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SCRIPTA Vol. 192, Feb. 2021, P1-6

Twin boundaries have enabled unique mechanical and physical properties in metallic materials. However, growth twins are rarely observed in Al due to its high stacking fault energy. In this study, we report on the synthesis of high-density growth twins in sputtered Al/Mg multilayer films. The formation mechanisms of incoherent and coherent twin boundaries are discussed. Besides, the incoherent twin boundary related structure, 9R phase, was also identified. This study highlights an effective method to elevate the twin /9R phase density in metals with high stacking fault energy.

SCRIPTA Vol. 192, Feb. 2021, P13-18

The fast development of various additive manufacturing processes is motivating research to improve microstructure control during laser assisted treatments, in order to spatially vary material properties on engineering components. In the present work, we demonstrate that a dual-pass laser treatment applied to a low carbon lath martensitic steel can simultaneously repair solidification or fatigue cracks, reduce surface roughness, and, most importantly, trigger phase transformations to create a more damage-resistant microstructure, namely, one that includes interlath austenite. We investigate these influences employing various scanning electron microscopy and transmission high energy X-ray diffraction analyses, mechanical characterization tests, and discuss the underlying mechanisms.

SCRIPTA Vol. 192, Feb. 2021, P19-25

A ferrite layer is formed between the outer intermetallics layers and the martensite substrate during the hot stamping process of Al-Si coated press-hardened steel (PHS). Unexpectedly, it is found here that the ferrite layer does not effectively prevent brittle cracks propagating continuously from the intermetallics to the martensite substrate. This leads to a high stress intensity factor (SIF) at the crack tip, therefore initiating highly localized shear deformation in the martensite substrate, degrading the bendability of the steel. A thinner Al-Si coating produces thinner brittle intermetallics and ferrite layers, and therefore shorter coating cracks and smaller SIF at the crack tip, thus improving the bendability. In addition, a thinner Al-Si coating also has a lower material cost while keeping similar paintability and corrosion resistance. The thin Al-Si coating could potentially change the current practice of Al-Si coating, impacting the global automotive industry.

SCRIPTA Vol. 192, Feb. 2021, P26-31

Fe-Mn-Al-Ni based shape memory alloys are regarded suitable for large scale applications due to relatively low materials costs and a good cold workability. The austenitic state consists of an A2 structured matrix with coherent B2 precipitates which are required to ensure thermoelasticity, supposedly due to a strengthening effect on the matrix. The martensitic state is based on the A1 structure. For a series of alloys, the martensite start temperatures were determined to be below ambient temperature, whereas for the same alloy compositions T0 temperatures of about 1000 °C are predicted for the transformation of the A2 matrix into A1 by means of CalPhaD-based calculations. It is shown by thermodynamic considerations that the undercooling of about 1000 K is caused by the resistance of the B2 precipitates (~NiAl with some content of Fe and Mn) to undergo the transformation to the L10 structure enforced by the martensitic transformation of the matrix.

SCRIPTA Vol. 192, Feb. 2021, P37-42

Elastic micromechanical fields are tracked for a nickel-based superalloy polycrystal via high energy X-ray diffraction microscopy (HEDM) and corresponding intragranular deformation metrics are determined with peak broadening analysis during cyclic high temperature loading. A low solvus, high refractory (LSHR) sample with 252 grains was subjected to uniaxial tension and held under fixed displacement while thermally cycling between 460 °C and 770 °C with intermittent HEDM characterization to study the grain average response to thermo-mechanical deformation. Elevated temperatures were found to allow for heterogeneous amounts of recovery amongst distinct grains within the sampled region, indicating complex grain interactions; the extent of recovery was greater at higher temperatures.

SCRIPTA Vol. 192, Feb. 2021, P43-48

Effects of solutes on saturation grain size and mechanical properties are investigated for the Cantor alloy and Ni-enriched variations ((CrMnFeCo)_xNi_1-x) with x=0.8, 0.4, 0.08 and 0. Indentation on coarse-grained and severely deformed states shows increasing hardness with increasing alloying content due to higher solid solution strengthening and Hall-Petch contributions. Nanoindentation strain rate jump tests reveal similar rate sensitivities of the deformed states without pronounced transient regimes. All compositions exhibit a history dependent softening indicating an unstable microstructure. The saturation grain size d_s after HPT deformation inversely correlates with the solid solution strengthening contribution, i.e. the higher Δτ the lower d_s.

