SCRIPTA Vol. 191, 15 Jan. 2021, P1-6

Due to virtually no solubility, He atoms implanted or created inside materials tend to form bubbles, which are known to damage material properties through embrittlement. Higher He density in nano-sized bubbles was observed both experimentally and computationally in Ni_(100−x)Fe_x-alloy samples compared to Ni. The bubbles in the Ni_(100−x)Fe_x-alloys were observed to be faceted, whereas in elemental Ni they were more spherical. Molecular dynamics simulations showed that stacking fault structures formed around bubbles at maximum He density. Higher Fe concentrations stabilize stacking fault structures, suppress evolution of dislocation network around bubbles and suppress complete dislocation emission, leading to higher He density.

SCRIPTA Vol. 191, 15 Jan. 2021, P7-11

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050°C and cooled at 5°C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

SCRIPTA Vol. 191, 15 Jan. 2021, P12-16

The interfacial temperature, microstructure and hardness distribution of the linear friction welded Ti-6Al-4V joints obtained at different applied pressures were investigated. The sound Ti-6Al-4V alloy joints were successfully fabricated below the β-phase transformation temperature. The increased pressure considerably decreased the welding temperature. The dynamically recrystallized ultra-fined α equiaxed grains of 0.2 μm were formed in the weld center of the joint by applying pressure of 550 MPa. Moreover, the hardness of the weld center was higher than that of the base material without the softened region in thermo-mechanical affected zone, which suggests the improved mechanical properties of the joints.

SCRIPTA Vol. 191, 15 Jan. 2021, P23-28

In this work, the nanometer-scale mass flow coupled to dislocation processes near the γ/γ′-interface during high temperature and low stress creep of a model Ni-base single crystal superalloy is investigated. In the early creep stages, the dislocation networks in the γ-phase at γ/γ′-interfaces attract γ-stabilizing elements like Cr, Co and in particular Re, resulting in compositional gradients close to the interface. At larger strains, where dislocations frequently cut into the γ′-phase, this local interfacial enrichment in these elements is no longer observed. The cutting dislocations take part of the segregated atoms away, whilst the remaining atoms are released and diffuse back into the γ-channels.

SCRIPTA Vol. 191, 15 Jan. 2021, P29-33

High entropy alloys (HEAs) are potential next-generation structural materials, yet accurate prediction of phase stability remains a challenge. We study two equimolar refractory high entropy alloys, NbMoTaTi–X (X = W, V). Ab initio calculations are performed to determine the vacancy exchange potential in both alloys. Results and experimental confirmation indicate that a zero/low vacancy exchange potential predicts phase instability in NbMoTaTiW, while elemental trends of the same predict segregation in single phase NbMoTaTiV. If these results hold true across other systems, vacancy exchange potential can serve as a rapid predictor of stability in the vast HEA compositional space.

SCRIPTA Vol. 191, 15 Jan. 2021, P46-51

A composite of two different medium entropy alloys (MEAs, i.e., CoCrFeNi and AlCoCrFe) was synthesized using ball milling and spark plasma sintering. The composite microstructure contains a homogenous distribution of fcc and bcc phases with submicron-sized grains and exhibits excellent microstructural and phase stability even after 100 h heat treatment at 800 °C. The composite provides a combination of high compressive strength, adequate plastic strain, and multiple strain-hardening stages at room temperature. This first exploratory study on a MEA composite can be used as a template to other systems and illustrates the feasibility of combining two or more MEAs.

SCRIPTA Vol. 191, 15 Jan. 2021, P62-66

Deformation twins in Mg–Gd alloys are investigated. It is found that the dominant twinning mode switches from {10-12} twinning to {11-21} twinning, with an increase in the Gd concentration in Mg. Our first-principles calculations suggest that the formation of the {11-21} twin in the Gd-rich alloy is triggered by the reduced {11-21} twin boundary energy, which results from strain relaxation along the twin boundary. Furthermore, this twinning mode is suggestive to be activated in other Mg alloys with a high concentration of solute atoms having larger size than Mg.

SCRIPTA Vol. 191, 15 Jan. 2021, P76-80

We report crystallographic orientation dependent fracture behavior in tantalum single crystals with the tensile axes oriented close to [100], [110] and [111] directions. Three tantalum single crystals were deformed in quasi-static, uniaxial tension and their fracture surfaces were characterized. To understand different deformation modes and failure mechanisms, crystal plasticity-finite element (CP-FE) simulations were performed. Both experiments and CP-FE simulations showed strong strain localization and shear banding in the ~[100] specimen, little rotation and profuse necking in the ~[110] specimen, and significant crystal rotations associated with shear-dominated behavior in the ~[111] single crystal. In addition, voids were observed in fracture surfaces of ~[100] and ~[111] single crystals while the ~[110] specimen was void-free. The failure processes of these single crystals showed that dislocation boundaries are necessary for void nucleation in pure tantalum. This work demonstrates strong effects of crystallographic orientations in failure behaviors.

SCRIPTA Vol. 191, 15 Jan. 2021, P81-85

Materials with the propensity to undergo irreversible deformation display an increasing resistance to progressive crack advance, i.e., stable crack growth. A one-parameter description of stable crack growth by a fracture toughness vs. crack advance (resistance) curve is possible under specific conditions. Here, experimental observations are reported to (i) demonstrate that the toughness enhancement due to crack advance in hysteretic materials may be “reversed” by partial unloading, which has implications on the evaluation of the resistance curve and its validity conditions, and (ii) provide an experimental verification of the theoretical insight into the mechanics of stable crack growth.

SCRIPTA Vol. 191, 15 Jan. 2021, P86-89

In addition to tuning the composition, precipitate, and grain microstructure of an alloy, manipulating dislocation structure provides an additional strategy of designing novel alloys. One promising dislocation structure, in the form of periodic dislocation arrays, conventionally forms in cubic alloys under cyclic loading. In this paper, we demonstrate new possibility of synthesizing periodic dislocation arrays in pure Mg under static loading, a simple processing route. This finding provides insight in advancing dislocation substructures, which promote the performance of engineering alloys.

SCRIPTA Vol. 191, 15 Jan. 2021, P96-100

The metastability of a typical non-equiatomic FeMnCoCr high-entropy alloy (HEA) system was demonstrated to be dormant upon co-doping with C and N interstitials, i.e., displacive γ-ε phase transformation disappears during tensile deformation. Interestingly, we found in the present study that the displacive phase transformation prevails again in the C-N co-doped HEA upon severe deformation via cold-rolling. The generation of ε-phase lamellae is found to be motivated by strain localization which facilitates the formation of shear bands. Such highly concentrated strains significantly intensify the local elastic fields to neighboring γ matrix and finally eke out the driving force to overcome the interstitials-enhanced energy barrier of ε-martensite transformation. The results suggest that the suppressed phase metastability is awakened, which inspires efforts to tune the phase metastability via modifying the strain state during alloy processing and gives new insights into the development of HEAs with desirable phase stability and mechanical properties.

SCRIPTA Vol. 191, 15 Jan. 2021, P107-110

The contribution of kinematic hardening to the overall strain hardening of a high entropy alloy (HEA) was investigated for the first time ever. It was assessed for a single-phase equiatomic CoCrFeMnNi alloy based on a study of the Bauschinger effect. The observed occurrence of high back stresses signifies substantial kinematic hardening. We attribute it to the low probability of cross-slip in this HEA which, however, rises in the process of straining. A model that captures the mentioned effects was proposed and validated.