Research On Superplastic Deformation Behaviors And Mechanisms Of As-extruded TiBw/TC4Composites

This paper is aimed to study the superplastic behavior of titanium boride whiskerreinforced TC4matrix composites, and try to reveal the deformation mechanism. Basedon the design of quasi-continuous network structure, composites with different volumefraction of reinforcement were fabricated by power metallurgy, and then deformed byextrusion. High temperature tensile test were carried out to study the superplastic behavioron Instron-1186. The morphology of fracture and flank of samples after superplasticdeformation were observed under OM and SEM. In addition, EBSD analysis was used tostudy the texture evolution and TEM analysis was used to discover the distribution ofdislocation during superplastic deformation.The optimum deformation parameters were obtained under high temperature tensiletest. Compared with the extruded TC4alloy, the optimum deformation temperaturereduced while the strain rate increased. At the same time, the tendency of necking duringdeformation decreased with the rising of volume fraction of reinforcement. Then, thestrain-rate-jump tests were conducted to determine the strain rate sensitivity in3TMCs.It turned out that the strain rate sensitivity reached its maximum value0.54at950℃. Atlast, the deformation activation energy of composites was obtained by calculating.Compared with TC4alloy with equiaxed structure, the extruded TC4alloy and TiBw/TC4composites stood a much higher activation energy with an average of448kJ/mol.According to fracture morphology analysis, the reinforcement could delay theoccurrence of the instability due to localized necking. Therefore, it could help grainboundary sliding process more smoothly. With the increasing of temperature anddecreasing strain rate, the grain boundary became softer and weaker, which strengthenedsliding process and enhanced in transformation.Flank morphology observation showed that the lamellar structure in extruded TC4alloy kinked and deflected during superplastic deforming, which resulted in a decrease inthe aspect ratio of lamellar, and finally helped to gain equiaxed structure after a largestrain. However, this effect brought structure differences among different regions, whichmade the transmission of localized necking harder. On the contrary, the lamellar structurein extruded TiBw/TC4composites transformed to equiaxed structure with a small strainby dynamic recrystallization progress. This phenomenon may account for less tendencyof necking during deformation and the large elongation composites got. With the increaseof strain, the volume fraction and the size of beta phase raised.Texture analysis indicated that a [0001]//ED texture of the original extrudedTiBw/TC4composites diffused during superplastic deformation. Nevertheless, a new [0001]//TD texture had been achieved in the process, which might have relationship withthe progress of phase transformation.It can be drew from the TEM analysis that the dislocation motion acted as anessential coordination mechanism during superplastic deformation in composites. Thegrains adjacent to the reinforcement had a higher dislocation density as a result ofdiscordance. Then, the nucleus of recrystallization grains formed at trigeminal grainboundary or the position where dislocation pile-up, which led to a mass of dynamicrecrystallization in composites.