This document summarizes research on refining the grain size of Alloy 617 through thermo-mechanical processing to enable creep studies. Alloy 617 is being considered for use in intermediate heat exchangers in nuclear plants due to its high temperature strength. The project aims to characterize the creep behavior of Alloy 617 at smaller grain sizes through cold working and heat treating a sample to reduce its grain size from the as-received 110 micrometers to the target size of 20 micrometers, achieving a final size of 5.1 micrometers. The processing establishes a recrystallization regime and hardness increases with smaller grain size due to impeding dislocation motion.

1. Presented: Boise State University Undergraduate Research Conference. (April 2014)
• Alloy 617 is being considered as an intermediate
heat exchanger (IHX) as part of the Next
Generation Nuclear Plant (NGNP) project.
• The alloy was selected due to its high
temperature strength and creep resistance.
• At elevated temperatures, with current grain
size (~110 μm), dislocation creep is the dominate
deformation mechanism.
• The creep behavior of Alloy 617 at smaller grain
sizes has not been thoroughly characterized.
• Project goal is grain size refinement of Alloy 617
through a thermo-mechanical process to enable
creep studies as a function of grain size.
Grain Size Refinement of Alloy 617
1. Introduction
Gary Mitchell1, Lance Patten1, Lejmarc Snowball1
1 Department of Materials Science and Engineering, Boise State University
Ti Kα1
Cr Kα1 Ni Kα1
SEM
10 μm
2. Background
• As grain size is reduced, it is expected that the
dominate creep deformation mechanism
transitions from dislocation to diffusion creep.
• Diffusion creep decreases overall creep
resistance.
• Idaho National Laboratory (INL) is interested in
evaluating the grain size at which diffusion creep
occurs. [2]
• Finding the transition range can provide INL
insight into acceptable grain size specifications.
• Refining grain size causes recrystallization.
• Figure (A) is a schematic depicting the
microstructural progression through recovery,
recrystallization, and grain growth cycles. [1]
• The recrystallization window seen in Figure (A)
has a direct correlation to annealing temperature.
• The targeted grain size is the point at which Alloy
617 fully recrystallizes before the onset of grain
growth.
• An Alloy 617 plate was cold rolled to 50% reduction
in thickness.
• When cold rolling a material, dislocations are
introduced.
• Dislocations act as nucleation sites for eventual
grain growth.
• Grain size was found using the Linear Intercept
Method using ASTM E112-96.
• Increasing cold work in an alloy decreases the
recrystallization temperature.
3. Approach
20 μm 20 μm
4a. Results
(A)
• The recrystallization regime for the
material was established: compare
figure (A) with figure (B).
• Figure (C) is the as-received sample
with an average grain size of 110 μm.
• Figure (D) had a 1,000°C heat
treatment for 1 hour resulting in an
average grain size of 5.1 μm.
• Recrystallization ends and grain growth
begins at approximately 1,000°C.
• Refining the grain size increases grain
boundaries and impedes dislocation
motion throughout Alloy 617.
• At full recrystallization the alloy is
harder than as-received (no cold work).
(D)
7. Acknowledgments
• We would like to thank Chad Watson,
Dr. Richard Wright (INL), Dr. Janelle
Wharry, Joe Croteau, and Dick Sevier for
their help with the project.
• This project was supported by Idaho
National Laboratory.
[1] R. W. J. Messler, The Essence of
Materials for Engineers, Jones &
Bartlett Learning, Massachusetts
2011.
[2] J. Croteau, A. Bateman, Y. Bhetwal, T.
Caldwell, J. Allen, E. Lindau, Boise
State University 2013.
6. References
4b. Results
• Titanium Nitride and Chromium
Carbide precipitates were observed,
seen in the images below.
• Which confirmed that Alloy 617 is
behaving normally.
• As-received material had a grain size of
approximately 110 μm.
• Target grain size was 20 μm, final grain
size is 5.1 μm.
• A grain size refinement method for
Alloy 617 was produced.
• A Hall-Petch relationship between
grain size and hardness will be created.
200 μm
5. Conclusions
4.1 4.2
4.9
4.1
3.7
3.3
2.9
2.6
2.3
2.1
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0 200 400 600 800 1000VickersHardness(GPa)
Temperature (°C)
Post Heat Treatment Hardness,
Vickers Hardness Versus Temperature
Pre-Heat Treatment Heat Treatment No Cold Work
(B)
200 μm
(C)
Recrystallization Zone