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Polycrystalline transparent magnesium aluminate ''spinel'' ceramics were fabricated by hot-pressing and hot isostatic pressing (HIPing) using commercially available MgO and Al 2 O 3 powders. Al 2 O 3 content of spinel was systematically changed that can be expressed as MgOÁnAl 2 O 3 with n ¼ 1:0, 1.5 and 2.0. UV/visible and near-IR wavelength region light reflection and transmission behaviors of the spinel ceramics were quantitatively correlated to their microstructure to account for the optical quality of the fabricated materials. The stoichiometric spinel ceramic with n ¼ 1:0 revealed a relatively poor optical transparency due to pronounced light scattering at the microcracked grain boundaries with a specular light transmission of $20{40% in the visible wavelength range. On the other hand, Al 2 O 3 rich compositions revealed a specular transmission of $40{60% in the same wavelength range with a high degree of transparency. Additionally, effect of chemical composition on the fracture toughness of spinel ceramics was investigated applying indentation and chevron notched specimen fracture toughness measurement techniques. The spinel ceramic with n ¼ 2:0 revealed the highest fracture toughness with a mean value of $2:02 MPaÁm 1=2 . Based on their optical and mechanical properties, potential of Al 2 O 3 rich non-stoichiometric polycrystalline spinel ceramics for engineering applications requiring high optical transparency and improved fracture toughness was addressed.

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