Radiation-induced phase transformations in MgAl 2 O 4 spinel
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Radiation-induced phase transformations in MgAl2 O4 spinel Ning Yu,a) Ram Devanathan, Kurt E. Sickafus, and Michael Nastasi Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 21 March 1996; accepted 17 March 1997)
Ion-irradiation was observed to transform MgAl2 O4 spinel first to a metastable crystalline phase and then to an amorphous phase at cryogenic temperatures. Elastic stiffening of 15% occurred upon formation of the metastable crystalline phase. A second transformation from the metastable crystalline spinel to an amorphous state was accompanied by elastic softening of 25% relative to unirradiated spinel. This phase transformation behavior in spinel appears to be different from that in intermetallic compounds where only elastic softening associated with radiation damage accumulation is observed. A two-stage radiation damage model is proposed to explain the observed phase transformations.
I. INTRODUCTION
Radiation-induced phase transformations are of particular importance because radiation provides a unique means to understand driving forces for solid-state phase transformations. Based upon an elastic instability argument, Cahn and Johnson1 suggested that solid-state amorphization should have a great similarity to the melting process. The suggestion was supported by the observation of Rehn et al.2 that a softening of the shear modulus by 50% accompanies ion-beam-induced amorphization in an intermetallic compound, Zr3 Al. Okamoto, Meshii, and Lam3 further developed a unified approach to solid-state amorphization and melting with a generalized form of the Lindemann melting criterion. The model indicated that solid-state amorphization can occur below the glass transition temperature as a result of accumulation of effective lattice displacement: point defects and chemical disorder. The molecular dynamics simulation by Devanathan et al. and Lam et al.4 further demonstrated the validity of the model in several intermetallic compounds in the Ni –Zr and Cu–Ti systems. The elastic softening accompanying amorphization has also been observed in ceramic materials, e.g., a –Al2 O3 .5 Many insulating ceramics show great resistance to amorphization under irradiation. In particular, MgAl2 O4 spinel exhibits impressive resistance to void formation and amorphization to 250 displacements per atom (dpa) under fast neutron irradiation6–9 and ion irradiation10 at elevated temperatures (.300 K). However, some recent studies11,12 have clearly demonstrated that spinel can be amorphized by ion irradiation at cryogenic temperatures where point defect mobility a)
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J. Mater. Res., Vol. 12, No. 7, Jul 1997
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is minimized. In the present study, we demonstrate that spinel undergoes a two-stage phase transformation process under
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