Flow behaviour of Mar M200 powder compacts during isothermal forging

Abstract

The high-temperature deformation behaviour of Mar M200 superalloy powder compacts, pressed below the γ′ solvus, has been examined. Compression tests were carried out at temperatures between 950 and 1200°C and at constant true-strain rates between 10⁻¹ and 10⁻⁵ S⁻¹. Superplastic behaviour was found at all the temperatures used and at strain rates below 10⁻² S⁻¹. The strain rate sensitivities at low strain rates are in the range 0.4–0.6 while at higher strain rates the exponents fall to between 0.1 and 0.3. In each range the exponent increases with temperature, indicating higher degrees of plasticity. Variations in strain-rate sensitivities are related to the various deformation mechanisms which are thought to contribute to flow. The effects of powder particle mesh size and size distribution on the flow properties of the compacts are also considered. Coarser mesh sizes produce the highest peak flow stresses at all strain rates and temperatures investigated. Compacts made predominantly from fine mesh powder but including the full range of particle sizes produce the lowest flow stresses. The interactions between flow and cracking at prior particle boundaries are discussed.