Ordering, deformation and microstructure in L1(0) type FePt

The long-range order (LRO) parameters for Ll(o) FePt have bren determined b y the X-ray diffraction method from powder specimens as functions of time a nd temperature from 773 K to near 1573 K. By and large, the ordering rakes place rapidly below T-c and reaches as high as 0.85 even at 773 K in the fi rst 30 min. The LRO value is about 0.81 near T-c (1573 K) before it drops a bruptly to zero at 1573 K. As a result, the order-disorder transformation i n FePt is concluded to be a first-order phase transformation. Deformation b ehavior in an Ll(o) type FePt alloy was investigated through both compressi ve and tensile deformation from room temperature (RT) to 1073 K. The negati ve temperature dependence of yield stress in this alloy contrasts with the positive dependence in Ll(o) type TiAl. The elongation increases exponentia lly with temperature and reaches similar to 6% at 873 K. The strain rate se nsitivity parameter against temperature is similar to those found in silver and copper, where the non-zero minimum is centered in a broad basin. This indicates that the temperature-dependent deformation in the range of RT to 1073 K is analogous to that of some face-centered cubic metals, but signifi cantly different from that of Ll(o) TiAl. The deformation structure investi gated by TEM shows that slip and tu inning are the two major deformation me chanisms. The identified slip systems include 1/2[110]{111}: [101]{111} and 1/2[112]{111}. The 112[112]{111} slip system, however, is only active at v ery low temperatures, e.g. 77 K. The twin system was identified as {111}[11 2] type. No pseudo-twinning was found in this alloy. The deformation below RT is mainly carried out by both superdislocations and ordinary dislocation s, while above 673 K, it is carried out mainly by ordinary dislocations. Th e morphology of these dislocations in the entire temperature range indicate s that the dislocations do not experience a high Peierls stress contrary to that observed in TiAl. No self-dissociation of superdislocations or APB cr oss-slip onto cube planes was observed under weak beam condtions. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserv ed.