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The hot deformation characteristics of a GH4169 superalloy are investigated at the temperature and strain rate ranges of 1193–1313 K and 0.01–1 s−1, respectively, through Gleeble-3500 simulator. The hot deformed microstructures are analyzed by optical microscopy (OM), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD) technology. The effects of deformation parameters on the features of flow curves and annealing twins are discussed in detail. It is found that the shapes of flow curves are greatly affected by the deformation temperature. Broad peaks appear at low deformation temperatures or high strain rates. In addition, the evolution of annealing twins is significantly sensitive to the deformation degree, temperature, and strain rate. The fraction of annealing twins first decreases and then rises with the added deformation degree. This is because the initial annealing twin characters disappear at the relatively small strains, while the annealing twins rapidly generate with the growth of dynamic recrystallized grains during the subsequent hot deformation. The fraction of annealing twins is relatively high when the deformation temperature is high or the strain rate is low. In addition, the important role of annealing twins on dynamic recrystallization (DRX) behaviors are elucidated. The obvious bulging at initial twin boundaries, and the coherency of annealing twin boundaries with dynamic recrystallized grain boundaries, indicates that annealing twins can motivate the DRX nucleation during the hot deformation.

Marginal-restraint mandrel-free spinning is an advanced technology for manufacturing ellipsoidal heads with large diameter-thickness ratios. Nevertheless, the spinning-induced residual stress, which greatly influences the in-service performance of spun heads, should be removed. In this study, the effects of annealing on the residual-stress relaxation behavior of 5052H32 aluminum alloy spun heads were investigated. It is found that the residual stress first rapidly decreases and then remains steady with the increase in annealing time at the tested annealing temperatures. The relaxation of the residual stress becomes increasingly obvious with the increase in annealing temperature. When the annealing temperature is less than 220 °C, there are no obvious changes in grain size. Moreover, the spinning-induced dislocations are consumed by the static recovery behavior, which decreases the residual stress during annealing. When the annealing temperature is approximately 300 °C, the broken grains transform into equiaxed grains. In addition, static recrystallization and recovery behaviors occur simultaneously to promote the relaxation of the residual stress. Considering the different stress relaxation mechanisms, a model based on the Zener-Wert-Avrami equation was established to predict the residual-stress relaxation behavior. Finally, the optimized annealing temperature and time were approximately 300 °C and 30 min, respectively.

In order to effectively improve the afterglow properties of CaAl2O4:Eu2+,Nd3+ phosphors, a series of Ca0.982–xAl2O4:0.012Eu2+,0.006Nd3+,xGd3+ (x = 0,0.012,0.024,0.036,0.048,0.060 mol) phosphors were prepared by a high-temperature solid-phase approach. Crystalline composition and microstructure were characterized by XRD, TEM, HRTEM, and XPS, luminescence properties were systematically analyzed by fluorescence spectra, afterglow decay curves and TL glow curve. Results show that all of Ca0.982–xAl2O4:0.012Eu2+,0.006Nd3+,xGd3+ phosphors belong to monoclinic CaAl2O4, without other cystalline phase. The blue emission at 442 nm is observed, which is assigned to the 4f65d →4f7 transition of Eu2+ ions. Doping with appropriate amount of Gd3+ ions (x = 0.036mol) significantly improves the afterglow properties of phosphors, but the excessive doping of Gd3+ induces the fluorescent quenching. The doping of moderate Gd3+ changes the traps states, the trap depth varies from 0.598 to 0.644 eV and the trap concentration is also greatly improved, thus significantly improving afterglow performance.

