Dynamic strain aging, deformation and fracture behaviour of the nickel base superalloy Inconel 617

Advanced Ultra Super Critical power plants (AUSC) with intended capabilities of higher efficiency and less emissions of CO2 compared with those of conventional power plants, require materials with high creep strength and good corrosion resistance. They are also required to withstand steam at high temperatures of 700-780 degrees C and pressures upto 35 MPa. Inconel 617 alloy is a candidate material for super heater and re-heater parts of boilers. In the present study, tensile behaviour of the Inconel 617 is investigated in the temperature range from RT to 900 degrees C at different strain rates from 5 x 10(-4) s(-1) to 1 x 10(-2) s(-1) and the deformed microstructures are characterised by optical and electron microscopy. Serrations are observed in the temperature range 300-700 degrees C at all the three strain rates which indicated the occurrence of dynamic strain aging (DSA). The amplitude of serrations increased with increase in the temperature and decrease in the strain rate. Temperature regime of DSA is confirmed to be from 300 degrees C to 700 degrees C by the occurrence of plateau in the yield strength, ductility minima and negative strain rate sensitivity in this regime. Activation energies for the serrated flow are found to be 65, 80 and 110 kJ/mol for the three types of serrations, namely B, (A +B) and C respectively. The controlling mechanism of the DSA is found to be diffusion of carbon through dislocation cores in the lower temperature range and diffusion of substitutional elements Cr and Mo in the higher temperature range. TEM studies revealed increase in the number of slip bands as well as interaction of dislocations with solute atoms with increase in the temperature up to 700 degrees C. Precipitation of carbides and subgrain formation was observed at 700 degrees C, at the ductility minima. SEM examination of the fracture surfaces revealed ductile fracture with dimples and facets at room temperature, ductile fracture with essentially dimples at 400-600 degrees C, at intense serrations in the flow curve. At 700 degrees C, at ductility minima, there was mixed mode of fracture with dimples, facets and intergranular cracks associated with some grains boundaries. At 800 degrees C and 900 degrees C, there was completely ductile fracture with large and deep dimples.