Zhao Yongqing

A Comparative Study of Various Flow Instability Criteria in Processing Map

Abstract Four instability criteria, namely Murty, Gegel, Malas and Prasad criteria, were compared, and the physical significance of parameters was analyzed in this paper. It is indicated that the instability map developed by Murty criterion is similar to Prasad criterion, showing an unstable flow at high strain rates. Murty criterion exhibits a little narrower unstable region. The instability maps developed by Malas criterion and Gegel criterion have similar shapes, and the unstable regions are wider than those Prasad criterion and Murty criterion. In addition to the unstable flow at high strain rates as predicted by Murty criterion and Prasad criterion, Malas criterion and Gegel criterion have a good ability for predicting unstable flow at high temperatures and low strain rates. The processing maps were validated by hot compression tests of Ti-22Al-25Nb alloy, and the possible causes of various instability were discussed.

Processing Map of One Kind of Metastable β Titanium Alloy

In order to exploit the potential hot deformation ability of metastable β titanium alloy Ti-B19 the isothermal compression behavior was investigated by a Gleeble-1500 thermal simulator in the temperature range of 750–1000 °C at an interval of 50 °C and strain rate from 0.001 to 10 s−1. The results indicate that the flow stress of the alloy increases with increasing of the strain rate at a given temperature, and decreases with the increment of temperature at a given strain rate. The processing map for the alloy obtained at strain of 0.6 can be divided into three domains. The first is the one in the temperature range of 700∼800 °C and the strain rate range of 0.001–0.1 s−1, with a peak efficiency of 42% at about 750 °C and 0.01 s−1. The flow curves in this domain present a single peak or oscillation in the flow curves before continuous softening appears. The second is the one in the temperature range of 800–1000 °C and the strain rate range of 0.001–0.1 s−1, with efficiency from 29% to 36%. The flow curves in this domain present a single peak or oscillation in the flow curves before reaching steady state. Recrystallized structures can be found, which is a typical dynamic recrystallization zone. The third one is a flow instable domain at the temperature lower than 800 °C and the strain rate above 0.1 s−1, or at the temperature higher than 800 °C and strain rate above 10 s−1. The adiabatic shear bands (ASBs) and flow inhomogeneous zone of β phases can be observed.

Microscopic Characterization of Semi-Solid Ti14 Alloy

The microstructural evolution and the element distribution of Ti14 (a+Ti2Cu) alloy during semi-solid compression were investigated. Moreover, the Ti2Cu precipitate behaviors were discussed in detail. The results show that the microstructure, the number and distribution of Ti2Cu precipitates have significant dependence on the process parameters. More Ti2Cu will precipitate on grain boundaries at a higher deformation temperature, and/or a lower strain rate as well as deformation ratio, and finally a network structure is formed after deformation at 1100 °C. The precipitation on grain boundaries is found to be mainly controlled by peritectic reactions. The elevated temperatures resulted in more liquid along the prior grain boundaries, and a Cu-rich region is formed, finally. the “coarse” grain boundaries appear during re-solidification.

Hot Deformation Microstructure and Mechanism of Ti53311S Titanium Alloy

Abstract The hot deformation behavior of Ti53311S titanium alloy in the temperature range of 880–1080 °C and strain rate range of 0.001-10 s −1 have been studied by hot compressive testing on a Gleeble-1500 simulator. The flow behavior was described by the hyperbolic sine constitutive equation, the activation energies in (α+ β ) region and in β region were calculated as 641 kJ/mol and 244 kJ/mol, respectively. The microstructure of Ti53311S was observed and analyzed using an optical microscope. The experimental results show that this alloy is sensitive to deformation temperature and strain rate, and the strain rate sensitivity exponent values ( m ) decreases with increasing of deformation temperature while the deformation activation energy ( Q ) increases. The leading deformation mechanism in the single phase β region is dynamic recover, while that in (α+ β ) region is grain boundary slipping and grain globularizing.

Discontinuous Yielding in High Temperature Deformation of Ti-5553 Alloy

The discontinuous yield behavior of Ti-5Al-5Mo-5V-3Cr-0.5Fe alloys, a new metastable β titanium alloy, was tested at 800∼860 °C and in strain rate range of 0.1∼10 s−1 on a Gleeble-1500 thermal Simulation testing machine. Results show that discontinuous yielding appears at the highest strain rate (10 s−1), and the magnitude of discontinuous yielding increases with increasing of the temperature. The above mentioned phenomenon is in agreement with Johnston and Gilmans dynamic yield theory, that is to say, the discontinuous yield behavior is related to the sudden multiplication of a great quantity of mobile dislocations in the alloy.

