Graduate Thesis Or Dissertation
 

The Effect of Isothermal Aging on the Development of Long-Range Order in Ni-based Alloys

Public Deposited

Downloadable Content

Download PDF
https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2n49t9400

Descriptions

Attribute NameValues
Creator
Abstract
  • Nickel (Ni)-based alloys with a high chromium (Cr) content are frequently employed in applications where elevated temperatures and/or corrosive environments are present due to their excellent corrosion resistance and mechanical properties. However, a gap exists around the formation, and kinetics of long-range order (LRO) Ni2Cr phase in both model binary Ni-Cr and ternary Ni-Cr-Fe alloys, as well as Ni-based commercial alloys, which may impact these desired properties. The transformation rate and impact on material properties of LRO in Alloy 690 (58min wt. % Ni - 27-31 wt. % Cr - 7-11 wt. % Fe), an alloy frequently used for the steam generator tubing in pressurized water reactors is of specific interest to this gap with respect to the nuclear industry. In this research, both binary and ternary model alloys will be studied to address the formation, mechanical behavior, and kinetics of LRO in Ni-based alloys. The following materials are researched; the commercial alloys are Alloy 690, Alloy 625, and Alloy 625 plus, the model binary Ni:Cr with atomic ratio shown and ternary Ni-Cr-Fe based alloys include: Ni:Cr 1.8, Ni:Cr 2.0, Ni:Cr 2.2, Ni:Cr 2.4, Ni:Cr 2.0 +5 Fe, Ni:Cr 2.0 +7 Fe, Ni:Cr 2.0 +10 Fe, Ni:Cr 2.2 +5 Fe, Ni:Cr 2.2 +10 Fe, Ni:Cr 1.8 +5 Fe, Ni:Cr 1.8 +10 Fe. Long term isothermal aging at temperatures 330, 360, 373 (binary only), 418, and 475 °C has been performed on these materials to address this gap. All alloys have been isothermally aged up to at least ~10,000 h. The four binary model alloys are water quenched post isothermal aging and the eleven ternary model and three commercial alloys are air cooled. Characterization of the effect of isothermal aging will be quantitatively and qualitatively compared between As-Received samples, the control group, and samples after isothermal aging experiments. Vickers microhardness XXXHV5/15 indentation measurements were used as an indirect quantitative method to gage LRO formation characteristics. Indentation testing results on alloys reveal increases in the measured hardness values, in general the trends are: 475>418>360>330 °C, stoichiometric>non-stoichiometric, 0>5>7>10 wt. % Fe, and binary>ternary>commercial alloys. Synchrotron X-ray diffraction (XRD) measurements were used as a direct quantitative and qualitative method to characterize the phases present in alloys and their evolution after isothermal aging experiments. Synchrotron-based XRD results of alloys reveal matrix lattice contraction and formation of Ni2Cr in model binary alloys. Our results highlight that changes in hardness correlate linearly with Ni2Cr precipitate size rather than phase fraction. We find that a precipitation hardening model for critical resolved shear stress with weakly coupled dislocations shows good agreement with the material property changes quantified from experimental measurements. The results show a clear link between Ni2Cr precipitate size and hardness. This trend held across all alloys with Ni2Cr formation regardless of Fe concentration. This important structure-property relationship can potentially help define Ni-Cr-Fe-based component lifetimes directly through an understanding of how Ni2Cr formation impacts mechanical properties as a function of Fe content. These findings offer key insight to the research community and warrant further investigation into the formation, mechanical properties, and kinetics of LRO in the Ni-based alloy.
License
Resource Type
Date Issued
Degree Level
Degree Name
Degree Field
Degree Grantor
Commencement Year
Advisor
Committee Member
Academic Affiliation
Rights Statement
Publisher
Peer Reviewed
Language

Relationships

Parents:

This work has no parents.

In Collection:

Items

Thumbnail Title Date Uploaded Visibility Actions
file details AerneNicholasSJ2023.pdf 2023-07-03 Public Download