On a ferroelastic mechanism governing the toughness of metastable tetragonal-prime ( t ′) yttria-stabilized zirconia

Abstract

This article investigates the toughness of yttria-stabilized zirconia (YSZ) with the tetragonal-prime ( t ′) structure. Such materials are used as durable thermal barriers in gas turbines. Their durability has been attributed to high toughness, relative to materials in the cubic phase field. Based on prior literature, a ferroelastic toughening mechanism is hypothesized and this assertion is examined by characterizing the material in the wake of an indentation-induced crack. Assessment by transmission electron microscopy, Raman spectroscopy and optical interferometry has affirmed the existence of a process zone, approximately 3 μm in width, containing a high density of nano-scale domains, with equal proportions of all three crystallographic variants. Outside the zone, individual grains contain a single variant (no domains) implying that the toughening mechanism is controlled by domain nucleation (rather than the motion of pre-existing twin boundaries). The viability of the ferroelastic toughening mechanism is assessed using a process zone model that relates the observed toughening to the stress/strain hysteresis accompanying domain formation. Based on the measured process zone size, the known tetragonality of t ′-7 wt% YSZ and the enhancement in toughness relative to cubic YSZ, consistency between the model and the observed toughening is demonstrated for a reasonable choice of the coercive stress.

Ferroelastic switching of doped zirconia: Modeling and understanding from first principles

“…Similar mechanisms may also be active in other materials, given that soft vibrational modes and tetragonal-cubic phase transitions are not a unique feature of zirconia. Even more importantly, the discussed dynamics supports the models that ascribe the durability of TBCs to a remarkable toughening mechanism based on ferroelastic switching: The reorientation of tetragonal domains under the stress field of a crack yields a residual stress field that shields the crack tip and increases the work of fracture [2,3]. Our calculations show that doping with YO 1.5 stabilizes the tetragonal structure, but also lifts the degeneracy between the various tetragonal orientations, so that ferroelastic switches may become a viable mechanism to absorb energy during crack propagation.…”

“…Indeed, excess doping eventually leads to stabilization of the cubic form with a much lower toughness [2]. In contrast thereto, recent work involving doping with Y 3+ and Ti 4+ has led to tetragonal alloys with superior phase stability and improved toughness [3].…”

“…Thus, most ZrO 2 materials are doped to control or suppress the transformation, as it is the case for state-of-the-art TBCs based on singlephase tetragonal zirconia stabilized with 8 ± 1 mol % YO 1.5 (8YSZ). The remarkable durability of 8YSZ is ascribed to its superior toughness [2] based on a ferroelastic domain switching mechanism [1,3]. However, 8YSZ is metastable as a single tetragonal phase and eventually decomposes into Y-rich cubic and Y-lean tetragonal domains, the latter susceptible to the disruptive monoclinic transformation [4].…”

“…2(b)]. In particular, this peculiar MEP provides critical insight in the mechanism that governs ferroelastic switching, i.e., the collective reorientation of both the anions and the lattice vectors that hitherto lacked an atomistic description and explanation [2].…”

The tetragonal–monoclinic, ferroelastic transformation in yttrium tantalate and effect of zirconia alloying

“…Ferroelastic toughening of metastable 7YSZ coatings has been demonstrated and is believed to be responsible for the unusual toughness of the 7YSZ coatings. [31] The attainable ferroelastic toughening is proportional to both the tetragonality and the coercive stress [31] so while the tetragonality decreases with increasing zirconia content it remains to be determined how the coercive stress varies and hence the attainable toughness.…”