Grain refinement stands out as the primary strengthening mechanism in magnesium alloys. Zr emerges as the most effective grain refiner for magnesium alloys in the absence of Al, Si, etc. Typically, Zr is introduced in the form of an Mg-Zr master alloy. The crucial factor for achieving effective grain refinement in magnesium alloys incorporating Zr lies in regulating the morphology of Zr elements in the Mg-Zr master alloy, distinguishing between particle Zr and solute Zr. This study presents the theoretical groundwork for grain refinement. Drawing upon the growth restriction theory and heterogeneous nucleation theory, the refinement mechanism of soluble Zr and particle Zr on magnesium alloys is discussed. The discussion also identifies the engineering application bottleneck associated with Zr-refined magnesium alloys. A comprehensive review of advancements in Zr-refined magnesium alloy research is conducted, encompassing particle Zr and solute Zr. This review highlights the synergistic design strategy proposed for Zr-refined magnesium alloys. Ultimately, the anticipated development trends for Zr-refined magnesium alloys is prospected.

Third-generation advanced high-strength steel (TG-AHSS) recently garnered significant attention in the field of materials science and the automotive industry. This study focuses on the composition design, heat treatment processing, and mechanisms underlying the strengthening and deformation of TG-AHSS. The principle of composition design for TG-AHSS is expounded based on thermodynamic stability. Furthermore, several representative heat treatment processes are interpreted by generalized stability (GS). The strengthening and deformation mechanisms of the TG-AHSS are summarized from the perspective of the thermo-kinetic connectivity arising from the GS and the thermo-kinetic correlation. Finally, considering concurrently thermodynamics and kinetics, the design strategy of the TG-AHSS was summarized and outlooked.

During the service, the turbine blades of aero-engines are subjected to a complex and ever-changing combination of temperature and stress, resulting in severe cyclic temperature/strain damages and thermal-mechanical fatigue (TMF) failures of the alloy. In this work, in-phase (IP) TMF tests under 600-1000^oC were conducted on a newly developed fourth-generation single-crystal superalloy. The alloy's fracture characteristics and comprehensive damage mechanisms were examined via SEM, EBSD, and TEM. The results showed that when the strain range increased, the fatigue life of the experimental alloy noticeably decreased, and the hysteresis loop clearly opened. Stress response behaviors shifted from cyclic softening at high temperatures and cyclic hardening at low temperatures into a dominant characteristic of cyclic stabilizing. The fracture surfaces of alloys displayed ductile features after fatigue fracture under various circumstances, and the area fraction of dimples reduced with increasing strain amplitude. When the strain amplitude was low, the alloy was mainly subjected to oxidation damage, accompanied with a certain degree of creep damage. In contrast, the dominant deformation mechanism of the alloy was dislocation slipping in γ matrix and Orowan by-passing through γ' particles. As the strain amplitude increased to higher levels, the alloy was subjected to severe plastic deformation damage, while the degree of oxidation damage had been alleviated. Under this condition, the interfacial dislocations could shear into the γ' phase with the generated stacking fault or anti-phase boundary. Notably, no recrystallization grains or deformation twins were formed in the DD91 alloy during the IP-TMF experiments at different mechanical strain amplitudes.

Wrought Ni-based superalloys are widely used in aviation and energy fields because of their excellent creep resistance, thermal stability, heat corrosion resistance, and oxidation resistance at high temperatures. The mechanical properties of wrought Ni-based superalloys are significantly affected by grain boundary precipitation. Among the grain boundary second phases, the topologically close-packed (TCP) phase is usually discovered in wrought superalloys with the addition of refractory metal elements. As a complex intermetallic compound with only tetrahedral interstices, the TCP phase is stacked with a high packing density of atoms, embodying low plasticity and high brittleness. Given these characteristics, the TCP phase tends to promote crack initiation and propagation during creep, thereby reducing the alloy's creep strength. Additionally, the formation of the TCP phase requires several refractory elements, thereby weakening the effect of the solid solution strengthening of the matrix. As a ubiquitous TCP phase in wrought superalloys, μ phases are represented by rectangular and parallelogram structural subunits, which are parallel to the basal plane of μ phases. Basal stacking faults (SFs) are the most common defects in the μ phase, and SFs with different stacking sequences will form different phases with corresponding structures and mechanical properties. The μ phases and their basal SFs in wrought Ni-based superalloy GH4151 were systematically studied by multifarious electron microscopy techniques, such as EDS and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of aberration-corrected TEM, revealing the structure and composition of the μ phase and the structure and distribution of numerous basal SFs in the μ phase. Based on the different arrangements of structural subunits, the basal SFs were divided into four types. Type I basal SF is equivalent to the μ phase with a layer of parallelogram structural subunit reversing to form two layers of microsymmetric structures, the reversed parallelogram structural subunit is symmetrical to the rectangular one; type II basal SF is equivalent to type I basal SF in the absence of a layer of the rectangular structural subunit, forming a C14 structure and microsymmetric structure; type III basal SF results from the absence of a layer of the parallelogram structural subunit in the μ phase, forming a complete Zr₄Al₃ phase; and type IV basal SF results from the absence of a layer of the rectangular structural subunit in the μ phase, forming a C15 structure. Among the four types of basal SFs, type II and type IV basal SFs form Laves phases, but the occurrence of the former is more than that of the latter. This finding is related to the stability of type II basal SF (C14 structure) over type IV basal SF (C15 structure) revealed by the first-principle calculations.

