Development of cobalt-base alloys for high temperature applications

Abstract

The plastic properties of cobalt base alloys show anomalous behaviour of yield strength around the order-disorder transition temperature. Superior high temperature strength of cobalt base alloys stems from intrinsic character of alloy, which restricts slip and dislocation. Design and development of strong alloys to fulfill technological potential is possible through spinodal decomposition by ternary systems, defect structure formation or lamellar termination densities at eutectic solidification front, and unlocking process on cross slipped (111) dislocation motion under high temperature compression etc. in Co - base alloys. Superplasticity of cobalt base alloys normally varies with temperature, strain rate and initial grain size. The formation of cracks and cavitations is of prime importance during either dynamic recrystallization or grain boundary sliding of superplastic deformation. High temperature performance of cobalt base alloys improves further on formation of microstructures with serrated grain boundaries. Hydrogen embrittlement in cobalt base alloys can not be combatted by boron doping and grain boundary protection. However, an oxidizing atmosphere reduces embrittlement by reducing exposure of bare cracked surface at rupture tip for moisture dissociation by oxidized layer formation. Introduction of iron and vanadium to cobalt base compounds improves boron-doping effects of grain boundary phenomenon and results in increment of hydrogen permeation resistance. Cobalt base multimetallic (CoCrAlY) coatings on superalloy substrates have been well known protection mechanism from high temperature and corrosion resistance. Yttrium forms adherent alumina layer on surface and increases coating performance to a further extent. Coating performance improves on introduction of additional elements to the quaternary composition as for example nickel, zirconium and silicon. The versatility of cobalt base systems around Curie temperature has been studied in thin film morphology of high-density magnetic recording media. Film or multilayer formation by electro deposition technique performs almost similar to sputter deposition techniques.