Structural Stability And Plastic Deformation Mechanism Of β-type Ti-V System Alloys

Titanium alloy is a novel twenty-one century metal after steel, magnesium alloy and aluminum. At present, there are hundreds of titanium alloys in the world. Titanium alloy has been widely used in building, medical, petrochemical and aerospace due to their low density, high specific strength, good corrosion resistance and excellent biocompatibility.Compared with other titanium alloys, β titanium alloys have the highest specific strength and excellent processing performance. Furthermore, β-Ti alloys are the most potential for the development of a novel titanium alloy with high strength and excellent ductility. The development of novel β titanium alloys with high strength and excellent ductility have been attracted much attention by governments around the world. In this paper, the research subjects are the Ti-V alloys prepared by arc melting and Ti-V-Nb-Zr alloys prepared by cold crucible levitation melting. The main measurement tools are electron backscatter diffraction and transmission electron microscopy. The mechanical properties and microstructure of samples were investigated at different deformation and heat treatment conditions. Both β phase stability and plastic deformation mechanisms were studied in Ti-V alloys and Ti-V-Nb-Zr alloys. The new β titanium alloys with excellent mechanical properties were developed by transformation-induced plasticity(TRIP), twinning-induced plasticity(TWIP) and grain refinement. The main conclusions are summarized as follows:1. Vanadium reduce diffraction intensity of athermal ω phase, significantly increase β phase stability and strongly inhibit stress-induced ω phase transformation in Ti-V alloys, thus, vanadium is strong β phase stabilizer in titanium alloys.2. The Ti-18 V alloy exhibits high yield strength 868.69 MPa and excellent total elongation 28.25%, because the Ti-18 V alloy plastically deforms via {332}<113> mechanical twinning and stress-induced ω phase transformation.3. The Ti-16 V alloy plastically deforms via stress-induced ω phase transformation; the Ti-18 V alloy deforms through {332}<113> mechanical twinning and stress-induced ω phase transformation; both the Ti-20 V and Ti-22 V alloys deform by {332}<113> mechanical twinning; the Ti-52 V alloy deforms via dislocation slip and {112}<111> mechanical twinning. The plastic deformation mechanisms depend on β phase stability, while β phase stability depend on vanadium content in Ti-V alloys.4. Oxygen increase the yield strength of Ti-V alloys and reduce its elastic modulus, because increased oxygen content lead to solid solution strengthening and enhance β phase stability in Ti-V alloys.5. The Ti-20V-0.034 O alloy plastically deforms via stress-induced ω phase transformation; the Ti-20V-0.276 O alloy via {332}<113> mechanical twinning. Although both the Ti-16V-0.036 O and Ti-16V-0.290 O alloys deform through stress-induced ω phase transformation, increased oxygen content significantly reduce the stress-induced ω phase content. Oxygen reduce diffraction intensity of athermal ω phase, enhance β phase stability and suppress stress-induced ω phase transformation, thus, oxygen is considered to be β phase stabilizer in titanium alloys.6. The Ti-16V-0.036O、Ti-20V-0.034O、Ti-16V-2Zr、Ti-20V-2Zr and Ti-16V-2Nb-2Zr alloys plastically deform stress-induced ω phase transformation; the Ti-20V-2Nb-2Zr alloy deforms {332}<113> mechanical twinning and stress-induced ω phase transformation.7. The Ti-16V-2Nb-2Zr alloys with different grain sizes plastically deform through stress-induced ω phase transformation; the Ti-20V-2Nb-2Zr alloys with different grain sizes deform via {332}<113> mechanical twinning. The relationship of yield strength and grain sizes obeys the Hall-Petch relationship in Ti-V-Nb-Zr alloys, and the Hall-Petch slopes in Ti-V-Nb-Zr alloys are much larger than that of 11.3 MPa mm1/2 in commercial pure titanium grade 2.8. The single variant of stress-induced ω phases were observed in Ti-16V-0.290 O, Ti-20V-0.034 O, Ti-16V-2Zr, Ti-20V-2Zr and Ti-16V-2Nb-2Zr alloys after tensile deformation. The new orientation relationships between β phase matrix and the single variant of stress-induced ω phase are [110]β∥[-12-10]ω,(3-3-2)β∥(-5052)ω and(-55-4)β∥(30-31)ω, and the observed habit plane of(3-3-2)β∥(-5052)ω for the stress-induced ω phase transition is different from that of(111)β∥(0001)ω often reported for the thermally induced ω phase transition.