Low-temperature superplasticity of VT22 titanium alloy

Characteristics of superplasticity (SP) of ultrafine-grained (UFG) sheets of VT22 titanium alloy with an average grain size of 0.3 mu m are studied. The UFG microstructure was obtained by severe plastic deformation by means of warm isothermal rolling at a temperature of 540 degrees C corresponding to the aging temperature of the VT22 alloy. The paper report on the results of experiments on uniaxial tension of UFG VT22 titanium alloy at the temperatures of 550 and 600 degrees C at strain rates within the range of 10(-4) s(-1) to 5.10(-3) s(-1). It has been shown that the formation of the UFG microstructure leads to a decrease of the temperature of manifestation of the superplasticity effect in the VT22 titanium alloy by 300 degrees C (the VT22 titanium alloy with a fine-grained structure with an average grain size d = 2 mu m has a traditional SP at T = 860 degrees C). It has been established that UFG VT22 titanium alloy displays superplastic properties at a temperature of 550 degrees C, and the maximum SP (a strain rate sensitivity factor m = 0.45, an elongation delta = 620%) is observed at 600 degrees C. At a temperature of 550 degrees C, the SP maximum is observed at strain rate of 10(-4) s(-1). An increase in the temperature to 600 degrees C results in a shift in the maximum values of SP towards higher strain rates. Microstructural and fractographic studies of the low-temperature SP effect of the UFG VT22 alloy revealed the absence of significant grain coarsening during superplastic deformation. The failure of the samples at the studied temperatures is likely due to cracking of the oxide film and the penetration of cracks into the sample during the tensile test.