have been found. Below the peak temperature T p glide on {110}-planes has been observed. The dislocations propagate by jumps. Once emitted from a source, they glide very rapidly over long distances before locking. In addition to glide on {110}-planes, close to T p glide on {112}-planes has been observed. In these planes the glide is viscous. It is concluded that the anomalous temperature variation of the CRSS in Fe 70 Al 30 can probably not be explained on the basis of the temperature dependence of the state of order.

Solidification of tetragonal Ni2B from the undercooled melt

The dendrite growth velocity during non-equilibrium solidification of tetragonal Ni2B crystals is measured as a function of undercooling on electrostatically levitated droplets. High-speed video imaging is applied to measure in situ the propagation of the solid-liquid interface apparent on the sample surface. To analyze the raw data, a three-dimensional model is developed to determine the growth velocity within the volume of the melt. Microstructure analysis is performed to resolve the pattern formation mechanism of tetragonal Ni2B crystals which differs significantly from the cubic case. The dendrite tips are found to grow perpendicular to the crystallographic {111} planes. As a consequence, the propagation of the solidification front on a macroscopic scale is determined by the lattice parameters of the tetragonal crystal structure. The dendrite growth velocities along the {111} normal directions are analyzed on a mesoscopic scale within a sharp-interface model.

Dendrite growth velocity in the undercooled melt of glass forming Ni50Zr50 compound

Abstract Glass-forming Ni50Zr50 intermetallic compound is containerless undercooled and solidified using electrostatic levitation. Large undercoolings up to ∆T = 300 K are achieved. The dendrite growth velocity of the congruently melting alloy is measured as a function of undercooling using a high-speed camera technique. The experimental data is analysed within a sharp interface theory. It is found that the driving force of crystallisation is controlling the growth kinetics at ∆T < 250 K but at larger undercoolings the growth kinetics is progressively controlled by atomic diffusion. This leads to a slowing down of the growth velocity. The maximum velocity and the temperature at which the maximum occurs (Tmax) are inferred from the dendrite growth velocity – undercooling relation. The relation of the temperature Tmax and the glass temperature fits into a general classification scheme for glass-forming systems. The kinetic and thermal undercooling terms are calculated within dendrite growth theory as a function of the total undercooling. At ∆T > 126 K, the kinetic undercooling dominates and increases rapidly with the undercooling ∆T. The maximum prefactor of the kinetic undercooling is plotted vs. the reciprocal temperature. Its temperature dependence is discussed.

Undercooling and rapid solidification of Cu84Co16 alloys under a static magnetic field

Bulk samples of Cu84Co16 composition were undercooled and rapidly solidified using the electromagnetic levitation technique under a static magnetic field of 2 T. A droplet-shaped microstructure was frozen in due to metastable phase separation in the undercooled liquid. A singular droplet size distribution was determined for the minority Co-rich phase, which differed from a bimodal one observed in the samples levitated without the static magnetic field. Such an effect of the magnetic field was found to be similar to that of reduced gravity of parabolic flights.

Containerless Undercooled Melts: Ordering, Nucleation, and Dendrite Growth

Electromagnetic and electrostatic levitation are applied to containerless undercool and solidify metallic melts. A large undercooling range becomes accessible with the extra benefit that the freely suspended drop is accessible directly for in situ observation. The short-range order in undercooled melts is investigated by combining levitation with elastic neutron scattering and X-ray scattering using synchrotron radiation. Muon Spin Rotation (µSR) experiments show magnetic ordering in deeply undercooled Co80Pd20 alloys. The onset of magnetic ordering stimulates nucleation. Results on nucleation undercooling of zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Metastable phase diagrams are determined by applying energy-dispersive X-ray diffraction of Ni-V alloys with varying concentration. Nucleation is followed by crystal growth. Rapid dendrite growth velocity is measured on levitation-processed samples as a function of undercooling ∆T by using high-speed video camera technique. Solute trapping in dilute solid solutions and disorder trapping in intermetallic compounds are experimentally verified. Measurements of glass-forming Cu-Zr alloy show a maximum in the V(∆T) relation that is indicative for diffusion-controlled growth. The influence of convection on dendrite growth of Al50Ni50 is shown by comparative measurements of dendrite growth velocity on Earth and in reduced gravity. Eventually, faceting of a rough interface by convection is presented as observed on Ni2B alloys.