Investigation using digital image correlation of Portevin-Le Chatelier Effect in Hastelloy X under thermo-mechanical loading

• High temperature uniaxial loading of un-notched and notched Hastelloy X samples.
• High temperature digital image correlation (DIC) measurements of strains fields in both cases.
• Identification of PLC effect from macroscale load–displacement measurements.
• Observation of PLC band strain fields by microscale DIC experiments.
• Investigation of PLC strain band origination from notch tips.

The plastic behavior of Hastelloy X subjected to thermo-mechanical loading was investigated in this work, and the phenomenon of dynamic strain aging was studied under isothermal conditions, using digital image correlation (DIC). The material of interest, Hastelloy X a Ni-based superalloy, is widely used in high temperature applications. It was seen that Hastelloy X exhibits serrated plastic flow behavior between temperatures of 300 °C and 700 °C, as a consequence of the dynamic strain aging effect, which, depending on temperature, manifests as type A/B or A and B oscillations on the uniaxial tension stress–strain curve, and is characterized by strain concentration bands propagating along the length of the specimen. The characteristics of Portevin-Le Chatelier (PLC) band internal strain distribution in uniaxial tension experiments were also investigated. The bands themselves were seen to have internal (sub)bands of two or three smaller strain jumps. An analysis of the strain increments induced by these bands was performed and clearly illustrates the inhomogeneous nature of the deformation – both spatially and temporally. In addition to the uniaxial behavior, biaxial stress states were investigated by studying PLC band formation near a notch in a double notched tensile geometry. Similar macroscopic results to the uniaxial case were seen while the strain bands themselves were seen to initiate in the notch region and travel away from the notches.