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

Abstract

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 °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 °C at different strain rates from 5 × 10−4 s−1 to 1 × 10−2 s−1 and the deformed microstructures are characterised by optical and electron microscopy. Serrations are observed in the temperature range 300–700 °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 °C to 700 °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 °C. Precipitation of carbides and subgrain formation was observed at 700 °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 °C, at intense serrations in the flow curve. At 700 °C, at ductility minima, there was mixed mode of fracture with dimples, facets and intergranular cracks associated with some grains boundaries. At 800 °C and 900 °C, there was completely ductile fracture with large and deep dimples.