Transformation toughening

Transformation toughening increases the crack-growth resistance by producing compressive residual stresses in the material during crack propagation. These are caused by stress-induced phase transformations, described in section 7.2.4. To achieve this, particles are added to the matrix that perform a phase transformation that results in a larger volume of the particles when a sufficient tensile stress is applied. 

Transformation toughening occurs when the zirconium oxide is in the metastable, tetragonal phase. This can be achieved by stabilising the tetragonal phase with another oxide. For example, by adding yttrium oxide (yttria, Y2O3) to zirconium oxide, the transformation temperature can be reduced to 

Additionally, compressive stresses can be imposed during cooling to favour the tetragonal phase energetically. 

Usually, this can not be ensured completely and a small fraction of cubic phase 

If we can induce a phase transformation ahead of a crack tip with the help of applied stress such that the transformed phase is tough, then we can increase the toughness of the material. This mechanism was originally discovered in zirconia [8]. Zirconia at 1 atmospheric pressure undergoes the following transformations  

The tetragonal to monoclinic phase is a diffusionless transformation. It is a shear process and is associated with a large volume increase. This transformation toughens the matrix [6,9]. 

Mechanism of transformation toughening, (a) shows the compressive stress field around crack tip, and (b) shows the stabilized zirconia particles. 

The effect of dilation strain is to reduce the stress intensity at the crack tip. The extent of the decrease in stress intensity is called the shielding factor Kj. This is related to the crack tip stress intensity K p by Equation 15.62. 

is the stress intensity due to applied stress. If the volume fraction of the transformable phase is Vf, and fhe widfh of the zone of transformation is w, then the shielding intensity factor is given by Equation 15.63 [10]. 

As in steels, martensitic transformation t- m is an instantaneous transformation, displacive in nature, which develops when temperature decreases. In pure Zr02, cooling transformation starts at about 950°C (point known as Ms) and reversible heating transformation occnrs beyond 1,150°C (A ). We can summarize the crystallographic aspects by saying that the stractirres t and m derive from the fluorine structure c by various distortions, the most important of which is the one associated with the t- m transition, with a shearing of = 9° parallel to the base plan of the array t to lead to an angle of the monoclinic cell P = 81°. 

Phenomenologically, we can write the toughness of a multiphase brittle material 

These effects result in microplasticity phenomena, which explain a non-hnear stress-strain behavior in the zones close to the transformation (for example, close to the edges of a microhardness indentation). 

c 8 MPam, it is the level of stress indndng the transformation that limits 7f  

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The transformation toughening mechanism has been most successfully exploited in materials where the phase transformation of interest is... 

Transformation toughening ceramics, 5 621-622 Transformed composition space, 22 330-331 Transforming particles ceramic—matrix composite reinforcement, 5 571—572 Transgene expression, controllable, 12 453 Transgenes, 12 452, 453... 

It should be noted that it is possible to produce fully stabilized bodies with much higher fracture strengths than listed here but this requires the use of fine particle size, chemically prepared powders (3). The use of this type of material involves a number of penalties both in cost and processability that may be prohibitive for a high volume automotive application. In addition to the type of partially stabilized body described here, two other basic types of partially stabilized bodies have been reported (4, ). Both are classified as transformation toughened partially stabilized zirconias and involve different processing techniques to obtain a body with various amounts of a metastable tetragonal phase. While the mechanical properties of these materials have been studied extensively, little has been reported about their electrical properties or their stability under the thermal, mechanical and chemical conditions of an automotive exhaust system. 

Honeyman-Colvin, P., Lange, F.F., Infiltration of porous alumina bodies with solution precursors strengthening via compositional grading, grain size control, and transformation toughening, J. Am. Ceram. Soc., 79(7), 1810-1814, 1996. 

N. Claussen, Transformation Toughening , in Concise Encyclopedia of Advanced Ceramic Materials (R. J, Brook, ed.), Pergamon Press, Oxford, Great Britain (1991). 

As mentioned previously, the main body of research on whisker-reinforced composites was concerned with alumina, mullite, and silicon nitride matrix materials. None the less, selected work examined zirconia, cordierite, and spinel as matrix materials.16-18 The high temperature strength behavior reported for these composites is summarized in Table 2.5. As shown, the zirconia matrix composites exhibited decreases in room temperature strength with the addition of SiC whiskers. However, the retained strength at 1000°C, was significantly improved for the whisker composites over the monolithic. Claussen and co-workers attributed this behavior to loss of transformation toughening at elevated temperatures for the zirconia monolith, whereas the whisker-reinforcement contribution did not decrease at the higher temperature.17,18... 

The followers of a phase transformation toughening (PTT) state that the change from a less densely packed crystalline structure (f)) to a more packed one (a) (i) promotes microvoiding in the earliest stages of the deformation and (ii) facilitates plasticity due to its exothermic character [72,78,193]. 

Zirconia Toughened Ceramics. Zirconia particles can be embedded in host matrices to form a variety7 of transformation-toughened... 

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