Influence Of Phase Volume Fraction On The Grain Refining And Superplastic Behavior Of A TC4Alloy By High-Pressure Torsion

High Pressure Torsion (HPT) is a unique processing method to obtain bulk materials with ultrafine-grains (UFG). HPT could significantly refine the grains and improve the mechanical and physical properties of materials, and it has become an attractive research field of materials science and engineering.HPT was performed on the TC4alloy with different volume fraction at room temperature for different turns in this paper. The characteristics of the microstructure and mechanical properties of as-processed TC4alloy were examined using optical microscope, transmission electron microscopy (TEM) and microhardness tester. The influences of HPT turns and volume fractions on the microstructure and microhardness of TC4alloy processed by HPT have been studied, and the refining mechanism of TC4alloy in the process of HPT was discussed. Superplastic tensile tests of the ultrafine-grained TC4alloy processed by HPT have been carried out in the temperature range of500～600℃. Mechanical behaviors and deformation abilities of SPD were analyzed, and its deformation mechanism was discussed. The main conclusions are as follows:(1) A cold-rolled TC4alloy sheet was subjected to three different heat treatments and different volume fractions of the two constituents, equiaxed a phase and lamellar (α+β) phase were attained. Quantitative measurements revealed that the volume fractions of the lamellar (α+β) decreased when temperature increased, being30%in TC4-1,53%in TC4-2and75%in TC4-3and the grain sizes of the equiaxed a phase were～7.0μm,～9.0μm and～9.5μm, respectively.(2) The results of the HPT tests showed that the microhardness increased as the numbers of HPT turns increased. Initially the distribution of microhardness along the radial direction was not homogeneous with lower values in the center and higher values at the edge. When the turns were higher than5, the microhardness gradually reached the saturation. The microhardness reached a maximum and the distribution along the radial direction tended to be reasonably homogeneous when the HPT was continued to20turns. The grain size decreased with increasing numbers of turns in HPT processing. After20turns HPT, the measured equilibrium grain sizes were～117nm in TC4-1,83nm in TC4-2and～77nm in TC4-3, which meaned grain refinement might be optimized as the lamellar (α+β) increased. Besides, as the fraction of the a phase decreased, the diameters of the areas of low microhardness gradually decreased from～3.0mm to～2.0mm and then to～.0mm. This was consistent with the inhomogeneous regions of the micro structure observed after HPT through20turns.(3) The microstructure observation of TC4alloy in the process of HPT revealed that:The lath structures with small misorientation are observed inside the grains after one turn HPT. When the turns are higher than5, the increase of misorientation means the lath structures gradually evolve into subgrains. At the same time, plenty of cellular structures are observed in the lath structures, which means the lath structures will be refined in longitudinal axis direction. As the turns go on increasing, more cellular structures evolve into subgrains and consequently dynamic recovery occurres, which forms the UFG with high-angle boundaries. All of these result in the grain refinement.(4) The results of the superplastic tensile tests show that the ultrafine-grained TC4alloy processed by HPT exhibited good low temperature and high strain rate superplasticity. The elongation of419%is obtained at an initial high strain rate of10-2s-1(T=600℃), and the elongation over300%is obtained even at a lower temperature of500℃(ε≤5×10-3s-1). The highest elongation of736%is obtained at600℃with an initial strain rate of5×10s-1. Besides, the UFG TC4alloy is sensitive to the deformation temperature and strain rate. The flow stress and elongation increased when temperature increased and decreased when the strain rate increased.(5) The deformation activation energies of the UFG TC4alloy are in the range of162-180kJ/mol over the temperature range(500-600℃), which are very close to the self-diffusion activation energy of grain boundary(130-169kJ/mol), indicating the main deformation mechanisms is grain boundary sliding controlled by grain boundary diffusion in the superplastic tensile deformation.