Creep Behavior of Fusion Zone and Base Metal of the Electron Beam Weldments of a Near-alpha Titanium Alloy

Abstract

The high temperature creep behavior of fusion zone (FZ) and base metal (BM) of the electron beam weldments of a near-alpha titanium alloy Ti-60 has been investigated. While the BM shows a fully transformed, coarse primary beta grains with large colonies consisting of aligned alpha lamellar, the FZ exhibits thin martensitic alpha' platelets in the columnar beta grains. The creep results show that the steady state creep rates of FZ follow the power-law creep, with the stress exponents obtained in the range from 5.6 (550 degrees C) to 5.9 (600 degrees C), and corrected activation energies of 309-352 kJ/mol; the stress exponents of BM exhibit different values when the creep testing stress and temperature alternate. The values of 2.4-3.2 are obtained when the stresses are under 220 MPa or the temperature is 550 degrees C, also an exponent of 6.6 is achieved at stresses above 220 MPa at 600 degrees C. The corrected activation energies of BM corresponding to the stress exponents are 123-161 kJ/mol (n=2.4-3.2) and 344 kJ/mol (n=6.6). The creep mechanisms of FZ and BM have been discussed in light of the creep microstructures, activation energies and the stress exponents. The creep mechanisms of FZ is the diffusion controlled dislocation climb, the creep of BM is controlled by 'solute drag' creep and dislocation climb when the stress and temperature are different. Transmission electron microscopy (TEM) observations of the dislocation structures of crept specimens are presented to give some supports for the creep behavior of FZ and BM. In addition to the creep mechanism of dislocation movement, the interface sliding has been found to play an important role during creep of FZ.