Fatigue Crack Growth Behavior in TC4-DT Titanium alloy with Different Lamellar Microstructures

Abstract This paper focused on the fatigue crack growth behavior of TC4-DT titanium alloy with different lamellar microstructures. Heat treatments were performed in order to prepare different microstructures with α lamella width and colony size. The results show that on the premise to guarantee adequate mechanical properties, the microstructures formed through air cooling from the β-phase field present better ability to resist fatigue crack propagation. This is attributed to the existing of bigger plastic deformation field in crack tip. It is also found that wide α lamellae and coarse α colony are beneficial to improve fracture toughness and resistance to fatigue crack propagation.

Effect of Temperature on Segregation and Deformation Mechanism of α+Ti2Cu Alloy during Semi-Solid Forging

Abstract The segregation behavior and the deformation mechanism of Ti14, a new typical burn resistant α+Ti2Cu alloy were investigated, during semi-solid forging at different temperatures. The results reveal that semi-solid temperature will affect the content and the distribution of liquid phase evidently. Upon increasing of temperature, the liquid phase distribution transforms from discontinuity to continuity at grain boundaries, and finally a type of net-structure is formed. Moreover, the macro-segregation of liquid and solid phases occurs during the semi-solid forging, which leads to a change of the main deformation mechanism from center to edge of the sample. The relationship of macro-segregation of liquid/solid phases and the main deformation mechanism was also discussed by a phenomenological model.

Deformation Mechanism and Microstructure Evolution of TLM Titanium Alloy during Cold and Hot Compression

Abstract The deformation mechanism and microstructure evolution of TLM titanium alloy were studied during cold compression at room temperature and hot compression at 850 o C under the condition of 0.001 s −1 strain rate. The results show that the deformation mechanisms and microstructure evolution under cold and hot compressions of TLM alloy are different. Twinning, stress-induced α “ phase transformations and dislocations slipping are the main deformation feature in cold compression, whereas the dislocations slipping, dynamic recovery (DR) and dynamic recrystallization (DRX) are the main deformation mechanisms in hot compression. The stress flow softening of TLM alloy in hot compression is related to DR and DRX. Meanwhile, the compressive strengths in cold and hot compression are 975 and 40 MPa, respectively. Compared with the cold compression, the compressive strength of hot compression is decreased by about 96%.

Effect of α Composite Layer on the Microstructure and Mechanical Properties of Ti-10V-2Fe-3Al Alloy

Abstract Oxygen enriched α composite layer with 82 μm thickness was formed on the surface of Ti-10V-2Fe-3Al alloy by the heat treatment at 830°C for 1 h in air and water quenching. The effects of α composite layer on the microstructure and mechanical properties of Ti-10V-2Fe-3Al alloy were investigated. The microstructure, the hardness and the elements distribution of α composite layer were studied. The results show that the hardness variation of α composite layer does not always display a decreasing trend from the edge to the matrix, but shows the law of high-low-high to stable trend, which is related with the distribution of elements (especially V and Fe) and microstructure evolution. The surface hardness of Ti-10V-2Fe-3Al alloy with α composite layer increases by 45%; meanwhile, the tensile strength and yield strength decrease by 5% only. Cracks will be generated firstly on the sample surface along the direction perpendicular to the tensile stress in tensile procedure, then expand in α composite layer and cross up to the matrix material. Fracture morphologies show the characters of ductility in matrix zone and brittle fracture in α composite layer zone. Stress induced phase transformation from β phase to α″ phase will take place during the tensile deformation.

Influence of Processing Parameters on the Deformation Behavior of Ti14 Alloy Containing a Small Volume of Liquid

Abstract The effects of the processing parameters on the deformation behavior of Ti14 alloy containing a small volume of liquid have been investigated through compressive tests. Experiments were conducted at deformation temperatures from 1000 to 1150 °C, strain rates from 5×10−3 to 5 s−1 and deformation degrees from 40% to 70%. The results show that deformation temperature and strain rate have significant effects on the peak flow stress. The flow stress decreases with increasing of deformation temperature and/or decreasing of strain rate. The response time required by deformation is affected by the strain rate and liquid fraction is controlled by the temperature. As the temperature increases, deformation mechanism transforms from sliding between solid particles (SS) to the flow of liquid incorporating solid particles (FLS), which improves the semi-solid deformation behavior. In addition, the flow stress decreases with increasing of deformation degrees during the compression which is associated with lubricating effect of liquid during semi-solid deformation.