<p>Structural metallic materials
exposed to energetic particle bombardments often experience various types of
irradiation-induced microstructural damage, thus degrading the mechanical
properties of the materials in form of irradiation hardening and embrittlement.
Nanostructured materials have shown better radiation resistance than their
coarse-grained (CG) counterparts due to the existence of abundant defect sinks,
such as grain boundaries, twin boundaries, and phase boundaries. However,
recently developed nanocrystalline (NC) steels show limited room-temperature
tensile ductility (< 1%), which may become a concern for their future
application for nuclear reactors. The focus of this thesis is to explore the
strength-ductility dilemma in modified 9Cr1Mo (T91) ferritic/martensitic (F/M)
steel processed by thermomechanical treatment (TMT) and surface severe plastic
deformation (SSPD) with an attempt to fabricate strong, ductile and radiation
resistant F/M steels. </p>
<p><b>Carbon partitioning</b>
between the quenched martensite and the other phases (bainitic ferrite or
retained austenite) is critical for enhancing the strength and ductility of T91
steel. The tensile properties of partially tempered (PT) T91 steel can be
tailored through introducing bainitic ferrite with high-density nanoscale
transition carbides and refined lath martensite. In addition, retained
austenite was introduced by increasing the carbon concentration of T91 steel to
0.6 wt.%. The carbon-modified steel processed by quenching partitioning (Q-P)
treatment exhibits an ultrahigh strength, ~ 2 GPa, with a uniform strain of ~
5% due to the existence of coherent carbides, ultrafine martensite and retained
austenite. </p>
<p>Meanwhile, surface mechanical
grinding treatment (SMGT) on T91 steel reveals that introducing <b>gradient
structures</b> on the sample surface contributes to a higher strength and an
improved plasticity than its homogeneously structured counterpart. The
deformation mechanism of the gradient structures was investigated with the
assistance of quasi <i>in situ</i> crystal orientation analyses. Furthermore, <i>ex
situ</i> He ion irradiation on the gradient T91 steel indicates that
radiation-induced damage, such as bubble-induced swelling and irradiation
hardening, were gradually mitigated by grain refinement from the sample surface
to the center, resulting in superior radiation resistance. The results obtained
from this thesis may facilitate the design and fabrication of strong, ductile
and radiation-tolerant F/M steels.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12719765 |
| Date | 17 November 2022 |
| Creators | Zhongxia Shang (9171533) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/MECHANICAL_PROPERTIES_AND_RADIATION_RESPONSE_OF_NANOSTRUCTURED_FERRITIC-MARTENSITIC_STEELS/12719765 |
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