Robert Günther

Heterogeneous nucleation and growth of the β(Ti) phase in the Ti–Al system—experiments and model calculations

The barrier to heterogeneous nucleation of the β(Ti) phase on TiB(2) and other borides has been evaluated using the plane to plane matching model. The results are compared to the known nucleation of the α(Ti) phase on the β(Ti) phase. According to this comparison, the barrier to heterogeneous nucleation of the β(Ti) phase on TiB(2) can be judged to be small. This is in agreement with inoculation experiments. The addition of a Ti-Al-TiB(2) master composite to a β(Ti) solidifying TiAl based alloy leads to a significantly refined microstructure. Microsegregations enable us to attribute this refinement to refined equiaxed β(Ti) dendrites. However, model calculations based on the hemispherical cap model predict that the refinement via heterogeneous β(Ti) nucleation should be more potent. First calculations indicate that structural imperfections of TiB(2) particles limit the nucleation site diameter. Thereby, the nucleation barrier is increased and the refinement is less pronounced.

Model calculation of inoculation and experimental verification in two alloy-systems

Abstract In order to clarify grain refinement mechanisms and optimize possible potent refiners in Mg and TiAl alloys, a simulation method for heterogeneous nucleation based on the free growth model by Greer is applied. The predictions of the model were improved by modifications taking into account violations of a perfect model system: Imperfections of inoculants and breakdown of stable spherical grain growth. The model predictions are compared with results from casting experiments and show a distinct improvement.

Modification of Magnesium Alloys by Ceramic Particles in Gravity Die Casting

A critical drawback for the application of magnesium wrought alloys is the limited formability of semifinished products that arises from a strong texture formation during thermomechanical treatment. The ability of second phase particles embedded into the metal matrix to alter this texture evolution is of great interest. Therefore, the fabrication of particle modified magnesium alloys (particle content 0.5–1 wt.-%) by gravity die casting has been studied. Five different types of micron sized ceramic powders (AlN, MgB2, MgO, SiC, and ZrB2) have been investigated to identify applicable particles for the modification. Agglomeration of the particles is revealed to be the central problem for the fabrication process. The main factors that influence the agglomerate size are the particle size and the intensity of melt stirring. Concerning handling, chemical stability in the Mg-Al-Zn alloy system, settling and wetting in the melt, and formation of the microstructure in most cases, the investigated powders show satisfying properties. However, SiC is chemically unstable in aluminum containing alloys. The high density of ZrB2 causes large particles to settle subsequent to stirring resulting in an inhomogeneous distribution of the particles over the cast billet.

Effects of Ceramic Inoculants and Intermetallic Phases on Hot Rolled AZ Magnesium Wrought Alloys

The role of ceramic particles, calcium and rare earth elements on magnesium alloys during solidification from the melt and after hot rolling has been studied by microstructural investigations, texture measurements and mechanical tests. Different ceramic inoculants like silicon carbide or zirconium diboride and two different rare earth elements (cerium and lanthanum) forming intermetallic compounds were used. Both, ceramic particles and intermetallic phases, modify the texture evolution during hot forming. The rolled alloys exhibited a basal fibre texture which is weakened by ceramic particles and intermetallic phases respectively. This weakening of the basal texture is capable of lowering the anisotropy of the yield stress and improving the formability of sheet material. Influences of the alloying contents on microstructure, texture evolution and mechanical properties will be discussed.