Superplastic Deformation of α′-Sialon

Abstract

Recent developments show that covalent ceramic materials such as silicon nitride may have superplastic forming (SPF) properties [1,2]. SPF allows large strains to be achieved under a low flow stress, with great potential as an industrial shaping technique. α′-Sialon is a promising material for SPF because of the presence of a transient liquid phase during sintering and its relatively stable grain size. In this paper, the results of an experimental investigation of the deformation behaviour of α′-sialon in compression will be presented. Samples are prepared by reaction sintering of a mixture of Al₂O₃, AlN, Si₃N₄ and Y₂O₃ at 1600 °C, in a 0.1 MPa nitrogen atmosphere, giving a density of 3.15x10³ kg/m³ (96 % TD). Samples are deformed in a graphite-based hot press, under a constant uniaxial stress of 30 to 100 MPa, at temperatures ranging from 1450 to 1600 °C. Upon deformation the material shows a moderate degree of work hardening with a strain-rate proportional to e-0.37±0.06. Variation in the degree of work hardening from sample to sample results in a large spread in the stress sensitivity. Assuming a power law type constitutive creep equation and taking the strain-rate at 0.5 % strain, the stress sensitivity(n) was found to be 0.94 at 1500 °C, 0.56 at 1550 °C and at 1600 °C, n = 0.2. The activation energy for creep is 900 kJ/mol and correlates with the activation energy for Y₃₊ diffusion which may imply that the pressure solution accommodation mechanism is controlled by the transport of this cation. SEM observations indicate show that virtually no grain growth occurs and that the grain shape remains equiaxed.