In view of developing novel alloys for applications in supercritical water-cooled reactor fuel cladding and in-core components, a 12%Cr reduced activation ferrite/martensite (RAFM) steel with good corrosion resistance and irradiation performance was developed. V and Ta were added to form fine MX type carbonitrides and enhance the high temperature creep rupture strength. Microstructure stability of the steel during long-term aging at 650 °C was studied experimentally combined with the simulation of Thermo-Calc and DICTRA software. The results show that the precipitates in the steel during long-term aging contain M23C6, MX and Laves phase. M23C6 carbides play a major role in the stabilization of the tempered martensite lath structure by exerting a large Zener pinning force as compared with MX and Laves phase. Adding V and Ta in the steel can not only promote MX precipitation, but also refine M23C6 carbides and thus improve the thermal stability of lath/subgrains, which is beneficial to the improvement of high temperature microstructure stability of the 12%Cr RAFM steel.

•A special γ′ morphological instability in a new Ni–Cr–Co-based P/M superalloy was studied.•Three heat treatments were applied to the alloy and microstructures were observed.•Microstructure of the alloy was homogenized by sub-solvus solution heat treatment.•Sub-solvus solution heat treatment influences morphology of γ′ fan-type structures.•Sub-solvus solution heat treatment makes γ′ fan-type structures regular and stable.The influence of the sub-solvus solution heat treatment on the microstructure, especially the γ′ morphology (γ′ fan-type structure), and microhardness of a new Ni–Cr–Co-based powder metallurgy superalloy was studied by means of field emission scanning electron microscopy (FESEM) and microhardness testing. The results show that sub-solvus solution heat treatment changes the microstructure of an as-forged alloy. It makes large primary γ′ phases at grain boundaries smaller and the distribution of secondary γ′ phases in the interior of the grains more homogeneous. Moreover, the grain boundaries widen because of the supplementary precipitate. The sub-solvus solution heat treatment before the super-solvus solution heat treatment does not change nucleation sites of the γ′ fan-type structures which precipitate during the super-solvus solution heat treatment. However, it influences the morphology of γ′ fan-type structures. Length distribution of the secondary γ′ dendrites in fan-type structures changes from a bimodal to a unimodal distribution, which means the lengths of the secondary γ′ dendrites become more uniform. Applying a sub-solvus solution heat treatment after the super-solvus solution heat treatment causes the secondary γ′ dendrites to be broken off in the fan-type structures and a refinement of the γ′ phases, and this improves stability of the γ′ phases.

The compressive deformation behaviors of a low carbon vanadium microalloyed steel and a medium carbon vanadium microalloyed steel were investigated at the temperatures from 900 °C to 1100 °C and strain rates from 0.005 s−1 to 10 s−1 on Gleeble-1500 thermo-mechanical simulator. It was found that increasing the carbon content of vanadium microalloyed steels decreased the flow stress at lower strain rates, whilst at higher strain rates carbon addition led to higher flow stress, especially at the initial stage of deformation. The flow stress constitutive equations of hot deformation were developed for the experimental steels; results showed that carbon addition has the trend to reduce the hot deformation activation energy. The dependence of the characteristic points under different deformation conditions on the Zener–Hollomon parameter and the relationship between critical strain (critical stress) and peak strain (peak stress) of the experimental steels were analyzed, and the results were in reasonable agreement with those reported before. The work hardening behavior of both steels was investigated and it was found that carbon addition can decrease the work hardening rate when strained at lower strain rates. Dynamic recrystallization analysis showed that carbon addition can accelerate the occurrence and the rate of dynamic recrystallization.

