Search Results (52)

This article evaluates the possibility of replacing creep-resistant magnesium Mg-Zn-RE-Zr alloys (EZ33) with Mg-Al-Ca-Sr alloys. (1) Background: Mg alloys with RE metals show excellent properties. Due to their high cost, new, more economical Mg alloys are being developed. Replacing RE metals with cheaper elements such as Al and Ca allows us to obtain high mechanical properties at elevated temperatures due to the tendency to form stable intermetallic phases. (2) Methods: Microstructure analysis (LM, SEM, TEM, and XRD) was performed and mechanical properties were tested at ambient and elevated temperatures. (3) Results: Increasing the Ca content and decreasing the Al content leads to the formation of a continuous skeleton of high-melting and brittle Ca-rich Laves phases and Sr-rich intermetallic phases and the formation of plate-like precipitates of the C15 phase inside the α-Mg solid solution. The crystallographic orientation of plate-like precipitates contributes to the blocking of dislocations in slip systems activated at elevated temperatures. Increasing the Ca and Sr content allows for the regulation of the Al concentration in the α-Mg, providing solution strengthening and stability of the α-Mg solid solution. These factors contribute to a significant improvement in creep resistance of Mg-Al-Ca-Sr alloys. (4) Conclusions: The strength properties and elongation at ambient temperature of the Mg alloys with Ca and Sr addition are comparable to those of the EZ33 alloy, and due to the presence of lighter alloying elements, a better specific strength is achieved. Ca-rich Mg-Al-Ca-Sr alloys exhibit better creep resistance at temperatures of up to 200 °C compared to the EZ33 alloy. Full article

This study examines the impact of aging at 600 °C on the tensile properties of 0.3NbFeCrAl alloy at various temperatures, including room temperature (RT), 300 °C, 350 °C, and 400 °C, as well as the corresponding changes in microstructure. Results demonstrate that as aging time increases, the grain size remains relatively stable, while the amount of precipitate gradually increases and becomes uniformly distributed. The tensile strength (R_m) also increases steadily with aging time, reaching its maximum after 1000 h of aging. This can be attributed to the precipitation strengthening effect of the Laves phase after 1000 h of aging. The yield strength (R_p0.2) remains constant when the specimen is stretched at room temperature, but gradually increases with both the stretching temperature and aging time. Additionally, the section shrinkage ratio (Z) exhibits a decreasing trend with aging time, except for stretching at room temperature. Nevertheless, Z remains above 50% in all other cases, indicating that the toughness of the aged specimens is maintained well. While fracture shrinkage is significant for tensile tests at RT, it exhibits minimal change with increasing aging time. Furthermore, a notable increase in the number of dimples and a decrease in their size is observed on the tensile fracture surface with aging. Full article

Alloying provides an effective approach to designing metallic materials with unique microstructures and enhanced performance. In this work, we developed a series of (CoCrNi)_100−xZr_x (where x = 1, 2, 3, 4, and 5) medium entropy alloys (MEAs) by vacuum arc-melting method. The effects of Zr addition on the microstructures and mechanical properties of (CoCrNi)_100−xZr_x MEAs were systematically investigated. Due to the negative mixing enthalpy of Zr with Co, Cr, and Ni, lamellar C15 Laves-phase precipitates formed within the ductile FCC matrix. As the Zr content increases, the alloys exhibit higher strength but become more brittle at room temperature. Among the (CoCrNi)_100−xZr_x MEAs series, the CoCrNiZr₃ MEA shows an excellent balance between strength and ductility, achieving a compressive yield strength of 610 MPa and a hardness of 249 HV, respectively, while maintaining a good ductility beyond 45%. Microstructural analysis using scanning electron microscope and transmission electron microscope suggests that this outstanding strength-ductility balance of CoCrNiZr₃ MEA arises from the synergistic effect of precipitation strengthening and solid solution strengthening. These findings not only provide deeper insight into the interaction between different strengthening mechanisms but also offer valuable guidance for designing high-performance multi-component alloys through strategic alloying. Full article

The microstructure, texture, and mechanical properties of Inconel 718 fabricated via hybrid wire-arc additive manufacturing (WAAM) with inter-pass forging, and the subsequent modified post-deposition heat treatment (PDHT), were investigated. The modified PDHT included homogenization at 1185 °C and double ageing at 720 °C, with furnace-cooling to 620 °C; this process was first used for Inconel 718 obtained via WAAM and inter-pass forging. In the as-printed material, two characteristic zones were distinguished, as follows: (i) columnar grains with a preferable <100> orientation and (ii) fine grains with a random crystallographic orientation. The development of static recrystallization induced via inter-pass forging and further heating during the deposition of the next (upper) layer provoked the formation of the fine-grained zone. In the as-printed material, particles of (Nb,Ti)C and TiN, and precipitates of a Nb-rich Laves phase that caused premature cracking and failure during mechanical testing, were detected. In the PDHT material, two zones were found, as follows: (i) a zone with coarse uniaxial grains and (ii) a zone with a gradient grain size distribution. PDHT resulted in the precipitation of γ″ nanoparticles in the γ-Ni matrix and the dissolution of the brittle Laves phase. Therefore, significant hardening and strengthening, as well as increases in ductility and impact toughness, occurred. Full article

