K. Morita

A high-strain-rate superplastic ceramic.

High-strain-rate superplasticity describes the ability of a material to sustain large plastic deformation in tension at high strain rates of the order of 10-2 to 10-1 s-1 and is of great technological interest for the shape-forming of engineering materials. High-strain-rate superplasticity has been observed in aluminium-based and magnesium-based alloys. But for ceramic materials, superplastic deformation has been restricted to low strain rates of the order of 10-5 to 10-4 s-1 for most oxides and nitrides with the presence of intergranular cavities leading to premature failure. Here we show that a composite ceramic material consisting of tetragonal zirconium oxide, magnesium aluminate spinel and α-alumina phases exhibits superplasticity at strain rates up to 1 s-1. The composite also exhibits a large tensile elongation, exceeding 1,050 per cent for a strain rate of 0.4 s-1. The tensile flow behaviour and deformed microstructure of the material indicate that superplasticity is due to a combination of limited grain growth in the constitutive phases and the intervention of dislocation-induced plasticity in the zirconium oxide phase. We suggest that the present results hold promise for the application of shape-forming technologies to ceramic materials.

Enhanced tensile ductility in ZrO2-Al2O3-spinel composite ceramic

Abstract For ZrO 2 –30 vol% Al 2 O 3 -30 vol% spinel, elongation-to-failure is evaluated by measuring the distance between the marker lines placed within the geometric gauge portion of tensile specimen. At strain rates of 10 −2 –10 −1 s −1 , elongations exceed 2000%; a maximum elongation of 2510% is recorded at a strain rate of 0.085 s −1 . Cavitation is significantly suppressed owing to stress relaxation at grain boundaries, which is caused by dislocation-induced plasticity in ZrO 2 grains.

Reply to: Comment on the role of intragranular dislocations in superplastic yttria-stabilized zirconia

Abstract This paper replies to a comment by Balasubramanian and Langdon on our studies of yttria-stabilized zirconia. Their conclusion conflicting with experimental facts may arise from contradictions in their analysis. Microstructural aspects indicate that the deformation occurs through grain boundary sliding rather than interface-controlled Coble creep.

Yield drop in high-strain-rate superplastic deformation of ZrO2-30 vol% MgAl2O4 spinel composite

The origin of a sudden yield drop in a tetragonal ZrO2 dispersed with 30 vol% MgAl2O4 spinel composite has been examined. The present ZrO2-spinel composite exhibits yield drop in superplastic flow at high strain rates of 0.2s−1 or greater, where the flow behaviour is characterized by a stress exponent of about 3.5 and a grain-size exponent of about 1.0. Experimental examination suggests that a sudden increase in the mobile dislocation density within spinel grains is responsible for the yield drop.

Evaluation of the threshold stress for creep deformation in a 3 mol% Y 2 O 3 -stabilized tetragonal ZrO 2 polycrystal

The origin of the increase in the stress exponent from n , 2.0 to with decreasing stress in 3 mol% Y 2 O 3 -stabilized tetragonal ZrO 2 has been examined. The present data show that the increase in the n value arises from the existence of a threshold stress that depends on the grain size and temperature. Careful examination of earlier creep data confirms that evaluation of the threshold stress is sensitive to the accuracy of the creep data and the value of n chosen for the compensation of the data. Inspection of the present results and some recent observations of the deformed microstructure suggests that the threshold stress is associated with intragranular dislocation motion.

Analysis of creep due to grain-boundary diffusion in hexagonal microstructures

For steady-state deformation caused by grain-boundary diffusion in hexagonal microstructures, the stress distribution on grain boundaries and the macroscopic strain rates are analysed by taking the effects of viscous grain-boundary sliding into account. The maximum normal stress and the extent of stress concentration are shown to decrease as the grain-boundary viscosity increases. For infinite viscosity and/or extremely small grain sizes, the distribution of the normal stress becomes uniform on grain boundaries. The strain rates are predicted by both the stress analysis and the energy balance method, and the two strain rates are consistent with each other. The predicted strain rates also decrease as the grain-boundary viscosity increases. The present analysis reveals that the grain-size exponent is dependent on the grain size and the grain-boundary viscosity: the exponent becomes unity for small grain sizes and/or high viscosity, while it is three for large grain sizes and/or low viscosity. Recent experimental observations that the strain rates of nano-sized grain are much lower than those predicted by grain-boundary diffusion are explained by the increasing contribution of viscous grain-boundary sliding with decreasing grain size.

Grain-boundary sliding in elongated microstructures during diffusion creep

In diffusion creep, the contribution of grain-boundary sliding to the overall strain os can be evaluated in arbitrary polycrystals, if the angular distribution of grain boundaries is known. A os value of 0.5 is obtained for two-dimensional (2D) equiaxed microstructures consisting of regular hexagonal grains, equiaxed grains grown from a Voronoi structure or grains having a circular distribution of grain-boundary angles. The os value is also evaluated for uniaxially deformed 2D microstructures, both diffusionally and uniformly deformed. For the former, the deformed microstructure is obtained by the simulation of microstructural evolution in polycrystals with straight grain boundaries. The os value increases gradually with increasing or decreasing strain and is larger in the diffusionally deformed microstructures than in the uniformly deformed microstructures for a given grain aspect ratio. The os value for three-dimensional (3D) polycrystalline microstructures is also obtained from an ellipsoidal distribution of grain-boundary angles. The resultant os value is 0.60 for 3D equiaxed polycrystals and increases gradually with increasing strain.

Rate of diffusion creep accompanied by grain boundary sliding in elongated microstructures

The instantaneous creep rate is evaluated for various two-dimensional elongated microstructures, when deformation occurs by grain boundary diffusion accompanied by grain boundary sliding. The strain contributions of grain boundary diffusion and grain boundary sliding are evaluated separatively, and the total creep rate is obtained as a sum of the two. The analysis for the diffusion on elliptical grain shows an increasing creep rate with grain elongation, while the creep rate for hexagonal microstructures increases or decreases depending on grain orientation. It is shown that the instantaneous creep rate calculated for hexagonal microstructures is reasonably consistent with the reported values in a steady state. The instantaneous creep rate in a polycrystal decreases with grain elongation, but the decrement is small enough to allow the application of the original constitutive equation to diffusion creep without significant modification.

Rate of creep due to grain-boundary diffusion in polycrystalline solids with grain-size distribution

For steady-state deformation caused by grain-boundary diffusion, the macroscopic creep rate is analysed for a three-dimensional polycrystal consisting of space-filling grains, by taking into account the effects of diffusional interaction between grains, viscous grain-boundary sliding and grain-size distributions. For regular polyhedral grains, the grain–grain interactions increase the degree of symmetry of diffusional field, resulting in a decrease of the effective diffusion distance. Meanwhile, both the viscous grain-boundary sliding and the grain-size distribution are found to decrease the creep rate. At decreasing grain sizes, the influence of the viscous grain-boundary sliding becomes increasingly important, which explains the recent experimental observations that the creep rates of nanosized grains are much lower than those predicted by grain-boundary diffusion. On the effect of the grain-size distribution, the upper-bound and lower-bound creep rates are estimated.