•The effects of Nb and C on hot flow behavior of Nb microalloyed steels were investigated.•Nb addition increases the hot deformation flow stress while C addition generates a softening effect.•Nb addition increases the hot deformation activation energy while C addition reduces the hot deformation activation energy.•Nb addition increases the characteristic strains while C addition reduces the characteristic strains.The compressive deformation behaviors of a C–Mn steel (0.36C–1.42Mn) and two Nb microalloyed steels (0.35C–1.41Mn–0.044Nb and 0.055C–1.42Mn–0.036Nb) were investigated at the temperatures from 900 °C to 1100 °C and strain rates from 0.005 s−1 to 10 s−1 on Gleeble-1500 thermo-mechanical simulator. It was found that the flow stress of the C–Mn steel is the lowest among the experimental steels, indicating that Nb microalloying in HSLA steels can effectively increase the hot deformation flow stress, and the 0.055C–1.42Mn–0.036Nb steel has a higher flow stress than that of the 0.35C–1.41Mn–0.044Nb steel, indicating that C addition generates a softening effect. The flow stress constitutive equations of hot deformation were developed for the experimental steels, the activation energy Q about 360 kJ/mol for the 0.055C–1.42Mn–0.036Nb steel was higher than that for the 0.35C–1.41Mn–0.044Nb steel (347 kJ/mol) and the C–Mn steel (278 kJ/mol). Characteristic points of flow stress for the three steels were analyzed. The results showed that Nb addition can effectively increase the peak strain and the steady state strain of steels, thus delay distinctly the occurrence of dynamic recrystallization, while C addition can reduce the peak strain and the steady state strain of Nb microalloyed steels, thus promote the occurrence of dynamic recrystallization.

Two medium carbon steel grades were used in the present investigation. One of them was microalloyed with vanadium. Both steel grades were subjected to hot compression tests on the Gleeble-1500 thermo-mechanical simulator in the temperature range of 900–1100 °C and strain rate range of 0.01–10 s−1. Constitutive relationships of both steels were investigated by the physically based approach incorporating the strain effect, which accounts for the dependence of Young׳s modulus and the self-diffusion coefficient of austenite on temperature. The accuracy and reliability of the equations was quantified by employing statistical parameters such as the correlation coefficient and absolute average error. The results showed that the proposed equations can predict the flow stress of the experimental steels with acceptable accuracy, thus may be an alternative method for predicting the flow stress in hot working.

•Microalloyed with V and V-Ta can promote the precipitation of MX instead of M2X.•The presence of delta-ferrite in microstructure affects the morphology of MX.•Two-phase separation of MX carbonitride was observed in 12Cr3WVTa steel.12%Cr reduced activation ferrite/martensite steels are promising candidate materials for good corrosion and irradiation resistance used for supercritical water-cooled reactor cladding and in-core components. V and Ta are considered to have improved the creep strength of high Cr steels by precipitating as MX phase. In this paper, a series of trial products microalloyed with V and V–Ta are produced, and the microstructure is characterized after quenching at 1050 °C and tempering at 780 °C by using TEM method to investigate the effect of these elements on the precipitation behavior of 12%Cr reduced activation ferrite/martensite steel. The results from both the experimental observations and thermodynamic and kinetic calculations reveal that V and V–Ta can promote the stable MX precipitation instead of M2X, thus increasing the volume fraction of M23C6. Two-phase separation behavior of the (Ta, V)(C, N) carbonitride into a Ta(V)C(N) phase and a V(Ta)N(C) phase in 12Cr3WVTa steel is observed and further discussed.

FGH4096 alloys with Hf contents of 0, 0.3 and 0.6 wt% were studied to clarify the effect of Hf on the carbides in these alloys. The carbides in FGH4096 alloys produced on HIPing and during high temperature tensile deformation were analyzed by SEM, TEM and EDS. The results indicated that carbides in HIPed alloys were mainly blocky MC and the previous particle boundaries (PPB) in the alloys were decorated by the carbides. ZrO2 cores with a cubic lattice were observed in part of the MC distributed at the PPB (PPB MC), while part of PPB MC in Hf modified alloys had an orthorhombic HfO2 core. Volume fraction of MC increases with increasing Hf content of the alloys. The formation of PPB MC was suppressed in the alloy with 0.3 wt% Hf, but more MC precipitated at the PPB when Hf content of the alloy was raised to 0.6 wt%. The amount of M23C6 delineations that precipitated at grain boundaries during aging treatment decreased with increasing Hf content. Besides, the formation of a film-like M23C6 at grain boundaries during tensile deformations at 650 °C and 750 °C were suppressed in the Hf modified alloys, which contributed a lot to the improvement of the tensile properties of FGH4096 alloys at elevated temperatures.

