Vol. 207 目录

SCRIPTA

This paper designs a new Co-rich and Ni-depleted Co_xCr₃₃Ni_67-x medium-entropy alloy. Among them, the content of Cr is controlled, so that the alloys studied have the same short range-order (SRO), and make the changes of Co and Ni content directly and efficiently reflected in the reduction of stacking fault energy. With the increase of Co content and decrease of Ni, the stacking fault energy of Co38Cr33Ni29 alloy is reduced to ∼12 mJ/m², and the deformation mode is changed from twinning induced plasticity (TWIP) to transformation induced plasticity (TRIP). At the same time, the tensile strength of 839 MPa and uniform elongation of 83% are obtained.

SCRIPTA

The pre-deformation effects of hydrogen embrittlement on nitrogen-doped duplex stainless steel were investigated using tensile testing. Hydrogen pre-charging and pre-straining were adopted, which increased the yield strength, but decreased the ductility. Silver decoration revealed that diffusible hydrogen was preferentially located in the ferrite or grain boundaries of the undeformed steel, whereas the 22% pre-strained steel had diffusible hydrogen in austenite. When deformed, the preferential plastic deformation path of the steel was in the ferrite; hence, the apparent diffusivity in ferrite decreased, while the austenite had a relatively high apparent hydrogen flux. Pre-straining suppressed macroscopic hydrogen diffusion and local diffusion in ferrite and austenite by increasing the dislocation density. The degree of the suppression of the local diffusion was more significant in ferrite than austenite. The changes in dislocation and hydrogen behaviors associated with pre-straining altered the cracking sites in the austenite and ferrite, and decreased hydrogen embrittlement susceptibility.

SCRIPTA

This study reports grain boundary (GB) energy calculations for 46 symmetric-tilt GBs in α-iron using molecular mechanics based on an artificial neural network (ANN) potential and compares the results with calculations based on the density functional theory (DFT), the embedded atom method (EAM), and the modified EAM (MEAM). The results by the ANN potential are in excellent agreement with those of the DFT (5% on average), while the EAM and MEAM significantly differ from the DFT results (about 27% on average). In a uniaxial tensile calculation of ∑3(11¯2) GB, the ANN potential reproduced the brittle fracture tendency of the GB observed in the DFT while the EAM and MEAM mistakenly showed ductile behaviors. These results demonstrate the effectiveness of the ANN potential in calculating grain boundaries of iron, which is in high demand in modern industry.

SCRIPTA

Multiple deformation mechanisms in high entropy alloy have been a notable method to overcome strength-ductility trade-off dilemma, but they usually occur in some severe conditions such as cryogenic temperature. Here, we triggered the multiple deformation mechanisms in room temperature tensile test by introducing “pre-twins” in CoCrFeNi high entropy alloy via cryogenic deformation and annealing. The deformation microstructures at strains of 1%, 5% and 15% were investigated by electron back scattering diffraction and transmission electron microscopy. The results demonstrated that at the strain of 5% or more, besides dislocation slip, microbanding and deformation twinning also occured due to the high flow stress facilitated by the pre-twins, which increased the working hardening ability, leading to a simultaneous increase of strength and ductility. The deformation process and the underlying mechanisms of the microbanding and twinning have been discussed in comparison with the CoCrFeNi high entropy alloy sample without twins.

SCRIPTA

Low temperature (1023 K) high stress (800 MPa) tensile creep behavior of the superalloy single crystal ERBO-1 (CMSX-4 type) is investigated. Three loading directions are compared: precise [001] and 15 ° deviations from [001] towards [111] and [011]. It is found that creep rates ε˙ scale as ε˙_{[001]→[111]}>ε˙_[001]>ε˙_{[001]→[011]} already in the early stages of creep (ε≤1%), where dislocation network formation and planar fault intersections cannot rationalize the observed rate effects. An analysis based on Peach-Köhler force calculations suggests, that fast creep rates are observed, when dislocations from two octahedral systems, which are required to react and form the leading part of a planar fault ribbon in the γ’-phase, experience similar driving forces. Creep data, micromechanical calculations and TEM results are in good qualitative agreement. From a technological point of view, the results show that while 15 ° deviations from [001] towards [011] can be tolerated, deviations towards [111] must be avoided.

Vol. 207, 15 Jan. 2022, 114284

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