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This study seeks to control the microstructure of Ti-6Al-4V alloy produced by a selective laser melting process using only heat treatment, while also improving the high temperature creep behavior. Initial microstructure observation showed a fully martensite structure for the as-fabricated specimen and a fine widmanstatten structure for the heat-treated specimen. Hardness tests indicated that the hardness of the material decreased somewhat, but the anisotropy of the mechanical properties (common in additive manufactured materials) disappeared. Compression tests at 500 °C indicated that the yield strength of the as-fabricated specimen was approximately 930 MPa, while in the heat-treated specimen it was approximately 557 MPa. High temperature compressive creep tests showed that the heat-treated specimen had relatively low creep strain and low steady-state creep rate (superior creep resistance) in all stress ranges, which is the opposite of the compressive yield strength results. After creep deformation, the microstructural evolution of the as-fabricated and heat-treated specimens was compared, and the microstructure changed much more extensively in the as-fabricated specimen. Correlations between microstructures, hardness, strength and creep properties were discussed based on these results.

This study clarifies the role of micro- and nano-TiC added to 316L stainless steel fabricated by the selective laser melting (SLM) process, an emerging additive manufacturing technology, in the microstructural evolution and mechanical properties. Directionally fine cellular dendrites and columnar grains formed during the fast solidification in SLM-processed stainless steel. Interestingly, the addition of TiC particles in the steel matrix significantly reduced the cellular and grain sizes after solidification and also disrupted the established directional structures, particularly for nanoscale TiC. The composite, particularly with nanoscale TiC, also exhibited greater room- and high-temperature compressive yield strengths than unreinforced steel, mainly because of the combined effects of grain-boundary strengthening and Orowan strengthening. The strengthening effect was well described by the Zener pinning model. The compressed surfaces suggest that TiC particles hinder crack propagation, and the TiC distribution was critical in improving the mechanical properties. The SLM process can tailor the microstructure across a rather limited length scale; hence, to better control the mechanical properties of the resulting products, compositing the relevant feedstock powder is a highly attractive strategy for developing components with novel structures and unique properties.

The high-cycle fatigue properties and fatigue damage mechanism of AlSi10Mg alloy manufactured by selective laser melting (SLM) were investigated. The influence of the post-heat treatment on the microstructure and high-cycle fatigue properties was also examined. The SLM-manufactured AlSi10Mg alloy was heat-treated under two different conditions, namely T6 and direct aging (DA). The results of the high-cycle fatigue tests, which were carried out at room temperature, imply that, under all three conditions (as-built, T6, DA), the fatigue strength decreases as the number of cycles increases along with the estimated endurance limit. Predominantly, the fatigue properties were influenced by the morphology and size of the Si particles, both controlled by adjusting the heat treatment. The alloy that was processed using DA heat treatment had superior fatigue properties due to its cellular structural morphology characterized by a significant amount of fine Si precipitates. Compared with the as-built and T6 specimens, the superior fatigue limit of the DA alloy is attributable to its outstanding strength characteristics. The reliability of the fatigue life of the DA alloy is relatively high. Fatigue fracture samples revealed that the DA alloy had evenly distributed dislocations along the cellular boundaries. The fine Si particles were found to play an important role in preventing fatigue crack propagation, thereby enhancing fatigue properties. The effect of the heat treatment on the microstructure and mechanical properties of the SLM AlSi10Mg specimens was investigated with a specific focus on fatigue strength. The results highlighted that an appropriate DA process could significantly enhance the fatigue performance of the AlSi10Mg alloy under investigation. The resulting characteristics are highly advantageous and are superior to those obtained with other conventional heat treatment processes.

