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The two-phase titanium alloy Ti–10 V–5Al-2.5Fe-0.1 B was taken as the experimental material, and thermal compression experiments were carried out at a deformation temperature of 770–920 °C and a strain rate of 0.0005–0.5 s−1. An Arrhenius model, a modified Johnson-Cook model, and an improved BP neural network model based on the sparrow search algorithm (SSA-BP) model were established to predict the high temperature rheological stress of the alloy. A comparison of the prediction accuracy of the three models was made. When the partial random data in the rheological curves was used for model building and relatively independent data were used for predicting the rheological stress, the SSA-BP model had higher prediction accuracy, which exhibits the highest mean square correlation coefficient (R2) value of 0.9992 and the lowest root mean square error (RMSE) and average absolute relative error (AARE) values of 1.3031, and 2.0947 %, respectively. The ability of three models to predict the rheological stress for the new process parameters was verified. Results show that the SSA-BP model still has better prediction ability, which exhibits the highest mean square correlation coefficient (R2) value of 0.9720 and the lowest root mean square error (RMSE) and average absolute relative error (AARE) values of 5.0099, and 6.0382 %, respectively. The predicted values of SSA-BP for the rheological stress were used to construct the hot processing map. Results show that the trend of the power dissipation factor (η) value from the hot processing map predicted by SSA-BP can well agree with the microstructure evolution of the alloy.

The paper studies the evolution of the structure, phase composition, and impact toughness of the Ti-6Al-3Mo titanium alloy after two types of processing. Mode I consists of heating and hot helical rolling, followed by quenching into water. In mode II, an additional air quenching is used between the rolling and quenching processes. This approach, combining two alloy cooling stages (in air and in water), makes it possible to control the recrystallization time and form a gradient structure over the cross-section of the bar stock. The processing of titanium alloy according to mode I leads to a decrease of the impact strength. An addition of an air-cooling stage (in mode II) leads to an insignificant decrease in the hardness and an increase of the alloy toughness. The treatment of the titanium alloy by modes I and II results in the formation of a structure containing large grains of the primary α-phase in a thin-plate structure of the secondary α-phase in the β-phase. It is shown that these structural states of the alloy affect the crack propagation under impact loading and change the amount of the energy spent on the complete failure of the samples. [ABSTRACT FROM AUTHOR]

Creep and fatigue properties of two-phase titanium alloys show strong dependence on microstructure, especially morphology of the α and β phases which can be controlled to certain extent by proper selection of hot working and heat treatment conditions. In the paper the creep behaviour of Ti-6Al-2Mo-2Cr alloy (VT3-1) at elevated temperature was modelled. Finite element analyses of primary creep stage were carried out taking into account some microstructural features of the two-phase alloy that were included in the physical model and different properties of α and β phases. In order to verify results of calculations distinct types of microstructure were developed in the alloy by heat treatment and creep tests were carried out at elevated temperature (450°C) at various stress levels. Based on the FEM simulations the effect of changes of some microstructure features on primary creep strain development was estimated.

Tensile stresses and hydrogen render strong influence on the titanic alloys propensity for delayed fracture. The protective film serves аs a barrier for penetration in hydrogen alloy. Therefore to study the stress effect on its structure and protective properties is of significant interest.The aim of this work is to research the tensile stress influence on the passivation, indexes of corrosion, protective film structure and reveal reasons for promoting hydrogenation and emerging propensity for delayed fracture of titanium alloy VТ22 in the marine air atmosphere.The fulfillеd research has shown that:- there is а tendency to reduce the passivation abilities of the alloy VТ22 in synthetic marine water (3 % solution of NaCl) with increasing tensile stresses up to 1170 МPа, namely to reduce the potential of free corrosion and the rate of its сhange, thus the alloy remains absolutely (rather) resistant;- the protective film consists of a titanium hydroxide layer under which there is the titanium oxide layer adjoining to the alloy, basically providing the corrosion protection.- the factors providing hydrogenation of titanium alloys and formation in their surface zone fragile hydrides, causing the appearing propensity for delayed fracture, alongside with tensile stresses are:- substances promoting chemisorbtion of hydrogen available in the alloy and on its surface;- the cathodic polarization caused by the coupling;- the presence of the structural defects promoting the formation of pitting and local аcidifying of the environment surrounding the alloy.

How tensile stresses and pH value influence the passivation, corrosion, and structure of a protective film of two-phase titanium alloy VT22 in 3% solution of NaCl is investigated with the help of potentiometry, potential and dynamical analysis, and electrical and chemical impedance spectroscopy. We simulate the process of electrical transfer through the protective film and we reveal that its structure is two-layered. The film consists of a layer based on titanium hydroxide under which there is a titanium oxide layer adjacent to the alloy and primarily it protects the alloy against corrosion. How tensile stresses and pH value influence the hydrogen saturation process and delayed fracture of titanium in an aqueous NaCl solution is investigated. [ABSTRACT FROM AUTHOR]

