Deformation characteristics of polysynthetically twinned (PST) crystals during creep at 1150 K

The creep deformation characteristics of a lamellar polysynthetically twinn ed (PST) crystal of the composition Ti-48 mol% Al was investigated as a fun ction of the lamellar orientation with respect to the compression axis and the applied stress. The creep resistance of hard PST orientation with the l amellar plates parallel or perpendicular to the compression axis was substa ntially higher than that of soft orientations with their lamellar plates or iented at intermediate angles to the compression axis. This fact could be a ssociated with the predominant deformation of the hard orientations by defo rmation modes with the slip plane inclined to the lamellar interfaces in co ntrast to the predominant deformation of the soft orientations by deformati on modes with the slip plane parallel to the lamellar plates. In the soft o rientations? mainly straight ordinary dislocations with the Burgers vector b = 1/2[110] aligned parallel to the lamellar interfaces were encountered i n domains with a high resolved shear stress as well as in domains with no r eserved shear stress. In the hard orientations? ordinary dislocations b = 1 /2[110] and superdislocations with a Burgers vector of the type b = 1/2[112 ] were observed. Despite a high resolved shear stress in certain oriented d omains, superdislocations of the type b = [101] were not found to pray a co nsiderable role during creep deformation under the investigated conditions. Cross twinning contributed to the deformation in favourably oriented varia nts, but twinning parallel to the lamellar interfaces was much more pronoun ced in the soft orientations leading to a substantial lamellar refinement. This microstructural hardening during creep results in the observed stress exponents of the soft orientations near unity at high stresses. A change in stress exponent from near unity at high stresses to about 9 at low stresse s occurs at about 200 MPa. A critical stress of about 200 MPa for parallel twinning is proposed as a reason for the change in stress exponent. (C) 200 0 Elsevier Science Ltd. All rights reserved.