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Hot compression tests of a hot isostatically pressed (HIPed) Ni based powder metallurgy (P/M) superalloy were carried out under various combinations of temperatures and strain rates. To bridge the relationship between stresses and strain rates, constitutive equations were established based on a hyperbolic sine Arrhenius equation, which yielded predicted stresses under the test conditions. It was found that the predict values fit the experimental values with good accuracy. Processing maps of the alloy under the test conditions were established; and the corresponding microstructures after test were examined to elaborate the workability of the alloy. It revealed that surface cracks occurred when strain was higher than 0.25, which initiated at the prior powder boundaries (PPBs) and propagated along the boundaries. The optimum hot working parameters for the alloy were proposed to beat the strain rate of 0.014 s
1 and 1075 °C.

I. INTRODUCTION

Ni based powder metallurgy (P/M) superalloys have been extensively used in gas turbine engines due to their excellent combinations of properties, including ultra-high strength, corrosion resistance, and superior fatigue strength at elevated temperature and aggressive environments.1,2 With the development of aerospace technology, gas turbine engine inevitably requests more outstanding performance of turbine disc materials, thus a new Ni based P/M superalloy with higher cobalt (Co) content and minor addition of hafnium (Hf) was devised. Increase of Co content in a certain range was reported to improve phase stability, lower the gamma solvus temperature, and reduce the stress induced by cooling and quenching.3–5 The addition of appropriate amount of Hf was considered to promote grain-boundary effects and formation of cellular c9 precipitation2 as well as enhance the compressive performance of Ni based superalloys.6 Currently, the fabrication of P/M superalloy turbine discs generally involves powder preparation, hot isostatic pressing (HIP), and hot forging (HF). However, the as-HIPed alloys are prone to suffer cracks during hot forging due to the existence of prior powder boundaries (PPBs)7,8 and hot extrusion is considered to be an effective way to break the continuous PPBs and improve the forgeability.7–9 Nevertheless, the superior deformation resistance and inferior plasticity of P/M Ni based superalloys make them

Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.384

extremely difficult to be extruded. In addition, the sensitivity and complexity in microstructure lead to the narrow processing window for forming the superalloy. As a result, the superalloys usually need to be deformed at controllable strain rate, appropriate reduction and temperature over 1000 °C.10 Hot deformation under these extreme conditions would cause catastrophic defects, such as cracks and flow localization, which arose from low processing temperature, large reduction, deformation friction, inappropriate strain rate, or local chill