DEVELOPMENT OF A NECKLACE MICROSTRUCTURE DURING ISOTHERMAL DEFORMATION AND ITS PROPERTIES RELATIVE TO UNIFORM MICROSTRUCTURES

A necklace microstructure containing a controlled amount of fine (i.e., 4 to 6-mu-m) grains along the boundaries of larger (i.e., 30 to 40-mu-m) warm-worked grains is intended to achieve a balance in mechanical properties for applications such as gas turbine disks. The development of this duplex grain microstructure critically depends on the starting microstructure, the strain and strain rate of the deformation process, and the subsequent heat-treatment conditions. These aspects of microstructural development were studied using isothermal compression tests on powder metallurgy (PM) ASTROLOY consolidated by "hipping" or extrusion. Tensile, creep, and stress-rupture (S/R) properties of the necklace microstructure were also evaluated and compared with those of uniform grain microstructures.