In situ Observations of Prismatic Glide in Ti 3 Al
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IN SITU OBSERVATIONS OF PRISMATIC GLIDE IN Ti 3 AI
MARC LEGROS, ALAIN COURET AND DANIEL CAILLARD CEMES-LOE/CNRS, 29, rue Jeanne Marvig, B.P. 4347, 31055 Toulouse Cedex, France
ABSTRACT In situ straining experiments are performed in Ti3 Al, in order to study the glide of 1/3 superdislocations in prismatic planes. Two different dislocation behaviours are observed, corresponding to two different antiphase boundaries (APBs) in two parallel prismatic planes. INTRODUCTION Ti 3A1 with the DO1 9 structure deforms by the glide of dislocations on prismatic, basal and pyramidal planes. Rather little is known about the micro-mechanisms which control dislocation glide and thus the mechanical properties. Deformation properties of pure HCP (hexagonal closepacked) metals have been studied extensively, e.g. in Mg, Be and Ti, but the effect of ordering from HCP on mechanical properties is largely unknown and can be revealed by studying Ti 3 Al. An in situ study of the plasticity of Ti 3AI polycrystals has thus been conducted, and the first results on the prismatic glide are reported in this article. The critical resolved shear stress (CRSS) for prismatic glide has been measured in single crystals by Minonishi 1, and Minonishi, Otsuka, and Tanaka 2. The CRSS is about 70 MPa at room temperature, and decreases with increasing temperature. Stress-strain curves are very similar in form to those of F.C:C metals in stage I deformation. Several microscopic observations (Minonishi et al 2, Thomas, Vassel and Veyssi~re 3, Court, Lofvander, Loretto and Fraser 4) have shown sujuerdislocations dissociated into two superpartials with the same Burgers vector 1/6, separated by an antiphase plane boundary (APB) ribbon in the prismatic plane. The dissociation width ranges between 6 nm and 10 nm. The substructure is dominated either by a high density of edge dislocations 1 or by a high density of rectilinear screw dipoles 2,4. According to Umakoshi and Yamaguchi 5, on the prismatic plane, two distinct APBs (I and H) can be defined according to the position of the corresponding cutting plane. Their energies are expected to be different, and estimates for Mg 3Cd give y1u/yI = 5. More detailed calculations have been performed for Ti 32AI by Cserti, Khantha, Vitek and Pope 6 givingy u/t1 = 9 with 7ju = 101 mJ-2 and 'y = 11.2 mJ- . These authors have also simulated the core configuration of screw superpartials bounding type I APBs. A core extended in the prismatic plane with a secondary spreading in the basal plane has been obtained, leading to a high Peierls frictional stress. It is similar to that obtained by Legrand 7 and Vitek and Igarashi 8 for pure titanium. High frictional forces along the screw orientation may also arise from covalent bonding, according to Court et als 4. Both types of frictional forces are consistant with the observations of numerous rectilinear screw dislocations. They however cannot explain the absence of screw dislocations which has been reported in other cases. The exact mechanism controlling the glide of dislocations in pri
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