This work focuses on a 165 ksi grade 20CrMoV high-strength OCTG steel. The influence of tempering temperature on the microstructural evolution and mechanical properties was systematically examined by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), room temperature tensile testing, and 0 °C Charpy impact testing. The quantitative relationships between the precipitation behavior of MC carbides and the strength–toughness combination were established. The results show that as the tempering temperature rises, strength gradually decreases while toughness improves continuously. An optimal balance of strength and toughness is achieved upon tempering at 620 °C, where the yield strength remains above 165 ksi and the 0 °C impact energy reaches 110 J. The synergistic action of molybdenum and vanadium promotes the dense precipitation of nanoscale MC and M3C carbides when tempered around 620 °C. These finely dispersed nanoprecipitates suppress a pronounced drop in dislocation density through a pinning effect, thereby intensifying precipitation strengthening and retarding a steep loss of strength. Meanwhile, the uniform distribution of nanoprecipitates favors a homogeneous stress distribution during impact loading, delaying the accumulation of geometrically necessary dislocations. A high fraction of high-angle grain boundaries effectively inhibits crack nucleation and propagation, consequently imparting a marked improvement in impact toughness. The findings provide a theoretical basis and practical guidance for process optimization and the precise tailoring of strength and toughness in 165 ksi grade high-strength OCTG steels.

65Mn steel is widely used, and its composition design is characterized by a high content of C and Mn, supplemented by a certain amount of Si element. In the process of hot rolling production, the oxidation burning loss of this steel is too large and the phenomenon of scale pressing is relatively serious. In this paper, the effect of Sb addition on the oxidation characteristics of 65Mn was innovatively analyzed. Continuous oxidation and isothermal oxidation experiments were carried out by thermogravimetry, and the characteristics of scale interface and Sb enrichment were observed by means of electron microscopy and electron probe. It was found that the addition of Sb element significantly inhibited the total oxidation weight gain of 65Mn steel. After the addition of Sb, the flatness of the high-temperature oxidation interface increased significantly. It was found that Sb element can increase the oxidation free energy of steel by using 700–900 °C isothermal oxidation experiment. The addition of Sb element is more beneficial to increase the oxidation resistance of steel under high temperature conditions, which is mainly because the surface enrichment of Sb element on interface that inhibits the outward diffusion rate of iron ions, and on the other hand prevents the liquefaction of high temperature fayalite phase into the matrix which accelerates the oxidation.The increase of Sb element can reduce the thickness of the scale during the hot rolling process, reduce the oxidation loss and improve the surface quality of the hot coil. However, after the precipitation of Sb element reaches solid solubility, it is easy to cause surface quality problems during continuous casting or hot rolling.

Inconel 617B superalloy (IN617B) is a prospective material for the next generation of ultra-supercritical thermal power generation units, with its high-temperature reliability being crucial to the performance of associated components. This study employs scanning electron microscopy (SEM), transmission electron microscopy (TEM), and chemical composition analysis to systematically investigate the microstructural evolution of IN617B during service at 850 °C and its impact on mechanical properties. The findings reveal the presence of three types of precipitates during the aging process: M23C6, M6C, and γ′. For aging times less than 500 h, the precipitation of M23C6 within the matrix significantly enhances the alloy's strength. Conversely, the formation of M6C at the grain boundaries markedly reduces impact energy. When the aging time surpasses 500 h, M23C6 tends to precipitate and grow at the grain boundaries. Concurrently, the stability of M23C6 within the matrix diminishes, leading to its decomposition and the subsequent formation of M6C and γ′ at the M23C6/matrix interface. This process effectively enhances the alloy's strength and maintains its stability. However, the precipitation of γ′ significantly inhibits the growth of M6C, creating a competitive dynamic between them for Ni. Throughout the aging process, the precipitation behavior of M23C6 within the matrix was crucial in enhancing the alloy's strength, while the impact energy was primarily influenced by the precipitation behavior of M6C at the grain boundary. The findings offer valuable insights for the regulation of the microstructure and properties of IN617B at elevated service temperatures.