Innovative, massive gas turbines have emerged as critical equipment for achieving the goals of energy conservation and the development of new clean energy sources. As the inlet temperature of industrial gas turbines continues to rise, the high-temperature capabilities of hot corrosion-resistant single-crystal turbine blades should be enhanced. This work investigates the effects of Ta on the microstructural stability and creep properties of hot corrosion-resistant Ni-based single-crystal superalloys with varying Ta contents (2Ta, 5Ta, and 8Ta) during long-term thermal exposure at 900^oC. The findings revealed that after various thermal exposure times, the addition of Ta had no observable influence on the size of γ' precipitates, but it considerably increased the cubic degree of γ' precipitates and continuously decreased the number of tertiary γ' precipitates in γ matrix. With the increase of Ta content from 2% to 5%, the volume fraction of γ' precipitates of 5Ta alloy is higher than that of 2Ta alloy except that which is close to each other when thermal exposure at 4000 h. In addition, the volume fraction of γ' precipitates increased was higher than that of 2Ta and 5Ta alloys, as the Ta content increased from 5% to 8% after various thermal exposure times. The creep lives of the three alloys at 900^oC and 275 MPa exhibited different trends as thermal exposure time increased; there was no obvious fluctuation in the 2Ta alloy after thermal exposure from 0 to 4000 h, but it significantly decreased after thermal exposure at 8000 h; the creep lives of the 5Ta and 8Ta alloys increased initially and then decreased, and the peak creep lives were 500 and 2000 h, respectively. With the addition of Ta, the steady-state creep rate continuously decreased, the creep life significantly increased, and the peak creep life shifted backward. Simultaneously, the degree and structural integrity of the rafted γ' precipitates after creep rupture increased steadily. Hence, the improvement of the creep life of the alloys was attributed to a combination of factors, such as the increase in the volume fraction of the γ' precipitates in the initial state of the creep, the increase in rafted structural integrity, and thickness of the rafted γ' precipitates during the creep process.

Owing to the excellent combination of specific strength and ductility, medium Mn steels (MMSs) with Mn contents of 3%-12% (mass fraction) are considered the most promising candidates for the third-generation advanced high-strength steel. The combination of excellent strength-ductility is mainly attributed to the active transformation-induced plasticity effect of the metastable retained austenite during deformation. Therefore, producing a considerable amount of retained austenite with reasonable stabilities in the steel by various heat treatment schedules is always important. In this study, granular- and lamellar-structured retained austenites were developed in a cold-rolled 0.15C-5Mn MMS by introducing a technical process of precontrolling ferrite recrystallization in the annealing schedule. The microstructures of the annealed samples were analyzed using SEM, EBSD, and TEM. The results show that duplex microstructures comprising various amounts of recrystallized ferrite and fresh martensite can be obtained in the cold-rolled MMS when controlling the occurrence of recrystallization at different intercritical temperatures by a preannealing process. When this microstructure is used for the final austenite reverted transformation annealing, the resultant ultrafine duplex microstructure with recrystallized ferrite and two types of heterogeneous retained austenite, i.e., lamellar and granular, is produced. The heterogeneous-structured austenite shows more sensitivity to increasing strain, i.e., various mechanical stabilities, which enable an excellent strength-ductility combination and reduced Lüders strain in the cold-rolled medium Mn steel.

