Twinning in L1o type Fe-Pt alloys

Previously, twin systems of [111] were reported in L1[sub 0] TiAl. No pseudo twinning was observed in TiAl. This is probably due to the fact that the pseudo twins, [111], [211] or [121], result in L1[sub 1] like structure which has a higher energy in TiAl. Nevertheless, it was suggested that the pseudo-twinning might be possible at high temperatures where a disordering occurs and the pseudo-twin energy becomes small. In current experiments, FePt alloy which was deformed at 600 C didn't show pseudo twinning. Since the order-disorder transformation occurs at 1,300 C, it is still likely that a high degree of disordering may occur above 1,100 C. Relatively easy twinning in FePt as compared to TiAl may be explained from two reasons. First, FePt may be characterized as having a metallic bonding while TiAl as having a covalent bonding. Also, the heat of formation indicates that a strong bonding exists in TiAl. The heat of formation for FePt is [minus]25 kJmol[sup [minus]1] (1,123 K) and that of TiAl is [minus]36 kJ[sup [minus]1]. Second, the twinning shear for FePt is 0.63 compared to 0.96 in TiAl. For these reasons, twinning shear in FePt would requires less energy than that requiredmore » in TiAl.« less