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Additive manufacturing (AM) of alloys has attracted much attention in recent years for making geometrically complex engineering parts owing to its unique benefits, such as high flexibility and low waste. The in-service performance of AM parts is dependent on the microstructures and grain boundary networks formed during AM, which are often significantly different from their wrought counterparts. Characteristics such as grain size and morphology, texture, and the detailed grain boundary network are known to control various mechanical and corrosion properties. Advanced understanding on how AM parameters affect the formation of these microstructural characteristics is hence critical for optimising processing parameters to unlock superior properties. In this study, the difficult-to-weld nickel-based superalloy Inconel 738 was fabricated via electron-beam powder bed fusion (EPBF) following linear and random scanning strategies. Random scanning resulted in finer, less elongated, and crystallographically more random grains compared to the linear strategy. However, both scanning strategies achieve unique high grain structure stability up to 1250 ℃ due to the presence of carbides pinning the grain boundaries. Despite significant difference in texture and morphology, majority of grains terminated on {100} habit planes in both linear and random built samples. The results show potential for controlling grain boundary networks during EPBF by tuning scan strategies. • Electron powder bed fusion Inconel 738 builds possess thermally stable grain structures up to 1250 ℃. • Change of scanning strategy from linear to random results in texture randomization and grain refinement. • The unique thermal stability can be due to the pinning of grain boundaries by carbides. • Grain boundaries mostly terminate on {100} habit planes irrespective of the scanning strategy. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) is well-known for its capability to produce and repair intricate parts e.g., Ni-based superalloy components used in aero-engine applications. However, a comprehensive understanding of the microstructure evolution and its link to properties remains elusive, with gaps in the through-process evolution of γ grains, γ', γ'', δ, and other phases in response to thermal cycling. The outstanding challenge here is to observe these transformations dynamically during the 3D printing process, as this usually requires complex in-situ measurements. Here we apply a different approach to systematically reveal the impact of thermal cycling on the solid-state microstructure evolution of IN718, via a combination of physical simulations and thermo-kinetic modelling. We replicate the thermal cycles IN718 experiences during laser directed energy deposition. Solid-state thermal cycles are shown to have minor effects on the grain morphology due to short holding times at critical temperatures, however, they do induce plastic strain accumulation. Significantly, we show how thermal cycling influences the evolution of γ', γ", and δ phases. While high initial peak temperatures inhibit the precipitation of γ', γ", and δ phases, prolonged thermal cycling to gradually decreasing peak temperatures promotes their precipitation. The δ phase forms along grain and twin boundaries, while both γ' and γ" predominantly precipitate in Nb-enriched regions, causing heterogeneities in hardness. Our results underpin the suitability of physical simulations for replicating superalloy AM, highlighting its potential as a tool to advance the current limited understanding of the microstructure and property evolution. [Display omitted] • Physical and thermo-kinetic simulation of solid-state thermal cycling in L-DED IN718. • Physical Gleeble experiments show excellent correlation with actual L-DED process. • Variation in γ′, γ′′, and δ precipitates correlates with hardness. • Solid-state thermal cycling affects γ′, γ′′, and δ morphology in individual layers. • Solid-state thermal cycling has negligible influence on grain morphology. [ABSTRACT FROM AUTHOR]

Ti-6Al-4 V is a popular alloy in additive manufacturing (AM) due to its applications in the biomedical implants and aerospace industries where the complex part geometries allowed by AM provide cost and performance benefits. Ti-6Al-4 V goes through a β → α' transformation after solidification which is known to experience variant selection, e.g., through the formation of clusters of variants which, when situated together, partially accommodate the strain of the phase transformation. During electron beam powder bed fusion AM, an in situ decomposition of α' martensite occurs during the cyclic reheating caused by melting successive layers, resulting in α + β microstructures. How variant selection influences the evolution beyond the initial rapid cooling remains an open question. Using 3D electron backscatter diffraction, we provide a clearer understanding without ambiguity from sectioning effects of how α' decomposes into microstructures with distinct morphologies and variant/intervariant distributions. We extract quantitative 3D information on the various intervariant boundaries networks formed in samples printed using three different electron beam scanning strategies. This shows that differing mechanisms during the decomposition result in a shift from self-accommodating clusters in an acicular microstructure, to either the preferred growth of six variants in a basketweave microstructure, or to a colony microstructure where variant selection is determined by prior-β grain boundaries. We propose a new representation of the misorientations arising from the Burgers orientation relationship, which we refer to as intervariant network diagram, to reveal how variant selection during the martensitic transformation and subsequent decomposition leads to the intervariant boundary networks observed. This holistic understanding of the microstructural evolution has the potential to allow tailoring of microstructures and properties for specific applications. [ABSTRACT FROM AUTHOR]