TiC–TiB₂ dual-phase nanoparticles were added into a Ni–Fe-based cast superalloy and their effects on the microstructure and mechanical properties were compared to those of a Ni–Fe-based superalloy with the addition of TiC nanoparticles. The addition of TiC nanoparticles led to the precipitation of a higher volume fraction of carbides. Compared to the addition of TiC, the addition of TiC–TiB₂ nanoparticles not only led to the precipitation of carbides but also promoted the formation of flaky borides and a reduction in the precipitation of the Laves phase. The strengthening effect of TiC–TiB₂ nanoparticles on the mechanical properties of Ni–Fe-based superalloys was stronger than that of TiC nanoparticles due to more secondary γ’ precipitates. This study provides valuable insights for selecting ceramic nanoparticles to increase the mechanical properties of cast Ni–Fe-based superalloys. Full article

High-temperature oxidation resistance is the major influence on the high-temperature service stability of refractory high entropy alloys. The oxidation behavior of lightweight Al_0.2CrNbTiV refractory high entropy alloy coatings with different dilution ratios at 650 °C and 800 °C deposited by high-speed laser cladding was analyzed in this paper. The oxidation kinetic was analyzed, the oxidation resistance mechanism of the Al_0.2CrNbTiV coating was clarified with the analysis of the formation and evolution of the oxidation layer, and the effect of the dilution rate on high-temperature performances was revealed. The results showed that the oxide layer was mainly composed of rutile oxides (Ti, Cr, Nb)O₂ after isothermal oxidation at 650 °C and 800 °C for 50 h. The Al_0.2CrNbTiV coating in low dilution exhibited better oxidation performance at 650 °C, due to the dense oxide layer formed with the synergistic growth of fine AlVO₃ particles and (Ti, Cr, Nb)O₂, and higher percentage of Cr, Nb in (Ti, Cr, Nb)O₂ strengthened the lattice distortion effect to inhibit the penetration of oxygen. The oxide layer formed at 800 °C for the Al_0.2CrNbTiV coating was relatively loose, but the oxidation performance of the coating in high dilution improved due to the precipitation of Cr₂Nb-type Laves phases along grain boundaries, which inhibits the diffusion of oxygen. Full article

The effects of heat treatment temperature on the microstructure and mechanical properties of Cu_0.3Cr₂Fe₂Ni₃Mn₂Nb_x high-entropy alloys (HEAs) were studied. Results indicate that in the as-cast state, an Nb₀ alloy is composed of a single FCC phase, and a Laves phase gradually forms as Nb content increases. After heat treatment at 800 °C, BCC solid solution phases rich in Cr, Fe, and Mn form in all alloys. The BCC phases in the Nb_0.2 and Nb_0.4 alloys decompose after heat treatment at 900 and 1000 °C, respectively, and the microhardness of the as-cast Cu_0.3Cr₂Fe₂Ni₃Mn₂Nb_x HEAs increases from 127 to 203 HV with increasing Nb content. After heat treatment, the microhardness of the alloys considerably improves, and the Nb_0.4 alloy has the highest microhardness after heat treatment at 800 °C (approximately 346 HV). After heat treatment at 900 and 1000 °C, the microhardness of the three alloys decreases. The yield strength of the as-cast Cu_0.3Cr₂Fe₂Ni₃Mn₂Nb_x HEAs increases with Nb content and shows a trend of first increasing and then decreasing with increasing heat treatment temperature. The strengthening mechanism of the heat-treated alloys is mainly attributed to the second-phase strengthening of the Laves phase and the solid solution strengthening of the BCC phase. Full article

The low hardness and poor wear resistance of laser-cladding 316L stainless steel impose significant constraints on its practical applications. In this study, a strategy for strengthening laser-cladding 316L stainless steel with WMoTaNb refractory high-entropy alloy as a reinforcement material is proposed. The results confirm that the coating primarily comprises a body-centered cubic (BCC) Fe-based solid solution, a network-distributed hexagonal Fe₂X (X = W, Mo, Ta, and Nb) Laves phase, and a diffusely distributed face-centered cubic (FCC) (Ta, Nb)C phase. The Fe-based solid solution distributes along columnar and fine dendrites, while the Laves phase and (Ta, Nb)C phase are in the inter-dendrites. The presence of a significant number of network Laves phases exhibiting high strength and hardness is the primary factor contributing to the enhancement of coating microhardness. The hardness of the composite coating is increased by nearly twice compared to that of the 316L coating, resulting in an improved wear resistance. The present work can shed light on designing and fabricating 316L stainless steel coating with enhanced hardness and wear resistance. Full article