Isothermal compression tests were carried out on a medium carbon vanadium microalloy steel (roughly Fe-0.33C-1.5Mn-0.1 V, wt%) by using a Gleeble-1500 simulator. Based on constitutive analysis including an Arrhenius term, activation energy for hot working was calculated and used to evaluate the rate-controlling mechanism of hot deformation. At low strain rates (0.1–1 s−1), the activation energy for hot working (287.4 kJ/mol) is very close to the austenite lattice self-diffusion activation energy, indicating that the rate-controlling mechanism is dislocation climb. While at high strain rates (10–30 s−1), the activation energy becomes very high (500.6 kJ/mol), and activation volume is better used under such conditions. Then, activation volume analysis based on both Schöck model and Kocks–Argon–Ashby model demonstrates that the rate-controlling mechanism at high strain rates is cross slip. That is, the rate-controlling mechanisms of hot deformation for the medium carbon vanadium microalloy steel at high and low strain rates are intrinsically different. Inspired by the findings above, processing map analysis based on dynamic materials model was further preceded and different peak domains of power dissipation efficiency in high and low strain rate regimes were found.

The dynamic recrystallization behavior of a medium carbon vanadium microalloyed steel was systematically investigated at the temperatures from 900 °C to 1100 °C and strain rates from 0.01 s−1 to 10 s−1 on a Gleeble-1500 thermo-simulation machine. The flow stress constitutive equation of hot deformation for this steel was developed with the activation energy Q being about 273 kJ/mol, which is in reasonable agreement with those reported before. Activation energy analysis showed that vanadium addition in microalloyed steels seemed not to affect the activation energy much. The effect of Zener–Hollomon parameter on the characteristic points of flow curves was studied using the power law relation, and the dependence of critical strain (stress) on peak strain (stress) obeyed a linear equation. Dynamic recrystallization is the most important softening mechanism for the experimental steel during hot compression. The dynamic recrystallization kinetics model of this steel was established based on flow stress and a frequently-used dynamic recrystallization kinetics equation. Dynamic recrystallization microstructure under different deformation conditions was also observed and the dependence of steady-state grain size on the Zener–Hollomon parameter was plotted.

The compressive deformation behaviors of a new 0.23C–1.50Mn–1.79Al (wt%) based microalloyed high-strength steel were investigated at the temperatures from 900 °C to 1100 °C and strain rates from 0.01 s−1 to 30 s−1 on a Gleeble-1500 thermo-simulation machine. The flow stress constitutive equation of hot deformation for this steel was developed with the activation energy Q being about 310 kJ/mol. Activation energy analysis showed that high Al addition in this steel seemed not to affect the activation energy much. A regression expression proposed by Medina and Hernandez [24] to predict the Q value of microalloyed steels was found to have a relative error 2.58% for this steel. The dynamic recrystallization (DRX) analysis showed that the DRX behavior of the experimental steel was evidently affected by both the deformation temperature and the strain rate. The dependence of steady-state grain size, the peak strain and the peak stress on Zener–Hollomon parameter of this steel was plotted and found that the values of Zener–Hollomon exponents of this steel was in reasonable agreement with microalloyed steels without high Al addition.

Based on the multi-component aspects of thermodynamics and diffusion, coarsening behavior of M23C6 (M = Cr, Fe, W) carbide at 650 °C in 12 %Cr-reduced activation ferrite/martensite steel has been investigated experimentally using scanning transmission electron microscopy, combined with DICTRA simulation. Both the experimental measurements as well as the simulations indicate that the interfacial energy of M23C6 carbide in this steel at 650 °C is probably 0.5 J m−2, and the coarsening rate of M23C6 carbide is very low. The influence of a change in Mn, V, and Ta content and temperature on the coarsening rate of M23C6 carbide is also investigated. The results show that the coarsening rate is increased by adding Mn and reduced by V and Ta addition, respectively, while an increase in the coarsening rate by an order of magnitude with increasing temperature per 50 °C between 600 and 750 °C. Precipitation of Laves (Fe2W) phase during aging has a negligible effect on the coarsening of M23C6.

A new grain topology-size relationship in three-dimensional (3D) polycrystalline microstructures has recently been established by considering the effects of non-random first nearest neighbor grains. In this contribution, a generalized form for this relationship is presented by considering the interactions of kth (k=1, 2, 3…) nearest neighbor grains, and large scale Monte Carlo-Potts model simulation is used to investigate the general neighborhood topological effect on the topology-size relationship. The results show that, unlike their first nearest neighbors (k=1), the topological correlations of 3D grains with their kth layers (k⩾2) of nearest-neighbors may have trivial effect on the topology-size relationship.

