Influence of High Current-Density Impulses on the Stress-Strain Response and Microstructural Evolution of the Single Crystal Superalloy CMSX-4

Abstract

Owing to the low formability of single-crystal nickel-based materials, single-crystal components are typically cast. Subsequently, a multi-stage heat treatment is carried out in order to partially compensate the dendrite segregation and to stabilize the precipitated γ'-phase. Such components possess a high resistance to creep at elevated temperatures. Since it is known that electrical impulses can be used to increase the formability of various materials, the potential of an electrical high current-density impulse treatment was evaluated for forming of the high strength nickel-based superalloy CMSX-4. Heat treated and pre-deformed specimens were loaded in compression and subjected to short pulses with a high-current density of 2.3 kA/mm 2. Depending on the microstructural state, the material demonstrated work hardening or softening as a consequence of the impulse treatment. In addition, experiments were carried out on crept specimens to test whether the current-impulse treatment can be used to reverse creep-related segregation of alloying elements (raft formation). It was possible to observe a change in the concentration of the elements in the γ/γ'-phase transition following the currentimpulse treatment. In particular, a local increase in the γ-forming elements Cr, Co and a decrease in the γ'-forming elements Ta, Ti was observed after the impulse treatment.

Electroplasticity Mechanisms in hcp Materials

“…[37] For the current pulse treatment, a high current pulse generator according to refs. [33,38] was employed (see Figure 2 left).…”

“…For instance, the effect of electron flow on dislocation mobility under electrical current pulses can either result in the formation of new Frank-Reed sources or in a decrease of the interaction energy of dislocations with the local (point) defects. The influence of the electron flow can be divided into three effects: (i) the initially elastic deformation gets partially replaced by plastic deformation after current impulse treatment, [32] (ii) additional dislocations with opposite sign are produced, which causes an annihilation of existing dislocations, [33] and (iii) new twins are formed either within the grains or at grain boundaries. [34] In the present study, the analysis of the mechanical properties depending on current density and the microstructural analysis after deformation with and without superimposed current pulse was conducted for three different hcp materials.…”