Uranium-molybdenum (U-Mo) alloys are critical for nuclear power generation and propulsion because of their superior thermal conductivity, irradiation stability, and anti-swelling properties. This study explores the plastic deformation mechanisms of gamma-phase U-Mo alloys using molecular dynamics (MD) simulations. In the slip model, the generalized stacking fault energy (GSFE) and the modified Peierls-Nabarro (P-N) model are used to determine the competitive relationships among different slip systems. In the twinning model, the generalized plane fault energy (GPFE) is assessed to evaluate the competition between slip and twinning. The findings reveal that among the three slip systems, the {110}< 111 > slip system is preferentially activated, while in the {112}< 111 > system, twinning is favored over slip, as confirmed by MD tensile simulations conducted in various directions. Additionally, the impact of Mo content on deformation behavior is emphasized. Insights are provided for optimizing process conditions to avoid gamma -> alpha '' transitions, thereby maintaining a higher proportion of gamma-phase U-Mo alloys for practical applications.

CF170 is an ultralow-carbon, cobalt-free maraging stainless steel with a tensile strength of 1700 MPa, making it an ideal material for high-load gears in aerospace robotic arms. However, these gears are subjected to a long ageing process during ion nitriding, resulting in remarkable variations in the austenite volume fraction (Va), which strongly affects the dimensional accuracy of the product. Therefore, accurately predicting Va is crucial for optimizing the chemical heat treatment process of CF170 steel. In this work, the nonisothermal and isothermal ageing process, combined with Kissinger analysis and the Johnson‒Mehl‒Avrami (JMA) model, proved that the nonisothermal kinetic model was applicable for the prediction of Va. Moreover, models of the preexponential factor K0 and maximum austenite volume fraction (Vmax) were proposed, and a kinetic model of austenite phase transformation after ageing at 482–593 °C for 0–6000 min was established, which predicted and controlled the microstructure within a large temperature and time range. The nucleation mechanism of austenite in CF170 steel was investigated by scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM). Nucleation was regulated by a shear mechanism, with nucleation occurring primarily at the grain boundary, packet, block, and subblock interfaces. The nucleation was the reversion of γ → α → γ, and the austenite inherited the crystal orientation features of the prior austenite, demonstrating the “austenite memory” phenomenon.

Precipitate was typically characterized by the transmission electron microscopy (TEM) or scanning electron microscopy (SEM) to analyze the relationship between material property and precipitate fraction. However, these procedures tend to be are always time consuming because of the complicated sample preparation process involved and the observation course. Particularly, sometime the precipitate can't be highlighted solely from the images easily. In this study, atomic force microscopy (ATM) was performed to characterize the morphology and fraction of the precipitate in this paper. Five kinds of materials were selected and prepared by electropolishing or vibration polishing method to display the precipitates on the sample surface. The experiment results prove that different types of precipitates with higher values for height on a sample surface could be clearly observed by AFM, and the images quality is highly relied upon the surface quality. The precipitate fraction can be calculated using the AFM micrographs and image photo post treated software (IPP). It was turned out that AFM is found suitable for observation almost all kinds of precipitates, and the precipitates can be easily separated from the images. Not only vibration polishing but also electrolytic polishing could offer a smooth surface for observing nanosized precipitates by AFM. A comparison of the fraction result obtained by AFM and precise microchemical analysis proves that the fraction measurement result obtained by AFM is acceptable.

A novel type of austenite–martensite dual-phase laminate steel with a yield strength of 2.2 GPa, ultimate tensile strength of 2.5 GPa, uniform elongation of 9.4% and total elongation of 12% was produced. Such unexpectable mechanical properties were mainly due to the synergistic strengthening of nanolaminate microstructure, high densities of dislocations and nanoprecipitates. The high uniform elongation at very high stress could be attributed to the strongly enhanced work hardening rate, mainly enabled by transformation-induced plasticity (TRIP) of the austenite reverted by annealing, as well as the cooperative deformation and stress/strain partitioning between the austenite–martensite stacked nanolaminate.

The effects of austenitizing temperature and holding time on oxidative decarbonization behavior of bare 22MnB5 steel sheet were investigated by OM, SEM, XRD, XPS, and room temperature tensile measurement. The oxidation kinetic model of 22MnB5 steel was calculated and revealed that early oxidation was controlled by chemical reaction, and later oxidation was controlled by elemental diffusion, for oxide layer structure from inside to outside of SiO2 + Cr2O3, wustite (FeO + Fe3O4), Fe3O4, and Fe2O3. With the increase in heating temperature (890-950 °C) and holding time (5-15 min), both oxidation and decarburization layers of 22MnB5 steel increased continuously, leading to a decrease in tensile strength and increase in elongation, making an increase in product of strength and ductility of about 1-4 GPa%. Based on the oxidative decarbonization and mechanical properties, the optimal heat treatment of 22MnB5 steel is 920 °C for 5 min.

