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Abstract
The effect of stepped tensile deformation at 850, 900, 950 °C on the elongation, microstructure, and mechanical characteristics of titanium alloy has been investigated. The stepped uniaxial tension (stepped-UT) was composed of the following three steps in sequence: constant speed tension, clearance stage, and maximum m superplasticity tension (MaxmSPT). Results showed that the maximum elongation of TC6 alloy between 850 and 950 °C through the Stepped-UT was 2053%, in which the first engineering strain of constant speed tension was 2.0, the following clearance time was 15 min, and the MaxmSPT was finally carried on until failure. And the optimal elongations obtained by the MaxmSPT and constant speed tensile method were 1347 and 753.9% at 850 °C, respectively. The true stress-strain curves showed the strain rate sensitivity index m of the alloy in the stepped-UT was higher than the one in the single step of the MaxmSPT. Moreover, the microstructure of TC6 alloys in the stepped superplastic deformation was observed and the grain refinement was found. The grain refinement and true stress-strain curves of TC6 alloys were all affected by preplanned engineering strain and temperatures. The results also showed the joint action of the dynamic recrystallization and static or meta-dynamic recrystallization refined the grains, improved the structure property, and induced the plasticity enhancement.
Highlights
It is widely believed that superplasticity is a characteristic of equiaxed, stable, fine-grained materials, consisting primarily of high-angle grain boundaries, when they are deformed in tension at relatively high temperatures
The microstructural evolution and superplastic properties have been reported for Ti-6Al-4V alloy (Ref 4-6), and the superplastic behaviors of titanium alloys have been investigated in some reports (Ref 7, 8)
He obtained the optimal elongation of 2300% for Ti-6.5AL-3.5Mo-1.5Zr-0.3Si alloy without grain refinement treating through the maximum m superplasticity deformation (MaxmSPD) tensile test at 900 °C, while their optimal elongations attained by the constant speed and constant strain rate tensile test were 1100 and 1147% at the same temperature, respectively
Summary
It is widely believed that superplasticity is a characteristic of equiaxed, stable, fine-grained materials, consisting primarily of high-angle grain boundaries, when they are deformed in tension at relatively high temperatures. Wang et al (Ref 17-19) introduced a novel concept of the maximum m superplasticity deformation (MaxmSPD) in which the maximum m value was always maintained through dynamic control and adjustment of deformation strain rate to insure the forming material in the optimum superplasticity state all the time. He obtained the optimal elongation of 2300% for Ti-6.5AL-3.5Mo-1.5Zr-0.3Si alloy without grain refinement treating through the MaxmSPD tensile test at 900 °C, while their optimal elongations attained by the constant speed and constant strain rate tensile test were 1100 and 1147% at the same temperature, respectively
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