Increasing static recrystallization (SRX) nucleation rate and decreasing grain growth rate are the main ways to obtain uniform and fine grain microstructure by annealing treatment. However, it is contradictory to raise the SRX nucleation rate and reduce grain growth rate during isothermal annealing treatment. This is because the increase of the SRX nucleation rate needs to raise annealing temperature, while the decline of grain growth rate needs to reduce the annealing temperature. To solve the contradiction, a novel method named cooling recrystallization annealing treatment (CRT) is designed and verified. For the CRT, the relatively high annealing temperature in the primary stage can promote SRX nucleation, and the gradually decreased annealing temperature can reduce the growth rate of SRX grains. Besides, an aged treatment is first carried out to precipitate high content of the δ phase before the CRT. Enough δ phases not only provide large numbers of sites for SRX nucleation but also limit the grain growth due to the pinning effect, especially in the primary high-temperature stage. The results show that the novel method can refine deformed mixed grain microstructure well when the suitable cooling rate, start and final recrystallization annealing temperatures are employed. The deformed mixed grain can be uniformly refined to an average size of 8.26 μm with the route of 900°C×12 h+1020°C→970°C×20 min.

The uniform refinement mechanisms and methods of deformed mixed and coarse grains inside a solution-treatment Ni-based superalloy during two-stage annealing treatment have been investigated. The two-stage heat treatment experiments include an aging annealing treatment (AT) and a subsequent recrystallization annealing treatment (RT). The object of AT is to precipitate some δ phases and consume part of storage energy to inhibit the grain growth during RT, while the RT is to refine mixed and coarse grains by recrystallization. It can be found that the recrystallization grains will quickly grow up to a large size when the AT time is too low or the RT temperature is too high, while the deformed coarse grains cannot be eliminated when the AT time is too long or the RT temperature is too low. In addition, the mixed microstructure composed of some abnormal coarse recrystallization grains (ACRGs) and a large number of fine grains can be observed in the annealed specimen when the AT time is 3 h and RT temperature is 980 ℃. The phenomenon attributes to the uneven distribution of δ phase resulted from the heterogeneous deformation energy when the AT time is too short. In the regions with a large number of δ phases, the recrystallization nucleation rate is promoted and the growth of grains is limited, which results in fine grains. However, in the regions with few δ phases, the recrystallization grains around grain boundaries can easily grow up, and the new recrystallization nucleus is difficult to form inside grain, which leads to ACRGs. Thus, in order to obtain uniform and fine annealed microstructure, it is a prerequisite to precipitate even-distributed δ phase by choosing a suitable AT time, such as 12 h. Moreover, a relative high RT temperature is also needed to promote the recrystallization nucleation around δ phase. The optimal annealing parameters range for uniformly refining mixed crystal can be summarized as: 900 ℃×12 h + 990 ℃×(40−60 min) and 900 ℃×12 h + 1000 ℃×(10−15 min).

Marginal-restraint mandrel-free spinning is an advanced technology for manufacturing ellipsoidal heads with large diameter-thickness ratios. The effects of spinning parameters on the forming accuracy of ellipsoidal heads have been studied, and optimized spinning parameters have been obtained. The microstructure evolution of a workpiece is usually very complicated in the spinning process. In this work, the influence of spinning parameters on the microstructures of two-pass spun ellipsoidal heads is studied. It is found that the forming angle and feed rate of the first pass, angle between passes, and feed rate of the second pass significantly affect the microstructures. Meanwhile, the evolution rule of the microstructures near the inner and outer surfaces of the spun parts is almost consistent. A large forming angle, large angle between passes, or large feed rate of the second pass are beneficial to obtain uniform microstructures. A small or large feed rate of the first pass reduces the microstructure uniformity. To improve the microstructure uniformity between the inner and outer surfaces, the optimized spinning parameters are determined.

