Flow and fracture behaviour of wire-reinforced superalloy composites during isothermal forging

Abstract

The forging behaviour of hot isostatically pressed nickel-base superalloy composites reinforced with 40% tungsten wire has been examined. Specimens were deformed by isothermal compression at temperatures from 1050 to 1180°C and at constant true strain rates of between 10⁻⁵ and 10⁻¹ s⁻¹. The loading direction was normal to the direction of fibre alignment. Peak flow stresses for the composites were up to four times higher than those for the non-reinforced matrix under similar working conditions. These differences are explained in terms of variations in microstructure and differences in directionality of flow in the two types of material. A forming limit of a geometric nature is described whereby damage is induced beyond a critical strain which depends on the volume fraction of fibres and their stacking arrangement. Damage in the form of voids and cavities was introduced at strains well below this critical strain. This damage occurred as a result of stresses at the tensile poles of the fibres normal to the loading direction and it became more extensive at fast strain rates and lower temperatures. Conditions which minimize the formation of voids at low strains are determined, based on an analysis of peak flow stress data for the composites and the non-reinforced material.