Non-stoichiometric magnesium aluminate spinel: microstructure evolution and its effect on properties

Abstract

Magnesium aluminate spinel is a material of interest for transparent armor applications. Owing to its unique combination of transparency to large portions of the electromagnetic spectrum and mechanical robustness, spinel is among the front runners for applications including transparent armor windows for military vehicles and space craft windows, missile radomes, and infrared windows. However, failure in such applications may lead to severe outcomes, creating motivation to further improve the mechanical reliability of the materials used. In this thesis, potential toughening mechanisms that utilize unique control over the evolution of second phase particles are explored. Al-rich spinel (MgO•nAl2O3) with a composition of n = 2 is investigated. First, it is demonstrated that precipitation of second phase Al2O3 from single phase spinel can be achieved by modifying the densification routines typically used to produce transparent spinel. Subsequent heat treatments in air and in vacuum result in varying amounts of precipitation, demonstrating that the single phase is stabilized by the creation of oxygen vacancies during densification, and a modified defect reaction for precipitation is proposed. The location of precipitation can be varied by controlling the reintroduction of oxygen, which is beneficial for toughening specific locations of material with complex shapes, such as toughening the surface of a curved missile radome. The fracture toughness ranges from 0.88 – 2.47 MPa√m depending on the local microstructure. Improved toughness within precipitated regions is due to increased crack tortuosity at phase boundaries. However, precipitation from the spinel matrix causes local volume contraction, creating porosity and residual tensile stresses in regions immediately adjacent to precipitated regions. The light scatter caused by porosity is detrimental to the transmission properties of the material, especially for precipitation layers greater than 60 m. The dissolution of second phase Al2O3 particles into a stoichiometric spinel matrix is also investigated. Complete dissolution of all Al2O3 demonstrates the capability to control the size of the second phase particles, limiting light scatter at phase boundaries. Furthermore, dissolution results in compressive, rather than tensile, stresses within the composite material. A maximum toughness of 4.34 MPa√m was measured in the two-phase composite compared to 2.26 MPa√m once complete dissolution had occurred. However, the toughness of the dissolved specimen is still an improvement from 1.72 MPa√m measured for single-phase, Al-rich spinel of the same overall composition as densified by traditional methods. The observed enhancement in toughness is attributed to a combination of residual stresses that arise from the coefficient of thermal expansion mismatch between particle and matrix, crack deflection caused by second phase particles, and the volume expansion as Al2O3 dissolves into the spinel matrix.