Forming and fracture forming limit diagrams are significant performance indexes for evaluating the formability of a material. In the present study, experimental and theoretical investigations of the forming and fracture behavior for precipitate-hardenable Inconel 718 superalloy, have been performed at different temperatures (Room temperature (RT) to 700 °C). Firstly, uniaxial tests have been conducted over a temperatures range and quasi-static strain rates (10−4-10−1s−1). Flow stress (tensile) has been found to be significantly affected by variation in test temperatures and strain rates. Further, yielding behavior of IN718 alloy has been predicted based on Hill'48 (r and σ based) and Barlat'89 criteria. Barlat'89 criterion has better predictability of yielding behavior for IN718 alloy at all test temperatures. Subsequently, experimental forming and fracture limit curves have been plotted at different temperatures using Nakazima test. Limiting true strains have been found to be increasing with test temperature for all deformation regions in forming limit diagrams. Marciniak Kuczynski (M-K) and Bao-Wierzbicki (B-W) model coupled with anisotropic yield criteria have been used for theoretical prediction of limiting and failure strains for IN718. The M-K and B-W models coupled with only Barlat'89 criteria have shown a good prediction ability of limiting strains with least root mean square error (RMSE) and average absolute error (AAE).

The effects of solution time and cooling rate on microstructures and mechanical properties of 2219 Al alloy for a larger spun thin-wall ellipsoidal head are investigated. It is found that the Al2Cu phases are greatly affected by the solution time. The content of Al2Cu phases first decreases and then stabilizes with increasing the solution time. Moreover, the effects of solution time on the transformation of precipitates are obvious. As the solution time is increased from 35 min to 60 min, almost all the precipitates transfer from GP zones to θ′′ phases. When the solution time is further increased to 110 min, some θ′ phases appear. Meanwhile, the size of θ′′ phases increases, while the density of θ′′ phases drops. These microstructure changes greatly influence the mechanical properties, i.e., the hardness and tensile strengths first increase and then slightly decrease with the continuous increase of the solution time. In addition, if the cooling rate is decreased, the precipitation of coarse equilibrium θ phases is enhanced, but the precipitation of strengthening phases (θ′′, θ′) is restricted. It significantly deteriorates the mechanical properties. Based on the comprehensive analysis, the suitable solution time and cooling method are about 60 min and water cooling.

A new thermomechanical process was applied to a near-β Ti-5Al-5Mo-5V-1Cr-1Fe alloy to induce twinning. The resulting combination of ultrafine grains and heat-treatment twins endowed the alloy with excellent tensile strength (1544 MPa) and elongation (13%). β-Field cross-rolling formed micro-sized β grains, and ultrafine α particles precipitated from pre-existing dense dislocation structures and martensites. α-Twinning occurred during heat treatment and continuously evolved with the ongoing heat treatment time. A precisely controlled heat treatment time enabled the distribution of nanosized twins throughout the α laths, simultaneously improving strength and ductility. The results of this study pave the way for the development of customised titanium alloys with significantly improved mechanical properties.

This work gives a comparison on the microstructural characteristics, textural discrepancies, and twinning behaviors of lamellar and equiaxed near β-Ti alloys during multi-pass cross rolling with a rolling reduction of 20 %, 50 % and 80 %. The results showed that the restoration mechanism of the alloy in β phase is strongly dependent on the α morphologies, and in comparison, strain path has weaker influences on the grain refinement of the β matrix. Therefore, the texture intensities of both α and β phases were weakened owing to the dynamic recrystallization (DRX) of the two phases in the equiaxed microstructure. While, with regard to the lamellar microstructure, dynamic recovery (DRV) of the β phase predominated, forming elongated β subgrains. Besides, the α and β matrix in lamellar microstructures obeyed the Burgers orientation relationship, which was gradually broken down until the final reduction. Lastly, the 1 1 ¯ 01 twinning exhibits a strong size effect. With the continuous DRX of α phases, the α-twinning is suppressed owing to progressive grain refinement. The activation of β-twinning, namely 332 〈 113 〉 and 112 〈 111 〉 , in near β-Ti alloys is heavily dependent on the deficient β-stabilizing elements and the local stress concentration. These findings provide an effective way to obtain ultra-fine grain microstructures of this alloy.

