Koji Kakehi

Tension/compression asymmetry in creep behavior of a Ni-based superalloy

Orientation and temperature dependence of yield stress or CRSS (Critical Resolved Shear Stress) and tension/compression anisotropy of the yield stress of CRSS have been shown by Shah and Duhl, Heredia and Pope, and Miner et al. Tension/compression asymmetry in the yield strength of Ni-based superalloys has been explained in terms of the core width effect. Shah and Duhl observed the tension/compression asymmetry in creep deformation, which is similar to that observed in the yield strength, and indicated that it can be attributed to cross slip and dislocation core-constriction mechanisms associated with octahedral slip. However, little is known about the mechanism of tension/compression asymmetry in creep. In the present study, single crystals of a Ni-base superalloy were subjected to tensile and compressive creep tests. Tension/compression asymmetry in creep behavior was examined in detail for each orientation.

Effect of primary and secondary precipitates on creep strength of Ni-base superalloy single crystals

Abstract The influence of primary and secondary precipitates in a single-crystal superalloy on creep strength was investigated at 700°C. In this study, the aging time and cooling rate were changed to alter the morphology of primary and secondary precipitates. Slow furnace cooling from the aging temperature was employed to obtain a clean matrix channel completely devoid of secondary γ′ precipitates. On the other hand, the forced-air quenched specimens included numerous superfine γ′ precipitates in the matrix channel. Since secondary γ′ precipitation can significantly decrease the net width of these matrix channels irrespective of primary γ′ sizes, the effect of secondary γ′ precipitation on creep strength was much larger than that of primary γ′ precipitates size. It was revealed that the net width of the matrix channel between precipitates could better account for the morphology effect of γ′ precipitates in Ni-based superalloys.

Effect of plastic anisotropy on tensile strength of single crystals of an Ni-based superalloy

Turbine blades are designed so that their primary orientation is within 10 to 15{degree} of the axis to insure a low modulus. The secondary dendritic direction ( direction) is usually randomly orientated with respect to the longitudinal direction of the turbine blade. The strengths of single crystals are influenced by the crystallographic orientations not only in the tensile direction but also in the normal direction of the specimen because a single crystal possesses intrinsic plastic anisotropy. The air-cooled turbine blades, which have a complicated hollow structure, are composed of sections of various thicknesses. Therefore, the mechanical properties of each blade section will depend on plastic anisotropy and the stress state as well as stress in the longitudinal direction. In previous studies, in an experimental single crystal alloy of an Ni-based superalloy, it has been revealed that {l_brace}111{r_brace} -type slip systems were activated during tensile tests. In this study, by using the experimental alloy which shows distinct active slip systems, the influence of crystallographic orientations and plastic anisotropy on the strength and ductility of single crystals of the Ni-based superalloy have been investigated on the assumption that the {l_brace}111{r_brace} slip systems operate.

The High Temperature Tensile and Creep Behaviors of High Entropy Superalloy

This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy FCC matrix and L12 ordered precipitates, so it is also named as “high entropy superalloy (HESA)”. The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. To the best of authors’ knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed.

The Effect of Post-Processes on the Microstructure and Creep Properties of Alloy718 Built Up by Selective Laser Melting

The selective laser melting (SLM) process was used to fabricate an Alloy718 specimen. The microstructure and creep properties were characterized in both the as-built and post-processed SLM materials. Post-processing involved several heat treatments and a combination of hot isostatic pressing (HIP) and solution treatment and aging (STA) to homogenize the microstructure. The experimental results showed that the originally recommended heat treatment process, STA-980 °C, for cast and wrought materials was not effective for SLM-processed specimens. Obvious grain growth structures were obtained in the STA-1180 °C/1 h and STA-1180 °C/4 h specimens. However, the grain size was uneven since heavy distortion or high-density dislocation formed during the SLM process, which would be harmful for the mechanical properties of SLM-fabricated materials. The HIP+ direct aging process was the most effective method among the post-processes to improve the creep behavior at 650 °C. The creep rupture life of the HIP+ direct aging condition approached 800 h since the HIP process had the benefit of being free of pores, thus preventing microcrack nucleation and the formation of a serrated grain boundary.

Plastic anisotropy of sheets with continuously varying anisotropic parameters and flow stress

A continuum mechanics model has been developed on the basis of Hills theory of orthogonal anisotropy for predicting global mechanical properties of sheets with a through-thickness texture gradient and strength gradient. By the present model, the globalr value and yield and flow stresses of the entire sheet can be predicted from the local anisotropic parameters, yield and flow stresses which are given as arbitrary functions of the through-thickness position of the sheet.