INTERACTION OF DEFORMATION TWINS WITH ALPHA(2) PLATES IN GAMMA-TIAL BASE ALLOY DEFORMED AT ROOM-TEMPERATURE

Intersections of deformation twins with alpha(2) plates in a two phase gamma-TiAl alloy deformed at room temperature have been investigated in detail by transmission electron microscopy (TEM). It has been found that two types of configurations, depending on the orientation of the intersection line, should be distinguished. For a type-I intersection with the intersection line parallel to [110](gamma), the deformation twins are blocked by the alpha(2) plate. At these intersections ordina ry glide dislocationswere emitted back into the gamma matrix on cube s lip planes, together with the production of twinning in gamma parallel to the gamma/alpha(2) interface. No shear transfer across the gamma/a lpha(2) interface was observed in this case. For a type-II intersectio n with the intersection line parallel to [011](gamma), the incident tw inning shear is usually accommodated by a-type slip on {<1(1)over bar 00>} planes in the alpha(2) plate. In one particular case, {<2(2)over bar 01>} twinning was observed in the alpha(2) plate at the intersecti on. The observations were analysed by taking into account all possible dissociations of a twinning partial (or a number of them) into two gl issile dislocations, or into one glissile dislocation (glissile in mat rix or plate) and a residual interface dislocation. This was done by c onsidering a number of conditions for dissociation of twinning partial s at the intersection and subsequent relaxation by slip. These conditi ons are similar as reported earlier for twin-twin intersections in the gamma phase, and for slip transfer at grain boundaries in metals and alloys. The analysis showed that for a particular dissociation not all conditions could be fulfilled at the same time. The combination of th e analysis and the observations showed the following order of priority of conditions: (i) a common line of intersection with the gamma/alpha (2), interface of the incoming twinning plane and the relaxation slip planes; (ii) the slip directions of the glissile product dislocations are consistent with the applied load, which was checked by taking into account the sign of the Schmid factor S; (iii) a zero or:small residu al interface dislocation, and (iv) a high value of \S\ (sometimes S wa s low).