FRACTURE OF A MANGANESE-MODIFIED TITANIUM TRIALUMINIDE ALLOY

Titanium trialuminides show limited tensile ductility and are known to fail by transgranular cleavage. Examinations of the fracture surfaces of a manganese-modified version of Al3Ti with the L1(2) crystal struc ture shows that fracture normally begins at the outside surface of a s ample by intergranular failure and only later becomes transgranular. T his is interpreted as an environmental influence weakening grain bound aries and initiating cracking. Cleavage cracks propagate subsequently through the grains with limited dislocation emission leading to only m inor stress relaxation. The dislocations produced are APB-dissociated super-dislocations which are mobile in the lattice. The limited extent of crack stress relaxation appears to be due to difficulties of opera ting dislocation sources. Two hypotheses may be suggested to explain t his: the extent of dissociation of the superdislocation partials is in sufficient for the individual dislocations to behave independently and the superdislocation behaves as a single dislocation of large Burgers vector; the core structure of each superpartial is insufficiently dis sociated to allow easy emission of elementary Shockley partial disloca tions. The low ductility of the material may also be related to the ve ry low density of mobile dislocations in the initial material.