A review on fundamental of high entropy alloys with promising high–temperature properties

Fundamental factors on formation mechanism of dislocation arrangements in cyclically deformed fcc single crystals

http://publish.illinois.edu/mechse-html/files/imported/sangid_maier_sehitogliu_ijp.20110408.4d9f59ff0c48a8.29327594.pdf

The role of grain boundaries on fatigue crack initiation - An energy approach

The discovery of the γ′-Co3(Al,W) phase with an L12 structure provided Co-base alloys with a new strengthening mechanism, enabling a new class of high-temperature material: Co-base superalloys. This review discusses the current understanding of the phase stability, deformation, and oxidation behaviors of γ′ single-phase and γ + γ′ two-phase alloys in comparison with Ni-base γ′-L12 phase and γ + γ′ superalloys. Relatively low stacking fault energies and phase stability of the γ′ phase compared with those in Ni-base alloys are responsible for the unique deformation behaviors observed in Co-base γ′ and γ + γ′ alloys. Controlling energies of planar defects, such as stacking faults and antiphase boundaries, by alloying is critical for alloy development. Experimental and density functional theory studies indicate that additions of Ta, Ti, Nb, Hf, and Ni are effective in simultaneously increasing the phase stability and stacking fault energy of γ′-Co3(Al,W), thus improving the high-temperature strength of Co-bas...

L12-Strengthened Cobalt-Base Superalloys

Nearly 90 per cent of service failures of metallic components and structures are caused by fatigue at cyclic stress amplitudes much lower than the tensile strength of the materials involved. Metals typically suffer from large amounts of cumulative, irreversible damage to microstructure during cyclic deformation, leading to cyclic responses that are unstable (hardening or softening) and history-dependent. Existing rules for fatigue life prediction, such as the linear cumulative damage rule, cannot account for the effect of loading history, and engineering components are often loaded by complex cyclic stresses with variable amplitudes, mean values and frequencies, such as aircraft wings in turbulent air. It is therefore usually extremely challenging to predict cyclic behaviour and fatigue life under a realistic load spectrum. Here, through both atomistic simulations and variable-strain-amplitude cyclic loading experiments at stress amplitudes lower than the tensile strength of the metal, we report a history-independent and stable cyclic response in bulk copper samples that contain highly oriented nanoscale twins. We demonstrate that this unusual cyclic behaviour is governed by a type of correlated 'necklace' dislocation consisting of multiple short component dislocations in adjacent twins, connected like the links of a necklace. Such dislocations are formed in the highly oriented nanotwinned structure under cyclic loading and help to maintain the stability of twin boundaries and the reversible damage, provided that the nanotwins are tilted within about 15 degrees of the loading axis. This cyclic deformation mechanism is distinct from the conventional strain localizing mechanisms associated with irreversible microstructural damage in single-crystal, coarse-grained, ultrafine-grained and nanograined metals.

P. Li

Papers

Fundamental factors on formation mechanism of dislocation arrangements in cyclically deformed fcc single crystals

Comparison of low-cycle fatigue behaviors between two nickel-based single-crystal superalloys

Unified factor controlling the dislocation evolution of fatigued face-centered cubic crystals

Effect of orientations on cyclic deformation behavior of Ag and Cu single crystals: Cyclic stress–strain curve and slip morphology

Effect of orientation on the cyclic deformation behavior of silver single crystals: Comparison with the behavior of copper and nickel single crystals