Processing and irradiation-driven evolution of refractory microstructures

Shah, Nachiket (Nachi)

Permalink

https://hdl.handle.net/2142/132647 Copy

Description

Title

Processing and irradiation-driven evolution of refractory microstructures

Author(s)

Shah, Nachiket (Nachi)

Issue Date

2025-11-21

Director of Research (if dissertation) or Advisor (if thesis)

Krogstad, Jessica

Doctoral Committee Chair(s)

Krogstad, Jessica

Committee Member(s)

• Bellon, Pascal
• Perry, Nicola
• Garg, Nishant

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Spark Plasma Sintering
• irradiation
• ceramics
• microstructure
• nanocrystalline
• irradiation induced grain growth

Abstract

The work presented in this document aims to understand the interplay between microstructural evolution and material performance of refractory metals and ceramics through two lenses: one driven by processing under non-equilibrium conditions, and the other driven by non-equilibrium or extreme environments experienced during service-like conditions (specifically, under irradiation). The broad goal is to understand how underlying mechanisms can drive microstructural development (and subsequently influence material properties and performance) under both these lenses, to help design more robust, optimized microstructures that have the potential to demonstrate superior performance. Note that ‘performance’ is contextualized, and tied to the application space for a given material system. In some cases, performance can be gauged through direct measurement and observation of enhanced material properties– for example, improvements in material properties like density and hardness – as covered in Chapter 2. On the other hand, when studying complex phenomena like irradiation, the “performance” of a materials response is more challenging to quantitatively define, and benefits from the observation of changes in microstructure over the course of external stimulus– as seen in Chapter 3. Thus, the dissertation comprises of two projects, with the focus shifting from microstructural changes driven by processing in Chapter 2, to those driven by service-like conditions in Chapter 3. Chapter 2 covers a project that attempts to leverage advances in materials processing techniques to optimize the microstructure of bulk refractory materials processed through ‘bottom-up’ methods. More specifically, work was done to implement the the Deformable Punch Spark Plasma Sintering (DPSPS) method to densify ultra-fine grained and nanocrystalline tungsten precursor powders into their bulk counterparts, while generally maintaining both precursor particle size as well as achieving appreciable final density. Tungsten demonstrates superior mechanical properties upon grain refinement; however, its inherent material properties and refractory nature make it very challenging to process into desirable microstructures. Tungsten powder precursors require significantly higher temperatures in order to consolidate into a bulk material with high density. Even leveraging additional driving forces of pressure and rapid ramp rate in advanced processing techniques like spark plasma sintering (SPS) generally require temperatures of 1400 ℃ and higher to achieve >95% bulk density. However, tungsten also has a relatively low re-crystallization temperature and exhibits significant grain growth at temperatures of 1100 ℃ and higher - making grain growth in the order of several microns inevitable even at the relatively lower SPS sintering temperatures. In the literature, microstructural control has been achieved via additives in the form of carbides or oxides that can help pin grains, or even via alloying in order to sinter precursors at lower temperatures. In this work however, grain refinement as well as higher densification was attempted to be achieved solely through changes in processing parameters to retain precursor material composition. While the overall methodology implemented under the design of experiments was largely an optimization process, with the primary objective of achieving high sintered density, insights into the mechanisms that may be influencing microstructure evolution under processing were also obtained through analysis of microstructure and measured properties. By varying the sintering parameters of temperature and pressure (and by association the deformation action of the punch), additional driving forces for densification like shear were used to successfully densify tungsten precursors to relative densities of ~ 95%, with significant measured hardness values. Due to these additional driving forces, the higher sintering temperatures were not needed - with a maximum applied temperature of 900 ℃ or 0.3Tm, and the ultra-fine grained character of the tungsten precursor powders was maintained due to no observable grain growth after sintering. Nanocrystalline precursors were also sintered to understand the impact of shear and high pressure on the sintering behavior and microstructural evolution of the densified samples. Thus the first project established that the DPSPS processing technique could be successfully utilized to obtain microstructures with desirable features such as a lack of porosity, as well as small grain size of the densified compact. Chapter 3 covers the second project, which is concerned with the consequences of achieving a desired degree of microstructural tuning - i.e. can the performance of a material be optimized or improved upon, for operation under extreme environments like under irradiation? The same processing tools of DPSPS were also used in this case to densify ceramics; however, via the introduction of a post-sintering annealing step, a nanoporous microstructure was obtained along with retention of nanocrystalline grain boundaries. This microstructure was considered to be highly suitable to test under irradiation-facing conditions. Nanocrystalline materials on account of their higher grain boundary density generally show higher resistance to irradiation driven damage by accommodating or annihilating irradiation induced defects at grain boundaries, resulting in lower defect cluster density compared to coarser-grained microstructures. However, nanocrystalline microstructures are highly susceptible to irradiation induced grain growth, which is observed at temperatures significantly lower than what is required for thermally driven equivalent grain growth. Even ceramic materials show significant grain growth under room temperature irradiation conditions. While the mechanisms behind this behavior are under continued study and debate, the interaction of grain boundaries with the collision cascade generated through high energy ions, as well as irradiation induced defects, are primarily responsible for grain boundary migration and instability. Additional defect sinks in the form of pores or nanovoids have also been shown in the literature to increase the irradiation resistance of the material by reducing the rate of defect accumulation and preferentially annihilating interstitial defects at room temperature. However, like nanocrystalline grain boundaries, their e!ciency reduces on account of dissipating under continuous irradiation conditions. Therefore, the ceramic microstructures generated via DPSPS and annealing can potentially help extend the onset of irradiation induced damage by leveraging both grain boundaries and nanopores as complementary defect sinks. To further understand this behavior, yttria stabilized zirconia (YSZ) was chosen as a model system, due to the extensive data in the literature already available for its response under irradiation conditions, as well as its known susceptibility for irradiation induced grain growth. Additionally, increasing the yttria content in YSZ has been shown to result in greater thermal stability, on account of the reduced surface energies as well as decreased diffusion coefficients. To explore whether these defects could also transfer to extend the stability of the nanoporous microstructure under irradiation, experiments were designed around the irradiation of YSZ samples processed via DPSPS at increasing yttria compositions. Observations from experiments revealed interesting results; although pores and grain boundaries are both considered to be equally e!cient defect sinks, the onset of pore closure was observed at significantly lower irradiation doses compared to the onset of grain growth. Furthermore, pore closure rates in nanocrystalline YSZ was shown to be more dependent on the relative grain size of the material rather than composition. Additionally, no significant changes were observed in the grain growth behavior of YSZ from compositions of 14.8 mol% to 50 mol% YO1.5 stabilized zirconia samples, with 57 mol% YO1.5 appearing to perform slightly better in delaying the onset of grain growth. While these results did not reveal the clear ‘winning’ strategy that could help extend the microstructural stability of these nanocrystalline materials under prolonged irradiation conditions, important trends in microstructural evolution as a function of the variables tested were realized. Future experiments can be implemented to develop microstructures that can not only be tailored to provide additional information about these non-equilibrium processes, but also fine-tune their response to certain irradiation conditions. This can be potentially achieved by making minor changes to both DPSPS and annealing processing parameters. Chapter 4 revisits the conclusions made from both the projects, and provides insights into potential future research directions.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132647

Copyright and License Information

Copyright 2025 Nachiket (Nachi) Shah

Owning Collections