The dynamic recrystallization (DRX) features and the evolution of the microstructure of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are investigated by hot-compression tests. The sensitivity of grain dimension and DRX behavior to deformation parameters is analyzed. The results reveal that the DRX features and grain-growth behavior are significantly affected by deformation conditions. The DRX process is promoted with a raised temperature/true strain or a reduced strain rate. However, the grains grow up rapidly at relatively high temperatures. At strain rates of o.1 s−1 and 1 s−1, a uniform microstructure and small grains are obtained. Due to the obvious differences in the DRX rate at various temperatures, the piecewise DRX kinetics equations are proposed to predict the DRX behavior. At the same time, a mathematical model for predicting the grain dimension and the grain growth behavior is established. To further analyze the DRX behavior and the changes in grain dimension, the hot deformation process is simulated. The developed grain-growth equation as well as the piecewise DRX kinetics equations are integrated into DEFORM software. The simulated DRX features are consistent with the test results, indicating that the proposed DRX kinetics equations and the established grain-growth model can be well used for describing the microstructure evolution. So, they are very useful for the practical hot forming of P/M superalloy parts.

The flow behavior and microstructure change of the Ti-55511 alloy are investigated by thermal compression experiments with stepped strain rates. The phase transformation features, the dynamic recrystallization (DRX) behavior of the β matrix, the dynamic spheroidization mechanism of the lamellar α phase and the evolution of the β sub-grain size are quantitatively analyzed. A unified constitutive model is constructed to characterize the hot deformation features of the Ti-55511 alloy. In the established model, the work hardening effect is taken into account by involving the coupled effects of the equiaxed and lamellar α phases, as well as β substructures. The dynamic softening mechanisms including the dynamic recovery (DRV), DRX and dynamic spheroidization mechanisms are also considered. The material parameters are optimized by the multi-objective algorithm in the MATLAB toolbox. The consistency between the predicted and experimental data indicates that the developed unified model can accurately describe the flow features and microstructure evolution of the hot compressed Ti-55511 at stepped strain rates.

The microstructural variation and high-temperature flow features of a Ti-55511 alloy in the β region are studied by utilizing double-stage compression with a stepped strain rate. The results demonstrate that the stresses in the latter stage of hot compression markedly reduce as the strain at the previous stage or the strain rate at the previous/latter stage drops. Moreover, the annihilation/interaction of substructures is promoted, and the distinct refinement of the dynamic recrystallization (DRX) in the β grain can be found. However, the coarsening of the β grain and the consumption of dislocation substructures are accelerated at high temperatures. Furthermore, the principal DRX nucleation mechanism of the Ti-55511 alloy during double-stage compression with a stepped strain rate in the β region is discontinuous DRX. Additionally, by using the microstructural variation characteristics related to the forming parameters, a physical mechanism equation is modeled to forecast the forming features, the DRX fraction, and the size of the β grain in the investigated alloy. The forecasted results are in accordance with the tested results, indicating that the established model can accurately forecast the microstructure variation and flow features of the studied alloy.

Metal sheet spinning is an advanced near-net forming technology for the manufacture of thin-walled ellipsoidal heads. The exact control of dimensional accuracy, however, is a considerable problem for spinning thin-walled parts with large diameter-to-thickness ratios. In this work, a marginal-restraint mandrel-free spinning process with two passes is proposed for the fabrication of thin-walled ellipsoidal heads without wrinkling. A finite element model is established and verified to study the influences of spinning parameters on the dimensional precision of thin-walled ellipsoidal heads. It is found that the spinning parameters considerably influence the deviations of wall thickness and contour characteristics. A small forming angle or small roller fillet radius during the first pass spinning, as well as the small angle between passes or high feed ratio during the second pass spinning, can improve the wall thickness precision. Meanwhile, as the forming angle or feed ratio is increased during the first pass spinning, the contour precision initially increases and then decreases. During the second pass spinning, the contour precision can be improved at a small angle between passes, whereas it deteriorates at a larger roller installation angle. The optimized spinning parameters are obtained and verified by experiments.

