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The complex thermal effects of laser direct energy deposition (L-DED) provide significant advantages for production of high-performance γ-TiAl alloy, whereas challenges still exist in microstructure tailoring and mechanical properties. This work focuses on the influence of intrinsic cyclic heat treatment (ICHT) on the evolution of microstructures and mechanical properties in different building height regions of an L-DED TiAl alloy thin-wall sample. The results indicate that ICHT induces discontinuous coarsening (DC) effects by consuming the lamellar interface energy of the initial lamellae, resulting in grain refinement. With the increase in building height, the thermal accumulation leads to an increase in the initial lamellar spacing, which weakens the DC effect. At the top, insufficient thermal cycles prevent fine grain growth and erode coarse grains, hindering overall grain refinement. Ultimately, the hierarchically heterogeneous microstructure forms in the L-DED TiAl alloy, including refinement of hierarchical lamellar colony structure decorated with fine (α 2 +γ) lamellar structure and (α 2 +6H-LPS) lamellar structure (6H-LPS refers to 6H long-period structure phase). Specifically, the bottom region exhibits the best combination of ultimate tensile strength (706 ± 33 MPa) and strain (0.68 ± 0.05 %). The high tensile strength can be attributed to the hindrance of dislocation movement by the fine lamellae and the 6H-LPS phase ,caused by the high cooling rate of L-DED, while the elongation is attributed to the reduction in crack propagation energy due to the presence of fine grains. • The in-situ microstructure evolution and performance enhancement in TiAl alloy were analyzed along the deposition direction. • Under rapid solidification and intrinsic cyclic heat treatment (ICHT), coarse lamellar colonies are significantly refined. • ICHT induced a bimodal grain distribution in thin-wall sample, and refined grains enhance TiAl alloy's mechanical properties. • The 6H long-period structure phase in coarse columnar grains may contribute to enhancing the performance of TiAl alloys. [ABSTRACT FROM AUTHOR]

Laser-directed energy deposition (L-DED) is considered as an effective repair method for damaged single-crystal (SX) superalloy turbine blades. However, controlling stray grains (SGs) and epitaxial growth is still the major problem. This study investigated the influences of pre-solution treatment and intrinsic heat treatment (IHT) on the formation of SGs and epitaxial growth of L-DED René N5 SX superalloy. The γ/γ' eutectics and coarse γ' precipitates at the inter-dendrite region can be eliminated after pre-solution treatment procedures before the L-DED experiment. A lower temperature in pre-solution treatment resulted in the retention of residual γ/γ' eutectics and coarse γ' precipitates. Thus, the aggravated element segregation within the local melt pool and interface collapse during the L-DED process can result in stray grains (SGs) formation. Improved pre-solution treatment procedures can also help reduce SGs caused by the carbides in the substrate. Besides, the SG formation in the deposited layers can be attributed to the recrystallization as a result of high thermal stress during the IHT in thermal cycles. The optimized pre-solution treatment procedure reduced the elemental segregation of the substrates , resulting in a larger primary dendrite arm spacing (PDAS) from the bottom of the deposited layers. Due to IHT during thermal cycles in the deposited layers, the equivalent heat treatment process promoted the re-precipitation and growth of γ' phase. Meanwhile, the optimized pre-solution treatment procedure decreased the size of the heat-affected zone (HAZ) caused by thermal cycles. In summary, by optimizing the substrates' pre-solution treatment procedures to eliminate the γ/γ' eutectics ,coarse precipitaes and decrease the carbides in the subsrtates, the formation of SGs resulting from substrate segregation and heterogeneous microstructure can be controlled. This work provides the potential guidance of substrate pre-treatment for L-DED repair of SX superalloys. [Display omitted] • Pre-solution heat treatment procedure is benefit to the control of stray grains (SGs) in the bottom of the molten pool. • SGs found in the deposited zone were strongly associated with recrystallization. • Improper pre-solution treatment procedure increased the length of heat-affect zone (HAZ). • Intrinsic heat-treatment (IHT) caused by thermal cycles led to different microstructure evolution of γ' precipitates in the sample. [ABSTRACT FROM AUTHOR]

The complex thermal history during the laser direct energy deposition (L-DED) can result in the inhomogeneous microstructure of the Ti-47Al-2Cr-2Nb (TiAl) alloy. This work investigates the grain structure and metastable microstructure evolution of L-DED TiAl samples with different deposit shapes of single-track, thin-wall, and bulk. Numerical simulations were performed for all samples to reveal the influence of thermal history on microstructural evolution. Among the three samples, the single-track sample has unique dendritic and cellular structures due to the highest cooling rate. Meanwhile, incompletely precipitated lamellar γ was found at the boundary and inside the α 2 grains. And residual B2 phase is also present in the single-track sample. As for the thin-wall and bulk samples, they exhibit fully lamellar structure, and contain only γ and α 2 phases at room temperature. During the L-DED process, the temperature gradient and cooling rate inside both samples decreased with the elevation of the deposition height. However, due to the different thermal histories, the temperature gradient and cooling rate do not change in the same way. This leads to a large difference in grain structure and metastable microstructure between the thin-wall and bulk samples. Also, these changes affect the mechanical properties of TiAl alloys. In summary, this study can provide potential guidance for a tailored microstructure in L-DED TiAl alloy. • Microstructure evolution of L-DED TiAl alloy was studied in three deposit shapes. • Different metastable microstructures were found at different building heights. • Grains in heat affected zone are influenced by cooling rate and thermal cycles. • The thermal history caused by the deposit shapes can determine the phase content. • The changes in microstructure of L-DED TiAl alloy are also reflected in hardness. [ABSTRACT FROM AUTHOR]

It is known that the reactive nitrogen atmosphere can effectively modulate the microstructure of laser powder bed fusion (L-PBF) of Ti6Al4V alloys. In the present work, high nitrogen contents of 15 and 25 vol% were applied in argon atmospheres to investigate the influence on microstructure and mechanical performances. The enhanced nitrogen solid solution was achieved with nitrogen content ranging 374 ppm and 456 ppm (parts per million) in the Ti6Al4V alloys, respectively. Besides, the EBSD (electron back-scattered diffraction) characterization shows the significant refinement of acicular martensite α′ and the EBSD phase reconstruction shows the interrupting epitaxial growth of the β phase with the increasing nitrogen content in the atmosphere. The KAM (kernel average misorientation) diagrams with an increasingly higher value indicate an increased micro-strain and dislocation density at higher nitrogen content in the atmosphere. Meanwhile, the high magnification TEM further confirms the martensite α' refinement and also the formation of β lamella at 25 vol% nitrogen content in L-PBF atmosphere. Such microstructure evolution can be mainly attributed to the more severe thermal cycling with a higher thermal gradient and cooling rate under the atmosphere with higher nitrogen content. By applying the 25 vol% nitrogen in the atmosphere, the L-PBF Ti6Al4V alloys exhibit a significant increase in maximum compressive strength of ∼1885 MPa and maximum compressive strain of ∼10%, respectively. The underlying strengthening mechanism can be attributed to the refinement of martensite structure, solid solution, and higher dislocation density. The present work shows that the atmosphere can effectively modulate the microstructure and improve the mechanical property of L-PBF alloys. [ABSTRACT FROM AUTHOR]