Narrow Results

The dynamic recrystallization (DRX) of TA15 (Ti-6Al-2Zr-1Mo-1V) titanium alloy during the hot deformation process was studied by the Cellular Automata (CA) model which is base on the dislocation density theory. To build the CA model, the dislocation density model, dynamic recovery model, nucleation model and grain growth model were introduced and developed. The influences of strain rate on the microstructure evolution and flow stress character were investigated which shows that high strain rate leads to later DRX appearance, high flow stress peak value, small mean size of recrystallizing grains(R-grains) and low DRX percentage, but they have the similar Avrami curve. The characteristic of DRX process in a modeling non-uniform temperature filed (NTF) has been studied. All the simulation results show good agreement with the pioneer's work and experimental results.

The inhomogeneity on texture, microstructure and magnetic properties of hot deformed (HD) disc shape R₂Fe₁₄B-typed magnets along radial and axial directions has been investigated. HD disc shape NdFeB magnet exhibits inhomogeneous texture, grain morphologies and magnetic properties not only along the radial direction but also along the axial direction. Higher magnetic properties can be found in the middle part of the HD magnet, due to the existence of larger volume fraction of plate-like Nd₂Fe₁₄B grains and the well c-axis alignment parallel to the pressing direction. In addition, particle-like and coarser grains are easily found not only in the bottom but also in the edge of the HD magnet, which may degrade the magnetic performance of the samples cut from those parts. Owing to the inhomogeneity of (00L) texture and grain morphologies along axial and radial directions, the optimal magnetic properties of B_r = 12.7 kG, H_ci = 5.1 kOe and (BH)_max = 39 MGOe can be obtained only in the middle sample of HD magnet with slight polishing the top and bottom surfaces.

The hot deformation behavior of powder metallurgy Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy was investigated by isothermal compression test at the wide temperature range of $700$–$1150^{\circ }$C, under an intermediate strain rate of 0.1 s${}^{-1}$, and with the sample deformation degree of 30%, 50% and 70%. Results suggested that the flow stress was very sensitive to the deformation variables and it decreased with increasing the deformation temperature. Flow localization occurred when the temperature was lower than $800^{\circ }$C, accompanied by the dynamic morphology changing of $\alpha$ phase. Dynamic recovery (DRV) features appeared when the temperature was over $850^{\circ }$C, while dynamic recrystallization (DRX) took a more important role than DRV for the alloy compressed at $900$–$1000^{\circ }$C.

The hot deformation behavior of Ti–3Al–5Mo–4.5V alloy with an ${\alpha }^{\prime \prime }$ martensitic microstructure was studied in the temperature range of 700–1000${}^{\circ }$C and strain rates range of 0.001–$1{\mathrm{\text{s}}}^{-1}$ up to a height reduction of 50%. The results show that an ultrafine equiaxed microstructure with an average grain size of $0.45\mu \mathrm{\text{m}}$ were successfully produced through thermomechanical processing of a martensitic starting microstructure. A processing map was successfully constructed and provides appropriate processing parameters for hot deformation which are located in the temperature range of 700–800${}^{\circ }$C and the low strain rate range of 0.001–$0.01{\mathrm{\text{s}}}^{-1}$. The instability domain mainly occurs at the strain rate higher than $0.05{\mathrm{\text{s}}}^{-1}$ for the whole deformation temperature, which should be avoided during practice. The flow softening mechanism of the alloy is determined to be continuous dynamic recrystallization and the unstable flow is caused by the macro-fracture and flow localization.