The phenomenological interplay in the segregation of phosphorus (P) and transition metal (TM) elements at grain boundaries (GBs) in steels has long been suspected to be the main contributor to temper embrittlement. However, many of the details remain unclear. Here, we investigate the segregation, co-segregation and cohesion effects of TMs (Co, Cr, Cu, Mn, Mo, Ni, Nb, Ti, V, W) along with P in various ferritic iron (α -Fe) GBs utilising density functional theory and simulations of kinetics. Our findings demonstrate that P is unlikely to cause intergranular fracture via weakened interfacial bonding when segregated by itself. Nevertheless, the stronger segregation binding of P compared to TMs can explain the ubiquitous presence of P segregated at GBs. We find that most P-TM interactions at ferritic GBs are repulsive and differ significantly from the corresponding interactions in the bulk. Due to the repulsive interactions and strong segregation binding of P, the enrichment of P over time at GBs leads to the depletion of prior-segregated cohesion-enhancing solutes at general GBs. Additionally, certain P-TM co-segregation combinations that are cohesion-lowering are energetically favoured at such GBs. We posit these mechanisms act in tandem as critical causalities of P-induced temper embrittlement in alloyed steels. Finally, we reveal a contradiction in the predicted cohesion effects of segregated P calculated in the Rice–Thomson–Wang theory of interfacial embrittlement compared to that as assessed by chemical bonding strength, calculated in the DDEC6 bond-order framework. These findings have important implications for GB engineering for interfacial cohesion. [Display omitted] [ABSTRACT FROM AUTHOR]

To gain insight into the origin of the ultra-high strength of ultrafine-grained (UFG) alloys, the solute clustering, precipitation phenomena, and microstructural evolutions were studied in an UFG Al-4.63Cu-1.51 Mg (wt.%) alloy (AA2024) processed by high-pressure torsion (HPT). The thermal analysis was performed using differential scanning calorimetry. The microstructures, internal microstrains and hardness following heating at a constant rate were characterised at room temperature using X-ray diffraction (XRD), transmission electron microscopy (TEM) and atom probe tomography (APT). The microhardness of the HPT processed sample initially increases following heating to 140 °C, and then remains unchanged on further heating to 210 °C. As the temperature increases up to 210 °C, the crystallite size calculated from XRD line broadening remains about 60–70 nm, while the dislocation densities remain in excess of 2 × 10 14 m −2 . A multimechanistic model is established to describe the strengthening due to grain refinement, dislocation accumulation, solid solution, precipitation, solute clusters and their segregation. The analysis reveals that solute clusters and lattice defects are key factors in HPT-induced strengthening of alloys, and illustrates the interactions between alloying elements, dislocations and grain boundaries enhance strength and stabilize ultrafine microstructures. Furthermore, for an HPT sample heated beyond 210 °C, the formation of nano-precipitates also contributes to hardness increment. The multimechanistic model for hardness contribution indicates the short-range order strengthening due to cluster-defect complexes is the dominant mechanism, which accounts for more than 40% of overall hardness. [ABSTRACT FROM AUTHOR]

[Display omitted] The segregation of transition metal elements to grain boundaries in steels plays a critical role in determining their cohesion. Here, we investigate the segregation, co-segregation, and cohesion effects of various transition metals (Co, Cr, Cu, Mn, Mo, Ni, Nb, Ti, V and W) to different grain boundary characters in ferritic-iron (α -Fe) through a systematic, brute-force style configurational analysis utilising density functional theory calculations. We demonstrate that differing grain boundary characters change not only transition metal segregation and co-segregation behaviours, but also their effects on cohesion. The effects of co-segregated solutes on cohesion can be substantially different from their summed individual parts. We show that solute-solute interactions at grain boundaries vary significantly as a function of grain boundary character. These interactions are shown to be substantially different from those that occur in the bulk. We introduce a novel quantitative method for assessing effects of segregated elements on interfacial cohesion through calculating the strength of bonds at a grain boundary in the DDEC6 bond-order framework. It is shown that work of separation quantities calculated through rigid separation of surfaces better captures the strength of bonding in most cases, and thus more accurately depicts intergranular fracture. Collectively, these results offer valuable insight towards rational grain boundary engineering in steels. [ABSTRACT FROM AUTHOR]

