No abstract

No abstract

In situ neutron diffraction experiments were conducted to monitor elastic microstrains during tension and creep of a directionally solidified (DS) superalloy PWA1422 and a single crystal (SX) alloy PWA 1484. The rationale was to obtain insight on internal stress evolution, and thereby improved understanding of deformation mechanisms. The misfit between the gamma (γ) and gamma-prime (γ') phases were determined as a function of temperature and deformation. The microstrains in the tension test were used to obtain the critical resolved shear stress (CRSS) of the phases. Results show a dramatic increase in the CRSS of the γ-phase compared to the bulk, and are consistent with dislocations bowing (Orowan type) between narrow γ-channels. A threshold stress is calculated for dislocation activity in the horizontal channel, and is suggested as a minimum stress for initiating any creep or rafting. Microstrains from the creep test at 900°C/425 MPa are consistent with the formation of dislocation networks, but the data do not suggest that the CRSS is exceeded for cutting of the γ'. Rather, rafting is suggested for the onset of tertiary creep. At 1092°C/120 MPa, the far-field creep strain exhibits an unusual behavior in the pre-steady-state low strain (<0.5%) regime. This is also accompanied with creep acceleration and decrease of elastic strain in γ'. The response is accompanied with the formation of a completely rafted microstructure. An explanation is provided in terms of deposition of actual misfit dislocations preferentially on the vertical interfaces.

The aspect ratio has a significant effect on buckling instability. In this study, we used molecular dynamics (MD) simulation to investigate the buckling instability mechanism of NiCoCr medium-entropy alloy (MEA) nanotubes under the compression process. Based on the compression results, we find that the buckling point and the critical buckling stress decrease with increasing aspect ratio. The critical buckling stress of the MD simulations resembles the Euler and Rayleigh-Ritz theories. The critical aspect ratio of 10 showed instability at NiCoCr nanotubes. In addition, we found that the compressive strength of single-crystalline specimens is much higher than that of polycrystalline specimens. Partial dislocations (PDs) dominated by stacking faults (SFs) and twinning structures are mainly distributed on the grain boundaries of small grain specimens and decrease with increasing grain size. We also found that polycrystalline structures tend to induce buckling instability. The wall thickness specimen of the 7 nm has confirmed similar compressive strength as the nanowire. The compressive strength decreases as the wall thickness decreases. More stress concentration and PDs distributed in the specimen when the wall thickness is reduced to 1 nm, and it is easier to induce buckling instability at NiCoCr MEA nanotubes.

No abstract