AGEING CHARACTERIZATION OF THE POWDER METALLURGY SUPERALLOY N18 Benjamin Flageolet, Patrick Villechaise, Mustapha Jouiad, José Mendez Laboratoire de Mécanique et de Physique des Matériaux ; Ecole Nationale Supérieure de Mécanique et d’Aérotechnique 1 Avenue Clement Ader ; 86960 Futuroscope Chasseneuil ; France Keywords: Ostwald ripening, ’ precipitates, ageing, particles Abstract The ageing process of the powder metallurgy superalloy N18 has been investigated. This alloy exhibits a two-phase microstructure with a long range L12 ordered multimodal ’ precipitation. N18 has been designed for high temperature turboengine disks applications. Superalloys however, are prone to microstructure evolution when sustained at high temperature since ’ precipitates are coarsening. A method based on scanning electron microscope (SEM) image analysis is developed to qualify and quantify the material ageing. Two kinds of ageing have been considered: material temperature exposure with and without mechanical loading application. A short term quantitative law considering precipitates size, ageing time and temperature is established. In addition a long term behavior in agreement with our measurements is suggested. Introduction Turboengine developments have highly enhanced aircraft performances, however gas temperature at the turbine entry remains the key to increased engine efficiency. The material mechanical resistance at high temperature, specially against creep and fatigue clearly remains the technological barrier. Nickel base superalloys have been developed and improved in the last three decades. These alloys are typically used in aeronautic industry for turbine blades and disks. They exhibit high mechanical properties in the temperature range 973-1373K. Because of numerous alloying elements (Al, Cr, Ti, Mo, W...) they combine high creep performance at high temperature and excellent oxidation and corrosion resistance. Their mechanical properties are due to the precipitation of a high volume fraction (~50-70%) of Ni 3 (Al, Ti) type precipitates ( ’), coherent with the matrix. The N18 alloy studied here is a polycrystalline powder metallurgy alloy designed for disks and elaborated by SNECMA Moteurs. This alloy is already used for military applications. In the framework of a program supported by the French research ministry this alloy is being evaluated for civil supersonic aircraft representative specifications. Such an application implies that the material used withstands fatigue loading at higher average temperature than military engines (700-750°C). Besides this, the temperature will be sustained for longer periods of time allowing nearly 30 000 flights (instead of 3000 flights for military engines). In contrast, the effective stress amplitudes applied are supposed to be lower than the one in military aircrafts The evaluation of N18 microstructure evolution during long time temperature exposure and its consequences on mechanical properties and its creep and fatigue durability is one objective of this program. Several works have been performed on the ageing and rafting process of single crystal superalloys such as AM1 or CMSX1. These works have shown that high temperature dwell time leads to ’ phase coarsening and coalescence. However very few studies were focused on the ageing of polycrystalline alloys with multimodal ’ precipitation. The aim of this study is to characterize the evolution of the fine ’ precipitation of N18 superalloy during long term temperature exposure. A special concern is given to the description of the phenomena involved in the ageing process considering various approaches issued from literature. An interpretation concerning the growth kinetic law of the ’ precipitation is proposed. Material The superalloy studied here is a polycrystalline powder metallurgy alloy. The nominal composition is given in Table 1[1]. N18 is hardened by the precipitation of ’ ordered L12 phase. The average grain size is around 15 µm. The standard thermal heat treatment applied to the alloy consists in a homogenisation of 4 hours at 1438K followed by a 50-100K/min cooling step and two final annealing treatments (24 hours at 973K + 4 hours at 1073K) leading to the precipitation of a large volume fraction of ’ particles within the austenitic solid solution. The ordered ’ phase is consisted of 3 different scales of precipitation (Figure 1): -Primary ( ’ I ) precipitation along grain boundaries. The average precipitate size is 2-5µm. -Secondary ( ’ II ) intragranular precipitation. The precipitates are mainly cubic and the mean edge length is 0.2 µm. -Tertiary ( ’ III ) intragranular precipitation. It is the finest population of precipitates. They are spherical and their diameter ranges from 0.02 to 0.05 µm. This population is known to highly influence creep properties. Element Ni Cr Al Ti Mo Co Hf B C Zr Fe Atomic % 54.42 12.3 9.15 5.11 3.77 14.82 0.16 0.083 0.075 0.018 0.11 Weight % 57.05 11.4 4.41 4.37 6.47 15.6 0.52 0.016 0.016 0.03 0.11 Table 1. N18 nominal composition 371 Superalloys 2004 Edited by K.A. Green, T.M. Pollock, H. Harada, TMS (The Minerals, Metals & Materials Society), 2004 T.E. Howson, R.C. Reed, J.J. Schirra, and S, Walston