Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Major/Program

Materials Science and Engineering

First Advisor's Name

Arvind Agarwal

First Advisor's Committee Title

Committee Chair

Second Advisor's Name

Benjamin Boesl

Second Advisor's Committee Title

Co-Committee Chair

Third Advisor's Name

Zhe Cheng

Third Advisor's Committee Title

Committee Member

Fourth Advisor's Name

Seung Jae Lee

Fourth Advisor's Committee Title

Committee Member

Keywords

Boron nitride nanotube, Metal matrix composites, Mechanical properties, Composite processing, In-situ mechanical characterization, Aluminum, Magnesium alloy, Titanium alloy

Date of Defense

6-26-2020

Abstract

Lightweight metals, such as Aluminum, Magnesium and Titanium, are receiving widespread attention for manufacturing agile structures. However, the mechanical strength of these metals and their alloys fall short of structural steels, curtailing their applicability in engineering applications where superior load-bearing ability is required. There is a need to effectively augment the deformation- and failure-resistance of these metals without compromising their density advantage.

This dissertation explores the mechanical reinforcement of the aforementioned lightweight metal matrices by utilizing Boron Nitride Nanotube (BNNT), a 1D nanomaterial with extraordinary mechanical properties. The nanotubes are found to resist thermo-oxidative transformations up to ~750°C, establishing their suitability for engineering metal matrix composites. Al-BNNT composites are fabricated by three classes of scalable processing approaches: powder metallurgy, solidification and plasma spray additive manufacturing. These processing techniques unravel metal-nanotube interactions in a vast processing space, such as state of metal (solid versus liquid), the timescale of interactions (10-3 to 10+3 s), range of temperatures (102 to 103°C) and pressures (10-1 to 10+1 MPa). Limited interfacial reactions between Al and BNNT are observed, which improve wetting, capture, bonding and stress-transfer in the composites. Consequently, remarkable mechanical reinforcement is achieved, with ~400% improvement in tensile strength, an order of magnitude jump in hardness, and up to two-fold enhancement of elastic modulus. Nanofiller assisted reinforcement of Magnesium alloys is challenging because of their low plasticity. Therefore, an architected, layered composite of AZ31 Mg alloy and BNNT is engineered to minimize embrittlement. In-situ double cantilever testing reveals effective crack bridging by BNNT, facilitated by reactive interface bonding.

Inspired by the importance of interphases, this work investigates the correlation between deformation mechanisms and chemical make-up of the composites. Ti-6Al-4V alloy is reinforced with BNNT and processed at two different temperatures (750 and 950°C) to induce varying degrees of interfacial reactions. Real-time imaging of deformation, in conjunction with high-resolution chemical mapping, provides insights into the synergistic strengthening of the alloy due to interphases and BNNT present in the microstructure. The holistic understanding of microstructure evolution and mechanics of stress-transfer advanced by this dissertation will be helpful for engineering lightweight BNNT-MMCs with superior mechanical performance.

Identifier

FIDC009007

ORCID

https://orcid.org/0000-0002-8437-5509

Previously Published In

  1. Pranjal Nautiyal, N. Denis, T. Dolmetsch, C. Zhang, B. Boesl, A. Agarwal, Interface Engineering and Direct Observation of Strengthening Behavior in Field-Sintered Boron Nitride Nanotube-Magnesium Alloy Composite, Advanced Engineering Materials, DOI: 10.1002/adem.202000170 (2020).
  2. Pranjal Nautiyal, C. Zhang, A. Loganathan, B. Boesl, A. Agarwal, High-Temperature Mechanics of Boron Nitride Nanotube “Buckypaper” for Engineering Advanced Structural Materials, ACS Applied Nanomaterials 2, 4402-4416 (2019).
  3. Pranjal Nautiyal, C. Zhang, B. Boesl, A. Agarwal, Non-Equilibrium Wetting and Capture of Boron Nitride Nanotubes in Molten Aluminum During Plasma Spray, Scripta Materialia 151, 71 (2018).
  4. Pranjal Nautiyal, A. Gupta, S. Seal, B. Boesl, A. Agarwal, Reactive wetting and filling of boron nitride nanotubes by molten aluminum during equilibrium solidification, Acta Materialia 126, 124 (2017).
  5. Pranjal Nautiyal, A. Loganathan, R. Agrawal, B. Boesl, C. Wang, A. Agarwal, Oxidative unzipping and transformation of high aspect ratio boron nitride nanotubes into white graphene oxide platelets, Scientific Reports 6, 29498 (2016).
  6. Pranjal Nautiyal, C. Rudolf, A. Loganathan, C. Zhang, B. Boesl, A. Agarwal, Directionally aligned ultra-long boron nitride nanotube induced strengthening of aluminum-based sandwich composite, Advanced Engineering Materials 18, 1747 (2016).

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