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SCRIPTA Vol. 190, Jan. 2021, P7-11

We present here a quantitative study of dislocation cross-slip, an essential thermally activated process in deformation of metals, in discrete dislocation dynamics (DDD) simulations. We implemented a stress-dependent line-tension model in DDD simulations, with minimal information from molecular dynamics (MD) simulations. This model allows reproducing in DDD simulations the probabilistic cross-slip rate calculated in MD simulations for Cu in a large range of stresses and temperatures. The implementation of an atomically-scale accurate cross-slip model allows simulating more accurately phenomena such as deformation softening, dislocation-precipitate interaction and dislocation patterning in DDD simulations.

SCRIPTA Vol. 190, Jan. 2021, P16-21

To recover the reduced ductility of cold-worked medium-Mn steels, a novel resetting process is proposed in this study. Through a simple heat treatment, the original microstructure of the steel is restored by the re-reversion of the Mn-enriched strain-induced martensite. The ductility of the reset steel is successfully recovered, and its strength is improved, relative to the steel prior to cold working. The present technique facilitates the use of a single sheet metal and a simple heat treatment process for an automotive component which originally comprised two different parts to compensate for the opposing attributes that it requires (e.g., having a high strength at the upper part and high toughness at the bottom part of a center pillar).

SCRIPTA Vol. 190, Jan. 2021, P27-31

The effects of initial twinned microstructure on the grain growth behavior of four sputtered nanotwinned copper alloys were compared. As-deposited films were characterized by transmission electron microscopy and the microstructural evolution of the annealed samples was investigated through electron backscatter diffraction. Texture showed a pronounced effect, where films with strong {111} textures exhibited abnormal grain growth that was not observed in a randomly textured film. Additionally, the stability induced by twin boundaries appears to be limited, as the excess energy generated by a large twin density could drive the early onset of abnormal grain growth and recrystallization.

SCRIPTA Vol. 190, Jan. 2021, P32-37

The effect of Lüders banding on hydrogen embrittlement (HE) susceptibility of medium Mn steels was investigated by tuning the degree of yield point elongation (YPE). It was found that the steel with a larger YPE was more susceptible to HE. A larger YPE led to a higher local flow stress and a higher fraction of strain-induced martensite, which in turns resulted in more H-induced cracks in the Lüders banding area and the consequent catastrophic failure. It thus suggests that Lüders banding and the associated localized deformation play a key role in influencing the overall HE susceptibility of medium Mn steels.

SCRIPTA Vol. 190, Jan. 2021, P40-45

The present work developed a new body-centered-cubic (BCC)-based Al_0.4Nb_0.5Ta_0.5TiZr_0.8refractory high entropy alloy with coherent precipitation. The formation of cuboidal nanoprecipitates with a size of 25~50 nm in an 873 K-aged alloy is ascribed to a moderate lattice misfit. A Zr₅Al₃ phase also coexists with B2 in a same particle shape due to similar chemical compositions, in which Ti is enriched in B2 but not in Zr₅Al₃. The BCC/B2 coherency would be deteriorated with increasing the aging temperature, leaving coarse Zr₅Al₃ alone. Besides, there exist two kinds of BCC phases due to the segregations of Nb/Ta and Ti/Zr.

SCRIPTA Vol. 190, Jan. 2021, P46-51

In this study, we developed an approach to additive manufacturing of three-dimensional (3D)-architected CoCrFeNiMn high-entropy alloys by direct ink writing combined with thermal sintering. The 3D-architected CoCrFeNiMn lattices exhibit the outstanding energy absorption capacity that expands the envelope of existing architected materials. Such a high-energy absorption ability originates from the bend-dominated deformation mode of the 3D-architecture as well as the fully-annealed homogeneous microstructure of equiaxed grains, which collectively lead to remarkable strain hardening upon deformation. Our study provides a new avenue for 3D-printing advanced architected materials with extreme mechanical energy absorption for a myriad of structural applications.

SCRIPTA Vol. 190, Jan. 2021, P63-68

Dislocation nucleation and interface sliding are two dominant plasticity events governing the mechanical behavior of metallic nanocomposites. Recent works have shown that both events can be closely related to atomistic interface geometries, however, how compositional factors, e.g., segregated hydrogen clusters, contribute both events are nearly unknown. Herein, we demonstrate that hydrogen clusters near misfit dislocation nodes can strongly suppress dislocation nucleation and interface sliding, while clusters at other positions will contribute somehow weaker effect on dislocation nucleation but facilitate interface sliding. These findings offer a rational atomistic mechanism for the effect of hydrogen clusters on interface facilitated plasticity.

SCRIPTA Vol. 190, Jan. 2021, P69-74

The strength-ductility trade-off dilemma is perennially problematic in the materials science community. In particular, the attainability of high tensile strength and large elongation is ambitious in alloys fabricated by powder metallurgy. Here, we demonstrate a powder-metallurgy-based fabrication route to achieve a high synergy of tensile strength and ductility through cold-consolidation of CoCrFeMnNi high-entropy alloy powder using high-pressure torsion followed by annealing. This approach has resulted in an exceptional synergy of high yield strength of 754 MPa with an ultra-high tensile elongation of 58% which has never been achieved in alloys fabricated by powder metallurgy routes. Additionally, the microstructure can be tuned by annealing treatment to achieve a range of strength and ductility that are highly sought after in industries for a specific application. The present fabrication route can be applied for fabrication of high-entropy alloy-matrix composites using alloys, metals, and ceramic powders to achieve controllable microstructure and eminent tensile properties.

