Mechanical Properties and Structural Superplasticity in Ultrafine Grained α-Titanium / Ti_xMe_y-Intermetallic Ti-8Fe-4Al, Ti-10Co-4Al and Ti-10Ni-4Al Alloys

摘要

Quasi eutectoide titanium-transition metal alloys with ultrafine-grained microstructures consisting of a-titanium solid solutions, matrix grain size of about 0.5 to 0.7 micron, and a dispersion of intermetallic FeTi (B2 type of structure), C0_2Ti and Ni_2Ti (cubic complex E9_9 superlattice structure) particles of 0.2 to 0.3 micron in size exhibit superplasticity at high-strain rates and superior strength properties. Strain-rate-sensitivity exponents ranging between 0.4 < m < 0.5 were achieved at high strain rates of 2·10~(-3) ≤ ε ≤ 5·10~(-2) s~(-1) and at medium temperatures from 625 to 775℃. Maximum elongations to failure of ε_(tot) ≈ 1100% were recorded. TEM observations revealed relatively high dislocation densities in the α-Ti(Al) grains, whereas the intermetallic particles are nearly free of dislocations. Activation analysis showed activation energies of the order of Q ≈ 205 ± 15 kJ/mol. The grain size exponent was determined to be p ≈ 2.1 ± 0.2. The obtained results reveal clearly that superplasticity at higher strain rates in these materials is due to grain boundary sliding accommodated by dislocation climb controlled by lattice diffusion of titanium. The strong increase in strength is caused by effective particle strengthening mechanisms.