The hot compressive deformation behaviour in 12Cr3WV steel was conducted on a Gleeble-1500 thermo-mechanical simulator at the temperature range of 1223–1373 K with the strain rate in the range of 0.01–30 s−1 and the height reduction of 60%. Based on the experimental results, strain compensated Arrhenius-type constitutive equations and an artificial neural network (ANN) model with a back-propagation learning algorithm were developed for the characterization and prediction of the high-temperature deformation behaviour in the steel. And then a comparative predictability of the constitutive equations and the trained ANN model were further evaluated in terms of the correlation coefficient (R), the average absolute relative error (AARE) and the relative error. For the constitutive equations, R and AARE were found to be 0.9952% and 3.48% respectively, while for the ANN model, 0.9998 and 0.58% respectively. The relative errors between experimental and predicted flow stress computed from the constitutive equations and ANN model were respectively in the range of −15.46% to 10.46% and −4.12% to 4.08%. Moreover, the relative error within ±1% was observed for more than 85% of the test data sets of ANN model, while only 32% of the test data sets for the constitutive equations. The results indicate that the trained ANN model is more efficient and accurate in predicting the hot compressive behaviour in 12Cr3WV steel than the Arrhenius-type constitutive equations.Highlights► Constitutive equations can obtain only a rough estimate for the flow stress. ► The three-layer BP ANN model can accurately track the experimental data. ► The developed ANN model has better capability to predict the flow stress.

By introducing the distribution probability of structural units in austenite containing alloying elements and considering its effects on phase transformation, this paper establishes a calculation model of distribution probability of structural units. A new valence electron structure (VES) parameter-transformation effect coefficient of alloying elements (HL), is defined and then studied both theoretically and experimentally. The relationship between the parameter HL and the multiplying factor (the quenching capability factor) of alloying elements is studied. The results indicate that the two parameters (HL and the quenching capability factor) have the same variation characteristic and substance feature. Therefore, the multiplying factor virtually expresses the relative quantity of structural units in the alloying elements-containing austenite.

A microstructure with a Weilbull grain size distribution (GSD) was generated and subjected to normal grain growth (NGG) by the Monte Carlo Potts method. The results show that the GSD keeps almost invariant during NGG. The Weibull function fits well the steady-state GSDs evolving from the Weibull and the Hillert distributions.

Grain topology–size relationships are of great significance for a better understanding of the polycrystalline microstructures and grain growth processes. However, experimental observations of the three-dimensional grain topology in real materials are extremely rare. In this paper, the topology and the size of 1215 three-dimensional grains in a real steel sample were quantitatively analyzed with the aid of serial sectioning technique. The results showed that a linear quantitative relationship exists between the number of the faces (F) and the mean tangent diameter of individual grains as theoretically predicted by DeHoff–Liu's model (Metall. Trans. 16A (1985) 2007). It can be written as F=2.0+(〈F〉-2) uT, where uT is the normalized mean tangent diameter, 〈F〉 is the mean value of F averaged over all the grains. However, the experimental relationship became F=a+bue+cue2 when the normalized sphere-equivalent diameter (ue) was used. Limitations of DeHoff–Liu's model and possible correlation between the two kinds of grain size parameters were also discussed in this paper.

A relationship between the growth kinetics and the topology of individual grains in three dimensions, as an equivalent to Von Neumann’s law in two dimensions, was derived theoretically. The relationship depicts that the changing rate of the surface area of an individual three-dimensional grain, other than that of grain volume, is independent of grain size and proportional to (F−Fc), where F is the number of grain faces, and Fc a constant. It should also be pointed out that the new theory is approximate in nature — perhaps an exact three-dimensional law does not exist at all. A modified Monte Carlo algorithm was newly developed by the authors to simulate the three-dimensional normal grain growth process including the quasi-steady stage. The simulated process obeys the power law kinetics with its growth exponent approaching 0.5, and agrees quantitatively well with the experimental observations of topological evolution during the three-dimensional grain growth process in various materials found in the literature. The topology dependency of individual grain growth rate in simulation agrees very well with that predicted by the above-mentioned new three-dimensional theory.