Pursing high coercivity‒remanence synergy is a common challenge for permanent magnets, particularly for the La/Ce-based RE‒Fe‒B (RE: rare earth) magnets that usually exhibit drastic magnetic dilution. Here we apply the Pr 80 Ga 20 grain boundary diffusion process (GBDP) to overcome the coercivity‒remanence trade-off of the LaCe-40 (La‒Ce/total RE = 40%) magnet and yield the record-high magnetic performance with H cj = 16.1 kOe, B r = 13.1 kG and (BH) max = 41.4 MGOe upon high La‒Ce substitution. One of the prominent merits of the PrGa GBDP is to assist the formation of anti-ferromagnetic RE 6 Fe 13 Ga phase at triple junctions and grain boundaries (GBs), which is observed for the first time within the RE‒Fe‒B diffusion system. The synergetic effects of Pr to enlarge the volume fraction of RE-rich phase and Ga to optimize its wettability also facilitate the formation of continuous non-ferromagnetic RE-rich phase at GBs, being another prominent merit of the PrGa GBDP. The anti-ferromagnetic RE 6 Fe 13 Ga and non-ferromagnetic RE-rich GBs to decouple neighboring ferromagnetic matrix grains, and the Pr-rich shell to strengthen local magnetocrystalline anisotropy jointly enhance the coercivity, as demonstrated by the microstructural characterization and micromagnetic simulation. The first order reversal curve and Kerr microscopy further reveal the impedance of the nucleation of reversal domains in the PrGa GBDP LaCe-40 magnet, transforming from multi-domain reversal to single-domain reversal. Moreover, Pr infiltration into the matrix grains increases the local magnetization and leads to raised remanence by 0.2 kG. The above work provides a clear picture of the synergetic effects between Pr and Ga towards fully exploiting paragenetic La‒Ce alloy and developing the low-cost commercial-grade permanent magnets. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • GB character controls vulnerability to hydrogen segregation and embrittlement. • Low energy, highly coherent GBs minimise H-accumulation, embrittlement. • H-H interactions controls amount of H segregation, accumulation at a GB. • Bonding analysis reveals fundamental atomistic nature of H-embrittlement at GBs. Here, the segregation and accumulation of hydrogen in Ni grain boundaries, and its effects on cohesion and tensile mechanical strength were studied by means of density functional theory simulations. Three model grain boundaries were considered: the Σ 3 (1 1 ¯ 1) [ 110 ] , Σ 5 (120) [ 001 ] and Σ 11 (1 1 ¯ 0) [ 113 ] , as representatives for the highly coherent twin, high energy random high angle, and "special" low energy highly coherent grain boundaries, respectively. Hydrogen segregation was found to be favourable in the Σ 5 and Σ 11 grain boundaries, but not in the Σ 3. Hydrogen accumulation studied via a comprehensive site-permutation analysis revealed the mechanisms for how H accumulation capacity varies as a function of grain boundary character. We show that the interfacial cohesion of boundaries can diminish by between 6.7–37.5% at varying levels of H-accumulation. The cohesion of the grain boundaries was analysed using a novel chemical bond-order based approach, enabling a quantitative atomistic determination of the fracture paths arising from hydrogen embrittlement. These simulations explain the details of why grain boundary character is the principal determinant of the likelihood of hydrogen segregation and accumulation, and hence their vulnerability to hydrogen-enhanced decohesion. This knowledge can be used in the design of thermomechanical processes to achieve grain boundary engineering for resistance to hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

Abstract: Most modern HSLA steels rely on the effect of Nb in steels to achieve the properties desired for a specific application. While the role of Nb in forming precipitates has been well characterized, its role in a solid solution is less well understood due to the difficulty of obtaining quantitative experimental data. In the current work, site-specific atom probe tomography was used to quantify the amount of Nb present at prior austenite grain boundaries in a commercial strip-cast steel, produced via the Castrip® process. This was compared to the amount of Nb found at ferrite–ferrite grain boundaries that had formed during the transformation from austenite to ferrite. With the interfacial excess Nb measured, thermodynamic calculations were carried out and compared to the change in transformation temperature obtained by dilatometry, with reference to a comparable Nb free, strip-cast steel. [Copyright &y& Elsevier]

