Yongqing Zhao

Microstructures of a burn resistant highly stabilized β-titanium alloy

Abstract Ti40 alloy (Ti–25V–15Cr–0.2Si) is a highly stabilized β-titanium alloy with good burn resistance. Its microstructures were examined and analyzed by OM, SEM, TEM, XRD and EDX. The results reveal that Ti40 is a single β-phase alloy. There are quad-point boundaries and a white zone around the grain boundaries in the as-cast structures, leading to difficulties in ingot breakdown. The recrystallization finishing temperature is 820°C, and the second recrystallization is at 1100°C. The subgrains emerge if solution treatment temperature is greater than 910°C and aging at 600°C, which leads to a good combination among tensile properties.

Constructing Processing Map of Ti40 Alloy Using Artificial Neural Network

Based on the experimental data of Ti40 alloy obtained from Gleeble-1500 thermal simulator, an artificial neural network model of high temperature flow stress as a function of strain, strain rate and temperature was established. In the network model, the input parameters of the model are strain, logarithm strain rate and temperature while flow stress is the output parameter. Multilayer perceptron (MLP) architecture with back-propagation algorithm is utilized. The present study achieves a good performance of the artificial neural network (ANN) model, and the predicted results are in agreement with experimental values. A processing map of Ti40 alloy is obtained with the flow stress predicted by the trained neural network model. The processing map developed by ANN model can efficiently track dynamic recrystallization and flow localization regions of Ti40 alloy during deforming. Subsequently, the safe and instable domains of hot working of Ti40 alloy are identified and validated through microstructural investigations.

The second phases in Ti40 burn resistant alloy after high temperature exposure for a long time

Abstract The Ti40 (Ti–25V–15Cr–0.2Si) alloy is a burn resistant β titanium alloy. Second phase precipitation after high temperature exposure for a long time has been studied. The second phases, i.e. Ti 5 Si 3 and α, precipitate from the β matrix after the Ti40 alloy has been exposed at high temperature for a long time. The Ti 5 Si 3 phase distributes discontinuously along the grain boundary if the exposure temperature is below 540°C. Exposed at 700°C for 100 h, the coarse Ti 5 Si 3 phase rapidly grows, its tensile properties obviously reducing after thermal exposure when conventionally forged. Coarse Ti 5 Si 3 and α phases form after exposure at 540°C for 100 h, which leads to serious decrease of tensile properties after thermal exposure for isothermally forged alloys. Ti 5 Si 3 precipitates distribute discontinuously along grain boundary in the conventionally forged alloys after creep exposure. There are also many coarse rod-like α phases in isothermally forged alloys after creep exposure.

Oxidation behavior of a burn resistant highly stabilized β titanium alloy

Abstract Ti40 alloy (Ti–25V–15Cr–0.2Si) is a highly stabilized β type burn resistant titanium alloy. Its oxidation behavior was studied. The weight gain increases greatly with the oxidation temperature, and then decreases when the oxidation temperature is over 1000°C. The oxidized products at 600°C are TiO2 and V2O5. If the oxidation temperature is greater than 700°C, TiO2 becomes the main product due to the vaporization of V2O5. The oxide scale cracks and peels off, if the temperature is over 900°C. SiO has been evidenced in the porous oxides if the temperature is over 800°C, and it vanishes as a result of scale spallation at 1050°C. A small amount of Cr2O3 has also been detected inside the scales and distributes non-continuously. Reasons of the fast oxidation are analyzed, and an oxidation model is suggested.

Liquid Segregation Phenomenological Behaviors of Ti14 Alloy during Semisolid Deformation

The liquid segregation phenomenon and its effect on deformation mechanism of Ti14 alloy in semisolid metal processing were investigated by thermal simulation test. Microstructure of depth profile was determined by cross-section quantitative metallography, and liquid segregation phenomenon was described by Darcys law. The results show that segregation phenomenon was affected by solid fraction, strain rate, and deformation rate. More liquid segregated from center to edge portion with high strain rate and/or deformation ratio as well as low solid fraction, which caused different distribution of dominating deformation mechanism. The relationship between liquid segregation and main deformation mechanism was also discussed by phenomenological model.

Intelligent method to develop constitutive relationship of Ti-6Al-2Zr-1Mo-1V alloy

Abstract The isothermal compression tests were carried out in the Thermecmastor-Z thermo-simulator at temperatures of 800, 850, 900, 950, 1000 and 1050 °C and the strain rates of 0.01, 0.1, 1 and 10 s −1 . The influence of deformation temperature and strain rate on the flow stress of Ti–6Al–2Zr–1Mo–1V alloy was studied. Based on the experimental data sets, the high temperature deformation behavior of Ti–6Al–2Zr–1Mo–1V alloy was presented using the intelligent method of artificial neural network (ANN). The results indicate that the predicted flow stress values by ANN model is quite consistent with the experimental results, which implies that the artificial neural network is an effective tool for studying the hot deformation behavior of the present alloy. In addition, the development of graphical user interface is implemented using Visual Basic programming language.

