EVOLUTION OF COMPOSITE PLATE AND LAMELLAR STRUCTURES IN NI RICH NIAL

Composite plate and lamellar structures were cultivated in binary NiAl containing 65.5 at. % Ni by means of heat treatment. This was accompl ished by exploiting the tetragonal martensitic (L1(0)) transformation both as a precursor for a primary peritectoidal transformation, and as a template for secondary precipitation. Specifically, ageing of the m artensitic lath structure allowed the following evolutionary progressi on: (i) Rapid stabilization in the Ni5Al3 regime, (ii) peritectoidal d ecomposition in the beta + gamma' phase field to form a lamellar struc ture above 700 degrees C, (iii) discontinuous coarsening of the lamell ar structure at longer ageing times, and (iv) the ultimate conversion to continuous plate precipitates aligned on the prior lath structure. The existence of a lamellar structure in the Ni-Al binary system has n ot, as far as is known, been reported before. The functionality of suc h composite structures in extrinsic toughening is considered.