With the continuous improvement of the pressure-bearing capacity of pressure vessels, higher requirements are put forward for the mechanical properties of materials. 40CrNi3MoV steel is a typical pressure-vessel steel, which mainly depends on carbide strengthening. To further improve its mechanical properties, Al and Cu were added to the material to form intermetallic compound precipitates for strengthening, then the precipitation behavior of NiAl and Cu and their effects on mechanical properties were studied. The size, composition, structure, and morphology of NiAl and Cu precipitates were characterized by SEM, TEM, and EDS. The distribution characteristics of precipitate-forming elements were characterized by three-dimensional atomic probe, and the mechanical properties of the experimental steel were compared. The results show that B2-NiAl precipitates coherent with the matrix are formed in the experimental steel after adding only Al. These NiAl precipitates are mainly precipitated at the grain boundaries and are large; after adding Cu, a bcc-ordered Cu-rich phase coherent with the matrix is precipitated. At this point, the large-scale precipitation of the NiAl phase at the grain boundaries is reduced and nanoscale NiAl precipitation is promoted in the crystal. The mismatch between the NiAl precipitates and the matrix lattice in the experimental steel is small, and the tensile strength increases by 200 MPa; The uniformly distributed, fine Cu-rich phase and the refined NiAl phase increase the resistance to dislocation, and the yield strength is also increased by 200 MPa. However, a large quantity of fine and high-density NiAl and Cu precipitates reduce the critical strain of cracks in the tensile process. Therefore, compared with the experimental steel with only Al added, the tensile strength is not improved.

Compared with selective laser melting (SLM), the micro-SLM (M-SLM) technology offers the advantages of small spot diameter (< 20 μm), high forming precision (20-50 μm), and surface roughness (R_a) of up 1 μm, which implies that the M-SLM technology provides great potential for promotion and application in communication electronics, biomedical, and other fields in the future. In this work, 316L stainless steel was prepared using M-SLM, and its tensile properties and fracture behavior were studied. The microstructures of transverse and longitudinal tensile specimens were also investigated. In addition, the fracture morphology was characterized and analyzed, and the grain orientation and grain-boundary-characteristic distribution in the near-section plastic-deformation zone were further analyzed using electron backscatter diffraction (EBSD). The results showed that the 316L stainless steel prepared by M-SLM had a cellular structure with a size of 100-300 nm inside the grains. The tensile fracture was dimple-shaped, and the average dimple diameter was 80-500 nm, which allowed the transverse average tensile strength of the 316L stainless steel to reach 692.1 MPa, the longitudinal average elongation after fracture was 54.6%, which were obviously better than that of the 316L stainless steel prepared using traditional SLM. The appearance of austenite Σ3 twin boundaries in the stretching process of the 316L stainless steel prepared by M-SLM was related to the grain orientation, which could more likely appear in grains with an orientation close to <111>. Further analysis indicated that the appearance of Σ3 grain boundaries blocked the connectivity of the special grain-boundary network. Statistical analysis of the coherent Σ3 (Σ3_c) and incoherent Σ3 (Σ3_ic) grain boundaries using the EBSD-based rectangular-section method revealed that the amount percentages of Σ3_c and Σ3_ic in the near-fracture region of the 316L transverse tensile specimen were approximately 43% and 57%, respectively. Meanwhile, the amount percentage of Σ3_c in the same region of the 316L longitudinal tensile specimen increased to approximately 70%. The increase in the coherent Σ3_c twin boundary reduced the total grain-boundary energy, which explained why the longitudinal tensile strength of the 316L stainless steel prepared by M-SLM was generally lower than the transverse tensile strength.

Titanium (Ti) has a strong sensitivity to oxygen atoms. Adding interstitial oxygen to pure Ti can greatly alter its mechanical behavior. Oxygen atoms increase strength and hardness while making Ti brittle. Therefore, controlling the oxygen content in Ti is extremely important. To better understand the influence of oxygen on the mechanical behavior of pure Ti, the plastic deformation behavior of nano-polycrystalline α-Ti with different interstitial oxygen content was studied. Molecular dynamic simulations were performed using the second nearest-neighbor modified embedded atom method and the charge equilibration (Q_eq) method to investigate the effect of O content, tensile temperature, and strain rate on the tensile mechanical properties and deformation mechanism of nano-polycrystalline α-Ti. Results indicate that the yield stress of nano-polycrystalline α-Ti increases with the increase of interstitial O content. {10\overline{\mathrm{1}}0}<10\overline{\mathrm{1}}2> deformation twin was observed when the O content is less than 0.3%, and twin growth was mediated by well-defined “zonal dislocations” at the twin boundary. Different activated slip systems were transformed and diversified when the O content is larger than 0.3%, that is, the prismatic, basal, and pyramidal <c + a> slip systems were simultaneously activated, and the dislocation type changed to edge dislocations. The plastic deformation of nano-polycrystalline α-Ti was mediated by dislocation and grain boundary. In addition, the mobility of the grain boundary increased significantly with the increase of tensile temperature and strain rate. The formation of new grains was accompanied by Ti_hcp→Ti_bcc→Ti_hcp phase transformation, which was due to the relative rotation of the grains. The number of new grains increased with the increase of strain rate. The current work reveals the mechanical properties and deformation mechanism of nano-polycrystalline α-Ti, which promotes the design, and development of Ti-based nano-structured alloys with superior mechanical properties.

