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Recent Trends in Superalloys Research for Critical Aero-Engine Components

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Recent Trends in Superalloys Research for Critical Aero-Engine Components gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn. RECENT TRENDS IN SUPERALLOYS RESEARCH FOR CRITICAL AERO-ENGINE COMPONENTS Luc Remy*, Jean-Yves Guedou** *Centre des Materiaux, Mines ParisTech, CNRS UMR 7633, B.P. 87, 91003 Evry Cedex, France ** Materials and Processes Department, Snecma, Safran Group, 77550 Moissy-Cramayel, France Abstract This paper is a brief survey of common research activity on superalloys for aero-engines between Snecma and Mines ParisTecb Centre des Materiaux during recent years. First in disks applications, the development of new powder metallurgy superalloys is shown. Then grain boundary engineering is investigated in a wrought superalloy. Secondly, design oriented research on single crystals blades is shown: a damage model for low cycle fatigue is used for life prediction when cracks initiated at casting pores. The methodology developed for assessing coating life is illustrated for thermal barrier coating deposited on AM I single crystal superalloy. Keywords: Nickel base superalloys, microstructure, low cycle fatigue, thermal barrier coatings, damage model 1. Introduction Turboengines for aircrafts represent a major challenge for mechanics and materials (Fig. 1). Major requirements are resistance to long duration operation under severe environmental and thermal - mechanical loading, lightness and reliability. Aero-engine manufacturers have therefore to develop high tech engineering methods, materials and lifing techniques. There is a continued improvement of aircraft engine performance with an increase of turbine entry temperature and pressure ratio. During the last three decades, the increase of Turbine Entry Temperature has been was approximately 15°C per year. This increase in performance is heavily relying on disks and blades in the high pressure turbine that are the most critical parts (Fig. 1). These components are made of nickel base superalloy and the increase in performance has been achieved first through material improvement. These alloys are strengthened by a high volume fraction of"( Nh (fi, AI) precipitates and the continuous trend has been to increase the volume fraction of strengthening precipitates. The content can reach about 0.50-0.55 in powder metallurgy alloys for discs and about 0.7 in cast directionally solidified single crystals for blades. A long cooperation has been developed between SNECMA and Centre des Materiaux, in particular in superalloy research for disks and blades. This has involved alloy development with powder metallurgy alloys for disks, and single crystals for blades, investigation of damage mechanisms in creep, low cycle fatigue (LCF) and thermal-mechanical fatigue (TMF), assessment of constitutive models and damage models over 30 years. This has involved interactions with mechanical engineering groups at SNECMA, ONERA and Mines ParisTech and the materials science group at ONERA. 596 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck a.nd B. Kuhn. SaM146 Figure 1: Aero-engines and a rotor with disk and blades made of superalloys. The present paper gives a brief survey of conunon research activity along these lines during recent years. Alloy development and optimization of microstructures will be first shown with the development of improved PM superalloys and the investigation of grain boundary engineering in alloys for disks. Design-oriented research will be then illustrated with the development and assessment of life prediction models for single crystals blades: low cycle fatigue life prediction for the substrate with cracks initiating at casting pores and prediction of spalling of thermal barrier coatings that are now deposited on blades in advanced high pressure stages. 2. Alloy development and optimisation of microstructnres. 2.1. Powder Metallurgy superalloy for disks. The decrease of NOx and C02 emissions and noise reductions on the short term, and increasing fuel saving levels for optimized acquisition and life cycle costs on the long term are major targets in modem gas turbine engines. These challenges are a major drift for higher overall pressure ratios and turbine entry temperatures. This results in higher temperatures and loadings for long durations for disks in high pressure compressors and turbines. Today such parts are made of a powder metallurgy (PM) superalloy N18 [1]. This alloy has a high volume fraction of"( precipitates (55%) and hence it has a high "( solvus temperature (ST) and a very narrow solution heat treatment window. In addition it is sensitive to quench cracking and prone to TCP (topologically close-packed) phase precipitation during long time exposure at 