Recent breakthroughs in nickel base superalloys

Abstract
Meanwhile the considerable amount of results acquired since more than sixty years in the study of this class of materials, the pre-eminence of nickel base superalloys in the gas turbine engineering, which is a domain in constant evolution, drives the significant progresses accomplished along the five last years. The knowledge, each day more precisely known, of the working conditions of the parts, the continuous increase of the computer capacity and the progressive sophistication of the designing methods end up in very high levels of requirements, progressively more complex, which induce the major routes of development. The particular accomplishments in three different domains of application will be described in this paper. Significant increases of performance have been acquired on directionally solidified turbine blades by an in-depth investigation of strain and damage mechanisms. Behaviour and ruin models take more and more into account the physical reality at each scale of observation of the crystalline structure response to coupled prompting of creep, fatigue and oxidation. On such a basis, a judicious choice of analytical compositions and overlays, with adapted conditions of manufacture enhance the use capability of the parts. In an other domain, decisive advantage has been taken of the variability of the forged materials devoted to disc applications. From the same chemistry of alloy, it is possible to optimize the structures according to various combinations of local stressing. This approach is funded on the disposal of efficient methods of microstructure prediction in relationship with the local rheological history. Precise control methods taking into account the complexity of the resulting microstructure allow the production of parts associated with narrow limits of life duration. At last, shall be discussed the significance of an adequate control of inclusion distribution and cleanliness in highly stressed superalloys in relationship with fatigue life duration. The problem more specific to superalloys produced by prealloyed powder metallurgy routes shall be analyzed and the solutions adapted to civil engine applications exposed.