SCRIPTA

Cold spray deposition (CSD) emerged as an additive manufacturing process in recent years. It features the highest efficiency among all metal additive manufacturing processes. In this work, the deposition mechanism of brittle reinforcing particles during the CSD of metal matrix composites (MMCs) was investigated. The rebound and fragmentation of brittle microparticles during deposition were investigated at nanosecond level and with microscale resolution through in-situ observation. For the first time, the velocity that determines the transition from particle rebound to particle fragmentation was identified. Herein, this velocity is defined as “fragmentation velocity”. The X-ray computed tomography on CSD deposits reveal that most of the diamonds in the deposits were fragments of the original particles and their volume fraction significantly depended on their impact velocities relative to the fragmentation velocity. Based on these findings along with microstructure characterizations, the deposition mechanism of brittle reinforcing particles during the CSD of MMCs is proposed.

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In the present study, the temperature dependence of martensite variant reorientation in stress-induced martensite aged at 423 K along the [110]_B2||[100]_L10-direction Ni₄₉Fe₁₈Ga₂₇Co₆ single crystals was investigated in compression. This martensite aging induces a rubber-like behavior with a reversible strain of up to -16.0% along the [001]_B2-direction in the temperature range from 248 K to 344 K, which is caused by L10-martensite variant reorientation. At higher temperatures between the forward and reverse transformation from 344 K to 382 K, the stress-induced martensite aged crystals demonstrate a two-stage stress-strain response with a reversible strain of up to -13.5%. The first stage is characterized by low critical stresses of σ_cr1 ≈ 1-15 MPa and an inverse elastocaloric effect (heat absorption) under loading. Superelasticity is observed in the second stage with σ_cr2 ≥ 100 MPa. In this temperature range, the martensite variant reorientation occurs through reverse and then forward martensitic transformation.

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Systematic investigation of microstructure characterization and mechanical property was performed on metastable Fe₄₆Co₃₀Cr₁₀Mn₅Si₇V₂ high-entropy alloy (HEA) with varying the grain size. The grain coarsening led to the decrease in stability of face-centered-cubic (FCC) phase, and consequently generated the formation of laminate-morphology hexagonal-close-packed (HCP) and butterfly-morphology body-centered-cubic (BCC) martensite. During the tensile test, a transformation-induced plasticity (TRIP) from the FCC to BCC via the intermediate HCP occurred, and the accelerated transformation rate of BCC increased the strain-hardening rate. Our results suggest that microstructural evolutions related to thermally-induced martensitic transformation and strain-hardening mechanisms can be well interpreted by understanding the FCC to HCP to BCC martensitic transformation.

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In the present study a novel approach to obtain functionally graded properties in an Fe-Mn-Al-Ni shape memory alloy is introduced. A single crystalline sample showing superelastic properties was aged in a partially martensitic state. As a result, the area aged in austenite is characterized by a changed superelastic hysteresis with different critical stresses for forward and backward transformation, whereas the properties in the area aged in martensite remained almost unchanged. Transmission electron microscopy studies revealed that the mean average size of nanometric β precipitates, which strongly influence the transformation temperatures and eventually the transformation stresses, is about 9 nm in the area aged in martensite and about 12 nm in the area aged in austenite. Based on these results it seems to be feasible to tailor Fe-Mn-Al-Ni components in a way allowing them to show locally different functional properties.

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The structure-property of a Ni-based superalloy fabricated by selective laser melting (SLM) was investigated under very high cycle fatigue (VHCF), and the subsurface cracking with fisheye pattern becomes a remarkable failure mode. Due to the presence of subsurface failure, the descending S–N curve property is represented. Based on 2D & 3D microscopic observation, a rough region with facets and inclusions, named as “multi-inclusion assisted facetted cracking zone (MAFCZ)”, is present within the fisheye. By combining electron backscatter diffraction analysis, the subsurface failure is related to grain size & orientation, microtexture and crystal defects. The crack is nucleated from the large grain slip with the highest Schmid factor under the assistance of the inclusions. The rough surface morphology with crack deflection reveals ultra-slow crack growth behavior within MAFCZ. Combined with failure modeling and fracture mechanics analysis, the subsurface failure mechanisms of SLM Ni-based superalloy under the VHCF are elucidated.

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Recently, a hexagonal phase has been reported in high carbon steels in several studies. Here, we show that the electron microscopy results used in these studies were erroneously interpreted. The extra-spots in the diffraction patterns and the odd contrasts in the high resolution images are not those a superstructure but result from double diffraction and streaking effects due to the presence of twins and stacking faults. We point out a similar unfortunate misunderstanding of these effects in papers reporting the existence of a 9R structure in aluminium or copper, or exotic forms of carbon in diamonds.

SCRIPTA

High-entropy alloys (HEAs) strengthened by coherent nanoparticles show great potentials for elevated-temperature structural applications, which however, generally suffer from a severe intergranular embrittlement when tested at intermediate temperatures. In this study, we demonstrated a novel “heterogenous columnar-grained” (HCG) approach that can effectively overcome this thorny problem. Different from the equiaxed counterpart which shows extreme brittleness along grain boundaries at 800 °C, the newly developed HCG-HEA exhibits an exceptionally high resistance to intergranular fractures originating from the unique grain-boundary characters and distributions. The presence of heterogenous columnar grain structure drastically suppresses the crack nucleation and propagation along with boundaries, resulting in an unusually large tensile ductility of ~18.4 % combined with a high yield strength of ~652 MPa at 800 °C. This finding provides a new insight into the innovative design of high-temperature materials with extraordinary mechanical properties.

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Pushing the performance limits of engineering systems aiming to enhance their efficiency and reduce emissions for a sustainable future requires new materials. Current alloy design strategies consider alloys as static systems, but their microstructure and composition continuously evolves in operando. This viewpoint is concerned with a new alloy design dynamic parameter that arises from information pertaining to the interactions of solutes with crystal defects obtained at the near-atomic scale. Recent technological breakthroughs in high-resolution characterization have enabled structural and compositional imaging at near-atomic scale providing new insights into such interactions. The role of these interactions on the continuously evolving microstructure and composition that governs failure in high temperature alloys, such Ni-, Co-based alloys and Ti-alloys is discussed. The development of a solute-defect database that accounts for these interactions, aiming to more accurately predict the mechanical performance of current and new alloys is necessary for more dynamic alloy design strategies.

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Nanograin materials have superior properties in mechanics, physics and chemistry. Here, we found a new phenomenon that nanograin formation spontaneously occurs in the process of ductile fracture for a titanium alloy, and the dominating mechanism is local severe-plastic-deformation (LSPD). The microstructure evolution during the entire process of monotonic tension was revealed to further understand the ductile fracture from plastic deformation to necking, and to final failure, especially in the post uniform deformation stage, in which the voids nucleate, grow and coalesce. The process of the LSPD can potentially provide a new concept and approach to design and produce high ductile materials, in which nanograin formation will consume massive strain energy to enable the large elongation after specimen necking in the post uniform deformation.

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Grain boundaries act to absorb and annihilate point defects generated by high-energy particles for polycrystalline nuclear materials. Thus, understanding the effect of irradiation of high-energy particles on the grain-boundary structural change is prerequisite to develop nuclear materials with desirable properties. It is expected that such an effect would be different for different GB characters. To explore the probability, we examine grain boundaries with twist and tilt components with the same misorientation relationship of [1 0 0]/45°in Au as a model system at a high-voltage transmission electron microscope with an acceleration voltage of 1250 keV. It is observed that, under the same irradiation condition, the tilt GB undergoes roughening while the twist one became faceted.