Grain size control during solutioning after forging of a powder metallurgy nickel base superalloy
2-year contract, remunerated
CEMEF Mines ParisTech, 06904 Sophia Antipolis
The objective of this work is to unravel the physical mechanisms by which overgrown grains can develop during solutioning after forging in a new powder metallurgy nickel base superalloy. The alloy is aimed at being used in the manufacture of critical turbojet engine components, which implies fulfilling very strict metallurgical specifications. The use of such alloys enables the operating temperature (and in turn energetic efficiency) of the engine to be raised and thus enables fuel consumption savings, with positive impact from the energetic and ecological point of views.
The solutioning treatment performed after forging aims at dissolving γ’-phase precipitates so that they can be formed again during subsequent controlled cooling to achieve optimal mechanical performances. During solutioning, grains become free to grow as the pinning force exerted by the dissolving precipitates vanish. In powder metallurgy alloys, grain growth is nevertheless usually limited by the presence of very small oxide particles at the so-called PPBs (prior particle boundaries, meaning the boundaries between the initial alloy powder particles). Under some circumstances, few grains are observed to grow to a very large extent (up to millimetric sizes, see the micrograph below), whereas the others stay in a typical 20-40 µm range. The aim of this work is to describe the metallurgical mechanisms leading to such very large grains and their dependency on the local thermomechanical history undergone while forging the piece.