2018 Volume 58 Issue 1 Pages 159-164
In twinned martensite, a metastable hexagonal ω-Fe phase always exists in the twin boundary region of the body-centered cubic (bcc) {112}<111>-type twin. The ω-Fe electron diffraction spots at the 1/3{112}α-Fe and 2/3{112}α-Fe positions have been treated as the twinning double diffraction effect previously. The ω-Fe spots fully cover the spots of the bcc matrix, twin and their double diffraction. Due to this, it is difficult to practically distinguish the ω-Fe diffraction spots from the sum of matrix + twin + double diffraction. Here, a simple method for observing the ω-Fe spots is introduced based on the twinning crystallographic analysis.
In this method, at first a [011] zone axis is found in twinned martensite, containing the diffraction spots of twin and ω-Fe (previously double diffraction spots). It is then confirmed that the twin plane is inclined to the incident electron beam by means of the dark field observation. The reciprocal <222>* direction (containing spots at 1/3{222} and 2/3{222} positions), is noted. A tilting is then performed keeping this direction un-tilted, i.e., tilting about this direction, to <112> zone axis. This requires about 30° tilting. If the ω-Fe spots are absent at the 1/3{222} and 2/3{222} positions when the zone axis reaches <112>, then an opposite tilt is performed (since there are two tilting directions: clockwise and counter-clockwise), then, the ω-Fe diffraction spots can be seen at the 1/3{222} and 2/3{222} positions. A large twinned martensite at the TEM specimen edge is better for tilting to avoid any overlapping.
