SCRIPTA Vol. 191, 15 Jan. 2021, P126-130

Atom probe tomography was used to characterise two low-Cu (< 0.04 at. %) model steels after exposure to long-term thermal ageing. Mn-Ni-Si-rich features were observed to form after as little as 20,731 h (∼ 2.4 years) of ageing. The composition of these features were compared to those predicted by thermodynamic models and the similarities and differences are discussed.

SCRIPTA Vol. 191, 15 Jan. 2021, P131-136

Poor high/medium-temperature phase stability and/or low room-temperature ductility are currently bottlenecks of refractory high-entropy alloys (RHEAs) that restrict their high-temperature applications. Here, we explored novel VxNbMoTa RHEAs with vanadium concentrations of 0–25 at.%. VxNbMoTa RHEAs exhibit a single body-centered cubic (BCC) structure with unprecedented phase stability at a wide temperature range from solidus down to 350°C. The grain structure of VxNbMoTa can be substantially refined when increasing the V concentration since higher V contents induce stronger solutal effects which contribute to higher growth restriction factors. Equimolar VNbMoTa exhibits the yield strength of 811 MPa at 1000°C that is superior to most RHEAs reported by far, and this alloy also shows excellent room-temperature ductility with the fracture strain > 25% and no strain-softening at high temperature, which is rarely seen in many popular RHEAs. These exceptional performances of VNbMoTa enable it to be a very promising material for high-temperature applications.

SCRIPTA Vol. 191, 15 Jan. 2021, P137-142

The influence of high magnetic field (HMF) during aging on the precipitation behavior of co-precipitates consisted of Cu-rich precipitates and Ni(AlMn) precipitates in multicomponent nano-precipitated steels was carefully investigated. It was indicated that HMF promoted the construction of hierarchical structures of Cu-rich and Ni(AlMn) co-precipitates. It was thought that HMF increased nucleation rates of secondary co-precipitates during aging because of the thermodynamics change, thus refining the secondary precipitates. Meanwhile, HMF increased the Ostwald ripening rate of primary particles during the further aging after tempering because of increasing diffusion of Cu, thus accelerating the coarsening process.

SCRIPTA Vol. 191, 15 Jan. 2021, P155-160

Grain refinement of additively manufactured titanium and titanium alloys can be promoted via adding foreign elements or particles, but it may lead to a reduction in ductility due to the formation of brittle intermetallic compounds. The present study shows that in-situ grain refinement of commercially pure titanium (CP-Ti) can be achieved through properly controlling the selective laser melting (SLM) parameters. It was found that higher input energy density worked in favour of grain refinement. Detailed microstructural characterization coupled with multi-physics simulation were performed to reveal the grain refinement mechanism. This was attributed to the intrinsic heat treatment (IHT) effect which resulted from the cyclic reheating inherent to the SLM process. As a result, the refined CP-Ti exhibited an exceptionally high ductility of 34.3 ± 0.5% without notable mechanical anisotropy. This work demonstrates the feasibility of utilizing thermal cycling of additive manufacturing (AM) to refine grains of metals without changing the composition.

SCRIPTA Vol. 191, Jan. 2021, P29-33

This work depicts how Nb addition to Ni-Ti-Hf alloys increases their ductility without significantly deviating functional performance due to the formation of eutectic constituents composed of soft β-Nb and B19’ Ni-Ti-Hf matrix phase. Crucial for designing high performing Ni-Ti-Hf-Nb alloys is considering consequences of the low solubility of Nb in the matrix and the varying solubility of elements in β-Nb on the transformation temperatures. A case study on a drawn Ni-Ti-Hf-Nb wire demonstrates that improved mechanical properties and high transformation temperatures can be achieved by this approach, increasing the possibility for wire drawing of high temperature shape memory alloys.

