Portevin–Le Chatelier (PLC) effect induced by different deformation mechanisms in Ni–25Mo–8Cr alloy during high-temperature tensile deformation

Uniaxial tensile testing explored the Portevin–Le Chatelier (PLC) effect in nickel-based superalloys featuring high Mo/Cr mass ratios, focusing on the influence of variations in the initial microstructure on the deformation behavior at room and elevated temperatures. Experimental results indicated that the PLC effect was observed solely in the high-temperature tensile curves. However, the deformation mechanisms and characteristics of the PLC effect varied with different initial microstructures. Solid solution (SS) and over-aged (OA) samples exhibited C-type serrations, while under-aged (UA) and peak-aged (PA) specimens, featuring short- and long-range ordered phases, respectively, exhibited A + B type serrations in their tensile curves. Microstructural evolution from the SS to the UA, PA and OA states changed their stacking fault energy (SFE), leading to a sequential transformation in the plastic deformation mechanisms during high-temperature tensile deformation: stacking fault (SF) → nanotwin → microtwin → SF. C-type serrations in the SS samples were associated with high solute-atom contents and SF formation. The PLC effects in the UA and PA samples were predominantly caused by solute atom pinning dislocations. Although precipitates and twins were not the primary drivers of the PLC effect, they impeded dislocation migration, exacerbated solute-atom segregation and enhanced dislocation pinning, generating A + B-shaped serrations. In the OA specimens, precipitated phases induced interfacial mismatch under thermal-force coupling. SF shearing of the precipitated phase and subsequent re-dissolution facilitated the formation of C-type serrations, whose PLC effect was induced by the combined action of dynamic strain aging (DSA), SFs of the matrix and diffusion-controlled pseudo-locking mechanisms.