This study investigated the effect of cryomilling on the creep properties of oxide dispersion strengthened steels. Milling temperatures during mechanical alloying (MA) were set as room temperature and −150 °C to manufacture ODS ferritic steels, K1 (room temperature) and K4 (−150 °C) with Fe–14Cr–3W–0.4Ti–0.3Y 2 O 3 (wt%) composition. Microstructural observation identified large, inhomogeneous grains in K1 (700 nm), and a relatively fine, homogeneous grains in K4 (303 nm). Some grain boundaries featured secondary phases. The dispersion strengthened phase, fine oxide particles, was finer and more homogeneously distributed in K4 (7 nm) than in K1 (22 nm). A compressive creep test was performed at 650 °C. The steady-state creep rate in an identical stress range was lower in K4 than in K1. The stress exponent (n) was similar in the two alloys ( n

Effect of gaseous hydrogen charging on the mechanical properties of additively-manufactured CrMnFeCoNi high-entropy alloy (HEA) strengthened by in-situ formed oxide was investigated. The results showed that SLM-built HEA exhibited yield strength of ∼729.6 MPa and has an excellent hydrogen embrittlement resistance due to high-hydrogen solubility and easy deformation twin formation.

This study conducted a quantitative analysis of the martensite/bainite (M/B; bainitic structure region) fraction of martensite-bainite steel using electron back-scatter diffraction (EBSD) analysis. The M/B fraction analyzed using the EBSD analysis method was then compared with phase fraction measurement results with an optical microscope (OM), field emission scanning electron microscope (FE-SEM), and field-emission transmission electron microscopy (FE-TEM). In addition, microstructure, tensile and high-cycle fatigue behaviors according to M/B phase fraction were investigated. Initial microstructural observation measured a prior austenite grain size (PAGS) of 24 μm (alloy A) and 11 μm (alloy B). Both alloys were observed to have martensite and bainite structures. XRD phase analysis of the two alloys identified an α-Fe peak expected to be martensite or bainite. Quantitative phase fraction of M/B using EBSD analysis measured M: 40.37% and B: 59.63% for alloy A, and M: 53.03% and B: 46.97% for alloy B. Tensile tests of the above materials confirmed that alloy B, which had finer PAGS and a higher martensite fraction, had greater yield strength (1423 MPa) and tensile strength (1826 MPa) that were approximately 200 MPa higher than alloy A. The yield strength was calculated based on the M/B phase fraction using EBSD and the measured microstructure factors, with a consideration of the prediction model. The calculation value was similar to the actual tested strength one. In the high-cycle fatigue test, alloy B, with its greater strength, had an approximately 200 MPa higher fatigue limit (1075 MPa) than that of alloy A. EBSD analysis of the fatigue crack initiation area confirmed that the M/B interface can act as a fatigue crack initiation site. Based on the above findings, tensile and fracture surface analyses were performed, and attempts were made to identify the tensile and deformation mechanism according to the M/B phase fraction.

Electron beam melting (EBM) has a lower cooling rate than other additive manufacturing processes (selective laser melting, laser metal deposition, etc.), so it is suitable for manufacturing titanium aluminide-based intermetallics. In this study, a gamma Ti-48Al-2Cr-2Nb alloy was manufactured using EBM, and its microstructure and high-temperature creep behaviors were investigated. Conventional Ti-48Al-2Cr-2Nb was also used for comparison and analysis. Initial microstructural observation confirmed that the EBM-built Ti-48Al-2Cr-2Nb alloy had a near-gamma (NG) structure, whereas the conventional Ti-48Al-2Cr-2Nb alloy had a fully lamellar (FL) structure. Room temperature and high temperature compression tests confirmed that the Ti-48Al-2Cr-2Nb alloy with the NG structure had lower strength in all temperature ranges, and the yield-strength anomaly phenomenon occurred in both materials. A 750 °C multi-step creep test confirmed that the EBM-built Ti-48Al-2Cr-2Nb alloy with lower strength had lower creep resistance as well. Microstructural observation after creep deformation confirmed that dislocation movement and mechanical twins were formed dominantly in the creep deformation of the material with the NG structure. For the material with the FL structure, diffusional creep in low-stress regions and large deformation at the colony boundary in high-stress regions dominated the creep deformation. Correlations between the microstructure, strength, and multi-step creep properties were discussed based on these findings.

