Precipitation sequences in rapidly solidified Allvac 718Plus alloy during solution treatment

Abstract

• A supersaturated solid solution and a higher composition uniformity of the γ-matrix in 718Plus result from the rapid solidification technology. • The growth/coarsening of η phase is mainly attributed to the stacking faults with 1 6 [ 211 ] γ Shockley partial dislocations. • The dissolution of η phase is achieved by the climbing of 1 4 [ 0001 ] η edge dislocations. • The Laves (C14) phase nucleates at MC carbides, promoted by the Cr segregation in the surrounding γ-matrix, and the Mo and Nb segregations at the edge of MC particles. Allvac 718Plus alloy is a compromising Ni-based superalloy at elevated-temperature of 704 °C, due to its combination of good mechanical properties and excellent structure stability. The precipitation behaviors during heat treatment in Allvac 718Plus alloy processed by rapid solidification technique were systematically investigated in this work. The evolution of secondary phases in rapidly solidified (RS) 718Plus alloy during annealing at 960 °C is: η + M C in 1 h, η + M C + Laves (C14) in 6 h, and M C + Laves (C14) in longer duration. The growth and dissolution behaviors of η phase are evidenced in RS samples and different types of dislocations at the η/γ interface are characterized. Based on these experimental observations, we examine the transformation mechanisms of γ→η and η→γ, which are related to the formation of stacking faults (SFs) with a 1 6 [ 211 ] γ Shockley partial dislocation and the climbing of 1 4 [ 0001 ] η edge dislocations at the η/γ interface, respectively. Both M C and Laves (C14) phases are enriched in Nb and Mo, with a higher level of Ti in M C carbides and Cr in the Laves (C14) phase. The precipitation of M C carbides is driven by the high concentration of C atoms in γ-matrix, while the Cr segregation promotes the later nucleation of Laves (C14) phase in γ-matrix around M C carbides, which is due to the Cr atoms rejection from M C carbides into the matrix.