Creep Deformation of Allvac 718Plus
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I.
INTRODUCTION
ALLVAC 718Plus is a newly developed Ni-base superalloy designed to maintain the processing characteristics of Inconel alloy 718 with a 328 K (55 C) improvement in elevated temperature capability over alloy 718. Alloy 718 use temperature peaks at about 923 K (650 C) due to the fact that it derives its strength from precipitation of c¢¢ rather than the more thermally stable c¢. A complete background on the development history and metallurgy of Allvac 718Plus can be found in Reference 1. In this study, we have focused on the creep deformation characteristics and deformation structures produced during creep over the temperature range of 923 K to 996 K (650 C to 732 C) and applied initial stress levels of 517 to 655 MPa. There are numerous publications within the open literature concerning the creep behavior of both polycrystalline disk alloys, as well as single crystal blade alloys. These two types of materials operate within significantly different temperature regimes and in the present study, we are interested in the types of deformation features found in the lower temperature disk-type alloys. Many of the published superalloy creep studies have attempted to analyze the creep behavior in terms of the classical phenomenologRobert W. HAYES, Principal Investagator, and MARYAM NASROLLAHZADEH, Researcher, are with the Metals Technology Inc., Northridge, CA 91324. Contact e-mail: [email protected] Raymond R. UNOCIC, Alvin M. Weinberg Fellow, is with the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted June 4, 2013. Article published online November 11, 2014 218—VOLUME 46A, JANUARY 2015
ical approach developed for pure metals and Class M solid solutions. This approach defines deformation at elevated temperature in terms of the following form. e_ ¼ ADGb=kTðb=dÞp ðr=GÞn
½1
where e_ is the steady-state or minimum strain rate, A is a constant dependent on structure, D is the appropriate diffusion coefficient and in terms of creep defines the activation energy for the creep process, G is the shear modulus, b is the Burger’s vector, k is Boltzmann’s constant, T is the absolute temperature, d is the grain size, p is the inverse grain size exponent, and n is the stress exponent. For systems deforming by dislocation creep Eq. [1] is generally simplified to the form. e_ ¼ Aðr=EÞn expðQ=RTÞ
½2
where Q is the activation energy for the rate-controlling creep deformation mechanism and all other terms are as defined above. According to the classical theories of creep, there are three well-established mechanisms which can be defined in terms of their temperature dependence, stress dependence, and grain size dependence.[2–4] For example, creep controlled by the sequential glide and climb of dislocations (as initially proposed by Weertman[5,6]) has an activation energy close to that for lattice self-diffusion, a stress exponent ranging from about 4 to 6 and is independent of grain size. Creep controlled by diffusion of atoms through the grains (as initially
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