On the creep behaviour of Ni based solid solution alloys from binary to quaternary

Abstract

On the creep behavior of Ni based solid solution alloys from binary to quaternary A power-law relationship between the steady state strain rate (𝜀̇) and imposed stress (σ) is well established so that the 𝜀̇∝𝜎^𝑛, at a fixed temperature, where n is termed the stress exponent. Additional microstructural terms influencing creep such as the grain size and stacking fault energy (γ) have been incorporated into the creep equation in a power-law form, such as 𝜀̇∝𝛾^𝑞. In the case of an intragranular dislocation climb controlled creep in a solid solution alloy, the creep rate has been expressed as 𝜀̇∝𝐷𝜎^5𝛾^3, where D is the appropriate diffusion coefficient, n~5 and q~3. However, an evaluation of the earlier creep data suggests that while q~3 is reasonable for pure metals, there is considerable uncertainty in the value of q for solid solution alloys. The current study focuses on characterizing the creep behavior and the value of q for Ni – xCo alloys, with x = 10, 33, and 60, where the addition of Co reduces the stacking fault energy. Following creep in Ni-Co alloys, the creep behaviour of CSSAs NiCoCr and NiCoCrFe was also investigated to probe potential changes in the creep mechanisms with the addition of alloying element. All alloys are single phase solid solutions with a face-centered cubic (f.c.c.) crystal structure. Prior to the creep, all alloys had a grain size d~100 μm. The observed stacking fault energies through weak beam dark field technique are 10 – 36 mJ m^-2 for Ni – 60 Co, 14 – 27 mJ m^-2 for NiCoCr, and 11 – 26 mJ m^-2 for NiCoCrFe alloys. At a creep testing temperature of 1015 K, the stress exponent n ~ 5 for binary Ni – Co alloys, suggesting that dislocation climb is the creep rate controlling mechanism. The cell structure observed through electron channel contrast imaging in the crept samples of the alloys is consistent with a dislocation climb mechanism. The stacking fault energy exponent q ~ 2 in Ni– (x) Co binary solid solution alloys. At 990 K, the creep rates of the NiCoCr and NiCoCrFe alloys were observed to be similar with a stress exponent n~5. Both the ternary and quaternary alloys showed significantly lower creep rates compared to the Ni – (x) Co binary alloys. Creep deformation did not cause any phase change in the NiCoCrFe alloy. Although the Ni – (x) Co alloys, NiCoCr, and NiCoCrFe alloys exhibited a similar stress exponent of n~5, the crept substructure in the NiCoCr and NiCoCrFe alloy showed planar band features. The possible causes for the observed differences in creep behavior between the Ni-Co binary alloys and the other concentrated solid solution alloys will be discussed.