Tensile Properties of Ni-Based Superalloy 720Li: Temperature and Strain Rate Effects
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EFFORTS to develop materials demanded by modern turbine engines that can withstand higher tensile, fatigue, and creep stresses at higher service temperatures are mainly centered around nickel-base superalloys because of their excellent mechanical and corrosion resistance properties at high temperatures. Improvisation of chemistry or processing methodology of alloys already in service, making them suitable for equally or more demanding application, is not uncommon. Alloy 720Li is one such advanced high-strength wrought alloy developed in the recent past for high integrity rotating components such as turbine discs by modifying the chemistry of alloy 720, originally developed for blade application in land-based gas turbines. Because of the high alloying element content, initially, alloy 720Li could be processed for turbine disc applications only through powder metallurgy route.[1,2] However, with optimization of melting processes and improved process control during metal working, processing of alloy 720Li through ingot metallurgy (cast and wrought) route is now possible and commercially viable. Hence, alloy K. GOPINATH and A.K. GOGIA, Scientists, are with the Project Office (Materials), DRDO, Hyderabad 500058, India. Contact e-mail: gopinath40@rediffmail.com S.V. KAMAT and R. BALAMURALIKRISHNAN, Scientists, are with the Defence Metallurgical Research Laboratory, DRDO, Hyderabad 500058, India. U. RAMAMURTY, Associate Professor, is with the Department of Materials Engineering, Indian Institute of Science, Bangalore560012, India. Manuscript submitted October 18, 2007. Article published online July 16, 2008 2340—VOLUME 39A, OCTOBER 2008
720Li is now considered to be one of the most advanced disc alloys to be processed through the cast and wrought route[3] for service temperatures in the range of 650 C to 730 C.[4] Processing and heat treatment adopted for alloy 720Li to achieve a fine-grained structure required for high-strength and low-cycle-fatigue (LCF) resistant turbine disc application is significantly different from that adopted for alloy 720 for creep-resistant blade application. While some issues related to processing of alloy 720Li[5] and mechanical properties such as LCF[6–8] and fatigue crack growth rates[7,9] of cast and wrought alloy 720Li have been addressed, not much information on the tensile behavior is available in open literature. In this article, we examine the effects of temperature and strain rate on tensile properties of alloy 720Li to gain detailed understanding of the underlying micromechanisms of deformation.
II.
MATERIAL AND EXPERIMENTS
Alloy 720Li, used for this study, was supplied by M/s Aubert and Duval (Les Ancizes, France) in the form of forged blocks of dimensions 150 9 150 9 100 mm in fully heat-treated condition. The alloy was triple melted sequentially through vacuum induction melting, electroslag refining, and vacuum arc refining and processed through ingot metallurgy route. The chemical composition of the alloy is given in Table I. The heat treatment involved solutionizing and two-stag
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