Transformation Plasticity of CeO₂‐Stabilized Tetragonal Zirconia Polycrystals: II, Pseudoelasticity and Shape Memory Effect

Abstract

A macroscopic shape memory effect is demonstrated by first deforming a CeO2‐stabilized tetragonal zirconia polycrystal (Ce‐TZP) between the Mb and As temperatures and then recovering the shape change by heating above Ab. Shape changes effected above Ab are immediately recoverable during unloading, giving rise to a pseudoelastic behavior. Deformation texture is reversible when the shape strain recovers. Both Mb and Ab, along with the associated temperature range for these effects, are depressed to lower temperatures by grain refinement. Prior deformation widens the gap between the transformation temperatures. These observations demonstrate that the shape accommodation in Ce‐TZP arises from twinning and elastic distortions. The operation of pseudoelasticity and shape memory effect is rationalized in terms of martensitic nucleation statistics, the stability of thermoelastic martensite, and internal stresses at the martensitic interface. The implications on transformation plasticity and transformation toughening are explored.

Shape Memory and Superelastic Ceramics at Small Scales

“…S4-S11. Some pillars did experience cracking (see (24) for more details), but only after many more cycles than have been previously reported in polycrystalline specimens (17). (24)) in which initial elastic loading of austenite is followed by forward transformation plateaus during the formation of martensite.…”

“…At strains of only about ~2%, cracking is observed after only a few transformation cycles (17). This is unlike shape memory metals such as Ni-Ti, which can access large strains up to ~8% and, at lower strain levels can be reversibly transformed up to millions of cycles (18).…”

“…Cracking has only been suppressed in prior work by completely eliminating the transformation itself, as for the fully stabilized zirconia ceramics in wide technological usage today (8). Some prior reports have suggested that shape memory properties may be possible in zirconia (17,(26)(27)(28), but these also reported failure at low strains (~1-2%) after only one or a few cycles.…”

In-situ studies on martensitic transformation and high-temperature shape memory in small volume zirconia

“…It is to be noted that there is a slight apparent difference in terms of transformation temperature between the two grains, which may simply be a resolution issue given the sparse nature of our temperature sampling in these experiments. In addition, it is worth noting that the transformation temperatures are sensitive to microstructure; for ceria doped ziiconia, a decrease of~40 C was reported when the grain size dropped from 5 to 0.8 mm [15]. The grains reported in this work are selected to be around 8 mm so that such grain size effects are minimized.…”

Microstructure, crystallization and shape memory behavior of titania and yttria co-doped zirconia

“…9,10 However, there is a general lack of observations of shape memory properties in Y-TZP, although they have been described in the related ceria doped zirconia systems for about three decades. 11,12 Due to the large ionic radius of yttrium, its diffusion in zirconia is slow, and Y-TZP therefore generally enjoys a low grain growth rate. Typical powder processed Y-TZP generally has submicron grains as well as an inhomogeneous Y 2 O 3 distribution.…”