Precipitate evolution characteristics of a Ni-based alloy containing tungsten during thermal exposure at 850 °C from 100 to 3000 h and its influence on the impact toughness were investigated. Transmission electron microscopy, scanning electron microscopy, and precise chemical analysis were used to analyze precipitate shape, type, composition, and growth mechanism. Experimental results proved that the μ, σ, M23C6, and M6C phases were formed and grew during thermal exposure at 850 °C. The μ phase was mostly formed within the grain or around the grain boundary in a strip-like or irregular shape and a stacking fault interior. M23C6 was formed at twin boundaries in a quadrilateral shape with one pair of parallel sides. However, at the grain boundary, the precipitate was mainly M6C and shaped in an irregular polygon. In addition, during long-term thermal exposure at 850 °C, M6C grew along the grain boundary and formed a chain-like structure. The μ phase can nucleate and grow independently or in clusters. Particularly, M6C and Ti(CN) or TiC can be the nucleation core for the μ phase. The precipitate size and quantity both increased with longer exposure time. Meanwhile, mechanical property testing indicated that the impact toughness worsened at the initial stages of aging, but remained relatively constant during thermal exposure from 500 to 3000 h. Microchemical phase analysis and EDS results showed that impact toughness deterioration for samples exposed to less than 500 h thermal treatment may be caused by the coarsening of grain boundary carbides and chemical composition fluctuation of the precipitate.

High-/medium-entropy alloys (H/MEAs) of face-centered-cubic-structured single phase usually suffer from a glaring drawback of low yield strength. Even worse, the trade-off emerges frustratingly between strength and ductility as strength increases. Here, the lamellar heterostructure (HS) is designed in an equiatomic ternary CoNiFe MEA by means of cold rolling followed by an incomplete recrystallization annealing. The lamellar HS consists of the soft recrystallized grains as well as severely deformed structures which are partly reserved. By comparison to the coarse-grained counterpart, the lamellar HS, shows a well enhanced yield strength-ductility synergy, together with an increased yield strength. This is ascribed to the hetero-deformation-induced (HDI) stress in HS during tensile deformation. Accordingly, the HDI strain hardening is induced, serving as an important addition to the conventional forest hardening. The HDI hardening is evidenced experimentally to account for a large proportion of global strain hardening. Furthermore, a fully recrystallized microstructure is obtained to show a simultaneous increase in both yield strength and ductility. The microstructures are evaluated in detail prior to and after tensile deformation by using the electron backscattered diffraction and transmission electron microscope observations. The mechanism for HDI strain hardening in various microstructures is analyzed to correlate to the evolution of microstructures in terms of the kernel average misorientation values, Schmid factor, and dislocation behaviors in response to plastic deformation.

Effect of microstructure size and type on the hardness for the duplex steel were disclosed by using of optical microscope (OM), scanning electron microscope (SEM) and nanoindenter for the samples hot compressed under different temperature with reduction of 10%, 30%, 50% and 70%. OM and SEM were used to measure the average martensite lamellar width, space and indenter morphology. nanoindenter test characterized the microstructure hardness for the samples under different process. Experiment results show that martensite hardness for the sample hot compressed at 950°C has larger diversity than that of sample hot compressed at 1200°C. The martensite hardness fluctuation range for the sample compressed at 950°C is almost from about 7GPa to 12GPa, while, for the sample compressed at 1200°C, the fluctuation range is basically from about 9GPa to 12GPa. However, the average hardness for the samples hot compressed at 950°C is comparably smaller, which is related with lower quench temperature. The larger martensite hardness fluctuation is mainly related with induced ferrite formation and finer martensite lamellar width. For the ferrite phase, the hardness fluctuation range is lower.

The effect of aging on transformation behavior of reverted austenite and impact toughness in Co-free maraging stainless steel were investigated via thermodynamic calculation, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Excluding the film-shaped austenite growing along the phase interface, other shapes of reverted austenite are evolved from the growth and aggregation of acicular austenite in the range of 300-600 °C. Under N-W orientation relationship, {111}γ grows inside the martensite lath along <100>α, and austenite merges in <100>α and <110>α simultaneously. Under K-S orientation relationship, the growth direction of {111}γ is 60° or parallel to <112>α. From 300 to 500 °C, Ni prefers to diffuse into η-Ni3Ti and matrix. The precipitation of Ni3Ti hinders the formation of reverted austenite and significantly deteriorates the toughness. Above 500 °C, due to the coarsening of Ni3Ti and the recovery of matrix, the resistance to the formation of austenite is obviously weakened, and then austenite plays a leading role in the improvement of toughness. When the aging temperature reaches 600 °C, the dissolution of Ni3Ti promotes the formation of austenite and η-Ni3Ti changes to γ'-Ni3Ti. The interaction between Ni3Ti and reverted austenite essentially depends on the diffusion behavior of Ni.

