Figure 1. Widmanstätten pattern in a Gibeon Meteorite (Berahas Etchant, 25X)[1]. Photomicrograph is the exclusive property of Aston Metallurgical Services Company, Inc. and is used by permission of Mr. Alan Stone (all rights reserved).
Count Alois von Beckh Widmanstätten is the man for whom these structures are named, but it was a curious scientist by the name of Thompson who, in the process of cleaning rust from a meteorite, first observed and recorded a strange new structure now most commonly known as a Widmanstätten pattern (Fig. 1).
Formally, a Widmanstätten structure is a microstructure resulting when steel is cooled (e.g., after welding) from extremely high temperatures faster than a certain critical rate. The structure is characterized by a geometric pattern resulting from the formation of a new phase along certain crystallographic planes in the parent phase. In general, it consists of ferrite and sometimes pearlite (Fig. 2) in what is often described as a cross-hatched pattern.
Figure 2. Widmanstätten Structure in SAE 4145H Material. Fine Widmanstätten ferrite and pearlite resulting from a hot forming process conducted well above A3. Islands of pearlite can also be seen.