Grain size effect on the mechanical properties of transparent spinel ceramics

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel

“…The fracture toughness of spinel varies between ~1.4–2.0 MPa·m ½ , with an apparent Hall–Petch dependency for coarse grains (greater than ~50 μm) . Lower toughness has been correlated with intergranular fracture (Fig.…”

“…Thus, strengthening grain boundaries should improve toughness. Strength decreases for intergranular fracture that persists in coarse‐grained microstructures . In fact, weak boundaries of large grains have been proposed to serve as critical surface flaws .…”

Grain boundary strengthening in nanocrystalline zinc aluminate

“…For instance, MgAl 2 O 4 has been the focus of several works targeting reduction of grain sizes to examine the effect on hardness. [3][4][5][6][7][8] Sokol et al reported a normal Hall-Petch relation when grains are reduced from micrometer sizes down to 17 nm, below such size, an inverse relation is observed, with a clear hardness reduction from 20.0 to 17.6 GPa. 6 Ryou et al have reported an increase in hardness with decreasing grain size for the same composition, reaching the maximum at a fairly similar grain size as that reported by Sokol et al,18.4 nm.…”

Enhanced fracture toughness in nonstoichiometric magnesium aluminate spinel through controlled dissolution of second phase alumina

“…In the absence of porosity and second phases, the toughness of spinel is relatively low, typically between 1.8 and 2.0 MPa√m. The utility of transparent spinel could be greatly improved if the toughness and subsequently, the strength, could be increased; typical strengths currently range from 250 to 400 MPa . One strategy to increase the toughness of ceramics is to add a second phase to activate mechanisms such as crack bridging, crack deflection, or thermal residual stress toughening, but in order to retain transparency, the size of the second phase must be significantly smaller than the wavelength of the light of interest.…”

“…The utility of transparent spinel could be greatly improved if the toughness and subsequently, the strength, could be increased; typical strengths currently range from 250 to 400 MPa. 4,[12][13][14] One strategy to increase the toughness of ceramics is to add a second phase to activate mechanisms such as crack bridging, 15,16 crack deflection, [17][18][19] or thermal residual stress toughening, 20 but in order to retain transparency, the size of the second phase must be significantly smaller than the wavelength of the light of interest. If the second phase is soluble in the matrix, then it should be possible to decrease its size to a value below the wavelength of light via an appropriate heat treatment that promotes dissolution.…”