Pore annihilation was investigated in the single-crystal nickel-base superalloy CMSX-4. HIP tests at 1288 °C/103 MPa were interrupted at different times, then the specimens were investigated by TEM, metallography and density measurements. The kinetics of pore annihilation was determined. The pore closure mechanism was identified as plastic deformation on the octahedral slip systems. A model describing the kinetics of pore closure has been developed on the base of crystal plasticity and large strain theory. Mechanical tests with the superalloy CMSX-4 and the Ru-containing superalloy VGM4 showed, that HIP significantly increases the fatigue life at low temperatures but has no effect on creep strength.
Data about the creep behaviour of metals and their alloys at temperatures close to the melting point are very limited. The reason is that most engineering alloys are used at temperatures below 0.6-0.8 of their melting point, so, Investigation of creep at higher temperatures has no practical relevance. For some special applications however it is important, in our case hot isostatic pressing (HIP) of single-crystal turbine blades cast from nickel-base superalloys. In order to remove porosity the blades are HlPed at temperatures between y'-solvus and solidus where superalloy has no strengthening y'-phase and therefore is very soft. For example, the Company Howmet Castings HIPs the superalloy CMSX-4 at the temperature 1288aC, which corresponds to a homologous temperature of about 0.97=1561 K/1612 K (solidus temperature). Therefore knowledge about the creep behaviour of CMSX-4 at this temperature and understanding of the creep mechanisms are necessary to model the kinetics of pore closure during HIP as well as to plan the Parameters of the HIP process.