The microstructure evolution and the corrosion feature of Al–xCu–yLi–Mg alloys (x:y = 0.44, 1.65 and 4.2) were systematically investigated under the same artificial aging conditions. The relationships between types of precipitates and mechanical performance, as well as electrochemical behaviors, were discussed. Our results show that different types of precipitates can be obtained in alloys with different Cu/Li mass ratios, which significantly influences the mechanical performance of the alloys and substantial corrosion behaviors. Specifically, the analogous corrosion evolution in the aging Al–xCu–yLi–Mg alloys was first ascertained to be derived from the growth mechanism of the precipitates at the grain boundary (GB). Moreover, a small number of GB precipitates can be obtained in the aged alloy with the lowest Cu/Li mass ratio, thereby resulting in the largest intergranular corrosion resistance. A higher proportion of the GB T1 phase in the continuous precipitates induces higher corrosion sensitivity in alloy with a high Cu/Li mass ratio.

Staggered spinning is an advanced method to manufacture thin-walled cylindrical parts. The microstructure evolution, which dominantly influences the overall performance, is very complex during the staggered spinning of Ni-based cylinder. In this work, the influences of feed rate and wall thickness reduction on the microstructures of a thin-walled Hastelloy C-276 cylinder during staggered spinning are investigated. It is found that the deformation of parts is highly inhomogeneous, and the microstructures are sensitive to the feed rate and wall thickness reduction during the staggered spinning. When the wall thickness reduction is small, the single slip is responsible for the deformation of grains. The deformation mechanism changes from single slip to cross slip with the increase of wall thickness reduction. Moreover, the uniform deformation of thin-walled Hastelloy C-276 cylinder can be obtained when the feed rate is about 0.8 mm/r or the wall thickness reduction is about 44.1%. These findings provide guidance for controlling the microstructures of thin-walled cylindrical parts during staggered spinning.

Active contour models (ACMs) have been widely applied in the field of image segmentation. However, it is still very challenging to construct an efficient ACM to segment images with intensity inhomogeneity. In this paper, a novel ACM guided by global and local signed energy-based pressure force (GLSEPF) is proposed. First, by computing the energy difference between the inner and outer energies of the evolution curve, a global signed energy-based pressure force (GSEPF) is designed, which can improve the robustness to initial curves. Second, a local signed energy-based pressure force (LSEPF) is introduced by computing the pixel-by-pixel energy difference within local neighborhood region, which can handle images with intensity inhomogeneity and noise. Finally, the global image information and the local energy information are used for the global and local force propagation functions, respectively. The global and local variances are used to automatically balance the weights of the GSEPF and the LSEPF, which can solve the problem of setting parameters. Meanwhile, a regularization term and a penalty term are applied to avoid the re-initialization process during iterations and smooth the level set function. Experimental results on different types of images demonstrate that the proposed model is more robust than the popular region-based and mixed ACMs for segmenting images with intensity inhomogeneity and noise. The code is available at: https://github.com/HuaxiangLiu/GLSEPF/.

The advanced modelling/simulating method and the effective optimization/controlling strategy relying on knowledge-based systems are highly demanded in industrial manufacturing of alloy components. In this work, based on the advantages of cellular automaton (CA) simulation and neural network-based model predictive control (NNMPC) methods, a material design framework is developed to optimize processing parameters for the designed target microstructures of alloys. In this framework, a matrix CA simulation method is developed to accurately and quickly describe the variations of microstructures with processing parameters. NNMPC, which is an effective control method for nonlinear and multi-objective system, is utilized to online optimize processing parameters according to the designed target microstructures. Based on the optimized processing parameters, the hot compressive deformation tests of a Ni-based superalloy are conducted to verify the effectiveness of the developed framework. The experimental results well agree with the simulated/designed ones, which implies that the developed material design framework can effectively optimize processing parameters for the designed target microstructures. Also, the developed material design framework is used to obtain the uniform and fine microstructures of a Ni-based superalloy.