In recent years, solidified structures in metal alloys driven by combined electromagnetic fields have received considerable attention. In this study, the effect of combined magnetic field (CMF) formed by pulsed magneto oscillation (PMO) and static magnetic field on the melt flow of Ga-20%In-12%Sn (mass fraction) and solidification structure of Al-7%Si alloy was investigated. Results of numerical simulations and flow experiments show that the flow pattern is single rolled in the direction parallel to the static magnetic field and double rolled in the direction perpendicular to the static magnetic field, which is different from the double-rolled flow pattern of the melt when the PMO is activated alone. The distribution and evolution of the electromagnetic forces in the melt under CMF are numerically simulated to explain the formation of the flow pattern. Moreover, the experimental results of solidification show that the grain size of the CMF-treated Al-7%Si alloy is smaller than that obtained when PMO is applied. Finally, the grain refinement mechanism of the Al-7%Si alloy under the influence of electromagnetic fields is discussed in relation to the effects of induced currents, electromagnetic forces, and forced flow based on the previously proposed mechanism of grain refinement in solidified metals driven by electromagnetic fields.

Lightweight structural materials with excellent strength and good ductility are extensively used in engineering applications. Although nanostructured Al alloys have relatively low density and high strength resulting in high specific strength, their application is severely limited due to their poor ductility. Recently, additive manufacturing (AM) techniques have been rapidly developed and complex lattice structures can be manufactured by AM. Here, a new composite containing titanium alloy lattice structure and nanostructured Al alloy was created. Selected laser melting is used to generate the TC4 three-dimensional lattice structure, which is subsequently hot extruded with the high-strength nanostructure Al₈₄Ni₇Gd₆Co₃ aluminum alloy. Tensile mechanical characteristics and fracture behavior were studied. The research results reveal that the TC4 lattice structure in the composite remains intact and the interface remains flat and clear, and the α and β phases are elongated along the extrusion direction to form a fine lamellar structure. There is a significant volume proportion of nanostructured intermetallic phases and nanocrystalline fcc-Al in the nanostructured aluminum alloy areas. The mechanical property test results reveal that the TC4 three-dimensional lattice structure has a clear limiting influence on fracture initiation and propagation in the nanostructured aluminum alloy region, resulting in good comprehensive tensile mechanical properties of the composite.

γ-TiAl is a family of promising structural materials with low density, high stiffness, and good oxidation and creep resistances at elevated temperatures. They can replace heavier nickel-based alloys at 600-800^oC; thus, they are used in the construction of low-pressure turbine blades for aeroengines, i.e., General Electric next-generation and leading edge aviation propulsion. Grinding is an important processing step in blade production to ensure the accuracy of assembling. However, the limited ductility and fracture toughness at room temperature and low thermal conductivity of γ-TiAl alloys narrow the parameter windows of the grinding process. Cracks often form on the surface when the processing parameters are not well controlled. Additionally, grinding greatly influences the surface integrity (i.e., roughness, microstructure, and hardness), which influences the mechanical properties, especially those of brittle γ-TiAl alloys that are sensitive to notch. Grinding depth is a major parameter in blade production because it influences quality and efficiency. Investigating the effect of grinding depth on the surface integrity and fatigue properties of γ-TiAl samples is necessary to optimize the grinding process and identify the major factors of surface integrity that guarantee optimal mechanical properties. In this work, cast γ-TiAl alloy (Ti-45Al-2Nb-2Mn-1B, atomic fraction, %) samples were ground with different depths. The surface integrity (surface roughness, microstructure, and microhardness) and fatigue properties of the samples were compared. Cracks were detected in samples ground to 0.5 and 1 mm depths, while no cracks were detected in samples ground to 0.2 mm or less depths, this is related to the tensile stress induced by temperature increase caused by deformation heat. With the increased grinding depth, the number and depth of grooves increased and for the surface roughness parameters, arithmetic mean deviation and 10-point mean roughness (R_z) increased, while skewness decreased. The γ + α₂ lamellae bended in the surface layer and layer thickness increased with the increased grinding depth. The microhardness initially decreased and then increased from the surface to the interior. The rotating bending fatigue life at 650^oC under a load of 440 MPa decreased with the increased grinding depth: it was > 10⁶ cyc at 0.05 mm grinding depth but dropped to ~10⁴ cyc at 0.2 mm grinding depth. Fracture surface analysis showed that the cracks mainly nucleated at the surface grooves caused by grinding, which resulted in stress concentration and reduced the fatigue life of samples ground to 0.2 mm depth. The fatigue life decreased with increasing R_z, but remained above 10⁶ cyc when R_z was less than 4 μm. A nonlinear relationship between fatigue life and R_z was shown.