Anomalous Work Hardening Behavior of a Single Crystalline Co-Base Superalloy

Abstract

The defect evolution associated with an anomalous work hardening behavior of a single crystalline quaternary Co-Al-W-Ta superalloy at 950 °C was investigated by transmission electron microscopy. As plastic deformation is initially confined to the γ matrix channels, a plateau arises in the stress-strain curve after yielding. At about 1% plastic strain, extensive shearing of the γ′ precipitates under superlattice stacking fault formation occurs leading to extreme work hardening rates up to 12 GPa and a total increase in stress of about 200 MPa. Additional investigations on the temperature and strain-rate dependence of the anomalous work hardening behavior reveal the significance of diffusion and segregation processes on the stress-strain curve and the work hardening behavior.

Influence of the γ′ Volume Fraction on the High-Temperature Strength of Single Crystalline Co–Al–W–Ta Superalloys

“…Unlike the insignificant yield strength anomaly, anomalous peaks of Rp,2.0 arose in VF40 and VF60 at 850 °C and in VF70 and VF80 at 750 °C. Since the anomalous work-hardening behavior in VF60 at 950 °C originated from extensive shearing of the γ′ phase and the formation of SFs [33], the stress-strain curves of the VF series at this temperature are compared in Figure 2c to rationalize the influence of the γ′ volume fraction and secondary phases. For the Group I alloys, the work-hardening was less obvious in VF0-VF40 and most significant in VF60.…”

“…In contrast, at higher stresses such as 350 MPa, shearing of γ precipitates readily happened in VF60 within a shorter time than at lower stresses, leading to a limited contribution of strengthening by rafting and the formation of a single creep rate minimum. In the case of VF70, its higher γ volume fraction of 81% than that of VF60 with 70% may have enabled more plastic deformation in the γ precipitates and a higher chance of SF interaction for hardening [31,33].…”

“…At a lower γ volume fraction of 55%, the highest creep strength was reached at high temperature and low stress, where rafting reduced the size of the vertical γ channels and the corresponding dislocation climbing along them [25]. Unlike in Ni-based superalloys, the deformation was mainly dominated by the shearing of γ precipitates under the formation of planar faults, such as anti-phase boundaries and stacking faults in Co-and CoNi-based superalloys because of their low stacking fault energy [2,[26][27][28][29][30][31][32][33]. It has further been found that increasing the γ volume fraction up to 80-90% can lead to even higher YS [16] and creep strength [20,34] in these superalloys.…”