Burn-resistant behavior and mechanism of Ti14 alloy

The direct-current simulation burning method was used to investigate the burn-resistant behavior of Ti14 titanium alloy. The results show that Ti14 alloy exhibits a better burn resistance than TC4 alloy (Ti–6Al–4V). Cu is observed to preferentially migrate to the surface of Ti14 alloy during the burning reaction, and the burned product contains Cu, Cu2O, and TiO2. An oxide layer mainly comprising loose TiO2 is observed beneath the burned product. Meanwhile, Ti2Cu precipitates at grain boundaries near the interface of the oxide layer, preventing the contact between O2 and Ti and forming a rapid diffusion layer near the matrix interface. Consequently, a multiple-layer structure with a Cu-enriched layer (burned product)/Cu-lean layer (oxide layer)/Cu-enriched layer (rapid diffusion layer) configuration is formed in the burn heat-affected zone of Ti14 alloy; this multiple-layer structure is beneficial for preventing O2 diffusion. Furthermore, although Al can migrate to form Al2O3 on the surface of TC4 alloy, the burn-resistant ability of TC4 is unimproved because the Al2O3 is discontinuous and not present in sufficient quantity.

Microstructure-Tensile Properties Correlation for the Ti-6Al-4V Titanium Alloy

Finding the quantitative microstructure-tensile properties correlations is the key to achieve performance optimization for various materials. However, it is extremely difficult due to their non-linear and highly interactive interrelations. In the present investigation, the lamellar microstructure features-tensile properties correlations of the Ti-6Al-4V alloy are studied using an error back-propagation artificial neural network (ANN-BP) model. Forty-eight thermomechanical treatments were conducted to prepare the Ti-6Al-4V alloy with different lamellar microstructure features. In the proposed model, the input variables are microstructure features including the α platelet thickness, colony size, and β grain size, which were extracted using Image Pro Plus software. The output variables are the tensile properties, including ultimate tensile strength, yield strength, elongation, and reduction of area. Fourteen hidden-layer neurons which can make ANN-BP model present the most excellent performance were applied. The training results show that all the relative errors between the predicted and experimental values are within 6%, which means that the trained ANN-BP model is capable of providing precise prediction of the tensile properties for Ti-6Al-4V alloy. Based on the corresponding relations between the tensile properties predicted by ANN-BP model and the lamellar microstructure features, it can be found that the yield strength decreases with increasing α platelet thickness continuously. However, the α platelet thickness exerts influence on the elongation in a more complicated way. In addition, for a given α platelet thickness, the yield strength and the elongation both increase with decreasing β grain size and colony size. In general, the β grain size and colony size play a more important role in affecting the tensile properties of Ti-6Al-4V alloy than the α platelet thickness.

Tailorable Burning Behavior of Ti14 Alloy by Controlling Semi-Solid Forging Temperature

Semi-solid processing (SSP) is a popular near-net-shape forming technology for metals, while its application is still limited in titanium alloy mainly due to its low formability. Recent works showed that SSP could effectively enhance the formability and mechanical properties of titanium alloys. The processing parameters such as temperature and forging rate/ratio, are directly correlated with the microstructure, which endow the alloy with different chemical and physical properties. Specifically, as a key structural material for the advanced aero-engine, the burn resistant performance is a crucial requirement for the burn resistant titanium alloy. Thus, this work aims to assess the burning behavior of Ti14, a kind of burn resistant alloy, as forged at different semi-solid forging temperatures. The burning characteristics of the alloy are analyzed by a series of burning tests with different burning durations, velocities, and microstructures of burned sample. The results showed that the burning process is highly dependent on the forging temperature, due to the fact that higher temperatures would result in more Ti2Cu precipitate within grain and along grain boundaries. Such a microstructure hinders the transport of oxygen in the stable burning stage through the formation of a kind of oxygen isolation Cu-enriched layer under the burn product zone. This work suggests that the burning resistance of the alloy can be effectively tuned by controlling the temperature during the semi-solid forging process.

Effect of Cu concentration on the semi-solid deformation behavior and microstructure of Ti–Cu alloy

The semi-solid compressive deformation behavior of Ti–Cu alloys was investigated by Gleeble-3500 hot simulator at the deformation temperatures ranging from 1273 to 1473 K with strain rates ranging from 5×10−3 to 5×10−1 s−1. The relationship between Cu concentration and flow stress was analyzed, and the deformation apparent activation energy was also calculated. The results show that Cu concentration has significant influence on the flows’ behavior of Ti–Cu alloys, especially at high semi-solid deformation temperatures. The Ti–14Cu exhibits the highest flow stress at 1273 and 1373 K, Ti–2.5Cu alloy exhibits the highest flow stress at 1473 K, and Ti–7Cu alloy shows the lowest flow stress at all tested temperatures, which corresponds to liquid fraction caused by varied Cu concentration and the deformation temperature. The difference in microstructure suggests that the shape and distribution of Ti2Cu precipitates are significantly affected by Cu concentration. The increase in Cu concentration leads to the growth and precipitation of acicular Ti2Cu along grain boundaries at high semi-solid deformation temperatures. The deformation apparent activation energy of Ti–14Cu alloy significantly decreases from solid deformation to semi-solid deformation owing to the change in main deformation mechanism from plastic deformation of solid particles to solid particles’ slippage and rotation of grain boundaries.