Creep curve behaviour of TiAl-8Ta intermetallic alloy

The creep behaviour of a new TiAl intermetallic alloy with 8 % in atom of Ta has been investigated under constant load and temperature conditions. The specimens have been creep tested in the temperature range of 700-850 °C and for specific applied stresses, in order to result in rupture times up to 3000 h. The alloy has been cast and properly cooled to enhance the massive transformation typical in such alloy. The sequent designed HIP treatments have been carried out to obtain a final convoluted alpha2 + gamma lamellar microstructure that ensures an adequate room temperature ductility, a critical parameter for intermetallic alloy applications. The material microstructure has been investigated through scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS) and X ray diffraction technique (XRD) to quantify the volume fractions of ?2 and ? phases and to link the microstructure evolution with the creep behaviour. To such aims the specimens have been analysed in the as-received conditions, after temperature exposure with no load applied, i.e. in the specimen heads, and in the gauges after ruptures. After depuration of the creep curves from the acceleration because of the true stress increase with strain, as it happens in constant load creep tests, an acceleration regime proper of the material has been put in evidence. Hence, at any temperature investigated, the alloy has not exhibited a real steady state regime, but a minimum in the creep rate with a significant primary and a dominant tertiary: applied stresses and minimum strain rates have been reported to follow a Norton relationships with exponents of about 7.8 and 4.9 in the temperature range boundaries of 700 °C and 850 °C, respectively. The creep alloy curves at different stresses and temperatures have exhibited the same shapes in plots epsilon vs. t/tR with tR equal to the rupture time, suggesting that the material has experienced the same creep strain correlated damage at any load and temperature. Interrupted creep tests at the minimum strain rates at 700 and 850 °C have been performed for microstructure investigations and comparison. Creep tests with load changes have been also run to have an insight on the nature of the deformation mechanism that controls the deformation.