Ordering, deformation and microstructure in L1 0 type FePt

Abstract

The long-range order (LRO) parameters for L1 0 FePt have been determined by the X-ray diffraction method from powder specimens as functions of time and temperature from 773 K to near 1573 K. By and large, the ordering takes place rapidly below T c and reaches as high as 0.85 even at 773 K in the first 30 min. The LRO value is about 0.81 near T c (1573 K) before it drops abruptly to zero at 1573 K. As a result, the order–disorder transformation in FePt is concluded to be a first-order phase transformation. Deformation behavior in an L1 0 type FePt alloy was investigated through both compressive and tensile deformation from room temperature (RT) to 1073 K. The negative temperature dependence of yield stress in this alloy contrasts with the positive dependence in L1 0 type TiAl. The elongation increases exponentially with temperature and reaches ∼6% at 873 K. The strain rate sensitivity 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 significantly different from that of L1 0 TiAl. The deformation structure investigated by TEM shows that slip and twinning are the two major deformation mechanisms. 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 very low temperatures, e.g. 77 K. The twin system was identified as {111}〈112] type. No pseudo-twinning was found in this alloy. The deformation below RT is mainly carried out by both superdislocations and ordinary dislocations, while above 673 K, it is carried out mainly by ordinary dislocations. The morphology of these dislocations in the entire temperature range indicates that the dislocations do not experience a high Peierls stress contrary to that observed in TiAl. No self-dissociation of superdislocations or APB cross-slip onto cube planes was observed under weak beam conditions.