SCRIPTA Vol. 191, 15 Jan. 2021, P173-178

We report that in contrast to the anticipation, the macroscopic mechanical strength of single crystals of the face-centered-cubic single-phase equiatomic Cr-Mn-Fe-Co-Ni high entropy alloy is not influenced by thermal annealing during which short-range ordering is expected to develop. Experimental evidence shows that hardness at room temperature, critical resolved shear stress at room temperature and at 77 K and activation volume values of the alloys do not significantly vary with the annealing temperature in the range from 900 to 1200°C. The stacking fault energies in the specimens deformed at 77 K following annealing at 1000 and 1200°C respectively are essentially identical to each other. These results clearly indicate that the impact of the formation of chemical short-range order on the macroscopic yield strength of the equiatomic Cr-Mn-Fe-Co-Ni high entropy alloy is insignificant, even if it exists.

SCRIPTA Vol. 191, 15 Jan. 2021, P185-190

Deformation-induced grain-growth in nanocrystalline materials is a widely-reported phenomenon that has been attributed to grain boundary (GB) processes. In this paper, we report on the opposite phenomenon, wherein a stable nanocrystalline (NC) Cu-Ta alloy undergoes a further refinement of the nano-grains during severe plastic deformation (SPD). SPD up to 250% results in a significant grain-size reduction despite the 350°C increase in temperature caused by the deformation process. Experiments and atomistic-simulations show that this unexpected grain-refinement is a direct result of well-dispersed Ta-nanoclusters throughout grain centers and along GBs acting as kinetic-pinning agents and suppressing GB processes that occur during recrystallization.

SCRIPTA Vol. 191, 15 Jan. 2021, P191-195

This paper presents a new model and open-source algorithm for reconstructing prior β phase orientation and grain shapes from measured α-phase electron backscatter diffraction data in α/β titanium alloys. It is based on the image segmentation model of Mumford and Shah, which includes a regularization term that helps create smooth boundaries and overcomes shortcomings of prior reconstruction techniques. Additionally, the new algorithm is resilient to noise. Our algorithm’s effectiveness is demonstrated on simulated and real world data.

SCRIPTA Vol. 191, 15 Jan. 2021, P196-201

Harmonic structure (HS), consisting of coarse-grained (CG) areas uniformly embedded in three dimensional continuously connected ultrafine-grained (UFG) areas, is considered as an effective microstructural design strategy to achieve enhanced strength and ductility in metallic materials. In the present study, HS designed non-equiatomic FeMnCoCr high-entropy alloy samples with tunable shell fractions (ranging from ~16% to ~70%) were successfully prepared via controlled mechanical milling and subsequent sintering. Microstructure observations suggested that the shell region was composed of fully recrystallized UFGs with a mean grain size below 1 µm. Tensile test revealed that the HS designed samples exhibited simultaneously enhanced strength and strain hardening capability than those of the homogeneous structured counterpart. Particularly, the ultimate tensile strength and uniform elongation of the sample with a shell fraction of ~70% were 1228MPa and 12.4%, respectively, demonstrating superior strength-ductility synergy. The underlying mechanisms responsible for the enhanced mechanical properties were discussed.

SCRIPTA Vol. 191, 15 Jan. 2021, P215-218

We have investigated the surface activated bonding (SAB) of deposited Al₂O₃ films by chemical vapor deposition under a short-time activated condition at room temperature. Although the surface energy for bonding of Al₂O₃ films was very low, that of Al₂O₃ film/sapphire bonding was approximately 1 J m^-2 and more than 2 J m^-2 for sapphire/sapphire bonding. Transmission electron microscopy showed an amorphous-like intermediate layer approximately 1 nm thick, observed at the bonding interface of Al₂O₃/Al₂O₃, but not in the bonding of Al₂O₃/sapphire, which suggests that the crystallinity of the Al₂O₃ film affects the bonding of Al₂O₃.

SCRIPTA Vol. 191, 15 Jan. 2021, P219-224

Interfacial segregation has been reported to play a critical role in the thermal-mechanical stability of nanocrystalline Mg alloys. Here we report Ag-segregation-assisted formation of nanocrystalline Mg-Ag alloy with high proportion of sub-grain boundaries during conventional rolling. The segregation structure is determined by dislocation configurations and subsequent strain field and misorientation of the sub-grain boundary. It indicates that the alloying elements, which induce <c+a> dislocations, would help to improve the stability of interfaces.