Creep of alpha-titanium at low stresses

Abstract

Low-stress creep behaviour of alpha-titanium has been investigated over the range of temperature from 823-1088 K (0.43-0.56 T_m), grain size from 34-443 μm and stresses upto 2.0MN/m². Resistance heated helical specimens were crept under their own weight and the deflections of individual coils were monitored in situ as a function of time by direct observation through a transparent vacuum chamber using a cathetometer. Analysis of the results has been made with the objective of establishing regimes of stress, temperature and grain size within which a specific creep mechanism controls deformation. Coble, Nabarro-Herring, Harper-Dorn and power-law creep mechanisms have been considered. It is shown that low temperatures and fine grain sizes are conducive to the dominance of Coble creep while Nabarro-Herring and Harper-Dorn creep processes respectively predominate in the intermediate temperature-intermediate grain size and intermediate temperature-coarse grain size regimes. In all these situations Newtonian viscous behaviour with negligible threshold stress has been observed. Analysis of the strain rate vs stress data above a transition stress, corresponding to the stress at which an upward deviation from linearity occurs, has established the operation of power-law creep. On the basis of the experimental results, Ashby- and Langdon-type creep mechanism maps have been constructed for alphatitanium.