Vacuum electron-beam welding (EBW) was used to join the precipitation-strengthened GH4169 superalloy and a new nickel-based superalloy IC10 to fabricate the turbine blade discs. In this study, a solid solution (1050 °C/2 h for GH4169 and 1150 °C/2 h for IC10) and different heat-exposure temperatures (650 °C, 750 °C, 950 °C and 1050 °C/200 h, respectively) were used to study the high-temperature tensile properties and microstructure evolution of welded joints; meanwhile, the formation and evolution of the second phases of the joints were analyzed. After EBW, the welded joint exhibited a typical nail morphology, and the fusion zone (FZ) consisted of columnar and cellular structures. During the solidification process of the molten pool, Mo elements are enriched in the dendrites and inter-dendrites, and that of Nb and Ti elements was enriched in the dendrites, which lead to forming a non-uniform distribution of Laves eutectic and MC carbides in the FZ. The microhardness of the FZ gradually increased during thermal exposure at 650 °C and reached 300–320 HV, and the γ′ and γ″ phases were gradually precipitated with size of about 50 nm. Meanwhile, the microhardness of the FZ decreased to 260–280 HV at 750 °C, and the higher temperature resulted in the coarsening of the γ″ phase (with a final size of about 100 nm) and the formation of the acicular δ-phase. At 950 °C and 1050 °C, the microhardness of FZ decreased sharply, reaching up to 170~190 HV and 160~180 HV, respectively. Moreover, the Laves eutectic and MC carbides are dissolved to a greater extent without the formation of γ″ and δ phases; as a result, the absent of γ″ and δ phases are attributed to the significant improvement of segregation at higher temperatures. Full article

This study offers valuable insights into the precipitation behavior of 13Ni maraging steels, emphasizing the role of molybdenum content in their microstructure, strengthening, and precipitate evolution. Precipitate morphology and crystallography were examined using a combination of high-resolution transmission electron microscopy and selected area electron diffraction. Strengthening mechanisms were assessed through Vickers hardness measurements. All the examined samples exhibited a lath martensite microstructure and displayed an increasing hardness over the aging time. The molybdenum content not only influenced the presence of retained austenite in the initial microstructure but also affected the type of precipitates formed during the early aging stages. Initially, Ni₃Mo precipitates were formed, succeeded by the formation of more stable Fe₂(Mo,Ti) Laves precipitates. The ultra-high strength of 13Ni maraging steels arises from the combination of the precipitate type and size distribution. The base composition of 13Ni maraging steels achieved a peak hardness of 798 HV1 through the precipitation of Laves Fe₂(Mo,Ti) phases ranging from 3 to 14 nm in diameter. Full article

Ten percent Cr steels are considered to be prospective materials for the production of pipes, tubes, and blades in coal-fired power plants, which are able to operate within ultra-supercritical steam parameters. The microstructural evolution of a Re-containing 10% Cr-3Co-3W steel with low N and high B content during creep was investigated at different strains at 923 K and under an applied stress of 120 MPa using TEM and EBSD analyses. The studied steel had been previously normalized at 1323 K and tempered at 1043 K for 3 h. In the initial state, the tempered martensite lath structure with high dislocation density was stabilized by M₂₃C₆ carbides, NbX carbonitrides, and M₆C carbides. At the end of the primary creep stage, the main microstructural change was found to be the precipitation of the fine Laves phase particles along the boundaries of the prior austenite grains, packets, blocks, and martensitic laths. The remarkable microstructural degradation processes, such as the significant growth of martensitic laths, the reduction in dislocation density within the lath interiors, and the growth of the grain boundary Laves phase particles, occurred during the steady-state and tertiary creep stages. Moreover, during the steady-state creep stage, the precipitation of the V-rich phase was revealed. Softening was in accordance with the dramatic reduction in hardness during the transition from the primary creep stage to the steady-state creep stage. The reasons for the softening were considered to be due to the change in the strengthening mechanisms and the interactions of the grain boundary M₂₃C₆ carbides and Laves phase with the low-angle boundaries of the martensitic laths and free dislocations. Full article

