| 其他摘要 | TiAl-based alloys have high specific strength, specific stiffness, excellent oxidation and creep resistance, which have been paid broad attentions to for years. However, the major factors limiting structural applications in this material are their ambient temperature brittleness and poor workability.
The hot deformation behavior and microstructure evolution process of as-cast Ti-47Al-2Cr-2Nb-0.2W-0.15B and Ti-47Al under different hot deformation conditions were investigated through single-step and multi-step isothermal compression tests. The experiments were conducted at 1100, 1150, 1200 and 1250oC on Gleeble3500 testing machine with strain rates of 0.001, 0.01, 0.1 and 1s-1. In multi-step compression tests, the deformation ratio in every pass was 8%, and the nominal total strain of the specimens was in the range of 30%~70%. In the experiments, the mechanisms of lamellar/colony boundary sliding and dynamic spheroidization process of the lamellar microstructure were observed; the associated flow softening behaviors of the alloys during isothermal, constant-strain-rate deformation were investigated.
From the research, it is concluded that the lamellar/colony boundary sliding and the associated buckling/kinking process initiated within the prior-alpha grains are the controlling factors in the initial stage of hot deformation. The frequency and severity of buckling/kinking increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression axis. Microstructures of the upset test specimens revealed the presence of fine, equiaxed grains resulting from the dynamic spheroidization process, which involves the breakdown of the lamellar structure during hot deformation occurring through a combination of events: shear localization along grain/colony boundaries; rotating and buckling of lamellae; formation of equiaxed particles on grain/colony boundaries, localized shear deformation bands and within lamellae oriented in hard direction. The different constitutive behaviour and microstructure evolution mechanisms in the two kinds of γ-TiAl alloys are also compared and discussed.
The hot deformation behavior, evolution of the microstructure and softening mechanisms of thin lamella Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy and duplex Ti-45.3Al-2Cr-2Nb-0.15B/Ti-47Al-2Cr-2Nb-0.2W-0.15B alloys have been studied on thermal simulator testing machine, with the aids of OM and SEM conducted. The influences of deformation temperature and strain rate on flow stress of the TiAl alloys in hot deformation process are discovered. The results indicated that the TiAl alloys is sensitive to the variety of temperature and strain rate, and the flow curves exhibited a peak stress followed by flow softening. The flow softening behavior is more obvious with the decrease of deformation temperature.
A hyperbolic sine constitutive equation has been established using the parameters of Zener-Holloman, and hot deformation activation energy and other deformation parameters have been obtained. The constitutive equations can perfectly describe the steady-state flow stress of the TiAl alloys and was verified by the experiment very well. Processing maps of the TiAl alloys were established on the basis of dynamic material model (DMM), and the high temperature deformation characteristics of the alloys have been analyzed using processing map in detail. The fracture mechanism at elevated temperature of the TiAl alloys with different composition and microstructure in this study were systemically investigated and discussed.
The hot deformation behavior of duplex Ti-45.3Al-2Cr-2Nb-0.15B under different cumulated deformation conditions was studied by simulating isothermal forging tests using 8mm diameter and 12mm height compressive specimens in order to find the appropriate pack-rolling processing parameters. The hot compressive experiments were conducted at 1150, 1200, 1250, and 1280oC on Gleeble hot simulator with nominal total deformation in the range of 30%~75% and strain rate of 0.2s-1. In the experiments, we selected different parameters of deformation times, reheating time between two deformation passes, and temperature losing process before deformation to simulate the conditions in pack-rolling process. The true stress vs. true strain curves and the dynamic recrystallization process during hot deformation were studied; the microstructure of the specimens was studied using optical microscopy and scanning electron microscopy (SEM); the deformation mechanisms of the specimens were investigated using transmission electron microscopy (TEM); and the features of hot deformation behavior of γ-TiAl based alloy under cumulated deformation conditions were summarized.
The TiAl sheets with thickness of about 3mm were prepared at 1200oC~1300oC by packed rolling from performs of forged slab. The microstructure of the sheets were studied using optical microscopy; the deformation mechanisms of pack rolled TiAl based alloy at elevated temperature were investigated by means of transmission electron microscopy (TEM). The experimental results show that, dislocation movement, mechanical twinning, dynamic recrystallization and static recrystallization occurred during pack-rolling and following reheating process between two rolling passes. Thus, homogenous and refined microstructure with mean size of about 10μm is formed.
The paper summarizes our successful effort of sheet rolling using atomized powder of gamma alloys. Emphasis will be placed upon impurity pick-up and control of the atomized powder, the influence of degassing/HIPing parameters on the soundness of the preform, as well as identification of optimal combinations of rolling parameters. |
修改评论