As-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets typically exhibit a higher coercivity than as-sintered single-main-phase Nd-Ce-Fe-B magnets with identical content of Ce. To clarify the microstructural features that are relevant to the coercivity, as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets are characterised by high-resolution magnetic force microscopy and atom probe tomography down to the atomic scale. Though inhomogeneous distribution of rare earth elements (Nd, Pr, and Ce) is observed inside 2:14:1 matrix grains, Ce atoms unexpectedly show similar partitioning behaviour (∼14–27% of rare earth element) at the edge of these grains in the vicinity of rare earth-rich grain boundary compared with as-sintered single-main-phase Nd-Ce-Fe-B magnets with a 20% Ce substitute of Nd and Pr. By contrast, the observation of stripe-liked magnetic domains pinned at the grain boundaries and a thin (Nd,Pr,Ce)-rich grain boundary with a composition of Nd 9.9 Pr 2.6 Ce 19.0 Fe 65.3 Co 1.9 Balance 1.3 (at.%) jointly indicates the enrichment of Ce in grain boundaries benefit to the magnetic isolation of neighbouring matrix grains in the as-sintered state. Unusual Ce-rich clustering regions are also observed in the grain boundary, which also contribute to the coercivity enhancement. The segregation of Ce, Co, and Cu and rare-earth-Cu clusters are only observed at the interface between rare earth-Cu rich phase and matrix phase while no obvious elemental segregation and clusters are observed at the interface between the rare earth-Ga rich phase and matrix phase, suggesting the occurrence of rare earth-Cu rich phase may be one of the reasons for achieving high coercivity. These findings offer fresh insights into the as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets. Image 1 • High intrinsic coercivity is achieved in the as-sintered multi-main-phase Nd-Ce-Fe-B magnets. • The formation of thin (Nd,Pr,Ce)-rich grain boundary in the as-sintered state. • The formation of Ce-rich clustering regions at grain boundaries in the as-sintered state. • The segregation of Cu at the interfaces between the rare earth-Cu rich phase and matrix phases. • No Ga segregation at the interfaces between the rare earth-Ga rich phase and matrix phases. [ABSTRACT FROM AUTHOR]

Abstract: Positron annihilation spectroscopy in two variants (coincidence Doppler-broadening CDB and lifetime spectroscopy LT) was used for the characterisation of severely deformed nanocrystalline 7075 Al alloy, with specific attention to the distribution of solute in the proximity of grain boundaries. The 7075 samples were deformed via the high pressure torsion (HPT) technique after solution treatment and quenching. The grain size at the end of the deformation was sub 100nm. The deformed samples have undergone 3 months of natural ageing post-processing. CDB and LT measurements consistently indicate that the fraction of trapped positrons in these samples is near to 90%. The analysis of CDB data shows that the environment of the positron traps is enriched with solute up to 50at.%, nearly evenly divided between Mg and transition metals (Zn and Cu). The CDB results indicate an enhancement of solute concentration at grain boundaries associated with HPT deformation. [Copyright &y& Elsevier]

Understanding the pathways and mechanisms of human tooth decay is central to the development of both prophylaxes and treatments, but only limited information is presently available about the initiation of caries at the nanoscale. By combining atom probe tomography and high-resolution electron microscopy, we have found three distinct initial sites for human dental enamel dissolution: a) along the central dark line (CDL) within carbonated apatite nanocrystals, b) at organic-rich precipitates and c) along high-angle grain boundaries. 3D maps of the atoms within hydroxyapatite nanocrystallites in sound and naturally-decayed human dental enamel reveal a higher concentration of Mg and Na in the CDL. The CDL is therefore thought to provide a pathway for the exchange of ions during demineralization and remineralization. Mg and Na enrichment of the CDL also suggests that it is associated with the ribbon-like organic-rich precursor in amelogenesis. Organic-rich precipitates and high-angle grain boundaries were also shown to be more vulnerable to corrosion while low-angle grain boundaries remained intact. This is attributed to the lower crystallinity in these regions. [ABSTRACT FROM AUTHOR]