A Simple Method for Observing ω-Fe Electron Diffraction Spots from < 112 >α-Fe Directions of Quenched Fe-C Twinned Martensite

In twinned martensite, a metastable hexagonal omega-Fe phase always exists in the twin boundary region of the body-centered cubic (bcc) {112} < 111 >-type twin. The omega-Fe electron diffraction spots at the1/3{112}alpha-Fe and 2/3{112}alpha-Fe positions have been treated as the twinning double diffraction effect previously. The omega-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 omega-Fe diffraction spots from the sum of matrix + twin + double diffraction. Here, a simple method for observing the omega-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 omega-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 omega-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 omega-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.