INTENSIVE PLASTIC BY TORSION OF EK61, EP975, AND EK79 NICKEL-BASED SUPERALLOYS

10.25712/ASTU.1811-1416.2023.03.011

Abstract

The influence of severe plastic deformation by torsion (SPDT) on the formation of a nanocrystalline (NC) structure is analyzed as applied to heat-resistant nickel based superalloys EK79, EP975 (hardening g¢-phase Ni₃(Al,Ti) and EK61 (hardening g¢¢-phase Ni₃Nb). According to the results of the study, it was shown that during HPT the initial coarse-grained structure in nickel alloys with various types of strengthening phase Ni₃(Al, Ti) and Ni₃Nb is transformed into a UFG structure of a duplex type with an average grain size of the matrix (γ-phase): for EK61 – 50 nm, for EK79 – 55 nm, and for EP975 – 40 nm. The microhardness after HPT increased by 85 % for the EK61 alloy, by 28 % for the EK79 alloy, and by 37 % for the EP975 alloy compared to the initial state. The conditions for obtaining an NC structure of a duplex type are determined. Studies of the effect of annealing at a temperature of 800 °C for 30 min on microhardness of EK61, EK79 and EP975 superalloys have been carried out. It has been established that the value of microhardness in the EK61 superalloy decreases by 19 %, while in the EK79 and EP975 alloys it increases by 29 % compared to the nanocrystalline state. Ii is probably due to coarsening of the grain size of the EK61 superalloy, as for the EK79 and EP975 superalloys it is due to the high thermal stability of the nanocrystalline structure and possible additional precipitation intragranular nanosized particles of the strengthening γ'-phase.