Phase Transformations

Phase Transformations

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Phase Transformations play a significant role in human daily life, as they represent the most applied technical process to improve macroscopic properties of functional materials such as mechanical hardness, electrical conductivity and magnetic behavior.

 
 
Figure 1: Field ion micrograph of a Cu-0.7 at % Fe alloy after heat treatment showing the precipitation of the thermodynamic equilibrium Fe-rich phase (arrow points toward the brightly imaging Fe-rich phase). The TAP-analysis volume is reconstructed in a 3D-image (a) shows the distribution and number density of the precipitates. The application of special data evaluation algorithm enables the unprecedented identification and analysis of these precipitates in (b).
 
 
Thermal ageing after or during casting of an alloy, for example, is the traditional procedure (hot or cold extrusion) to strengthen materials. In particular, the precipitation of a second phase in a cast alloy affects its mechanical strength to a large extent. However, to understand this mechanism called precipitation-hardening, knowledge of the structure, spacing, size, shape, composition and distribution of the decomposed microstructure, i.e. the decomposition parameters, is essential, in particular in the early stages of phase separation. These phenomena are investigated in our group in super saturated alloys such as Cu-Fe, Cu-Co, Cu-Ti, Ni-Ti, Ti-Al. We are also studying transformation phenomena in metallic glasses such as ZrNiAl and PdNi(Cu)P.
 
           
Figure 2: Field ion micrograph of a Cu-0.7 at % Ti alloy after heat treatment showing the precipitation of the darkly imaging equilibrium Cu4Ti phase (as indicated by the arrows). The reconstructed TAP-analysis volume is shown in (a), and (b) is rotated by 90° clockwise with respect to (a). These images show clearly the preferential alignment of the precipitates along specific crystallographic directions, i.e. in this case the (100) of the f.c.c. lattice.