Low yield strength is the bottleneck of the face-centered cubic-structured high-entropy alloys (HEAs). Here, the strategy of hetero-deformation-induced (HDI) plasticity is applied by hetero-structuring to induce strengthening and strain hardening for a simultaneous increase in both yield strength and ductility in a Fe50Mn30Co10Cr10 HEA. The coarse-grain (CG) microstructure is a dual phase consisting of face-centered cubic (γ) and hexagonal close-packed (ε) phases, along with phase transformation from γ to ε to happen during tensile deformation. The hetero-structure (HS) was designed, besides recrystallized γ and ε, specifically to reserve a part of deformed γ after cold rolling followed by incomplete recrystallization. Yield strength increases from 200 MPa in CG to 760 MPa in HS, while uniform elongation (i.e., ductility) increases from 35% to 38%. The tensile load-unload-reload testing showed the ceaselessly presence of hysteresis loop during each unload-reload cycle. Both the residual plastic strain and HDI stress were measured with tensile strains in both HS and CG, providing solid evidence of the effect of HDI plasticity. To be specific, the HDI stress is found to account for a large proportion of global flow stress in HS as compared to that in CG. It turns out that the HDI plasticity facilitates both HDI strengthening and HDI strain hardening, which play the crucial role in enhancing strength and ductility. The microstructural origin of HDI plasticity in HS was ascribed to plastic incompatibility at hetero-interfaces of among varying grains as evidenced by the evolution of Schmid factor and KAM values as well.

Aging is an important heat treatment process for maraging stainless steel. During aging, the interaction between intermetallic compounds and dislocations has a significant impact on the mechanical properties of the material. In this article, the effect of aging temperature on the precipitation behavior and mechanical properties of Fe–Cr–Ni maraging stainless steel was studied by means of a three-dimensional atom probe (3DAP) and high-resolution transmission electron microscopy (HRTEM). The results show that coherent Ni3(Ti, Al) precipitates by heterogeneous nucleation from Ni/Ti/Al coclusters formed at defects (mainly dislocations) in the range of 350–450 °C, which leads to a rapid increase in strength and a sharp decrease in toughness. At 450–500 °C, the interface between Ni3(Ti, Al) and the matrix gradually changes from coherent to semicoherent, and an increase in the equivalent precipitate radius is accompanied by a decrease in their density. The tensile strength of the material reaches its peak value, and the toughness is improved. Within the range of 500–600 °C, Ni3(Ti, Al) continues to grow, aggregate and coarsen, and the amount of reverted austenite increases significantly. Both effects lead to a prominent decrease in the tensile strength and a substantial increase in toughness. The Ni3(Ti, Al) grows axially along the <111> dislocations through tube diffusion. The radial growth is parallel to [110]α and [11‾2]α, and the process is affected by the climbing and slipping of misfit edge dislocations. The strength increment of precipitates with different sizes formed at the peak aging temperature is calculated according to the critical transition radius, and the superposed results of the cutting and bypassing mechanisms during the yielding stage are found to be consistent with the experimental data, indicating that the two mechanisms work simultaneously. The findings provide a more accurate method for predicting the yield strength of materials.

The thermal processing parameters is very important to the hot rolling and forging process for producing grain refinement in lightweight high-manganese and aluminum steels. In this work, the high temperature deformation behaviors of a low-density steel of Fe30Mn11Al1C alloyed with 0.1Nb and 0.1V were studied by isothermal hot compression tests at temperatures of 850–1150 °C and strain rates between 0.01 s−1 and 10 s−1. It was found that the flow stress constitutive model could be effectively established by the Arrhenius based hyperbolic sine equation with an activation energy of about 389.1 kJ/mol. The thermal processing maps were developed based on the dynamic material model at different strains. It’s shown that the safe region for high temperatures in a very broad range of both deformation temperature and deformation strain and only a small unstable high deformation region, located at low temperatures lower than 950 °C. The deformation microstructures were found to be fully recrystallized microstructure in the safe deformation region and the grain size decreases along with decreasing temperature and increasing strain rate. Whereas the deformation microstructures is composed by grain refinement-recrystallized grains and a small fraction of non-recrystallized microstructure in the unstable deformation region, indicating that the deformation behaviors controlled by continuous dynamic recrystallization. The Hall Petch relationship between microhardness and the grain size of the high temperature deformed materials indicates that high strength low-density steel could be developed by a relative low temperature deformation and high strain rate.

