Papers by Keyword: Flow Softening

Abstract: The hot deformation behavior of Ti-6Al-4V alloy with transitional microstructure over temperature 800°C~950°C and strain rate ranges of 0.001~10s^-1 has been studied by Gleeble-3500 hot working simulation testing machine. The flow softening of stress-strain curves is resulted from the spheroidization of transitional microstructure, dynamic recrystallization and superplasticity. Both temperature and strain rate are important factors affecting the deformation behavior. Flow instability induced by adiabatic shear bands occurs at 800-880°Cand 0.32-10 s^-1. With the increasing of strain rate and decreasing of temperature, the degree of strain localization increases. The optimum working region of Ti-6Al-4V alloy with a transitional microstructure is at 820-910°C and 0.001-0.1 s^-1.

Abstract: This paper discusses the microstructural changes during hot deformation, mainly dynamic nucleation and growth of a precipitates, occurring in the b metastable titanium alloy Ti-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe). The effect of process variables on flow response and microstructure evolution during hot working of the alloy with initial bimodal microstructure was established using isothermal hot-compression tests. Testing was conducted at 4 strain rates between 0.001 and 1 s⁻¹ and 5 temperatures between 720 °C and 850 °C, on material with prior b grain size of 450 μm, a nodular size of 3 µm and a known primary a-phase fraction. All flow curves exhibited a peak stress followed by moderate flow softening in the two-phase domain. Flow softening was interpreted in terms of deformation heating and substructure or texture evolutions. The dependence on strain rate and temperature of the kinetics of dynamic a-phase nucleation during straining is complex and appears to be of second-order importance compared to the effects of strain. This suggests that the nucleation and growth of a phase in the temperature range between 720 °C and 990 °C results from a mixed-mode displacive-diffusional transformation, similar to the austenite / ferrite transformation above the Ae3 temperature reported by some authors.

Abstract: The plastic deformation behaviors of 7050 Al alloy were investigated by compression tests at temperatures ranging of 250°C
450°C under constant strain rates of 0.01s−1, 1s−1 and 10s−1. The results showed that all the flow curves consisted of three stages, i.e. strain-hardening, strain-softening and steady state-strain. Initially, the stress rises steeply at microstrain deformation, and then increases at a decreased rate, followed by a strain-softening until a steady state stress. The stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener-Hollomon parameter in an exponent-type equation, 1 19 7.202 σ p 80.71 sinh (1.64 10 Z) = ⋅ −  × − ⋅ −  . Elongated grains with serrations developed in the grain boundaries were observed; the dynamic recrystallization (DRX) occurs with increasing temperature and dislocation density, and the shape of grain at steady state is nearly equiaxial. It can be concluded that the DRX phenomenon is sensitive to the temperature and the dynamic flow softening is mainly as the result of dynamic recovery and DRX.

Abstract: This paper presents the true stress - strain curves and data analyses of a Ni-containing TiAl and its reference alloy based on the isothermal compression tests at 1000°C and 0.01 - 1.0s^-1 strain rates. The results show that the minor Ni addition makes the flow softening coming sooner and therefore significantly lowers the peak stress. Those effects, in addition with a better balance between the work hardening and flow softening during hot deformation, improve the steady state flow behavior of TiAl. The Ni-influence mechanisms are also suggested based on the TEM observation of dislocation configurations and lamellar breakdown during the deformation.

Abstract: Prediction of final microstructures after high temperature forming of Ti-6Al-4V alloy was´attempted in this study. Using two typical microstructures, i.e., equiaxed and Widmanstätten microstructures, compression test was carried out up to the strain level of 0.6 at various temperatures (700 ~ 1100°C) and strain rates (10^-4 ~ 10²/s). From the flow stress-strain data, parameters such as strain rate sensitivity (m) and activation energy (Q) were calculated and used to establish constitutive equations for both microstructures. Then, finite element analysis was performed to predict the final microstructure of the deformed body, which was well accorded with the experimental results.