Effect of liquid-forming additives on low-temperature superplastic deformation of alumina

It has recently been reported that in the Y203-stabilized tetragonal zirconia phase (TZP) system the addition of a small amount of transition-metal oxides (e.g. CuO, MnO and ZnO) which form a liquid or amorphous grain boundary phase significantly enhances superplasticity [1, 2]. In CuO-added 2Y-TZP abrupt changes have been found in the stress exponent and activation energy across the melting temperature, which are consistent with the proposed deformation mechanism that the superplasticity in these cases is diffusion-controlled below the melting temperature and is interface-controlled above that [1]. Similar deformation characteristics have also been observed in an 80 wt % 2Y-TZP/A1203 composite doped with liquid-forming additives [3]. The deformation of transition-metals-doped alumina has been studied before [4-10]. Due to the coarse-grained (usually of the order of 10/~m) samples used in these studies, deoformation has always been conducted in the creep range with strain rates <10 -5 s -1 and to small strains. Creep enhancement by the dopants has generally been observed, especially in alumina simultaneously doped with compensating dopants, e.g. Ti4++ Cu 2+ [4], Ti 4+ + Fe 2+ [5] and Ti 4+ + Mn 2+ [6]. The influence of the transition-metal additives in these studies has been interpreted in terms of defect chemistry [10], although liquid phases may be present in systems doped with Ti4++ Cu 2+ [4]. We have recently shown that, when doped with 2 mol % liquid-forming additives containing compensating dopants, a commercial alumina powder can be sintered to a dense ultrafine-grained (0.3/~m) ceramic at a temperature as low as 1070 °C [11]. Such a fine-grained microstructure should enable us to conduct the deformation study in the superplastic flow region at relatively llow temperatures. In this letter we report the superplastic deformation behaviour of low-temperature sintered alumina doped with 2mo1% liquid-forming additives which shows a greatly enhanced superplasticity and a similar temperaturedependent characteristic to that found in CuOadded 2Y-TZP. A commercial high-purity (> 99.99%) alumina powder (TM-D, Taimei Chemicals, Tokyo, Japan) was used as the starting material, which was subsequently doped with a liquid-forming additive (0.9mo1% TiO 2+0.9mo1% C u O + 0 . 1 m o t % B203 + 0.1 mol % MgO). Details of the material preparation were described in [11]. Specimens with densities greater than 98.5% theoretical (which was achieved by sintering at 1070 °C for i h) were used in