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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]

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]

Fluctuations in energy distribution during additive manufacturing (AM) can result in spatial and temporal thermal transients. These transients can lead to complexities, most significantly when alloys with multi phases are subjected to AM. Here we unveil such complexities in a duplex stainless steel, where we report an unanticipated formation of a Ni-Mn-Si rich phase at grain boundaries and a local fluctuation in Cr and Fe concentrations in regions close to grain boundaries, providing Cr-rich precursors for Cr 2 N formation after laser powder bed fusion (LPBF). The formation of these phases is believed to be due to severe thermal gyrations and thermal stresses associated with LPBF resulting in a high-volume fraction of ferrite supersaturated with N and Ni, and a high density of dislocations accelerating diffusion and phase transformations. [Display omitted] [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]