The intermediate-temperature (473–973 K) deformation behavior of a nickel-based superalloy is researched by uniaxial tensile experiments. It is observed that the flow characteristics and the Portevin–Le Chatelier (PLC) effects are significantly affected by the thermo-mechanical parameters. The serrated flow features are obvious under the testing conditions. A stacked auto-encoders (SAEs) network model is proposed for predicting the flow behaviors of the researched superalloy. The architecture of the established SAEs model is optimized layer by layer. The best number of hidden layer is 3, and the nodes per hidden layer are 15, 20 and 50, respectively. The excellent prediction ability suggests that the developed SAEs model can well reconstruct the intermediate-temperature flow behavior involving the PLC effects of the researched superalloy.

In authors’ previous work [Mater. Charact. 141 (2018) 212–222], it was found that the heterogeneous deformed microstructures can be replaced by the relatively homogeneous recrystallized grains through an annealing treatment. However, there are still some relatively large recrystallized grains. To find the reasons for the formation of large grains, some new annealing treatment tests were done, and the cellular automation (CA) simulations were carried out in the present work. The experimental results showed that the microstructural evolution during annealing treatment is significantly affected by the content of δ phase. So, the effects of δ phase on the nucleation and growth of grains are carefully considered in the CA model to accurately simulate the microstructural evolution behavior. By the CA simulation, it is found that the dislocation density rapidly decreases due to the nucleation of static recrystallization (SRX) and the growth of dynamc recrystallization (DRX) nuclei at the early stage of annealing. The high initial dislocation density can provide the high velocity for the growth of DRX nuclei, which is responsible for the formation of coarse grains. However, the growth rate of SRX nuclei is relatively small due to the low dislocation density and pinning effects of δ phase.

Staggered spinning is a cost-effective method to manufacture axisymmetric components, especially for the ultrathin cylindrical parts. However, due to the complex material flow behavior during staggered spinning of Ni-based thin-walled cylindrical parts, the accurate control of geometric precision is still a great challenge. In this work, a three-dimensional (3D) finite element model (FEM) is developed to investigate the effects of staggered spinning parameters on the dimensional accuracy of a thin-walled Hastelloy C-276 cylindrical part. It is found that the deviations of wall thickness and inner diameter are sensitive to spinning parameters. Based on the results from the finite element analysis, the optimized spinning parameters are obtained for the studied thin-walled cylindrical part, i.e., an appropriate wall thickness reduction range is 30%–40% during the single pass spinning. Meanwhile, the effects of feed ratio on the uniformities of wall thickness and inner diameter are significant, and a suitable feed ratio is 0.8 mm/r. Finally, the industrial three-roller staggered spinning experiments are carried out to verify a fact that the developed FEM, as well as the optimized spinning parameters, can be effectively used for the staggered spinning of thin-walled Hastelloy C-276 cylindrical parts.

Hot compression experiments of a Ti55511 titanium alloy are conducted to study the phase transformation characteristics and dynamic recrystallization (DRX) behavior. It is found that the fraction of α phase decreases with increasing the strain rate or deformation amount. Meanwhile, the original lamellar α phases easily transform into the spheroidal and bulk α phases at larger deformation amounts or higher strain rates. Most of α phases disappear and the final microstructures are mainly β phases when the temperature is over 800 °C. The DRX degree rises with raising the deformation amount or decreasing the strain rate. However, the DRX behavior becomes weaken with raising the deformation temperature. α phases are evenly distributed around β phases during hot compressive deformation, which not only restrains the growth of β phases but also promotes the DRX process of β phases. In addition, the geometric DRX takes place within α phases, while the continuous DRX occurs within β phases during the hot compression.

In this study, a strategy based on the particle swarm optimization (PSO) algorithm is developed to control the microstructures of a Ni-based superalloy during hot forging. This strategy is composed of three parts, namely, material models, optimality criterions, and a PSO algorithm. The material models are utilized to predict microstructure information, such as recrystallization volume fraction and average grain size. The optimality criterion can be determined by the designed target microstructures and random errors. The developed strategy is resolved using the PSO algorithm, which is an intelligent optimal algorithm. This algorithm does not need a derivable objective function, which renders it suitable for dealing with the complex hot forging process of alloy components. The optimal processing parameters (deformation temperature and strain rate) are obtained by the developed strategy and validated by the hot forging experiments. Uniform and fine target microstructures can be obtained using the optimized processing parameters, which indicates that the developed strategy is effective for controlling the microstructural evolution during the hot forging of the studied superalloy.