SCRIPTA Vol. 192, Feb. 2021, P55-60

A β-solidifying TiAl alloy with a refined microstructure and ultra-fine yttrium-rich precipitates that was consolidated by spark plasma sintering was investigated by scanning and transmission electron microscopies. Two categories of yttrium-rich precipitates, Y₂O₃ and YAl₂, with sizes ranging from tens to hundreds of nanometers were found to be dispersed within the matrix uniformly. The crystallographic relationship between the two precipitates and the γ-TiAl matrix was determined by the selected area electron diffraction and high-resolution transmission electron microscopy. This spark plasma sintered TiAl alloy was characterized by a uniform duplex microstructure, and it exhibited excellent tensile properties at room temperature. Typically, specimen sintered at 1150°C has an ultimate tensile strength of 1113.1 MPa, yield strength of 881MPa, and a moderate plastic elongation of 1.41%.

SCRIPTA Vol. 192, Feb. 2021, P61-66

Molecular dynamics (MD) simulations are performed to investigate the interaction of a migrating grain boundary (GB) with interstitial Frank loops, using a set of GBs along the [100] tilt axis in Cu as model systems. Our results show that the interaction is directly related to the misorientation angle. Specifically, three modes of interaction are observed as a function of misorientation angle. The sink efficiency of a GB, namely, the capability of a GB to absorb the Frank loops, increases with misorientation angle. The above trends are explained based on the differences in the structural units comprising the GB. Additionally, the transition angle between different modes can be altered by varying the local GB structure.

SCRIPTA Vol. 192, Feb. 2021, P78-82

The microstructural origins of highly localized elastic strain concentrations in polycrystalline microstructures under monotonic loading are studied using grain-scale, in situ digital image correlation and crystal plasticity finite element method. It is shown that the locations of exceptionally high elastic strain concentrations in the microstructure depend on particular crystallographic and morphological orientations of grains and less so on crystalline details of their local neighborhood. Based on these results, we discuss how topological and crystallographic features of annealing twin boundaries can increase the likelihood of slip band initiation throughout the microstructure of polycrystalline Ni-base superalloys.

SCRIPTA Vol. 192, Feb. 2021, P83-88

Twinning nucleation in a medium Mn austenitic steel containing precipitates is studied at low strain ~ 0.02, using transmission electron microscopy. In the near vicinity of the precipitates, unfaulting dislocation reaction is favored, while twinning is activated farther from the precipitates. Twin nucleation follows a hybrid mechanism, involving creation of stacking faults through classical dislocation dissociation, while those stacking faults subsequently overlapped following a non-classical alternated stacking fault pair mechanism to create a three-layer twin nucleus. The observed twinning behavior is interpreted from an energy barrier variation viewpoint within the matrix.

SCRIPTA Vol. 192, Feb. 2021, P106-110

Static strain aging behavior of cold-rolled 16Cr-5Ni supermartensitic stainless steel was investigated after it had been aged for 30 min at 400°C. In uniaxial tensile tests of 20% cold-rolled specimens, increase of yield strength to ~300 MPa and decrease of strain hardening rate were observed in the aged specimen, compared to the as-rolled specimen. In interrupted tensile tests, delayed martensitic transformation was observed in the aged specimen. Dilatometry analysis detected volume shrinkage during the 30 min holding period at 400°C indicating partitioning of carbon (C) from α’ martensite to austenite. The clustering of C in α’ martensite was confirmed by impulse internal friction technique with observation of Cottrell atmosphere formation. 3D atom probe tomography analysis revealed partitioning of C atoms into austenite and clustering of C atoms in the α’ martensite. The remarkable increase of yield strength was attributed to redistribution of C atoms during the aging treatment.

SCRIPTA Vol. 192, Feb. 2021, P115-119

The grain boundary character distribution (GBCD) in an austenitic stainless steel produced by additive manufacturing (AM) in both as-built and annealed conditions was studied. Relatively fine grains and a non-fibre texture was achieved by AM, and as-built structure showed a high population of Σ3 boundaries. A five-parameter GBCD analysis revealed that the microstructure is mostly dominated by highly incoherent Σ3 boundaries. The grain boundary network also consisted of random high angle, coherent Σ3s terminating on (111) planes with a pure twist character, and tilt Σ9 boundaries. The findings show prospects for the possibility of engineering the grain boundary network of materials in-situ, via the stress and heat induced by the thermal cycles during AM.