A system of microstructure-dependent fatigue failure models that allows calculating the number of stress cycles to failure of smooth specimens in the absence of data on the fatigue resistance of the material is proposed. Fatigue life is represented as the sum of the number of stress cycles to the initiation of a microstructurally short crack with a depth equal to one grain size and the number of stress cycles to the instant the growing physically small and long crack reaches the depth taken as a fatigue failure criterion. To fill the models, it is necessary to test standard specimens under monotonic short-term tension to determine the elastic modulus, Poisson’s ratio, and the proportional limit and to analyze the microstructure and texture of the material to determine the Taylor factor, the magnitude of the Burgers vector, and the distance between neighboring parallel slip planes in the lattice, depending on which slip system is activated relative to the tension direction. The models are applicable to metals and alloys in which a fatigue crack is initiated along a persistent slip band in a surface grain under high-cycle loading. The models are validated against fatigue test data for flat specimens of different two-phase titanium alloys (VT3-1, IMI 834, Ti–6Al–4V, VT6, VT16, VT22, VT23, LCB) with different types of microstructure (bimodal, globular, fine-grained β -transformed) subject to symmetric in-plane bending. The plotted S – N curves are in a satisfactory agreement with experimental data. [ABSTRACT FROM AUTHOR]

In this paper, a set of mechanism based unified viscoplastic constitutive equations have been established for two-phase titanium alloys with initial lamellar microstructure, which models the softening mechanisms of the alloys in hot forming conditions. The dislocation density, rotation and globularization of lamellar α-phase and their effects on flow behaviour can also be modelled. The values of material constants in the equation set have been calibrated, according to stress-strain curves and globularization fractions of lamellar α-phase obtained from compression tests at a range of temperatures and strain rates, using a genetic algorithm (GA) based optimisation method. Based on the determined constitutive equations, flow stress and globularization evolution of Ti-17 and TA15 alloys at different temperatures and strain rates were predicted. Good agreements between the experimental and computed results were obtained.

A new method to determine the major-minor sequence of microstructural features influencing mechanical properties of two-phase titanium alloys was proposed in a ternary microstructural system composed of equiaxed α phase, coarse α plates and residual β matrix in this investigation. The triplex heat treatment in (α+β) two-phase field was carried out on two-phase titanium alloys to change the relative contents of three microstructural features. The microstructure-property relationships described in the form of three-dimensional curved surface were established by interpolating property data and corresponding content data of these microstructural features in a stereoscopic triangular coordinate system. The major-minor sequence of microstructural features was determined by comparing the angles between contour lines of property surfaces and coordinate axes. The main microstructural feature influencing alloy's properties in a binary microstructual system was determined by superimposing the vertical section of a three-dimensional property surface along one coordinate axis and its corresponding major-minor sequence distribution. Validated using Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si alloy, the method is generally applicable to all two-phase titanium alloys or other alloys having a ternary microstructural system. [ABSTRACT FROM AUTHOR]

Abstract: An internal-state-variable based self-consistent constitutive model was proposed for unified prediction of flow stress and microstructure evolution during hot working of wrought two-phase titanium alloys in both single-beta region and two-phase region. For each constituent phase of titanium alloys, a set of constitutive equations incorporating solution strengthening, Hall–Petch effect, dislocation interaction, and dynamic recrystallization were developed using internal state variable method. The effect of second phase on recystallization was modeled by considering particle stimulated nucleation and exerting drag force on boundary migration. The constitutive equations of constituent phases were implemented into a viscoplastic self-consistent scheme to predict the overall response of the aggregate. Predictions of the model are in good agreement with experimental results of the Ti–6Al–4V alloy and IMI834 alloy. The model can reproduce many features of the hot working of two-phase titanium alloys, including the dependence of flow stress on temperature, strain rate and alloying elements; the increase of strain rate sensitivity with temperature; the stress and strain partitionings between alpha and beta phases; the relatively high apparent activation energy in two-phase region, the decrease of recrystallization kinetics with temperature in two-phase region; and the decrease of recrystallized grain size with Zener–Hollomon parameter in beta working. [Copyright &y& Elsevier]

This paper presents the results of investigations of the microstructure and mechanical properties of two-phase α+β titanium alloys after different heat treatment. The influence of the morphology of lamellar microstructure and phase composition on the tensile properties and fracture toughness of the alloys was studied. Static tensile, hardness and fracture toughness tests and microstructure investigations were performed. It was noticed that the cooling rate from the β-phase range and ageing conditions had an effect on the microstructure parameters, volume fraction and chemical composition of the β-phase, and has a significant effect on the mechanical properties of the alloys tested. [Copyright &y& Elsevier]

In this paper, a set of mechanism based unified viscoplastic constitutive equations have been established for two-phase titanium alloys with initial lamellar microstructure, which models the softening mechanisms of the alloys in hot forming conditions. The dislocation density, rotation and globularization of lamellar α-phase and their effects on flow behaviour can also be modelled. The values of material constants in the equation set have been calibrated, according to stress-strain curves and globularization fractions of lamellar α-phase obtained from compression tests at a range of temperatures and strain rates, using a genetic algorithm (GA) based optimisation method. Based on the determined constitutive equations, flow stress and globularization evolution of Ti-17 and TA15 alloys at different temperatures and strain rates were predicted. Good agreements between the experimental and computed results were obtained. [ABSTRACT FROM AUTHOR]