The durability of thermal barrier coatings (TBCs) can dictate the life of the hot section engine components on which they are applied. In this study, we examine the microstructural degradation of air plasma sprayed ZrO 2 -8 wt.% Y 2 O 3 TBCs with a low-pressure plasma sprayed CoNiCrAlY bond coat on an IN 738LC superalloy substrate. Thermal cyclic tests were carried out in air at 1100 °C with a 1-, 10-, and 50-h dwell period, proceeded by a 10-min heat-up and followed by a 10-min forced-air-quench. Microstructural analyses were carried out to document the growth of the thermally grown oxide scale, the depletion of the Al-rich β-NiAl phase in the bond coat, and the population and growth of micro-cracks near the YSZ/bond coat interface. Evolution in these microstructural features was examined with respect to the lifetime of TBCs. A lifetime approximation model was developed, via modification of Paris Law, based on the experimental data. The model predicted the TBC lifetime within 10% of the experimental lifetime.

The mechanical properties and crystallization behavior of new Fe–Ni–Cr–Si–B-based bulk glassy alloys were investigated. The suitability of the continuous roll casting method for the production of bulk metallic glass (BMG) sheets in such alloy systems was also examined. BMG samples (Fe–Ni–Cr–Si–B, Fe–Ni–Zr–Cr–Si–B, Fe–Ni–Zr–Cr–W–Si–B) in amorphous strip, cylindrical, and sheet forms were prepared through melt spinning, copper mold casting, and twin roll strip casting, respectively. Fe–Ni–Cr–Si–B alloy exhibited compressive strength of up to 2.93 GPa and plastic strain of about 1.51%. On the other hand, the Fe–Ni–Zr–Cr–Si–B, composite-type bulk sample with diameter of 2.0 mm showed remarkable compressive plastic strain of about 4.03%. The addition of zirconium was found to enhance the homogeneous precipitation of nanocrystalline less than 7 nm and to develop a hybrid-composite microstructure with increasing sample thickness. Twin roll strip casting was successfully applied to the fabrication of sheets in Fe–Ni–Cr–Si–B-based BMGs. The combined characteristics of high mechanical properties and ease of microstructure control proved to be promising in terms of the future progress of structural bulk amorphous alloys.

A study has been made to investigate the fatigue properties of a high-strength spring steel in relation to the microstructural variation via different heat treatments. Rotating–bending fatigue and fatigue crack growth (FCG) tests were conducted to evaluate the fatigue properties, and a transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) unit was used to characterize the tempered microstructure. The results indicate that the fatigue endurance σf increases with increasing tempering temperature, reaching a maximum at 450°C, then decreases. The increase of σf is mainly attributed to the refined distribution of precipitation, together with the structural uniformity of tempered martensite. The softening of tempered martensite due to excessive precipitation accounts for the decrease of σf. By contrast, the FCG results show an insensitivity of the stage-II growth behavior to the microstructural changes for the whole range of tempering temperature tested. The insensitivity is interpreted in terms of the counterbalancing microstructure-dependent contributions to the FCG behavior.

An investigation was carried out to study the microstructural influence on the acoustic emission (AE) behavior during the tensile and fracture tests of an Al Li 8090 alloy, and to correlate it with the deformation mechanisms of two different microstructures (the δ ′ microstructure and the S ′ + δ ′ microstructure), and of two different orientations ( L and S ). For the L -oriented specimens, continuous emissions were mostly produced in the δ ′ microstructure due to the shearing of coherent δ ′ precipitates, while in the S ′ + δ ′ microstructure the burst type emissions were dominant mainly due to the shearing or microcracking of large incoherent S ′ precipitates as well as the intergranular failure. The S -oriented specimens of both microstructures showed more burst emissions than the L -oriented ones, which resulted from the frequent occurrence of intergranular delamination in the S -oriented specimens. The K AE values measured by AE method were well agreed with the K IC values, implying that the K AE could be considered as a quantitative and reliable parameter to evaluate the fracture toughness of a material.