Refractory high-entropy alloys (RHEAs) are among the promising candidates for the design of structural materials in advanced nuclear energy systems. The effects of Cr, V, Ta, and Ti elements and ball milling on the microstructural evolution and mechanical properties of model RHEAs were investigated. The results show that W-rich BCC1 and Ta-rich BCC2 solid solution phases were generated after a long milling duration. After high-temperature sintering, the (Cr, Ta)-rich phase associated with the Laves phase was observed in the Cr-containing model RHEAs. In addition, a high level of Ti, Ta, and V contents promoted the in situ formation of oxide particles in the alloys. Complex TiTa₂O₇ and Ta₂VO₆ oxide phases were identified by TEM, which suggests a solid-state reaction of Ti-O, Ta-O, and V-O subjected to high-energy ball milling. The oxide particles are uniformly dispersed in the BCC matrix, which can result in dispersion strengthening and the enhancement of mechanical properties. Full article

The Inconel 718 alloy clad coating was successfully prepared via pulsed laser deposition. The effect of pulse frequency on the evolution of microstructure, hardness and tribological properties of the as-deposited samples were analyzed via scanning electron microscopy (SEM), microhardness tester and ball-on-plate tribometer. The results showed that with the decrease in pulse frequency, the cooling rate of molten pool increases gradually, which effectively refines the γ-(Ni, Cr, and Fe) dendrites and restrains Nb segregation. Hence, the morphology of the brittle Laves phase changed from long chained to granular and its volume fraction decreased from 6.59% to 2.41%. The hardness of the coating increased from 261 HV_0.1 to 297 HV_0.1 and the tribological property also improved simultaneously. The friction coefficient decreased from 0.2387 to 0.2066, and the wear rate decreased from 27.30 × 10⁻⁴ mg·N⁻¹·m⁻¹ to 19.15 × 10⁻⁴ mg·N⁻¹·m⁻¹. It can also be observed that the wear area of the counterpart, Si₃N₄ ball, increased from 2.016 mm² to 2.662 mm². The increase in the hardness and tribological property were attributed to the grain refining strengthening. Full article

This paper mainly used database technology, machine learning, thermodynamic calculation, experimental verification, etc., on integrated computational materials engineering. The interaction between different alloying elements and the strengthening effect of precipitated phases were investigated mainly for martensitic ageing steels. Modelling and parameter optimization were performed by machine learning, and the highest prediction accuracy was 98.58%. We investigated the influence of composition fluctuation on performance and correlation tests to analyze the influence of elements from multiple perspectives. Furthermore, we screened out the three-component composition process parameters with composition and performance with high contrast. Thermodynamic calculations studied the effect of alloying element content on the nano-precipitation phase, Laves phase, and austenite in the material. The heat treatment process parameters of the new steel grade were also developed based on the phase diagram. A new type of martensitic ageing steel was prepared by selected vacuum arc melting. The sample with the highest overall mechanical properties had a yield strength of 1887 MPa, a tensile strength of 1907 MPa, and a hardness of 58 HRC. The sample with the highest plasticity had an elongation of 7.8%. The machine learning process for the accelerated design of new ultra-high tensile steels was found to be generalizable and reliable. Full article

Concentrating solar power (CSP) and thermal energy storage (TES) based on molten salts still lacks economic feasibility, with the material investment costs being a major drawback. Ferritic stainless steels are a comparatively cheap class of materials that could significantly contribute to cost reductions. The addition of aluminum to ferritic steel can result in self-passivation by forming a compact Al₂O₃ top layer, which exhibits significantly higher corrosion resistance to solar salt compared to the Cr₂O₃ surface layers typically formed on expensive structural alloys for CSP and TES, such as austenitic stainless steels and Ni-base super alloys. However, to date, no ferritic stainless steel combining Al₂O₃ formation and sufficient structural strength is available. For this reason, cyclic salt corrosion tests under flowing synthetic air were carried out on seven Laves phase-forming, ferritic model alloys (17Cr2-14Al0.6-1Nb2.6-4W0.25Si), using “solar salt” (60 wt. % NaNO₃ and 40 wt. % KNO₃). The Al content was varied to investigate the influence on the precipitation of the mechanically strengthening Laves phase, as well as the impact on the formation of the Al-oxide top layer. The W and Nb contents of the alloys were increased to examine their influence on the precipitation of the Laves phase. The salt corrosion experiments demonstrated that simultaneous self-passivation against a molten salt attack and mechanical strengthening by precipitation of fine Laves phase particles is possible in novel ferritic HiperFer^{SCR (salt corrosion-resistant)} steel. Microstructural examination unveiled the formation of a compact, continuous Al₂O₃ layer on the surface of the model alloys with Al contents of 5 wt. % and higher. Furthermore, a stable distribution of fine, strengthening Laves phase precipitates was achieved in the metal matrix, resulting in a combination of molten salt corrosion resistance and potentially high mechanical strength by a combination of solid solution and precipitation strengthening. These results show that high-strength ferritic alloys are suitable for use in CSP applications. Full article