CRITICAL ISSUES OF POWDER METALLURGY TURBINE DISKS

P/M superalloy disks obtain their final strength by appropriate heat treatments; the maximum attainable strength depends on the rapid cooling rate from the solution annealing. A rapid quench of a large disk forging can cause two problems, surface cracking and shape distortion.In the past,many attempts employ the finite element code to model and to predict temperature evolution and induced stress distribution in a large turbine disk. The major difficulty was the correct description of alloy behavior; particularly the thermomechanical properties and the failure criteria of material during the cooling. High temperature fatigue resistance is always the key requirement for disk materials. New methodology of residual life management emphasizes the initiation as well as the propagation of the cracks developed under the service conditions. One of major challenges to P/M superalloys is the time-dependent behavior of fatigue cracking, which relates to the well-known SAGBO (stress-assisted grain boundary oxidation) phenomenon.A great effort has been done to understand the micro-mechanism of time-dependent fatigue crack propagation resulted in the second generation of P/M superalloys. Further improvement on temperature capability of disk alloys at rim area may lead to the idea of dual-property disks.Different grain structures at different portions of a large disk are possible,as the property requirements for different locations are different. This goal is achievable if the thermal history at specific disk locations can be controlled to develop desirable microstructures and properties.Some suggestions on the future direction of research efforts will be discused.

ENVIRONMENTAL DAMAGE OF CAST Ni-BASE SUPERALLOY K17

Rupture and tensile properties of K17 before and after thermal exposure in vacuum,air and hot corrosion environments have been investigated.The results show that vacuum exposure es- sentially does not affect the rupture lives but it results in the decrease of high temperature ten- sile strength and increases of rupture and high temperature tensile ductilities;air or hot corro- sion environmental exposure results in the degradation of alloy:severely reduce the rupture and high temperature tensile properties,hot corrosion environmental exposure being more harmful.The former causes grain boundary oxidation and depletion of Cr,and the later makes the sulphurization at grain boundaries.This results in the concentration of S and de- pletion of Cr at grain boundaries and,consequently,weakens the grain boundaries.

Unusual W-shaped Galvanic Cell Model of ISCC of α-brass in Neutral Mattsson's Solution

Intergranular stress corrosion cracking (ISCC) of α-brass in neutral Mattsson's solution was found to be controlled by an unusual 'W'-shaped galvanic cell whose cathode is the grain boundary oxide film (G.B.0. film) and surface film and the anode is fresh metal at the cracked tip on both sides of the G.B.0. film. Redox reactions involved in the cell have been proposed here. According to this mdel, initidtion of ISCC is caused by the rupturing of surface film along grain boundaries, thus forming a galvanic cell. Propagation of ISCC resulted from alternate advances of G.B.0. film and dissolution on both sides of G.B.0. film caused by the effect of electrochemical reaction. This work developed an effective approach to investigate the embrittlement process at the tip of the crack, by increasing the length of the embrittlement region through constant strain test and distinguishing the morphology and the nature of the corrosion products by optical microscopy and scanning electron microscopy (SEH).