对比研究了锻态0. 15C5Mn钢和0. 15C5Mn2Al钢在室温下和750 ℃准静态拉伸条件下的力学性能,并对微观组织利用SEM和EBSD进行表征.研究结果表明,A1的加入引起了室温下的微观组织结构的不同,含铝钢在室温下的组织中存在很少量铁素体,导致含铝钢强度低;锻态0.15C5Mn钢和0.15C5Mn2Al钢在750 ℃下分别获得了90.5％和101％的伸长率;经750 ℃拉伸变形后0.15C5Mn钢获得马氏体组织,A1元素的添加扩大了双相区,使0.15C5Mn2Al钢在双相区拉伸变形,最终得到铁素体+马氏体双相组织,双相区变形使0.15C5Mn2Al钢具有较高的伸长率,降低了抗拉强度.

The high temperature tensile test and microstructure characterization of 0.10C5Mn2Al steel with strain rates 10-2/s at 750 ~ 850 ℃ are carried out. The results show that the elongation of 0.10C5Mn2AI steel strain rate 10-2/s at 750 °C ,800 °C and 850 ℃ is 320% ,570% and 730% respectively to obtain superplasticity. In the process of superplastic deformation, both austenite and ferrite grains grow due to the combination of deformation and high temperature and the reformation is the main cause of grain growth.

The effects of heat treatment on the microstructure evolution was studied in regards to austenite nucleation and grain growth. It was found that the austenite nucleation and matrix recrystallization kinetics of samples annealed at 675 °C for different times were revealed, implying a strong interaction between the ferrite matrix and austenite was revealed. The recrystallization of the matrix during annealing provided favorable conditions for austenite nucleation and growth, and the formation of austenite during this process reduced the matrix recrystallization kinetics, thus delaying the recrystallization process of the matrix around the austenite grains. The statistical results for the austenite grain size under different annealing temperatures indicated that the average grain size of the austenite slightly increases with increasing of the annealing temperature, but the austenite with the largest grain size grows faster at the same temperature. This difference is attributed to the strict Kurdjumov Sachs (KS) orientation relationship (OR) between the austenite grains and the matrix, because the growth of austenite with a strict KS OR with the matrix is often inhibited during annealing. In contrast, the austenite maintains a non-strict KS OR with the matrix and can grow preferentially with increasing annealing temperature and time.

We report deformation bands ahead of the crack tip in a CrCoNi alloy under impact loading, and analyze the effects of deformation twinning on strain localization in the form of shear banding. Due to low stacking fault energy, upon high-strain-rate deformation this medium-entropy alloy forms twins with thickness and spacing of only a few nanometers. These nano-twins are embedded dynamically not only ahead of any developing shear band to retard its advance, but also in its interior to work harden. This dual effect helps to suppress runaway shear banding instability that instigates failure and contributes to the energy absorption as recorded in instrumented Charpy tests.

A hot-rolled medium Mn (0.2C5Mn) steel is annealed at 650 °C to produce an ultrafine-grained duplex microstructure with different austenite volume fractions by austenite reverted transformation (ART) annealing, and the orientation relationship strictly obeys K–S orientation relationship before deformation. Tensile tests are carried out in a temperature range from − 196 to 400 °C to examine the effects of the austenite volume fraction and the deformation temperature on the tensile properties and the austenite stability. Microstructural observations reveal that the metastable austenite gradually transformed into α-martensite, which is controlled by the deformation strain, the temperature and the austenite volume fraction. Both strain hardening behavior and ductility of the studied steel are dependent on austenite volume fraction and deformation temperature significantly. The stress–strain curves of ART-annealed 0.2C5Mn steel assume an S shape and a very large work hardening rate of about 10 GPa is obtained at liquid nitrogen deformation temperature. Based on the experimental data, a quantitative relation is proposed to describe the ductility dependence on both the austenite volume fraction and its mechanical stability.