Hot deformation mechanisms in metastable beta titanium alloy Ti-10V-2Fe-3Al

The mechanisms of hot deformation in the beta titanium alloy Ti - 10V - 2Fe - 3Al have been characterised in the temperature range 650-850 degreesC an d strain rate range 0.001-100 s(-1) using constant true strain rate isother mal compression tests. The beta transus for this alloy is similar to 790 de greesC, below which the alloy has a fine grained duplex alpha+beta structur e. At temperatures lower than the beta transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, ei ther continuous flow softening or oscillations are observed at lower or hig her temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650-750 degreesC and at strain rat es lower than 0.01 s(-1), the material exhibits fine grained superplasticit y marked by abnormal elongation, with a peak at similar to 700 degreesC. Un der conditions within this domain, the stress-strain curves are of the stea dy state type. The apparent activation energy estimated in the domain of fi ne grained superplasticity is similar to 225 kJ mol(-1), which suggests tha t dynamic recovery in the beta phase is the mechanism by which the stress c oncentration at the triple junctions is accommodated. Second, at temperatur es higher than 800 degreesC and strain rates lower than similar to0.1 s(-1) ; the alloy exhibits large grained superplasticity, with the highest elonga tion occurring at 850 degreesC and 0.001 s(-1); the value of this is about one-half of that recorded at 700 degreesC. The microstructure of the specim en deformed under conditions in this domain shows stable subgrain structure s within large beta grains. Third, at strain rates higher than 10 s(-1) and temperatures lower than 700 degreesC, cracking occurs in the regions of ad iabatic shear bands. Also, at strain rates above 3 s(-1) and temperatures a bove 700 degreesC, the material exhibits flow localisation. (C) 2001 IoM Co mmunications Ltd.