SCRIPTA Vol. 190, Jan. 2021, P80-85

The effect of elemental segregation on local hardness and microstructural evolution introduced by high strain-rate deformation in a CrMnFeCoNi high entropy alloy was investigated. Mn and Ni elemental segregation to interdendritic boundaries occurs during the solidification process and is intensified by dynamic deformation. Local hardness is reduced in the Mn and Ni enriched areas, which may be due to the increase of local stacking fault energy that changes the deformation mechanisms. This leads to the alternation of soft and hard regions and the structure is effective in hindering the propagation of adiabatic shear bands that would toughen the material.

SCRIPTA Vol. 190, Jan. 2021, P97-102

An Al-Si-Cu-based cast alloy was modified with transition metal elements Zr, V and Ti to achieve enhanced mechanical performance at elevated temperatures. Multiple microstructural characterization techniques were used to investigate the solidification segregation behavior of these transition metal elements and their partitioning to the primary intermetallic phases and secondary precipitates. Three transition metal elements were shown to segregate into the dendrite center to varying degrees. Two primary intermetallic phases enriched with different amounts of transition metal elements were identified as D0₂₂ and D0₂₃ phases. Zr was observed to concentrate much more than V and Ti in the nano-size secondary L1₂ precipitates, which formed in the matrix during post-solidification heat treatment.

SCRIPTA Vol. 190, Jan. 2021, P108-112

The effect of hydrogen on the twinning evolution in a twinning-induced plasticity (TWIP) austenitic steel was investigated at different strain levels. The presence of hydrogen decreased the twin thickness and increased the twin density in individual twin bundles, in comparison with the hydrogen-free condition. This mechanism, reported for the first time, was termed hydrogen-enhanced densified twinning (HEDT). Based on these experimental observations, the effect of hydrogen on twinning was analyzed from the three stages of nucleation, growth and stability. HEDT may elevate the local stress, and thus significantly affect the hydrogen embrittlement (HE) of TWIP steels.

SCRIPTA Vol. 190, Jan. 2021, P113-117

We investigate the onset of detwinning in magnesium alloy Mg-3Al-1Zn under continuous loading with real-time in situ synchrotron X-ray diffraction. Detwinning of the {101-2} extension twins activated both by tension parallel to and compression perpendicular to the c-axis fibers is explored. The experimental results reveal that detwinning of the {101-2} extension twins occurs immediately upon unloading, regardless of whether the twins are activated by tension parallel to or compression perpendicular to the c-axis fibers.

SCRIPTA Vol. 190, Jan. 2021, P121-125

A dislocation network which formed during selective laser melting (SLM) of a Ni-base superalloy was analyzed using scanning transmission electron microscopy (STEM). This network traverses an ordered Gamma'-phase domain, in between two adjacent Gamma-solid solution regions. The Gamma’-phase region has formed when two Gamma’-phase particles have started to coalesce, trapping the dislocation network in this ordered region so that it formed two dislocation families with pairs of anti-phase boundary (APB) coupled super partial dislocations. The network features are presented and unusual features (twist character and low APB energies), not previously reported, are discussed.

SCRIPTA Vol. 190, Jan. 2021, P131-135

Stress reduction creep tests conducted on a Cu-Cr-Nb alloy (GRCop-84) at 923 K have confirmed local dislocation climb to be the rate-controlling deformation mechanism. The activation energy of creep was measured to be consistent with that of self-diffusion in Cu matrix. An internal back stress of approximately -9 MPa was identified to act on the rate-controlling dislocations, which is believed to be the sum of the back stress for dislocation-particle interaction and the forward stress for dislocation-dislocation interaction. This back stress, however, does not lead to a true threshold in the framework of thermally activated dislocation motion.

SCRIPTA Vol. 190, Jan. 2021, P141-146

Al alloys, despite their excellent strength-to-weight ratio, cannot be used at elevated temperatures because of microstructural instability owing to grain growth and precipitate coarsening, thus, leading to a drastic loss in their strength. In this work, we have attempted to address the issue of grain growth by introducing in-situ formed polymer derived ceramics in an Al-Mg alloy. A stable grain structure with minimal loss in hardness when exposed to 450°C and 550°C for 1 hour was obtained due to the particle pinning of the grain boundaries by the Zener mechanism.

SCRIPTA Vol. 190, Jan. 2021, P158-162

We report a map of irradiation-induced recrystallization and polygonization in a CrFeCoNiCu high-entropy alloy obtained by a precession electron diffraction technique. This orientation map enables the visualization of depth-dependent recrystallization at a glance and reveals the effects of irradiation dose and material entropy. Discontinuous dynamic recrystallization preferentially occurs near the surface, where the irradiation dose is low, but radiation-enhanced diffusion dominantly occurs leading to dislocation climb even at room temperature. Interestingly, in the high-entropy phase with a relatively low stacking fault energy, discontinuous dynamic recrystallizastion is suppressed due to the lower stored energy and grain boundary diffusivity compared low-entropy one.

SCRIPTA Vol. 190, Jan. 2021, P179-182

During quenching of aluminium alloys from the solutionising temperature vacancies are partially conserved as excess vacancies, partially lost to vacancy sinks, the exact fractions depending on the cooling rate. Positron lifetime measurements in samples from interrupted quenching experiments reveal that vacancies are lost during cooling down to 200 °C, after which solute atoms start to form clusters down to 20 °C. Slow cooling leads to 1 to 2 orders of magnitude lower excess vacancies than fast cooling. Since quenched-in vacancies are crucial for natural ageing (NA) in Al-Mg-Si alloys it is surprising to find just small differences between the NA hardening kinetics after different quenches. Specifically, hardening rates differ only in the initial stage (<100 min), after which they are almost identical for NA up to ~1 year. This suggests that interactions between vacancies and early-stage solute clusters help equalising the free vacancy fractions in differently quenched samples.