650°C. Therefore a collaborative program was undertaken between Snecma, ONERA and Centre des Materiaux to develop a new PM superalloy with the following specifications: capability of supersolvus solutioning which implies a reduced "( content, higher creep and fatigue resistance with respect to N18 up to 700°C, increased strain hardening, density lower than 8.35 Kg.dm-3 [2]. The "( content decrease has therefore to be counterbalanced by the improvement of strengthening of both matrix and precipitate. A careful balance between Cr, Mo and W was optimized to achieve matrix strengthening and avoiding TCP phases, using the new PHACOMP method; the Co content was used mainly to decrease the solvus temperature. Then in order to strengthen the"( phase, the ratio (Ti+Nb+Ta)/AI (in at%) was increased from 0.57 in alloy N18 to about 1. The content of minor elements was kept in the 597 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepois, T. Beck and B. Kuhn. range 150-320 wt ppm for C, 150-200 wt ppm for C, 0.3 wt% for Hf and 600 wt ppm for Zr when added [2]. A total of 22 experimental heats were processed with alloys Nl8 and Rem~ 88. Electrodes of each grade were machined from vacuum induction melted ingots of 3.2 kg. Pre-alloyed powders were produced by the rotating electrode process (REP). The phase transformation temperature (solvus, incipient melting) were measured through differential thermal analysis (DTA) analysis. Powders were extruded in subsolvus conditions (ST-25°C). Screening tests consisted in metallography, tensile tests and creep tests. From 22 alloys, 5 remain after these tests and two of these were chosen for more detailed analysis [2]. Alloy Ni Co Cr Mo w A1 Ti Nb Hf B c Zr Nl8REP Bal. 14.9 12.4 3.8 - 9.1 5.1 - 0.13 0.09 0.07 0.02 SM048 Bal. 14.9 12.3 3.6 4.0 3.2 4.4 0.8 0.3 0.01 0.03 - SM043 Bal. 12.2 13.3 4.6 3.0 2.9 3.6 1.5 0.25 0.01 0.015 0.05 Table 1. Chemical composition ofalloys (wt%) Two alloys were selected SM043 and SM048, and provided by Aubert & Duval (composition is given in Table 1). Powders were produced using an argon atomization industrial facility (Aubert & Duval). They were then processed using the industrial route as for Nl8 PM alloy: sieving (<53 Jl.lll), container filling and hot isostatic pressing (1150°C). Two bars were extruded at Snecma quite below the solvus temperature. The amount of"( phase was computed as 43% and 48% in alloys SM043 and SM048 respectively. Alloy density was 8.34 and 8.31 Kg.dm·3 respectively (instead of 8.00 for Nl8). Complete solutioning was possible due to the decrease in solvus temperature. stress MPa 10000 f--- • SM043-&so·c f--- A. SM043-7So•c • SM048-650"C r--- e SM048-750"C N18-650"C 1000 - r- .. -..::: 1--. 100 10 100 1000 10000 Time,h Figure 2: Creep times to 0.2% strain for new PM superalloys compared with alloy N18. 598 gth Liege Conference : Materials for Advanced Power Engineering 2010 edited by J. Lecomte-Beckers, Q. Contrepoi.s, T. Beck and B. Kuhn. ~ Quasi-cleavage Pseudo-stress amplitude, MPa ,. 1-400 r-+-il-+tttt11r-+-t-t+H+Hr---i SM043 SM048 N18 1200 +--+-t-ffittt+l--t-+i-+tH+t----i I • 100) +-"""tod-ffittt+l--t-+i-+tH+t----i Inclusion - -' (". 10000 1(XX)OO 100)00) Number of cycles ' Figure 3: Low cycle fatigue at 650°C of new PM superalloys compared with alloy NIB. Initiation sites by quasi-cleavage or at ceramic inclusions (SM043 ). SMO alloys with a medium grain size (45-60 ~)exhibit similar tensile properties with fine grain size N18. However tailored chemistries for matrix strengthening, and medium grain size result in a major improvement in the creep times to rupture and even more in creep times to 0.2% creep strain (Fig.2). This yields a 100°C increase in temperature capability. Fatigue properties are of major importance for disks, but PM superalloys are known to be very sensitive to inclusions [3, 4] which requires good fatigue crack growth resistance [5] for damage tolerance and to increase inspections intervals. Only a limited number of LCF tests have been completed on smooth specimens under strain controlled conditions (zero to maximum strain, at 0.5Hz). At 650°C SM043 exhibits a higher life than N18 (average): crack initiation often occurs in grains along quasi-cleavage facets and less often on ceramic inclusions (Fig.3). These alloys seem to have a reduced sensitivity to inclusions as compared to alloy N18 but this has to be confirmed on a large number of specimens. Fatigue crack propagation remains fairly good, nearly as good as for alloy N18 [2, 6]. The LCF resistance is the major criterion for disk life. Therefore the alloy SM043 was selected as the new superalloy for disks. An extensive study in underway to optimize the microstructures and fatigue properties, both LCF and fatigue crack propagation of this alloy. 2.2. Evaluation of Grain boundary engineering in alloys for disks.
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