Creep cavity growth from tritium-induced helium bubbles in nickel
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I.
INTRODUCTION
THEgrowth of intergranular cavities during creep has attracted considerable attention over many years, primarily because the interlinkage of cavities is the dominant failure mode of most metals under creep conditions. This dedicated effort has gradually yielded an improved understanding of the basic processes involved. Attempts to extend that understanding into broadly predictive theoretical descriptions have met with limited success, due in part to experimental and analytical ambiguities resulting from the coexistence of nucleation and growth processes. Cavity nucleation is now known to be continuous ~ throughout creep in most cases, but is not yet well understood and therefore introduces substantial interpretative uncertainty when linked with cavity growth. Efforts to overcome these difficulties in recent years have centered on the concept of bypassing the nucleation step by implantation of "precreep" voids or gas bubbles which then serve as favored sites for cavity growth. Notable examples of this approach are given by Nix and c o - w o r k e r s 2'3'4 using steam bubbles and by Dyson and c o - w o r k e r s , 5'6'7 who introduced voids by a pre-strain/sintering technique. Both of these techniques avoid the complications of radiation damage which attends both ion injection and neutron irradiation. Significant progress has been made using the above methods, but none has proven ideal. For example, the use of steam has been criticized on the grounds that it may significantly modify cavity surface energetics, 8 and cavities formed by pre-strain tend to be irregular in size and distribution and do not fully suppress new nucleation. 9 The approach pioneered in the present investigation, "tritium J. M. MINTZ is Staff Scientist, Sandia National Laboratories, Division 8443, Livermore, CA 94550. A . K . MUKHERJEE is Professor, Department of Mechanical Engineering, The University of California-Davis, Davis, CA 95616. This paper is based on a presentation made in the symposium "Crack Propagation under Creep and Creep-Fatigue" presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee. METALLURGICALTRANSACTIONS A
aging", overcomes most objections by using inert helium as a bubble former. As no high energy particles are involved, radiation damage is also avoided. Two primary mechanisms have been proposed to account for cavity growth during creep. One of these considers plastic deformation to be the dominant process and is thus referred to as "plasticity control." The other is diffusive in nature and may be either "grain boundary diffusive control" or "surface diffusive control," depending on which of these serial processes is rate limiting. Coupled models have also been developed which postulate a role for both primary mechanisms, although for a given set of conditions one only will typically be dominant. An excellent, comprehensive summary o~ proposed models has been given by Cocks and Ashby. The tritium aging technique has been applied
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