<p>Gradient structured (GS) metallic
materials have shown unique properties including the synergy of high strength
and good ductility, improved fatigue and wear/friction resistance etc. One of
the severe surface modification technique, surface mechanical grinding
treatment (SMGT), has been proven an effective method for the generation of
gradient structures in metallic materials. Most of Ni-based superalloys are
precipitation strengthened and with an extraordinary combination of high
strength, ductility and resistance to oxidation at high temperatures. The
precipitation behaviors of these materials are sensitive to their initial
microstructures. This thesis focuses on the microstructure evolution and
mechanical behaviors of two types of gradient Ni alloys. </p>
<p>GS Hastelloy C-22HS and Inconel
718 (IN718) Ni-based superalloys were fabricated through the SMGT technique.
The gradient structures consist of nanograined (NG) or nanolaminate (NL)
surface layer and the subsurface layers with deformation twins. <i>In situ </i>compression test results reveal
that intergranular back stress may contributes to the high work hardening capability
of the GS C-22HS alloy. Mo-rich thick grain boundaries (GBs) formed in the
gradient C-22HS samples after heat treatment. <i>In situ</i> micropillar compression studies coupled with molecular
dynamics (MD) simulations reveal that the Mo-rich thick GBs are stronger
barriers than conventional thin GBs to the transmission of dislocations,
leading to significant strengthening. Furthermore, the formation of thick GBs
also contributes to the improvement of thermal stability of nanograins in the
C-22HS alloys. The gradient microstructures in the IN718 alloy changed the
precipitation behavior and thermal stability of nanograins in the alloy. The studies
on precipitation behaviors of GS IN718 alloy reveal that η phase formed in the
severely deformed surface NG layer after annealing at 700 <sup>o</sup>C. Thermal
stability studies show that NG IN718 alloy with grain sizes smaller than the
critical value of ~ 40 nm is thermally more stable than their coarse-grained
counterpart. The underlying mechanisms of strengthening and improved thermal
stability of the gradient Ni-based superalloys are discussed based on
transmission electron microscopy studies and MD simulations. This work suggests
that tailoring the gradient microstructures may lead to the discovery of
metallic materials with novel mechanical and thermodynamic properties. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12202826 |
| Date | 28 April 2020 |
| Creators | Jie Ding (8771438) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Microstructure_and_thermo-mechanical_properties_of_gradient_nickel_alloys/12202826 |
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