Creep Behavior of Additively Manufactured, Oxide Dispersion Strengthened, and Refractory Multi-Principal Element Alloys

This study investigates the influence of processing methods and oxide dispersion strengthening (ODS) on the creep behavior and deformation mechanisms of several multi-principal element alloys (MPEA). The effect of a BCC crystal structure composed of refractory elements on the creep behavior of MPEAs is also investigated. The materials examined are equiatomic wrought and additively manufactured (AM) CrCoNi, equiatomic AM ODS CrCoNi, and wrought Nb45Ta25Ti15Hf15. Constant stress creep tests were performed on all materials to determine stress exponents and activation energies. Stress reduction creep tests were conducted on AM CrCoNi and AM ODS CrCoNi to characterize activation areas and subsequently their rate-controlling deformation mechanisms during steady state creep. Scanning electron microscopy (SEM) was utilized to characterize initial microstructures, atomic compositions, and fracture surfaces. Pre- and post-creep dislocation structures were characterized by transmission electron microscopy (TEM). Stress exponents of 4.5 ± 0.2, 5.9 ± 0.1, 6.5 ± 0.1 were determined for wrought CrCoNi, AM CrCoNi, and AM ODS CrCoNi, respectively. These results indicate that the stress dependence of the creep behavior is different for each of these FCC MPEAs and thus they have unique creep deformation mechanisms. Activation energy ranges of 240 – 259 kJ/mol, 320 – 331 kJ/mol, and 335 – 367 kJ/mol were found for wrought CrCoNi, AM CrCoNi, and AM ODS CrCoNi, respectively. The increasing activation energy ranges reflect the improved temperature resistance of these FCC alloys. The Nb45Ta25Ti15Hf15 exhibited creep properties superior to the investigated FCC MPEAs at all but the lowest values of applied stress, with its creep deformation behavior being attributed to thermally activated and stress assisted dislocation glide.The AM material exhibited superior creep resistance and inferior creep ductility compared to wrought alloy. This difference was attributed to the AM material having a higher percentage of Cr-rich oxides and a lower percentage of low angle grain boundaries (LAGB). Three types of Cr-rich oxides were observed via TEM/STEM-EDS in the AM material, leading to dislocation interactions with these oxide particles as well as contributing to the nucleation of voids leading to fracture. Essentially no oxides were found in the wrought material. The AM material has a lower LAGB density than the wrought material, leading to fewer dislocation pile-up stress concentrations in the wrought material due to the propensity to propagate deformation more readily through LAGBs. The steady state dislocation structures of the AM and wrought material consist of individual curved dislocations with dislocation multijunctions and jogs. Compared to its non-ODS counterpart, AM ODS CrCoNi has superior creep resistance at all tested stresses and temperatures. The excellent creep resistance of AM ODS CrCoNi was attributed to the interaction of mixed character dislocations with oxide particles. The creep ductility of AM ODS CrCoNi was higher than its non-ODS counterpart due to a large percentage of low angle grain boundaries associated with its columnar grain structure. It was also found that creep ductility is significantly greater at the highest applied stress of 200 MPa compared to lower stresses. The steady state dislocation structure of AM ODS CrCoNi consists of long arrays of dislocations which is different than that observed in non-ODS CrCoNi which consists of individual curved dislocations. Operational activation areas were determined to be 920 b2 for AM ODS CrCoNi and 960 b2 for AM CrCoNi. These operational activation areas are indicative of forest dislocation controlled steady state creep, which is consistent with the dislocation structures observed in these alloys. The operational activation area of AM CrCoNi is significantly higher than the values reported for CrMnFeCoNi, revealing a difference in rate controlling mechanism. The quinary alloy is rate limited by a combination of dislocation interactions with forest dislocations and the solid solution matrix, and the simplified ternary alloy is controlled solely by forest dislocation strengthening. Similarly, the operational activation area of AM ODS CrCoNi is significantly higher than the values reported for other ODS alloys, indicating that the volume fraction of oxides in AM ODS CrCoNi is too low to effectively limit the rate of dislocation motion, and revealing that the thermal activation of dislocation glide is instead limited by forest dislocations. Creep behavior in Nb45Ta25Ti15Hf15 was characterized by both the power law creep model and the thermally activated dislocation glide model. Utilizing the power law creep model, a stress exponent of 1.2 was measured at low applied stresses suggesting diffusion controlled creep occurs in this regime. A stress exponent of 5.7 was measured at high applied stresses implying dislocation climb controlled creep in the high stress regime. However, an exponential relationship provides a better fit of the creep data, suggesting that the conventional phenomenological power law approach does not accurately describe the creep behavior in this material. The exponential approach is consistent with the thermally activated and stress assisted dislocation glide description of the rate controlling mechanism for creep deformation in Nb45Ta25Ti15Hf15. Brittle intergranular fracture was observed as a result of Hf-oxide formation at grain boundaries. This brittle intergranular fracture decreased creep ductility, creep life, and creep rate, especially at low applied stresses where extended exposure to oxygen resulted in more extensive brittle intergranular fracture