Juraj Lapin

Microstructure and Mechanical Properties of a Cast Intermetallic Ti-46Al-8Ta Alloy

Microstructure and mechanical properties of a new cast air-hardenable intermetallic Ti-46Al-8Ta (at.%) alloy are studied. Primary solidification phase, solidification path and high temperature phase equilibria are determined using quenching during directional solidification (QDS) experiments combined with microstructural analysis. Vickers hardness, tensile, compression and creep properties are presented. The effect of short-term high temperature exposure on room temperature (RT) ductility of uncoated specimens is evaluated and discussed. Creep properties such as minimum creep rate, time to 1 % creep deformation, fracture time and creep fracture mechanisms are presented and compared to other TiAl base alloys developed for industrial applications. Long-term high temperature microstructural stability and the effect of ageing on Vickers hardness and RT tensile properties are presented. Based on the achieved experimental results, the potential of this new alloy to fulfill industrial specifications is discussed.

The development of a microgravity experiment involving columnar to equiaxed transition for solidification of a Ti-Al based alloy

The authors are members of the integrated project Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS), funded within the European Framework (FP6). One of the aims of IMPRESS is to develop new alloys and processes for the casting of TiAl-based turbine blades for the next generation of aero and industrial gas turbine engines. Within IMPRESS, two related issues have been identified during the primary solidification stage, namely, segregation and the columnar-to-equiaxed transition (CET). The authors have set out to isolate the effects of thermo-solutal convection, by designing a microgravity experiment to be performed on a European Space Agency platform. This experiment will investigate the CET formation during solidification. It is planned to use a sounding rocket providing a microgravity time of approximately twelve minutes. The results of this microgravity solidification experiment will be used as unique benchmark data for development and validation of new computational models of TiAl solidification. This in turn will produce accurate models and ultimately new robust industrial processes by project partners in the aerospace industry. The evolution of the design of the microgravity experiment is discussed and the results of preliminary ground reference experiments are presented. Future plans and objectives for the project are also highlighted.

The effect of multiaxial stress state on formation of rafts in CMSX-4 superalloy during creep

The effect of multiaxial stress state caused by a notch effect on microstructure degradation of single crystal nickel based superalloy CMSX-4 was studied during creep at a temperature of 950 °C, under constant nominal stress of 120 MPa for 500 and 1000 h. In order to minimize surface oxidation effects on microstructure degradation and notched geometry of specimens, all experiments were performed under protective atmosphere. The geometry of creep specimens was designed with the aim to obtain defined interval of principal stresses, which is sufficiently large to get reliable microstructure response during tensile creep experiments. Flat specimens with “U-type” notch were designed using elastic finite element method (FEM) calculations. Microstructure analysis of the crept specimens showed that notches affect degradation processes of the studied superalloy significantly. FEM elastic-plastic analysis was performed to simulate creep of the notched specimens. Material creep behaviour was integrated into calculations by a subroutine using equations derived from experimentally measured data for cylindrical creep specimens tested at various constant nominal stresses. Magnitude, distribution and orientation of principal stresses resulting from the specimen loading were calculated and their effect on formation of rafts was described.

Columnar to Equiaxed Transition in Peritectic TiAl Based Alloy Studied by a Power-Down Technique

Columnar to equiaxed transition (CET) was studied in a peritectic TiAl-based alloy with chemical composition Ti-45.1Al-4.9Nb-0.25C-0.2B (at.%). Solidification experiments were conducted in a Bridgman-type apparatus using cylindrical moulds made of high-purity Y2O3. The methodology containing appropriate etching and observations under flat light in stereo-microscope was used to identify the morphology of primary β phase grains and position of CET in the samples˰ All samples prepared by power down-technique showed sharp CET. The position of the CET measured from the beginning of the sample depends on the applied cooling rate and increases from approximately 65 to 115 mm by decreasing cooling rate from 50 to 15 K/min. Based on terrestrial experiments, the future work focused on microgravity and hypergravity CET experiments and numerical modeling is proposed. A Bridgman furnace front tracking method will be applied in future work to complement the experimental results here as part of the European Space Agency GRADECET programme. This modeling will input directly into planned microgravity and hypergravity CET experiments.

Conditions for CET in a gamma TiAl alloy

The solidification of gamma TiAl alloys is of interest to the aerospace and automotive industries. A gamma TiAl multicomponent alloy: Ti-45.5Al-4.7Nb-0.2C-0.2B (at. %) has been the focus of a study to investigate the solidification conditions that led to a Columnar to Equiaxed Transition (CET) in a directional solidification experiment where traditional Bridgman solidification was combined in series with the power down method. In this paper, a numerical modelling result (a locus plot of columnar growth rate and temperature gradient) from this experiment is superimposed onto CET maps generated using an established analytical model for CET from the literature. A parametric study is carried out over suitable ranges of nucleation undercooling and nuclei density values. The predicted CET positions are compared with the experimentally measured CET position. Reasonable agreement is found at low levels of nuclei density. The paper concludes with estimates for the solidification conditions (nuclei density and nucleation undercooling) that led to the CET.

Microsegregation Induced Inhomogeneity of Coarsening of γ’ Precipitates in a Nickel-Based Single Crystal Superalloy

Microsegregation induced inhomogeneity of the coarsening of cuboidal γ’(Ni3(Al,Ti)) precipitates was studied in a single crystal nickel-based superalloy CMSX-4 at temperatures ranging from 850 to 1000 °C and for an ageing time up to 5000 h. Experimental results showed a significant statistical difference in the size of the γ’ precipitates between the dendrites and interdendritic region. The achieved results are analyzed and discussed from the point of chemical heterogeneity, activation energy for coarsening, time exponent, effective diffusion coefficient and γ/γ’ interfacial energy which control coarsening kinetics of the cuboidal γ’ precipitates within the dendrites and interdendritic region.