Forging behaviour of superalloy compacts and composites

The forging behaviour of hot isostatically pressed nickel-base superalloy (Mar M200) compacts, reinforced with 40% volume fraction of tungsten wires, has been examined and compared with that of the non-reinforced alloy matrix. Subscale forging blanks, 10 X 10 X 15 mm high for the composite and 9.65 mm high by 6.35 mm in diameter for the matrix, were deformed by means of isothermal compression at temperatures in the range from 950°C to 1l00°C ± 2°C and at constant true strain rates between 10- 4 S-1 and 1 S-1 ± 2%. Loading of the composite specimens was transverse to the wires' alignment. It is shown that the alloy matrix flows superplastically above 1000°C for strain rates below 10- 3 s- 1. In addition, compacts prepared from the finer mesh size portion of the alloy powder exhibit lower flow stresses. Under identical working conditions, peak flow stresses for the composite specimens are up to four times those for the non-reinforced matrix. Furthermore, the strain rate sensitivity of the composite is approximately half that of the superplastic matrix. A rationalization of these differences is presented. Forming limit criteria for the composite are examined. A model is considered which predicts an upper bound forming limit beyond which severe damage is introduced into the composite. In the present case, the maximum strain achievable is predicted and shown to be approximately 0.34. At lower strains, formability is shown to be further limited by void nucleation at the tensile poles of the wires, normal to the loading direction. Means of improving the formability, which is controlled by the strength of the wire/matrix interface relative to that of the matrix, are discussed.