Transformation Hardening

The phase diagram of the Zr02-Mg0 system shows, on the Zr02 side, the three polymorphic phases that exist in Zr02 (here, the solid solution of MgO in Zr02 is shown, indicating the three polymorphs of Zr02 at 0 % MgO). 

The elastic modulus (a) and Vickers hardness (b), indented with a 200 N load on polished surfaces, are shown in Fig. 5.55. The lines increase linearly with the increasing fraction of Z1O2. Such a trend may reflect the absence of spontaneous microcrack formation. 

The critical stress-intensity factor, as a function of the total zirconia content, and relative toughness, as a function of the m-Zr02 content, when the t-Zr02 is excluded, are shown in Fig. 5.56. 

The relative increase in toughness during transformation toughening is usually expressed [24] as  

More on the t-Zr02 m-Zr02 Martensitic transformation in TZP ceramics is found in the work of Yin in his discussions on the thermodynamics, crystallography and kinetics of this transformation. The solid transformation in pure zirconia is  

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If local stresses exceed the forces of cohesion between atoms or lattice molecules, the crystal cracks. Micro- and macrocracks have a pronounced influence on the course of chemical reactions. We mention three different examples of technical importance for illustration. 1) The spallation of metal oxide layers during the high temperature corrosion of metals, 2) hydrogen embrittlement of steel, and 3) transformation hardening of ceramic materials based on energy consuming phase transformations in the dilated zone of an advancing crack tip. 

The automotive industry, which turned to laser transformation hardening of engine and other drive train components, is now seriously investigating the solar hardening process,... 

Determination of the temperature distribution induced by laser, electron, or plasma beam sources is relevant in operations such as surface transformation hardening of metals, drilling, cutting, annealing, shaping, and micromachining. Descriptions of beam-generating devices as well as discussions of applications are available [7-15]. 

Nitride layers are produced after treatment in a salt bath or in a gas atmosphere by inward diffusion, usually below 600°C. Unlike transformation hardening, which is effected by lattice distortion as a result of embedded carbon atoms, nitriding and boriding provide the surface with an exceptional increase in hardness by the formation of an intermetallic bonding layer. These bonding layers also increase the general corrosion resistance, but they are so thin that they can break by mechanical point loading. 

Figure 4.17 shows an example transformation performed when HETA is applied to an unprotected code. It presents a program code divided irrto two BBs, from in-stmctions 2 to 5 and 6 to 10 in the hardened code, where the first is of has NT type X and the second has NT type A. The left colrrrrm shows the original program code, while the right colutrm shows the transformed hardened program code. 

Both processes, disorder-order transformation hardening and precipitation hardening, are reversible, that is, the material can be brought back to the nonhardened state by a homogenization heat treatment. With both processes it is mostly referred to as age-hardening. As opposed to these,... 

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