An accurate constitutive model is essential for analyzing deformation behavior of material and reliable numerical simulations in metal forming processes. In this study, hot tensile tests of Inconel 718 alloy have been conducted over a wide range of temperatures (300-973 K at an interval of 100 K), strains (0.01-0.3 at an interval of 0.01) and quasi-static strain rates (0.0001, 0.001, 0.01 s−1). Flow stress behavior is significantly affected by test temperatures and strain rates. Microstructure characteristics of deformed test specimens have been examined using scanning electron microscope and electron backscatter diffraction (EBSD). The fractography study revealed that fracture is mix-mode type, i.e., ductile and brittle. Subsequently, EBSD analysis shown that dynamic recrystallization mechanism is more pronounced at a higher temperature. Furthermore, four constitutive models, namely modified Cowper–Symonds, modified Johnson Cook, modified Zerillie-Armstrong and integrated Johnson Cook–Zerillie-Armstrong (JC-ZA) models have been investigated for flow stress prediction. Capability of models has been evaluated based on the correlation coefficient (R), average absolute error (Δ) and its standard deviation (δ). Accurate prediction of flow stress behavior is found by integrated JC-ZA model with R = 0.9873, Δ = 2.44 and δ = 4.08%.

In authors' previous work [Mater. Charact., 141(2018)212–222], it was found that the heterogeneous deformed microstructures can be replaced by the relatively homogeneous recrystallized grains through a single annealing treatment. However, there are still some relatively large recrystallized grains. In this study, two-stage annealing treatment tests for the deformed samples are designed to better refine the grains and improve the homogeneity. Results showed that the two-stage annealing treatment can better refine grains and improve the microstructural homogeneity compared with single annealing treatment. Thus, the more uniform distribution of grains is obtained. Based on the new experimental findings, the mechanisms for refining and homogenizing the deformed coarse grains are discussed. It is found that the δ phase can easily precipitate around the boundaries of recrystallization grains in the first annealing stage when the temperature is 900°C. The precipitated δ phase can effectively inhibit the grain growth. However, with increasing annealing time, the precipitation rate of δ phase gradually decreases due to the decreased Nb element content. So, the pinning force becomes smaller and smaller, and the follow-up formed recrystallization grains grow to a relatively large size. Therefore, the holding time of first stage annealing treatment has a great influence on the final grain size and the content of δ phase. During the second stage annealing treatment, the increasing nucleation rate induced by the higher temperature and the pinning effects of δ phase make the uniform and fine grains. The feasible parameters are as follows: the first stage annealing treatment is 900°C for 9–12 h and the second stage annealing treatment is 980°C for 60 min.

The precipitation behavior of α phase in a β-quenched Ti-55511 alloy during high-temperature compression is investigated. The influences of deformation parameters on the flow characteristics, microstructural evolution, as well as the precipitation behavior of α phase, are discussed. Results show that the flow stress features and the dominant softening mechanism are sensitive to the deformation parameters. At low strain rates or high deformation temperatures, the main softening mechanism is dynamic recovery. But, the dynamic recrystallization becomes the dominant softening mechanism at high strain rates or low temperatures. At low strain rates, the continuous strengthening, induced by the pinning effect of α precipitates, happens at the late period of deformation. Distribution of α precipitates is not greatly affected by deformation parameters. In the region near β grain boundary, α precipitates are densely distributed with few orientations. While in the central region of β grain, the α precipitates are distributed a bit more sparsely with several scattering orientations. In addition, with increasing deformation temperature, the content, aspect ratio and size of α phases decrease. However, with increasing strain rate, the content, aspect ratio and size first increase and then decrease. Furthermore, the peak value of aspect ratio appears at 0.01 s−1.