Understanding the stress-resisting creep and hot tensile deformation in ODS superalloys

In this investigation, the creep and hot tensile properties of two oxide dispersion strengthened (ODS) superalloys, Inconel MA754 and Inconel MA6000, have been studied with the objective of obtaining a unified understanding of the creep and stress-strain behavior of these materials. Tensile and creep tests were conducted in tension over wide, overlapping ranges of stress, strain rate and temperature. MA754 was tested at temperatures of 1033K, 1255K, and 1366K. MA6000 was tested at 1033K.; Testing of MA754, a nickel-chromium solid solution strengthened by yttria (Y{dollar}sb2rm Osb3{dollar}) dispersoids, showed that the stress-strain and creep deformation are equivalent at steady state. By virtue of this equivalence, it was found that the resisting stress to hot deformation is equal to the proportional limit measured at the appropriate strain rate and temperature. Evidence is given showing that the resisting stress arises from an attractive interaction between the hard dispersoids and the dislocation and that the mechanism governing high temperature deformation is self diffusion controlled recovery as is observed in simpler systems.; For MA6000, strengthened by coherent Y{dollar}spprime{dollar} (Ni{dollar}sb3{dollar} (Al,Ti)) precipitates as well as yttria dispersoids, the grain texture of the material was found to be a pivotal behavioral factor. Tensile testing over a strain rate range of 10{dollar}sp{lcub}-2{rcub}{dollar} to 10{dollar}sp{lcub}-7{rcub}{dollar} sec{dollar}sp{lcub}-1{rcub}{dollar} showed that little work hardening is observed and the yielding behavior is strongly strain rate dependent. As the strain rate was decreased from 10{dollar}sp{lcub}-2{rcub}{dollar} sec{dollar}sp{lcub}-1{rcub}{dollar}, yield drop behavior was observed at strain rates down to 10{dollar}sp{lcub}-5{rcub}{dollar} sec{dollar}sp{lcub}-1{rcub}{dollar} below which necking behavior was observed. Consistently, the fracture behavior was also found to change with strain rate from non-ductile, crystallographic fracture at 10{dollar}sp{lcub}-2{rcub}{dollar} sec{dollar}sp{lcub}-1{rcub}{dollar} to ductile, transgranular fracture at strain rates greater than 10{dollar}sp{lcub}-5{rcub}{dollar} sec{dollar}sp{lcub}-1{rcub}{dollar}, with mixed mode fracture at intermediate strain rates. The yielding, fracture and yield drop behavior are attributed to lattice rotation as a result of the {dollar}langle110rangle{dollar} texture of the material. The data from the tensile tests and from creep tests performed at 1033K in a similar deformation range were compared and found to be equivalent at steady state. The implications that these findings have for identifying the resisting stress in MA6000 are discussed.