Peculiar oxidation behaviors of additively manufactured (AMed) Inconel 738 (IN738) Ni-based superalloy, influenced by AMed microstructures and the reactive element hafnium (Hf), were investigated. This study compared the oxidation behaviors of Hf-added and Hf-free direct-laser-deposited IN738. Hf induces microstructural heterogeneity and significant elemental partitioning between the dendrite cores and interdendritic regions in direct-laser-deposited Ni-based superalloys. The effects of Hf on the oxidation behavior of IN738 were examined, revealing that Hf enhances oxidation resistance by promoting early Hf oxide peg formation and the development of a protection layer. The formation of interconnected Hf channels originating from partitioned Hf was observed as oxidation progressed. The channeling structure facilitated the transport of Hf from the base alloy to the surface and contributed to the formation of a protection layer made of Hf oxide. The Hf-driven protection layer effectively suppressed oxidation compared to alloys without Hf. The synergy between Hf and the AMed microstructure by directed energy deposition was crucial for this enhanced oxidation resistance.

This study examines the effects of trace Zn addition on the microstructure, tensile properties, and precipitation behavior of a high-speed-extruded Mg-5Bi-3Al (BA53, wt.%) alloy. While this alloy exhibits excellent extrudability and high strength, its low ductility and prolonged aging time to achieve peak hardness limit its commercial viability. To address these limitations, 0.5 wt.% Zn was added to the BA53 alloy to enhance its ductility and reduce its peak-aging time without compromising its strength. Subsequently, BA53 and Mg-5Bi-3Al-0.5Zn (BAZ530) alloys were extruded at 400 degrees C with a die-exit speed of 40 m/min. Microstructural analysis revealed that trace Zn addition had minimal impact on grain size or Mg3Bi2 particle distribution but reduced the maximum texture intensity of the high-speed-extruded alloy. The BAZ530 alloy demonstrated a 38% increase in tensile elongation compared to the BA53 alloy, with no reduction in tensile yield strength, primarily due to the activation of non-basal slip systems. In addition to improving ductility, Zn addition significantly reduced the peak-aging time at 180 degrees C from 128 h for the BA53 alloy to 16 h for the BAZ530 alloy, an eightfold decrease, while achieving comparable peak hardness values. This acceleration is attributed to the substitution of Bi atoms by Zn atoms in the Mg3Bi2 phase, which lowered the formation enthalpy and increased nucleation rates. Overall, our findings demonstrate that trace Zn addition effectively enhances tensile elongation and accelerates precipitation in high-alloyed Mg-Bi-Al alloys.

We investigated the prevention of hot cracking in a non-weldable Ni-based superalloy by utilizing a passivation layer formed through laser-based directed energy deposition. The addition of Hf to the Ni-based superalloy facilitated the formation of a passivation layer consisting of Hf oxide on the surface of the as-deposited sample. An increase in the amount (thickness) of the passivation layers with increasing Hf content resulted in a reduction in hot cracking occurrence. Specifically, the addition of 2.5 wt% Hf led to the formation of uniformly distributed fine oxides without inducing hot cracks. This passivation layer effectively inhibited the formation of coarsened Mo oxides, which typically occur during oxidation in the liquid state and contribute to the hot cracking of Ni-based superalloy. During oxidation in the liquid state, Hf diffuses out to the surface and forms a passivation layer, thereby suppressing the formation and growth of oxides to sizes large enough to induce hot cracking.

We report on the effects of Ru addition on the formation of topologically close-packed (TCP) phases in a Ni-based superalloy during high-temperature exposure. The Ru addition reduces the lattice misfit between the γ and TCP phase, unexpectedly promoting nucleation of the TCP phase. Furthermore, the γ matrix is destabilized by Ru, while the stability of TCP phase is less affected according to the prediction of an electronic structure map. The results of this work indicate that γ destabilization is another important factor for the role of Ru in TCP phase formation.

Herein, the evolution of long-period stacking ordered (LPSO) phases in the as-cast Mg-6Gd-1Zn-0.6Zr (wt.%) alloy are investigated via transmission electron microscopy (TEM) and atom probe tomography (APT). The TEM results reveal that two types of LPSO phase (a bulky interdendritic phase and a plate-like matrix LPSO phase) are formed in the as-cast sample. Most of the LPSO phases are confirmed to be of the 14H type, with a smaller proportion being of the 18R LPSO. Further, the APT results reveal that the composition of the interdendritic LPSO phase is closer to that of the ideal 14H phase compared to the matrix LPSO phase, and both the interdendritic and matrix LPSO phases exhibit a Gd/Zn ratio of 2.5, thereby indicating a deficient Zn content compared to the ideal 14H phase (i.e., 1.3). In addition, the influence of the LPSO phases on the deformation behavior is investigated at different compressive plastic strains using electron backscatter diffraction (EBSD) analysis to reveal twinning and slip behavior during deformation. The results indicate that the LPSO phase induces additional work hardening in the late stage of deformation via the suppression of {101¯1} compressive twinning and the activation of non-basal slip systems.

Ultrafast ultrasonic techniques using picosecond/femtosecond lasers show promise for the nondestructive evaluation at fine spatial resolutions because the induced ultrasonic waves have an extremely high frequency range, from GHz to THz. However, most existing applications are based on the linear feature variation of the measured ultrafast ultrasonic waves; this is limited by the linearity assumptions in lock-in detection schemes adopted for ultrafast ultrasonic measurement. This study proposes a technique to measure ultrafast nonlinear ultrasonic waves using a femtosecond laser and a modified lock-in detection scheme. The advantages of the proposed technique are as follows. (1) The developed technique can measure nonlinear ultrasonic waves induced by a femtosecond laser; (2) linear and nonlinear ultrasonic responses can be decomposed and measured using a modified lock-in detection scheme; and (3) the decomposed nonlinear ultrasonic waves can be used for effective micro defect detection and microstructure characterization at sub-micrometer scale. The proposed technique was used to successfully perform validation tests on micro crack detection in a silicon wafer and microstructure characterization of an additively manufactured Ti-6Al-4V sample.

Metal additive manufacturing (AM) enables rapid customization of complex parts. However, it leads to forming of columnar grain structures which give the AM parts anisotropic properties. In this study, we propose a pulsed laser-assisted AM (PLAAM) technique for in-situ grain refinement of Ti-6Al-4V parts. A nanosecond pulsed laser was focused onto a melt pool to generate a favorable environment for the promotion of fine equiaxed grains. The PLAAM technique provided an average prior-β grain size of 549.6 μm, compared to that of 1297 μm provided by the conventional AM technique. Moreover, the maximum value of multiples of uniform distribution of the β phase decreased from 16 to 7.7 when using the PLAAM technique, which indicates a weakened crystallographic texture. These changes confirm that the proposed PLAAM technique promotes finer and more equiaxed prior-β grains. Furthermore, because the proposed technique is a non-contact technique, it can be applied to existing processes without adjusting tool paths.