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Evolution for the dynamic recrystallization (DRX) volume fraction of Ti-5Al-5Mo-5V-3Cr-1Zr near β titanium alloy during hot deformation was characterized by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. To determine the equation parameters, a series of thermal simulation experiments at the temperature of 1023–1098 K and strain rate of 0.001–1 s‒1 to the true strain of 0.7 were conducted to obtain the essential data about stress σ and strain ε. By further transforming the relationship of σ versus ε into the relationship of strain hardening rate dσ/dε versus σ, two characteristic strains at the beginning of DRX (critical strain εc) and at the peak stress (peak strain εp) were identified from the dσ/dε-σ curves. Sequentially, the parameters in the JMAK equation were determined from the linear fitting of the different relationships among critical strain εc, peak strain εp and deformation conditions (including temperature T, strain rate ε ˙ $\dot \varepsilon $ and strain ε). The as-obtained JMAK equation was expressed as XDRX=1-exp[-0.0053((ε-εc)/εc)2.1], where εc=0.6053εp and εp=0.0031 ε ˙ $\dot \varepsilon $ 0.0081exp(28,781/RT). Finally, the JMAK equation was implanted into finite element program to simulate the hot compression of thermal simulation experiments. The simulation predictions and experimental results about the DRX volume fraction distribution showed a good consistency. [ABSTRACT FROM AUTHOR]

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. • Precipitation behavior of α phase in a β-quenched Ti-55511 alloy during hot compression is studied. • At low strain rates or high deformation temperatures, the main softening mechanism is dynamic recovery. • Dynamic recrystallization is the dominant softening mechanism at high strain rates or low temperatures. • α precipitates are distributed a bit more sparsely with several scattering orientations within β grains. • With increasing deformation temperature, the content, aspect ratio and size of α phases decrease. [ABSTRACT FROM AUTHOR]

Abstract: Ti-5Al-5Mo-5V-1Cr-1Fe near β titanium alloys containing a thick and thin lamellar α phase were hot compressed at 700–800°C and strain rates of 10−3–10−1s−1. The influence of lamellar α on alloy deformation and microstructure evolution was investigated. The great impediment of thick lamellar α to plastic flow led to high deformation resistance, even though lamellar rotation caused a stress reduction at the beginning of continuous flow softening. In contrast, the integral and translational motion of thin lamellar α without apparent lamellar rotation presented a lower impediment to plastic flow. Moreover, the corresponding alloy exhibited low deformation resistance and gentle continuous flow softening. Furthermore, ultra-fine grained Ti-55511 with an α/β grain size<0.5 - 1.0 μm was prepared by introducing the thin lamellar α at a low deformation temperature to prevent the substantial dissolution of α. The thin lamellar α structures were easily fragmented into fine α grains. The ease with which dislocations were cut across the thin lamellar α enhanced the dislocation density in the small β regions among thin, disperse lamellar α regions, which further led to the formation of β subgrains by the slipping-climbing of dislocations. Subsequently, fine β grains formed through the subgrain rotation mechanism. [Copyright &y& Elsevier]

The precipitation orientation of α phases in a Ti-55511 alloy during the heat treatment and the subsequent hot deformation is investigated. It is observed that the β grains prominently influence the precipitation process of α phases, i.e., for the neighbouring β grains without similar {110} pole, the precipitation orientation of grain boundary (GB) α phases (α GB) keep {0001}//{110}β1 or {0001}//{110}β2 to maintain the burgers orientation relationship (BOR) with the neighbouring β grains. For the neighbouring β grains with similar {110} pole, the {0001} poles of α GB phases precipitate parallel to this similar {110} pole and maintain BOR with the neighbouring β grains. For the neighbouring β grains with multiple similar {110} poles, the {0001} poles of α GB phases precipitate along these similar poles and maintain BOR with the neighbouring β grains. Besides, the interface instability nucleation and sympathetic nucleation are the main nucleation modes of the GB widmanstatten α phase (α WGB), which primarily precipitate within β grains with the similar orientation of α GB phases. The intracrystalline widmanstatten phase (α WI) precipitate in a single β grain with 12 precipitation orientations. The BOR between α WI phases and β grains is destroyed during the subsequent hot compressive deformation. • Precipitation orientation of α phases in Ti-55511 alloy during heat treatment plus hot deformation is studied. • Precipitation orientation of GB α phases maintains the BOR with neighbouring β grains along different poles. • Interface instability and sympathetic nucleations are the main nucleation modes of GB widmanstatten α phases. • The intracrystalline widmanstatten phases (α WI) precipitate in a single β grain with 12 precipitation orientations. • The BOR between α WI phases and β grains is destroyed during the subsequent hot compressive deformation. [ABSTRACT FROM AUTHOR]